AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
- Joined
- Jan 19, 2014
- Messages
- 24,861
- Reaction score
- 12,209
Ok one more time;
I am not a 318 hater.
Page-1; As for the 318
I get that some of you are on a tight budget, and just wanna use what you got.
So let's say you have a stock low-compression LA, and you just want a modest streeter. Let's say you just freshened it up, with new rings and what-not, and
You measured the flat-top, no-eyebrow pistons, at .054 in the holes =10.6cc;
and, You're gonna use the .028 gasket at 6.8cc, and
your stock unmilled heads are 74cc.
That totals 91.4 cc
And your stock bores come to a swept of 652.27, so
the Scr comes to (652.27 +91.4)/ 91.4= 8.1 good deal.
This combo has no quench, so about the highest Effective-Dcr that you can run with iron heads, still on pump gas, is ~8.0.
So; what Ica on the cam, can we run at say 800ft elevation?
Well it turns out that, even the Ica of the stock cam (Ica of 50*), can only get you 137psi@ E-Dcr of 6.97. This will get you a pretty low VP of 112.
And performance cams don't start until the Ica's get into the high 50s
So lets try an Ica of 58*.
The Wallace comes back with an E-Dcr of 6.72@127psi/99VP... IMO this is a VERY weak bottom end.The engine has lost a chitload of low-rpm power/torque.
The only way past this now, is with higher pressure or a bigger volume. But we're sticking with the 318 for now; and you don't want to put hi-compression pistons into it. and even if you wanted to, the selection is very small. And So then, your choices are;
1) to shave a bunch off the heads, which will require a matching cut to the intake, or
2) get some heads with smaller chambers. So what are you gonna do? Next;
So, that cam with the 58* Ica wants an Scr of 9.8 for an E-Dcr of 7.84 @160psi and a VP of 124. That's the best it can be.
To get to an Scr of 9.8 will require a maximum chamber size of 74.1cc. and less (10.6 deck + 6.8 gasket)= heads of ~57cc. That's doable. So I chose heads.
Now, lets go look at that cam with the 58* Ica on it. Here's just one of many possibilities;
262/270/110 in at 107; I like it.
So to recap, what did we buy? Besides the usual; just a pair of small closed-chamber heads, and a cam-kit.
and the results are;
The Q is now .028 + .054=.082 so just barely out of the danger zone. The pressure is up to 160, The E-Dcr is 7.84, and the VP is 124,about the best it can ever be (at 58* Ica) with this combo. And it will need to run 91gas at WOT.
This is as cheap a build as yur ever gonna find.
Any bigger cam than that 262* is gonna make less pressure, and the engine will start to lose low-end torque, which at a VP of 124, is already pretty low. To get off the line briskly with that small amount of VP, your combo will want at least 3.23s, and I like the 2800TC.
Ok so this is an extra expense. You cannot run the stock ~2200TC with 2.76s forget that. So you are looking at probably OVER $1000 in additional costs to make this a fun combo.
Now,
if you had ordered up some zero deck pistons, your total chamber volume would have been 78.8 with the stock heads, and your Scr would have been
(652.27 +78.8)/78.8=9.28 ........ and you could have run the next smaller cam with the same VP, and have run 89gas at WOT!
So tell me, would custom pistons installed, be more or less money, than the closed chamber heads. I mean in this case it doesn't much matter which way you do it, cuz the end result is close to the same.
Page two; the 360
Ok so now we know that the best VP you can get the 318 up to, with an Ica of 58*, is about 124.
So where does the stock 360 come in at?
it has a swept of 737.21, and the total chamber volume comes in at around IDK say 104cc, so the Scr comes to 8.0 and IIRC the the 2bbl cam is 252/260/112, so the Ica comes in around 54*, and thus, the Wallace kicks out a VP of 119 @131psi/E-Dcr of 6.75.....So....... the bottom-end is already nearly as strong as the hot318 (124VP). The cam is a 252*, versus the 262* in the 318, so the 360 cam is about 1.5 cam-sizes smaller..... but the 360 has 42! more cubes, which is plus13%. This lo-po 360 will still like more than 2.76s but she can run less stall.
