NC Engine Builder
Grumpy Old Man
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A 5.2 magnum in that truck, with about a 2.94 rear gear, would be a riot! Good economy, and still pretty fast
Bore size versus fuel mileage. 273 versus 5.2 magnum
- Thread starter HankRearden
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AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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Here are some of my tricks, that I have used in the past and that I would try in your case;
I would Run closed chamber alloy heads, a short period cam, and set the Squish tighter than you think should be possible; such that;.
The Dcr and cylinder pressure are way higher than you think that your fuel can stand. and then
reduce loading, by running a tiny primary carb, with delayed secondaries; and
finally Run the Right gear combo so that the engine never has to work hard, and
run an overdrive to bury the cruise rpm as low as possible.
Get yourself a dash-mounted, dial-back, timing retard box; and with it, get the ignition timing optimized and then just lean the carb out until just before it won't keep a steady speed any more.
Put an override-able throttle stop on it that just allows your required roadspeed, on the flat; and do not override it when climbing hills. Yes the car will slow while climbing, but just hold her steady, and she will recover when going over the top.
Fuel-economy will take care of itself
Consider this; the major part of the energy transfer of the expanding gasses to the crank, should be happening in about the first inch of the piston down travel, so, during the rest of the stroke the expanding gasses are just chasing after the falling piston. If you get the ignition timing set right, bore size will not play a negative role, in fact bigger is better.
Consider that with a 3.91 bore 200psi makes a force of 2400pounds, but that same 200psi in a 3.63 bore makes only 2070pounds; a difference of ~14%. This allows you to run the same percentage less cruise gear before the power pukes, making final-drive ratios of less than 2.0 possible.
Would I try this with a 273?
Yes and no.
>Yes, but not at ultra low rpm.
>No because,IMO the 273 bores do not have enough swept volume to make sufficient pressure with , at the very low rpm, that overdrive will allow, That is to say, good luck hitting 190psi at cranking speed. and
alloy heads have bigger valves that don't especially like small bores.
And finally, I have already built an engine like this at 367 cubes.
The thing is this; if your body style requires 40 hp to cruise at 65 on the flat, you cannot change that. So it matters not a whole lot what engine you use except as to the internal friction that subtracts from fuel economy. So it behooves you to get the rpm down as low as it goes, and still make the 40hp.
The 360 will get down to 1400rpm and still pull a steady 60/65mph
I ran that with 3.23s, an A833od, and GVod strapped to the back; FD of 1.788
I would Run closed chamber alloy heads, a short period cam, and set the Squish tighter than you think should be possible; such that;.
The Dcr and cylinder pressure are way higher than you think that your fuel can stand. and then
reduce loading, by running a tiny primary carb, with delayed secondaries; and
finally Run the Right gear combo so that the engine never has to work hard, and
run an overdrive to bury the cruise rpm as low as possible.
Get yourself a dash-mounted, dial-back, timing retard box; and with it, get the ignition timing optimized and then just lean the carb out until just before it won't keep a steady speed any more.
Put an override-able throttle stop on it that just allows your required roadspeed, on the flat; and do not override it when climbing hills. Yes the car will slow while climbing, but just hold her steady, and she will recover when going over the top.
Fuel-economy will take care of itself
Consider this; the major part of the energy transfer of the expanding gasses to the crank, should be happening in about the first inch of the piston down travel, so, during the rest of the stroke the expanding gasses are just chasing after the falling piston. If you get the ignition timing set right, bore size will not play a negative role, in fact bigger is better.
Consider that with a 3.91 bore 200psi makes a force of 2400pounds, but that same 200psi in a 3.63 bore makes only 2070pounds; a difference of ~14%. This allows you to run the same percentage less cruise gear before the power pukes, making final-drive ratios of less than 2.0 possible.
Would I try this with a 273?
Yes and no.
>Yes, but not at ultra low rpm.
>No because,IMO the 273 bores do not have enough swept volume to make sufficient pressure with , at the very low rpm, that overdrive will allow, That is to say, good luck hitting 190psi at cranking speed. and
alloy heads have bigger valves that don't especially like small bores.
And finally, I have already built an engine like this at 367 cubes.
The thing is this; if your body style requires 40 hp to cruise at 65 on the flat, you cannot change that. So it matters not a whole lot what engine you use except as to the internal friction that subtracts from fuel economy. So it behooves you to get the rpm down as low as it goes, and still make the 40hp.
