Compression Ratio

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Got to remember that OP is just wanting to 4bbl and mild cam to a running 318 and maybe and some cr down the road when rebuilding and probably don't want be living on the edge of detonation.
 
Has a lot to do with what the end use of the motor is. Not going to use as high of a comp ratio on a road race motor as you are on a drag race motor.

And with that, Dan you want a cruiser with a little zip. Your way over thinking this stuff. Use the KISS method. When you start with all this bench racing crap, its outside of your stated wants in an Abody.

The more performance you want the more money it’s going to cost and the more **** your break, the more it cost. You can do a simple home rebuild for less than 2grand or can spend 25 grand or more for a 600HP plus small block.
 
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Dan the man said:
For my goal, do I really need more than a 9.5:1 cr? I'm not going for big power, but I do want a improvement over stock.
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I wouldn't push it last thing you want is pinging mess of an engine. For what your doing I'd shoot for 8.8-9:1. Is another .5 gonna be that much greater ?
 
my plan is Rusty Rat Rods guide to hot rod bliss... chrysler engineers did a nice job on those 340's ... keep it simple and enjoy
 
Dan the man said:
Yes. I'm just trying to get a plan in advance so that I have some kind of a idea of what I'm wanting to do
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Judging by the rest of your threads, you're having a hard enough time buying a car. I'd focus on that before getting caught up in a hypothetical engine build.
 
Dan the man said:
Yes. I'm just trying to get a plan in advance so that I have some kind of a idea of what I'm wanting to do
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To me when rebuilding a street car engine shouldn't go any lower that 8.8:1 but only go higher if the cam your usings needs it which most mild cams are fine with 8.8:1.

Street/sttip racing totally different but are your gonna run more cr than the quality of gas you plan on using.
 
Your hands are fairly tied when it comes to cr there's some that know how to push the limits.

For the rest of us Look at what the average guy can run iron heads and agressive cam 9.5-10:1 with aluminum heads 10.5:1 but since we're talking milder cams were talking 8.8-9.5:1, like I said before is adding .5 cr gonna make that much of a difference to risk a pinging mess of an engine. So if on the mild hassle free side probably want 9:1 at best.
 
Hey Dan, here's where the waters get muddy, lol.

You say to yourself, self, I just want a cruiser with a little kick. You plan out a great strategy, you build your engine. You start driving your car. It's fun!!

...but in 6 months the smiles wanes and you wish you had MO POWER!!!

:rofl: :rofl::rofl:
 
I think the 360 Magnums were 9-1? If I were in the OP's shoes, I'd buy one, rebuild it with a reground cam and a performer intake, drop it in an A body with some 3.55's and have fun.
 
justinp61 said:
I think the 360 Magnums were 9-1? If I were in the OP's shoes, I'd buy one, rebuild it with a reground cam and a performer intake, drop it in an A body with some 3.55's and have fun.
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I’d agree with that. I’m building a mild la 360 with stock heads, cheap headers, airgap intake, and a nice carb. Also with 3.55’s. The magnum 360’s are really cheap and good builders for a fun a-body.
 
Dan the man said:
For my goal, do I really need more than a 9.5:1 cr? I'm not going for big power, but I do want an improvement over stock.
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9-1 is just fine. Pump gas friendly and you can still us a very small cam. Honestly, don’t focus on the ratio any. The biggest power gain is within the cylinder heads. That doesn’t mean you should run out and put trick flows on top an upgraded 340 cam and a 8.8 ratio.

The percentage gain/loss of compression ratios is general and build dependent.
 
Dan the man said:
For my goal, do I really need more than a 9.5:1 cr? I'm not going for big power, but I do want a improvement over stock.
Click to expand...
with what I can glean from your posts, no. you should be just fine with something in the the 9.5 area.

realistically, probably the best build you could do would be a magnum 360 with a +1 cam, light head work and a dual plane manifold with 750ish carb. that would be *plenty* for what you are and would meet your criteria almost perfectly.

it wouldn't cost a fortune to build, parts are inexpensive and easily available, it'll run on pump gas all day long and produce decent power while being reliable.
 
junkyardhero said:
with what I can glean from your posts, no. you should be just fine with something in the the 9.5 area.

realistically, probably the best build you could do would be a magnum 360 with a +1 cam, light head work and a dual plane manifold with 750ish carb. that would be *plenty* for what you are and would meet your criteria almost perfectly.

it wouldn't cost a fortune to build, parts are inexpensive and easily available, it'll run on pump gas all day long and produce decent power while being reliable.
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Frackin- A!!!!!

But he’s been told that already many times over.
He won’t do ****.
 
rumblefish360 said:
Frackin- A!!!!!

But he’s been told that already many times over.
He won’t do ****.
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figure if we all keep telling him the same thing that eventually he'll come down out of orbit and land feet down with his head screwed on right.
 
junkyardhero said:
figure if we all keep telling him the same thing that eventually he'll come down out of orbit and land feet down with his head screwed on right.
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No… because he keeps changing the path of wants.
 