Ok so to compare apples to apples lets put those 57cc chambers on this 360 with an .039FellPro gasket, and install that same 262* cam. I get a total chamber volume of about 82cc so the Scr climbs to 10.0
Whoa, the pressure climbs outta sight to 169psi with a very strong, tire-frying VP, of 149. But you cannot run 169psi with iron heads @WOT, on pump gas, so we need less pressure or a bigger cam. So, keeping the pressure to 160, the Wallace spits out an Ica of 64* at 10/1, with a VP of 134. Badaboom.
So to recap; the cam is now two sizes bigger than the 262 in the 318, but the 360 is using the same heads. With a VP now of 134, this is 134/124=8% higher, so you can use either the stock stall with a lil more gear, or the stock gears with a lil more stall.
So now, lets say that with this 10/1 360, you get away with a 2800 and the 2.76s for the same low speed performance as the 318. What are your additional costs?
Again, the 57cc heads and the cam kit. but not BOTH the gears plus TC; only one of. So about $400 to $500 cheaper in the big picture; LESS the cost of a 360 short block.
But hang on this 360 combo is running a cam that is two sizes bigger, and it has plus 42 cubes,................ costs less money and I estimate about 20% more power, with the same typical bolt-ons.....
so you decide;
But hang on; If you are used to your stock 318 at a VP of 112, and you drop in an engine, no matter what it's size, that has a VP of 134, You will be shocked. 134/112 is ~20% more powerful right off the get-go. All other things being equal, that's like swapping in 20% more gear, or from 2.76s to 3.30s, or from 3.23s to 3.88s....
Or running the stall up the rpm band to 20% more torque, say from 240ftlbs to 288.
So just maybe, you can get away with BOTH the stock gears and the stock stall, maybe.
So, IDK if you guys are getting this;
Same money in the engine, but less in the TC/gears, for ~20% more torque off the get-go, and IDK guessing 50/60more hp over the top.
Now if you already own the hi-stall and appropriate gears, well ok, now I get that you gotta buy the 360 short, for additional money. But think about it; shouldn't you be willing to spend $250ish for a good 360short, to get 50/60 horsepower? cuz that's the only additional cost.
And that is the point I'm trying to make; namely $250/50hp= 5 bucks per horsepower! You just can't get any cheaper than that, and you get all the additional benefits like a powerful VP, loafing down the hiway, passing filling stations, and running cheaper gas.
page 3, you don't care about 50/60 hp
Ok then, why are we even talking here
Page 4, you just want a lil more performance
Ok I get that, then just slam a 2800 on the back of the 318crank and some gears in the backend, and yur done. That's the cheapest fastest bestest way to get performance from, not just the 318, but any combo.
The Stall will let the engine slingshot up to a higher rpm where more power is available, and the gears will let the engine run thru the powerband more quickly and more often. This is always a killer starting place.
Lemmee tell you; running 2800TC and 3.55s, even a 2bbl smoggerteen seems pretty peppy.
Lemmee lay it out;
Suppose your 318 puts down 200 ftlbs at a stall of 2200. And suppose you are running a 904/727 with a 2.45 low gear, and 2.76s out back. That means, into the rear axles, the 318 is pushing 200 x2.45 x2.76=1352 ftlbs.
Now suppose you install a 2800 and 3.55s. The new torque at the higher rpm, might be
224 x2.45 x3.55=1948.. That is 44% more torque at zero mph, before adding any TM(Torque-Multiplication) that happens inside the TC. That's mega-sweet.
But you say what about a 4bbl?
What about it? If the tires don't spin, then the 4bbl will take a long time to open. If you open the secondaries too soon, before the engine can use the additional airflow, the only thing that will happen is that all 4 barrels will now share whatever airflow the engine wants, instead of just the two. Even if you time it right, if the tires don't spin, the carb only starts off, by allowing 1 horsepower more, then two, then three and so on, until the peak power is reached, and then the power starts to decline. Suppose that with your 2 bbl/ or primaries only, peak airflow happens at 3000 rpm. With 2.76s, that will be about 29 mph @20% slip. But with 3.55s, maybe 22 mph. So the 4bbl is dormant or at least not making additional power, until the car is moving right along in the 22 to 29 mph range depending on the gears. But the stall and gears start multiplying as soon as the car begins to move.