The 360 will get down to 1400rpm and still pull a steady 60/65mph
I ran that with 3.23s, an A833od, and GVod strapped to the back; FD of 1.788
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273 all the way. If you can find one. What does that Arrow pickup weigh? Plenty of power to pull mountains. I've gotten mid to high 20's mpg from a 9:1 273 with the stock intake and a 72 340 TQ. And that was just swapping parts on my brothers 67 Barracuda. Mind you that is with 3,000 pound, 904 auto trans, 3.23 geared 8 3/4 rear, and midwest plains on a long trip. 273's are cheap to build, probably just have to polish the forged crank, good rings and pistons, good valve job, small solid cam with springs, and electronic ignition. Maybe a Street Demon carb on any intake. I would not use a mileage intake, 273's like to rpm and don't need to be limited. No cracked heads, small ports that will flow with stock 360 heads with minimal work. Good torque, and power on top. The Magnum intake valves are too large for the 273 bore.
5.2 Magnum. A close second, I never got to play with one yet, although I do have one. Might not have to bore and re-piston the Magnum. Both have the right compression, but the 273 will have quench at that compression. The catch will be head condition and available roller cam for better mpg.
5.2 Magnum. A close second, I never got to play with one yet, although I do have one. Might not have to bore and re-piston the Magnum. Both have the right compression, but the 273 will have quench at that compression. The catch will be head condition and available roller cam for better mpg.
mgoblue9798
Well-Known Member
I would build the 5.2, deck the block to get pistons up to deck height, cut the magnum heads .020 to help more with compression. Find a 90-95 model 518 or 500 overdrive if automatic, if standard maybe a 5spd from a dakota.
Pick your gear and tire to set cruise rpm at about 1800 rpm. OEMs are using compression and low rpm cruise to eek as much efficiency out of their engines as possible.
I would run a small thermoquad if mileage is the primary concern. Not engine related, but a tonneau cover was worth almost 1 mile per gallon on a dakota driving on the interstate. Electric fan will help in town mileage.
Pick your gear and tire to set cruise rpm at about 1800 rpm. OEMs are using compression and low rpm cruise to eek as much efficiency out of their engines as possible.
I would run a small thermoquad if mileage is the primary concern. Not engine related, but a tonneau cover was worth almost 1 mile per gallon on a dakota driving on the interstate. Electric fan will help in town mileage.
Wonder how a 273 would work with a magnum heads and the rest like your combo? Or even the whole efi and all.
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I have thought about it, but the larger magnum valves scare me on a 273 bore. In the end, I don't think I'd change either one. I think they are both great engines with a few mods.
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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for fuel mileage only, here's my thinking;
static compression ratio doesn't mean much because
at hiway speed, your V8 is heavily throttled; and so the effective compression ratio will be very low.
Take a smog 318 for instance; at 8.1 Scr, with the stock cam, the Dynamic C/R comes to about 7.0@138psi. But throttled at 65=2240 with 2.76s, the engine will never get a full 318/8=~40 cubes of air@ per cylinder, at normal density.
If it takes 35hp to go 65 mph, it matters not how big the engine is, the throttle will always be open just far enough to make the required hp. Your cam timing events will be far more important.
However, the more chamber volume your engine has, the more surface area it will have and so the greater the possibility that some fuel will never make it into the combustion event. You can overcome some of that by ensuring the ignition timing occurs at the proper time. But with the stock-type distributor, this is nearly impossible, cuz at 2240 rpm, my 11.3/1, 367 wants 56* to 60*.
So then, IMO, as for fuel mileage only, the tune is far more important than the actual bore/stroke/or Scr, than anything else.
Part of the tune is the engine-temperature at cruising speed. The hotter she runs (within reason), the less chance there is of some fuel going out the exhaust, not having taken part in the combustion event. This gets more and more important as the cam gets bigger. In this case, extra EFFECTIVE stroke allows more time for the event while the exhaust valve is still closed.
Going back to the smog 318, with the stock cam(240/248/112), in at split overlap, the effective stroke is 2.86inches. Closing the LSA to 108, with no other changes, moves the effective stroke to 2.93inches. installing a 262/110 cam changes the Effective stroke to 2.68.
However, more importantly is what is happening to the power stroke, which changes from 122*@112Lsa to 126*@108Lsa to just 114* of Extraction with the performance cam; thus showing a very significant loss of time for power extraction.
Increasing the stroke with the 262/110 cam from 3.315 to 3.58 changes the Effective stroke to 2.91, good deal. the power extraction degrees does not change, but the distance does and so the theory is that more energy will be given to the crank during power extraction.