Oooooooo, ssssshhhhhock collar…..

(Add in a drooling Homer Simpson.)
 
Dan the man said:
For my goal, do I really need more than a 9.5:1 cr? I'm not going for big power, but I do want a improvement over stock.
Click to expand...
Pressure is heat is power. The more pressure your engine has as long as it doesn't detonate, the more power it can make. The problem is that in stock configuration the smog teens barely make 135 psi. as soon as you install a cam with a later-closing intake, the pressure falls , and it falls fast. .
For a streeter, with a 2.76 gear, and a 2000 stall, that teener will never wake up until mid to late 30s mph
If you want performance , with a stock compression 318 streeter, leave the cam alone. Get you some big rear gears and a 2800 stall and a 4bbl/dual exhaust and burn rubber.
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The street engine doesn't care about it's Static Compression Ratio. It only cares about peak pressure and when it occurs, and how much load is on the crank. The engine cannot begin to make pressure until the stinking intake valve is closed and not leaking. The pressure is determined by something called the dynamic compression ratio, which is what is left of your Scr after the intake closes.
IIRC the cam in the stocker is 240/248/112, which in at 108 the Ica will be 48*
the Wallace says that at 500ft elevation, this is likely to produce 133 psi @ 7.8 Compression ratio, which, as you know, is abysmal.
Now say with no other changes, you install a 262/110 cam. This will change the Ica to 57* and the Wallace says that the pressure will drop to 124psi. So then, your bottom end has gone from bad to piss-poor. This translates as a huge performance loss below about 3000/3500. But yeah, if you wait until ~4000, the cam will eventually wake the engine up. The problem is that with 2.73 gears and stock TC, you'll be going 43mph!, and no the 4bbl will not make this up.... because the 2bbl is good to at least ~3000 all by itself. So it don't matter how you time the secondaries, they will not be contributing power over what the primaries could do on their own, until after ~3000.
Now, on 87 gas, that teener could easily run the pressure up to 150/155psi. To get there with the stock cam would take an Scr of about 8.75. To get there with the 262, would require the Scr to be 9.2
Ok, then imagine the bottom end power loss going from 133psi to 124. Now imagine going from 124 to 153.
The Wallace includes a calculation to help you understand this, called the P/V index The higher it is, the more fun the bottom end will be. Conversely, the lower it is the more of a lazy-dog the engine will be.
the Stock 318 has a P/V of 111.... and you may know what that feels like...... lol
Now still at 7.8Scr and with the 262 cam, the P/V index drops to 97, but
pumping the Scr up to 9.2, the P/V now climbs to 120.
What do the numbers mean?
Well from 111 to 97 is a loss of 13%, and from
97 to 120 is a gain of 23.7%
So these are numbers your butt-dyno is gonna really feel. But recall that these numbers are ALL ABOUT low-rpm power. If you install a 3500TC, none of this is gonna matter., NONE of it.

What increasing compression ratio does not tell you, is how the engine reacts to changes in throttle openings.
The stock 318 is a lazy engine. When you are cruising along at say 2400 rpm and you floor it, chit takes a while to get moving. Why is that? Because at 7.8Scr, she has a total combustion chamber volume of 96cc, and when the piston starts "sucking" on the carb, that big chamber is like a big rubber band, it takes a long time to get the air moving. But at 9.2Scr, the chambers is about 16.5cc smaller which is 17%, which is a lot! And so the air is a lot more willing to get moving. This makes for snappier throttle response that you can really relate to.
>New thought; Suppose your stock teener makes the same 300ftlbs at 2000 as at 2800. At 2000 this is 114hp. But at 2800 this is 160hp. That is an increase of 40%. There is NOTHING that you can do to your stock 318 short of maybe supercharging it, that can generate this size of power increase. So then this should be your very FIRST "bolt-on". Couple that with a 4bbl for another say 10ftlbs at 2800, and that would be 165hp
> New thought; those 300ftlbs, by the time they come out to the rear axles, in 2.45 first gear with say 3.23s, to be fair, will get you 2374 ftlbs to share between the two tires. Now slap some 4.10s on there, and this becomes 3013 . Now stall it up to 2800 and add the 4bbl and it looks like 3113 ftlbs. But hang on, this does not reflect the 40% horsepower increase in the stall rpm. Now the last time I tried to presemt this power increase, in an understandable context, I got thoroughly slapped for it. So I ain't doing it again. But obviously the extra power is gonna do something at the rear wheels that the modest 100 ftlbs does not speak to.
> so in conclusion: as I said;
If you want performance , with a stock compression 318 streeter, leave the cam alone. Get you some big rear gears and a 2800 stall and a 4bbl/dual exhaust and burn rubber. Leave the spare tire at home. With the spare not there, you might as well leave the jack home too. Since no one will passenger with me, I took out the back seat too (72 stinking pounds of of folding boat-anchor).
 