But you say, what about the headers?
Well firstly they don't make a specific number, their contribution is a percentage of the whole. and they work over a specific rpm band, and depend a whole bunch on the overlap of the cam to do their best work. So at 2200 stall, they are just waking up; and the 2.76s are like the snooze button on your alarm clock; the headers are forced to nap by those 2.76s, until the Rs get up. Those headers are not worth much until the cam starts to wake up, so say 3000rpm again. and the 262/270/110 cam, with it's very modest overlap of 46 degrees, is just beginning to invite the headers, at say 3500.
So then, now you know; if you can't have cubic inches off the line, then the next best thing is to have stall, and then gears.
I got no dog in this hunt. I don't care which option you go with. Analyze my message, and just do whatever seems right for you.
One more thing;
page 4; fuel efficiency
For highest fuel mileage potential , wait;
I gotta tell you something; typically our A-bodies need less than 35hp to blow down the road at 65 mph; some A's are a lil better than others. 35 hp at .5 pounds of fuel/per hp/per hour, is 17.5 pounds of fuel per hour, which at 6 pounds per gallon is 2.9 gallons . At 65 mph this is 22.4 mpg.
If you can do better than that, good for you. At PartThrottle, it's hard to match that.
Now; having said that, the equation to figure this, does not care how big or how small the engine is; it only knows the engine has a carb on it. And it doesn't much care about rpm; except that the engine has to make a minimum of 35 horsepower,lol. So if you gear your engine too slow, the carb might be in danger of going wide open on the primaries, which might or might not be a bad thing, it depends on your carb. But if you gear your engine too fast, then the throttles may be too far closed, and now the engine's internal friction and such gets to cost you money.
So you gotta put the Rs in a sweetspot.
The smaller the engine is, the higher the rpm will need to be to make the 35hp at a reasonable throttle opening.
The bigger the engine is, the lower the rpm can be, and still make the 35 hp at a reasonable throttle opening.
However, when running a performance cam, the rpm window shrinks, and has to increase, to keep the engine above an rpm where the engine becomes inefficient, or where your ignition timing system can no longer supply adequate advance.
Ok NOW back to;
For highest fuel mileage potential, the cylinder pressure needs to be as high as possible and peak combustion pressure needs to occur at a very specific place in the crank's position relative to AFTER TDC.When you get all these things right, you will be rewarded with phenomenal hiway mpgs, well beyond the 22.4 mpgs.
And sooooooooo, consider a minimum overlap, and a loooong power extraction period. These two together point to a small period cam. To get the bottom end back, you need the cubes. To get the top end back, you need the heads, and as much Effective overlap as is possible. But if you have Scr, you can sacrifice some compression duration.
To understand compression duration as it pertains to fuel-economy;
When the engine is throttled, as it is when cruising, the engine cannot draw in a full load of oxygen. So that means that the cylinder pressure will be only get to be a tiny fraction of what it can be, at WOT. But it is the cylinder pressure that drives the car ahead, so the more Scr that you have then the less throttle-opening it will take.
Summary;
Unfortunately, it's very hard to do all these things with one engine. But I have found that the bigger the small block is, the easier it is to hit all the bases. Especially; performance with economy.
I'll guess that street-performance starts with an absolute hp at around 250hp. But that engine needs to have a strong bottom-end, else you have added expenses of stall and gear. and then fuel-economy goes away. To keep the fuel economy up, the bottom-end needs to be VERY strong,and the rpm kept low. To accomplish that will takes cubes and/or pressure. But the 318 is very pressure challenged, principally because of the small amount of swept cylinder volume; 652cc is quite small. And and as the cam gets bigger, the effective stroke shrinks fast, making the swept ever smaller. And so the total chamber volume, compared to the swept gets ever bigger, driving the Dcr ever lower. That's a bad thing. And as the engine moves to a higher elevation, the Effective Dcr falls even lower.I mean these things are the same for all engines; it's just that the smaller your starting place, the more quickly these things happen.
VP
Really, the best way to look at the street engine is by studying it's VP.