In this regard, the 340 cam is terrible for fuel economy because the power extraction with the cam in at split overlap, is only 106* and the Effective stroke is just 2.52".. Closing that same cam up to 108Lsa, would get you 122* of extraction and an Effective stroke of 2.66" ..
to recap;
For fuel economy;
getting the correct amount of fuel into the chambers at the right time, and getting ALL of it to contribute to power production, at as low an rpm as is possible, with the correct Ignition Timing....... is the key. The smaller the chamber the better, and the hotter it runs, is also better.
-----------------
In case you doubt my thinking;
I had an Eddy-headed 367 @11.3Scr running 207*F, that on a certain test trip, made 32mpgs.... with overdrive gears turning 75=1850rpm. The bore is 4.045. The stroke is 3.58. The cam was a Hughes HE2430AL with specs of 270/280/110 and @223@.050
With the stock Distributor there was no way to give it the timing it wanted, so I installed a dial-back timing module with a maximum range of 15*, and so a maximum available amount of ~60*.
The point is this;
You don't absolutely HAVE to sacrifice cubes for hiway fuel-economy. You just have to tune the crap out of whatever you got. However the more cubes you have, the more gas she will use getting up to cruising speed. And, your cam-timing will play a very significant role; especially the exhaust extraction.
Happy HotRodding
static compression ratio doesn't mean much because
at hiway speed, your V8 is heavily throttled; and so the effective compression ratio will be very low.
Take a smog 318 for instance; at 8.1 Scr, with the stock cam, the Dynamic C/R comes to about 7.0@138psi. But throttled at 65=2240 with 2.76s, the engine will never get a full 318/8=~40 cubes of air@ per cylinder, at normal density.
If it takes 35hp to go 65 mph, it matters not how big the engine is, the throttle will always be open just far enough to make the required hp. Your cam timing events will be far more important.
However, the more chamber volume your engine has, the more surface area it will have and so the greater the possibility that some fuel will never make it into the combustion event. You can overcome some of that by ensuring the ignition timing occurs at the proper time. But with the stock-type distributor, this is nearly impossible, cuz at 2240 rpm, my 11.3/1, 367 wants 56* to 60*.
So then, IMO, as for fuel mileage only, the tune is far more important than the actual bore/stroke/or Scr, than anything else.
Part of the tune is the engine-temperature at cruising speed. The hotter she runs (within reason), the less chance there is of some fuel going out the exhaust, not having taken part in the combustion event. This gets more and more important as the cam gets bigger. In this case, extra EFFECTIVE stroke allows more time for the event while the exhaust valve is still closed.
Going back to the smog 318, with the stock cam(240/248/112), in at split overlap, the effective stroke is 2.86inches. Closing the LSA to 108, with no other changes, moves the effective stroke to 2.93inches. installing a 262/110 cam changes the Effective stroke to 2.68.
However, more importantly is what is happening to the power stroke, which changes from 122*@112Lsa to 126*@108Lsa to just 114* of Extraction with the performance cam; thus showing a very significant loss of time for power extraction.
Increasing the stroke with the 262/110 cam from 3.315 to 3.58 changes the Effective stroke to 2.91, good deal. the power extraction degrees does not change, but the distance does and so the theory is that more energy will be given to the crank during power extraction.
In this regard, the 340 cam is terrible for fuel economy because the power extraction with the cam in at split overlap, is only 106* and the Effective stroke is just 2.52".. Closing that same cam up to 108Lsa, would get you 122* of extraction and an Effective stroke of 2.66" ..
to recap;
For fuel economy;
getting the correct amount of fuel into the chambers at the right time, and getting ALL of it to contribute to power production, at as low an rpm as is possible, with the correct Ignition Timing....... is the key. The smaller the chamber the better, and the hotter it runs, is also better.
-----------------
In case you doubt my thinking;
I had an Eddy-headed 367 @11.3Scr running 207*F, that on a certain test trip, made 32mpgs.... with overdrive gears turning 75=1850rpm. The bore is 4.045. The stroke is 3.58. The cam was a Hughes HE2430AL with specs of 270/280/110 and @223@.050
With the stock Distributor there was no way to give it the timing it wanted, so I installed a dial-back timing module with a maximum range of 15*, and so a maximum available amount of ~60*.
The point is this;
You don't absolutely HAVE to sacrifice cubes for hiway fuel-economy. You just have to tune the crap out of whatever you got. However the more cubes you have, the more gas she will use getting up to cruising speed. And, your cam-timing will play a very significant role; especially the exhaust extraction.
Happy HotRodding
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