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AJ/FormS said:
Pressure is heat is power. The more pressure your engine has as long as it doesn't detonate, the more power it can make. The problem is that in stock configuration the smog teens barely make 135 psi. as soon as you install a cam with a later-closing intake, the pressure falls , and it falls fast. .
For a streeter, with a 2.76 gear, and a 2000 stall, that teener will never wake up until mid to late 30s mph
If you want performance , with a stock compression 318 streeter, leave the cam alone. Get you some big rear gears and a 2800 stall and a 4bbl/dual exhaust and burn rubber.
_______________________
The street engine doesn't care about it's Static Compression Ratio. It only cares about peak pressure and when it occurs, and how much load is on the crank. The engine cannot begin to make pressure until the stinking intake valve is closed and not leaking. The pressure is determined by something called the dynamic compression ratio, which is what is left of your Scr after the intake closes.
IIRC the cam in the stocker is 240/248/112, which in at 108 the Ica will be 48*
the Wallace says that at 500ft elevation, this is likely to produce 133 psi @ 7.8 Compression ratio, which, as you know, is abysmal.
Now say with no other changes, you install a 262/110 cam. This will change the Ica to 57* and the Wallace says that the pressure will drop to 124psi. So then, your bottom end has gone from bad to piss-poor. This translates as a huge performance loss below about 3000/3500. But yeah, if you wait until ~4000, the cam will eventually wake the engine up. The problem is that with 2.73 gears and stock TC, you'll be going 43mph!, and no the 4bbl will not make this up.... because the 2bbl is good to at least ~3000 all by itself. So it don't matter how you time the secondaries, they will not be contributing power over what the primaries could do on their own, until after ~3000.
Now, on 87 gas, that teener could easily run the pressure up to 150/155psi. To get there with the stock cam would take an Scr of about 8.75. To get there with the 262, would require the Scr to be 9.2
Ok, then imagine the bottom end power loss going from 133psi to 124. Now imagine going from 124 to 153.
The Wallace includes a calculation to help you understand this, called the P/V index The higher it is, the more fun the bottom end will be. Conversely, the lower it is the more of a lazy-dog the engine will be.
the Stock 318 has a P/V of 111.... and you may know what that feels like...... lol
Now still at 7.8Scr and with the 262 cam, the P/V index drops to 97, but
pumping the Scr up to 9.2, the P/V now climbs to 120.
What do the numbers mean?
Well from 111 to 97 is a loss of 13%, and from
97 to 120 is a gain of 23.7%
So these are numbers your butt-dyno is gonna really feel. But recall that these numbers are ALL ABOUT low-rpm power. If you install a 3500TC, none of this is gonna matter., NONE of it.

What increasing compression ratio does not tell you, is how the engine reacts to changes in throttle openings.
The stock 318 is a lazy engine. When you are cruising along at say 2400 rpm and you floor it, chit takes a while to get moving. Why is that? Because at 7.8Scr, she has a total combustion chamber volume of 96cc, and when the piston starts "sucking" on the carb, that big chamber is like a big rubber band, it takes a long time to get the air moving. But at 9.2Scr, the chambers is about 16.5cc smaller which is 17%, which is a lot! And so the air is a lot more willing to get moving. This makes for snappier throttle response that you can really relate to.
>New thought; Suppose your stock teener makes the same 300ftlbs at 2000 as at 2800. At 2000 this is 114hp. But at 2800 this is 160hp. That is an increase of 40%. There is NOTHING that you can do to your stock 318 short of maybe supercharging it, that can generate this size of power increase. So then this should be your very FIRST "bolt-on". Couple that with a 4bbl for another say 10ftlbs at 2800, and that would be 165hp
> New thought; those 300ftlbs, by the time they come out to the rear axles, in 2.45 first gear with say 3.23s, to be fair, will get you 2374 ftlbs to share between the two tires. Now slap some 4.10s on there, and this becomes 3013 . Now stall it up to 2800 and add the 4bbl and it looks like 3113 ftlbs. But hang on, this does not reflect the 40% horsepower increase in the stall rpm. Now the last time I tried to presemt this power increase, in an understandable context, I got thoroughly slapped for it. So I ain't doing it again. But obviously the extra power is gonna do something at the rear wheels that the modest 100 ftlbs does not speak to.
> so in conclusion: as I said;
If you want performance , with a stock compression 318 streeter, leave the cam alone. Get you some big rear gears and a 2800 stall and a 4bbl/dual exhaust and burn rubber. Leave the spare tire at home. With the spare not there, you might as well leave the jack home too. Since no one will passenger with me, I took out the back seat too (72 stinking pounds of of folding boat-anchor).
Click to expand...


This should be made a sticky. Very good post AJ.
 
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