Typically, the VP runs from a low of about 100, to a high of around 160. The higher the better for ALL cases, except;
1) if you HAVE to, but don' want to, run alloy heads because of excessive pressure or
2) you can't get up into the high pressure, because your design can't support it or cists ridiculous money, or
3) the number gets so high that your chassis cannot handle it anyhow, and you end up with a tire-fryer.
Ok so here's my opinion on the V/P breakdown;
120 or less is weak
130 is barely adequate
140 is where the fun begins
150 is leaving fun and getting into excessive tirespin
As we have seen;
it's hard to get a 318 with iron heads past a VP of 124. And if you try, it will get expensive in a hurry, because of the lack of high-compression pistons, and/or the cost of all the machining.
Whereas a 360 can be made to practically fall together at nearly any V/P between say 135/iron and 155/alloys.
But you say; where does the 340 fit?
Great question, but, just try and find one. They only had a 5/6-year run, so not many produced, and guys hang onto them dearly. But, IMO, they are the exact right size for a 3400 pound A-body. They are big enough that you can slide a decent cam into without having the bottom end go away, and compared to a 318; you can run a lil less cam, a lil less stall, and a lil less gear. It already has enough head, and already has bigger valves, and those 4.04" holes make room for it all.
Guys say that 340s are nothing but bored-out 318s, and you can put all the 340 stuff onto the 318, and now yur up to speed. Well, IMO, that's like saying when a man and a woman get married, the two become one, and now they can do double what they did before; not even close.
The truth is that whatever you do to the 318, the same doing on a 340 will always be 22 cubes bigger. I mean you can say what you want, to convince yourself the world is a ball, but that don't make it true. And so the 340 will forever, on the street, be that much ahead of the 318.
IMO, the 360 is the ideal A-body engine for the street, but I sure wouldn't lose sleep if I couldn't find a 340. The 340 IMO, is a good bridge.
You gotta remember, that the street is a different world than dragracing, and the two are far apart. On the street, torque is king. Or Torque-Multiplication.
But when hiway travel is involved, you gotta go back to actually having real torque.
And every time you take off from a stop, torque is what gets you moving. Before you go all technical on me, what I mean by torque is low-rpm power. If you don't got some, take-off is sluggish. If you don't got some, passing is sluggish. What good is 300 horsepower at 5000 rpm, when you are taking off from a light at 2000stall? Hmmm.
Now the vast majority of the life of your engine will be, taking off and running up to some Second Gear cruising rpm; am I right? How much time will your street-engine actually spend, during it's lifetime, at 5000 rpm?
You might drive three hundred miles on a weekend, or say 5 hours at 60 mph, or all day at 30 mph; and not accumulate more than a minute or two, actually at 5000 rpm, right? Two minutes at 5 seconds a pop, is 24blasts, during which the tach is NOT actually stuck at 5000 for the whole time either. That comes to one blast for every 12.5 miles driven. That would ,IMO, be insane, so you get what I'm saying. So why are we camming our engines for peak power at 5000 rpm, and sacrificing all other modes, to have it? IDK, it seemed a good idea at the time................ I , personally, pulled that 292 out of my engine within a few weeks, and stuck a cam 3 sizes smaller in my engine, and was way happier............ cuz I had been paying dearly, for having the pleasure of the occasional wide open blast in three gears, far away from the cops. Now I had some killer take-off torque to deal with. But I digress. You might not be like me. You might be willing to drive what amounts to a top-end racer, on the street, over and over to 40, or 50 or 60 mph, burning up all the gas in your tank, time after time after time. Sure that 3500stall, 4.30-geared 318 is a lot of fun. No doubt. But all you can do with it is drive around endless blocks of city streets. You might as well have a slanty under the hood, with a lil nitrous, for when the mood hits.
Most of us streeters, have other bases to hit; specifically touring a couple of hours from home to get some ice-cream, or going to visit a distant family member, or passing the occasional filling station. Or you know; leaving it in Drive for a while, and listening to the tunes.
So 4.30s and a 3500 stall, for most of us, are not on the table; and so, lack of torque comes back to haunt. But you say why not just but a stroker in it then and run straight hiway gears? ...................... That would be a topic for someone else to expound upon, I have no experience with them.lol.
editing
I am not a 318 hater.
Page-1; As for the 318
I get that some of you are on a tight budget, and just wanna use what you got.
So let's say you have a stock low-compression LA, and you just want a modest streeter. Let's say you just freshened it up, with new rings and what-not, and
You measured the flat-top, no-eyebrow pistons, at .054 in the holes =10.6cc;
and, You're gonna use the .028 gasket at 6.8cc, and
your stock unmilled heads are 74cc.
That totals 91.4 cc
And your stock bores come to a swept of 652.27, so
the Scr comes to (652.27 +91.4)/ 91.4= 8.1 good deal.
This combo has no quench, so about the highest Effective-Dcr that you can run with iron heads, still on pump gas, is ~8.0.
So; what Ica on the cam, can we run at say 800ft elevation?
Well it turns out that, even the Ica of the stock cam (Ica of 50*), can only get you 137psi@ E-Dcr of 6.97. This will get you a pretty low VP of 112.
And performance cams don't start until the Ica's get into the high 50s
So lets try an Ica of 58*.
The Wallace comes back with an E-Dcr of 6.72@127psi/99VP... IMO this is a VERY weak bottom end.The engine has lost a chitload of low-rpm power/torque.
The only way past this now, is with higher pressure or a bigger volume. But we're sticking with the 318 for now; and you don't want to put hi-compression pistons into it. and even if you wanted to, the selection is very small. And So then, your choices are;
1) to shave a bunch off the heads, which will require a matching cut to the intake, or
2) get some heads with smaller chambers. So what are you gonna do? Next;
So, that cam with the 58* Ica wants an Scr of 9.8 for an E-Dcr of 7.84 @160psi and a VP of 124. That's the best it can be.
To get to an Scr of 9.8 will require a maximum chamber size of 74.1cc. and less (10.6 deck + 6.8 gasket)= heads of ~57cc. That's doable. So I chose heads.
Now, lets go look at that cam with the 58* Ica on it. Here's just one of many possibilities;
262/270/110 in at 107; I like it.
So to recap, what did we buy? Besides the usual; just a pair of small closed-chamber heads, and a cam-kit.
and the results are;
The Q is now .028 + .054=.082 so just barely out of the danger zone. The pressure is up to 160, The E-Dcr is 7.84, and the VP is 124,about the best it can ever be (at 58* Ica) with this combo. And it will need to run 91gas at WOT.
This is as cheap a build as yur ever gonna find.
Any bigger cam than that 262* is gonna make less pressure, and the engine will start to lose low-end torque, which at a VP of 124, is already pretty low. To get off the line briskly with that small amount of VP, your combo will want at least 3.23s, and I like the 2800TC.
Ok so this is an extra expense. You cannot run the stock ~2200TC with 2.76s forget that. So you are looking at probably OVER $1000 in additional costs to make this a fun combo.
Now,
if you had ordered up some zero deck pistons, your total chamber volume would have been 78.8 with the stock heads, and your Scr would have been
(652.27 +78.8)/78.8=9.28 ........ and you could have run the next smaller cam with the same VP, and have run 89gas at WOT!
So tell me, would custom pistons installed, be more or less money, than the closed chamber heads. I mean in this case it doesn't much matter which way you do it, cuz the end result is close to the same.
Page two; the 360
Ok so now we know that the best VP you can get the 318 up to, with an Ica of 58*, is about 124.
So where does the stock 360 come in at?
it has a swept of 737.21, and the total chamber volume comes in at around IDK say 104cc, so the Scr comes to 8.0 and IIRC the the 2bbl cam is 252/260/112, so the Ica comes in around 54*, and thus, the Wallace kicks out a VP of 119 @131psi/E-Dcr of 6.75.....So....... the bottom-end is already nearly as strong as the hot318 (124VP). The cam is a 252*, versus the 262* in the 318, so the 360 cam is about 1.5 cam-sizes smaller..... but the 360 has 42! more cubes, which is plus13%. This lo-po 360 will still like more than 2.76s but she can run less stall.
Ok so to compare apples to apples lets put those 57cc chambers on this 360 with an .039FellPro gasket, and install that same 262* cam. I get a total chamber volume of about 82cc so the Scr climbs to 10.0
Whoa, the pressure climbs outta sight to 169psi with a very strong, tire-frying VP, of 149. But you cannot run 169psi with iron heads @WOT, on pump gas, so we need less pressure or a bigger cam. So, keeping the pressure to 160, the Wallace spits out an Ica of 64* at 10/1, with a VP of 134. Badaboom.
So to recap; the cam is now two sizes bigger than the 262 in the 318, but the 360 is using the same heads. With a VP now of 134, this is 134/124=8% higher, so you can use either the stock stall with a lil more gear, or the stock gears with a lil more stall.
So now, lets say that with this 10/1 360, you get away with a 2800 and the 2.76s for the same low speed performance as the 318. What are your additional costs?
Again, the 57cc heads and the cam kit. but not BOTH the gears plus TC; only one of. So about $400 to $500 cheaper in the big picture; LESS the cost of a 360 short block.
But hang on this 360 combo is running a cam that is two sizes bigger, and it has plus 42 cubes,................ costs less money and I estimate about 20% more power, with the same typical bolt-ons.....
so you decide;
But hang on; If you are used to your stock 318 at a VP of 112, and you drop in an engine, no matter what it's size, that has a VP of 134, You will be shocked. 134/112 is ~20% more powerful right off the get-go. All other things being equal, that's like swapping in 20% more gear, or from 2.76s to 3.30s, or from 3.23s to 3.88s....
Or running the stall up the rpm band to 20% more torque, say from 240ftlbs to 288.
So just maybe, you can get away with BOTH the stock gears and the stock stall, maybe.
So, IDK if you guys are getting this;
Same money in the engine, but less in the TC/gears, for ~20% more torque off the get-go, and IDK guessing 50/60more hp over the top.
Now if you already own the hi-stall and appropriate gears, well ok, now I get that you gotta buy the 360 short, for additional money. But think about it; shouldn't you be willing to spend $250ish for a good 360short, to get 50/60 horsepower? cuz that's the only additional cost.
And that is the point I'm trying to make; namely $250/50hp= 5 bucks per horsepower! You just can't get any cheaper than that, and you get all the additional benefits like a powerful VP, loafing down the hiway, passing filling stations, and running cheaper gas.
page 3, you don't care about 50/60 hp
Ok then, why are we even talking here
Page 4, you just want a lil more performance
Ok I get that, then just slam a 2800 on the back of the 318crank and some gears in the backend, and yur done. That's the cheapest fastest bestest way to get performance from, not just the 318, but any combo.
The Stall will let the engine slingshot up to a higher rpm where more power is available, and the gears will let the engine run thru the powerband more quickly and more often. This is always a killer starting place.
Lemmee tell you; running 2800TC and 3.55s, even a 2bbl smoggerteen seems pretty peppy.
Lemmee lay it out;
Suppose your 318 puts down 200 ftlbs at a stall of 2200. And suppose you are running a 904/727 with a 2.45 low gear, and 2.76s out back. That means, into the rear axles, the 318 is pushing 200 x2.45 x2.76=1352 ftlbs.
Now suppose you install a 2800 and 3.55s. The new torque at the higher rpm, might be
224 x2.45 x3.55=1948.. That is 44% more torque at zero mph, before adding any TM(Torque-Multiplication) that happens inside the TC. That's mega-sweet.
But you say what about a 4bbl?
What about it? If the tires don't spin, then the 4bbl will take a long time to open. If you open the secondaries too soon, before the engine can use the additional airflow, the only thing that will happen is that all 4 barrels will now share whatever airflow the engine wants, instead of just the two. Even if you time it right, if the tires don't spin, the carb only starts off, by allowing 1 horsepower more, then two, then three and so on, until the peak power is reached, and then the power starts to decline. Suppose that with your 2 bbl/ or primaries only, peak airflow happens at 3000 rpm. With 2.76s, that will be about 29 mph @20% slip. But with 3.55s, maybe 22 mph. So the 4bbl is dormant or at least not making additional power, until the car is moving right along in the 22 to 29 mph range depending on the gears. But the stall and gears start multiplying as soon as the car begins to move.
But you say, what about the headers?
Well firstly they don't make a specific number, their contribution is a percentage of the whole. and they work over a specific rpm band, and depend a whole bunch on the overlap of the cam to do their best work. So at 2200 stall, they are just waking up; and the 2.76s are like the snooze button on your alarm clock; the headers are forced to nap by those 2.76s, until the Rs get up. Those headers are not worth much until the cam starts to wake up, so say 3000rpm again. and the 262/270/110 cam, with it's very modest overlap of 46 degrees, is just beginning to invite the headers, at say 3500.
So then, now you know; if you can't have cubic inches off the line, then the next best thing is to have stall, and then gears.
I got no dog in this hunt. I don't care which option you go with. Analyze my message, and just do whatever seems right for you.
One more thing;
page 4; fuel efficiency
For highest fuel mileage potential , wait;
I gotta tell you something; typically our A-bodies need less than 35hp to blow down the road at 65 mph; some A's are a lil better than others. 35 hp at .5 pounds of fuel/per hp/per hour, is 17.5 pounds of fuel per hour, which at 6 pounds per gallon is 2.9 gallons . At 65 mph this is 22.4 mpg.
If you can do better than that, good for you. At PartThrottle, it's hard to match that.
Now; having said that, the equation to figure this, does not care how big or how small the engine is; it only knows the engine has a carb on it. And it doesn't much care about rpm; except that the engine has to make a minimum of 35 horsepower,lol. So if you gear your engine too slow, the carb might be in danger of going wide open on the primaries, which might or might not be a bad thing, it depends on your carb. But if you gear your engine too fast, then the throttles may be too far closed, and now the engine's internal friction and such gets to cost you money.
So you gotta put the Rs in a sweetspot.
The smaller the engine is, the higher the rpm will need to be to make the 35hp at a reasonable throttle opening.
The bigger the engine is, the lower the rpm can be, and still make the 35 hp at a reasonable throttle opening.
However, when running a performance cam, the rpm window shrinks, and has to increase, to keep the engine above an rpm where the engine becomes inefficient, or where your ignition timing system can no longer supply adequate advance.
Ok NOW back to;
For highest fuel mileage potential, the cylinder pressure needs to be as high as possible and peak combustion pressure needs to occur at a very specific place in the crank's position relative to AFTER TDC.When you get all these things right, you will be rewarded with phenomenal hiway mpgs, well beyond the 22.4 mpgs.
And sooooooooo, consider a minimum overlap, and a loooong power extraction period. These two together point to a small period cam. To get the bottom end back, you need the cubes. To get the top end back, you need the heads, and as much Effective overlap as is possible. But if you have Scr, you can sacrifice some compression duration.
To understand compression duration as it pertains to fuel-economy;
When the engine is throttled, as it is when cruising, the engine cannot draw in a full load of oxygen. So that means that the cylinder pressure will be only get to be a tiny fraction of what it can be, at WOT. But it is the cylinder pressure that drives the car ahead, so the more Scr that you have then the less throttle-opening it will take.
Summary;
Unfortunately, it's very hard to do all these things with one engine. But I have found that the bigger the small block is, the easier it is to hit all the bases. Especially; performance with economy.
I'll guess that street-performance starts with an absolute hp at around 250hp. But that engine needs to have a strong bottom-end, else you have added expenses of stall and gear. and then fuel-economy goes away. To keep the fuel economy up, the bottom-end needs to be VERY strong,and the rpm kept low. To accomplish that will takes cubes and/or pressure. But the 318 is very pressure challenged, principally because of the small amount of swept cylinder volume; 652cc is quite small. And and as the cam gets bigger, the effective stroke shrinks fast, making the swept ever smaller. And so the total chamber volume, compared to the swept gets ever bigger, driving the Dcr ever lower. That's a bad thing. And as the engine moves to a higher elevation, the Effective Dcr falls even lower.I mean these things are the same for all engines; it's just that the smaller your starting place, the more quickly these things happen.
VP
Really, the best way to look at the street engine is by studying it's VP.
Typically, the VP runs from a low of about 100, to a high of around 160. The higher the better for ALL cases, except;
1) if you HAVE to, but don' want to, run alloy heads because of excessive pressure or
2) you can't get up into the high pressure, because your design can't support it or cists ridiculous money, or
3) the number gets so high that your chassis cannot handle it anyhow, and you end up with a tire-fryer.
Ok so here's my opinion on the V/P breakdown;
120 or less is weak
130 is barely adequate
140 is where the fun begins
150 is leaving fun and getting into excessive tirespin
As we have seen;
it's hard to get a 318 with iron heads past a VP of 124. And if you try, it will get expensive in a hurry, because of the lack of high-compression pistons, and/or the cost of all the machining.
Whereas a 360 can be made to practically fall together at nearly any V/P between say 135/iron and 155/alloys.
But you say; where does the 340 fit?
Great question, but, just try and find one. They only had a 5/6-year run, so not many produced, and guys hang onto them dearly. But, IMO, they are the exact right size for a 3400 pound A-body. They are big enough that you can slide a decent cam into without having the bottom end go away, and compared to a 318; you can run a lil less cam, a lil less stall, and a lil less gear. It already has enough head, and already has bigger valves, and those 4.04" holes make room for it all.
Guys say that 340s are nothing but bored-out 318s, and you can put all the 340 stuff onto the 318, and now yur up to speed. Well, IMO, that's like saying when a man and a woman get married, the two become one, and now they can do double what they did before; not even close.
The truth is that whatever you do to the 318, the same doing on a 340 will always be 22 cubes bigger. I mean you can say what you want, to convince yourself the world is a ball, but that don't make it true. And so the 340 will forever, on the street, be that much ahead of the 318.
IMO, the 360 is the ideal A-body engine for the street, but I sure wouldn't lose sleep if I couldn't find a 340. The 340 IMO, is a good bridge.
You gotta remember, that the street is a different world than dragracing, and the two are far apart. On the street, torque is king. Or Torque-Multiplication.
But when hiway travel is involved, you gotta go back to actually having real torque.
And every time you take off from a stop, torque is what gets you moving. Before you go all technical on me, what I mean by torque is low-rpm power. If you don't got some, take-off is sluggish. If you don't got some, passing is sluggish. What good is 300 horsepower at 5000 rpm, when you are taking off from a light at 2000stall? Hmmm.
Now the vast majority of the life of your engine will be, taking off and running up to some Second Gear cruising rpm; am I right? How much time will your street-engine actually spend, during it's lifetime, at 5000 rpm?
You might drive three hundred miles on a weekend, or say 5 hours at 60 mph, or all day at 30 mph; and not accumulate more than a minute or two, actually at 5000 rpm, right? Two minutes at 5 seconds a pop, is 24blasts, during which the tach is NOT actually stuck at 5000 for the whole time either. That comes to one blast for every 12.5 miles driven. That would ,IMO, be insane, so you get what I'm saying. So why are we camming our engines for peak power at 5000 rpm, and sacrificing all other modes, to have it? IDK, it seemed a good idea at the time................ I , personally, pulled that 292 out of my engine within a few weeks, and stuck a cam 3 sizes smaller in my engine, and was way happier............ cuz I had been paying dearly, for having the pleasure of the occasional wide open blast in three gears, far away from the cops. Now I had some killer take-off torque to deal with. But I digress. You might not be like me. You might be willing to drive what amounts to a top-end racer, on the street, over and over to 40, or 50 or 60 mph, burning up all the gas in your tank, time after time after time. Sure that 3500stall, 4.30-geared 318 is a lot of fun. No doubt. But all you can do with it is drive around endless blocks of city streets. You might as well have a slanty under the hood, with a lil nitrous, for when the mood hits.
Most of us streeters, have other bases to hit; specifically touring a couple of hours from home to get some ice-cream, or going to visit a distant family member, or passing the occasional filling station. Or you know; leaving it in Drive for a while, and listening to the tunes.
So 4.30s and a 3500 stall, for most of us, are not on the table; and so, lack of torque comes back to haunt. But you say why not just but a stroker in it then and run straight hiway gears? ...................... That would be a topic for someone else to expound upon, I have no experience with them.lol.
editing
AJ
