cardo0
A/F ratios and EGTs
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Joined: Oct 2004
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From: Pipe Creek TX
A/F ratios and EGTs
The most efficent engines I have built will have exhaust gas temps @ 1350 -1450 on a 300 rpm/sec. pull, with the air/fuel around 13-1 , and BSFC of .40 . These have given the most HP per cubic inch.
Now, I would like to know what needs changed, or what to attack first, when an engine acts cold? Example - You lean the engine until power is loss , and find the EGTs are only 1100 -1200 degrees. BSFC at .57 and A/F of 14-1. This kind of engine is usually down about 50-100 hp from what you thought it should make.
I have changed camshafts,advanced and retarded the cam ,changed intakes and exhaust systems. You would think increasing the volumetric efficency would make a difference , but it doesn't.
I begining to think the heads are controling more than I understand.
Anyone have the same outcomes?
Now, I would like to know what needs changed, or what to attack first, when an engine acts cold? Example - You lean the engine until power is loss , and find the EGTs are only 1100 -1200 degrees. BSFC at .57 and A/F of 14-1. This kind of engine is usually down about 50-100 hp from what you thought it should make.
I have changed camshafts,advanced and retarded the cam ,changed intakes and exhaust systems. You would think increasing the volumetric efficency would make a difference , but it doesn't.
I begining to think the heads are controling more than I understand.
Anyone have the same outcomes?
Burning Brakes
Joined: Feb 2004
Posts: 958
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From: shawnee ks
Something is not right. When you lean an engine your egt should be hotter and your air/fuel leaner on the edge of too lean.A fatter engine will have a lower egt.My engine runs 1450 to 1600 on the top end on lean out.
Racer
Joined: Feb 2004
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From: Wet Coast
Originally Posted by big632
Something is not right. When you lean an engine your egt should be hotter and your air/fuel leaner on the edge of too lean.A fatter engine will have a lower egt.My engine runs 1450 to 1600 on the top end on lean out.
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Compression ratio, A/F ratio, and timing affect EGT. Optimum spark timing to create peak torque/power at a given A/F will correspond to minimum EGT, but "lean" (near stoic.) mixtures need a little more timing than a rich "best power" mixture, which is typically near 12.5:1.
Engines just convert the heat of combustion of the fuel into work at the crankshaft. The lower the EGT the more of the fuel's energy is being used to create useful work. Higher CRs create greater thermal effciency by having greater EXPANSION ratio, so more work is done before the exhaust vavle opens, and all other things being equal, a higher CR should see lower EGT.
The low BSFCs, EGTs, and power you are seeing could be due to lean misfire. Lean misfire may not be bad enough to feel, but it will show up in the numbers.
You are right that the heads are the key to power. That's why modern engines have such high specfic output with relatively modest valve timing, which yields excellent torque bandwidth. Back in the old days, big cams were just a crutch for the lousy flowing heads and inlet manifolds. They made more top end power, but really killed the bottom half of the torque curve.
Duke
Engines just convert the heat of combustion of the fuel into work at the crankshaft. The lower the EGT the more of the fuel's energy is being used to create useful work. Higher CRs create greater thermal effciency by having greater EXPANSION ratio, so more work is done before the exhaust vavle opens, and all other things being equal, a higher CR should see lower EGT.
The low BSFCs, EGTs, and power you are seeing could be due to lean misfire. Lean misfire may not be bad enough to feel, but it will show up in the numbers.
You are right that the heads are the key to power. That's why modern engines have such high specfic output with relatively modest valve timing, which yields excellent torque bandwidth. Back in the old days, big cams were just a crutch for the lousy flowing heads and inlet manifolds. They made more top end power, but really killed the bottom half of the torque curve.
Duke
Le Mans Master
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From: Las Vegas - Just stop perpetuating myths please.
A/F ratio and peak EGT curves don't match.
Ok now i think i c what ur asking. The peak EGT curve doesn't match the A/F curve. And yes u can lean to a cooler EGT. Really simple idea that less fuel > less combustion > lower chamber temps. Piston aircraft are now leaning the fuel injected engines for greater fuel economy and cylinder/piston longivity (OK say engine lasts longer). Carb'd piston a/c are much more difficult to lean this way because most are updraft carbs with the intake charge already heated to vaporize the fuel (o what do ya call it) by passing through the oil sump.
Piston a/c have a mixture control to lean for low air density at hi altitudes while timing is always constant. The trick is to run rich for climb pwr and then pull back the mixture passing through peak EGT to run about 100*F lean peak for economy.
I know cars have water cooled mtrs but i though this might help.
cardo0
Piston a/c have a mixture control to lean for low air density at hi altitudes while timing is always constant. The trick is to run rich for climb pwr and then pull back the mixture passing through peak EGT to run about 100*F lean peak for economy.
I know cars have water cooled mtrs but i though this might help.
cardo0
Last edited by cardo0; Oct 31, 2004 at 01:48 AM. Reason: typo
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Joined: Oct 2004
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From: Pipe Creek TX
Originally Posted by big632
Something is not right. When you lean an engine your egt should be hotter and your air/fuel leaner on the edge of too lean.A fatter engine will have a lower egt.My engine runs 1450 to 1600 on the top end on lean out.
You leaned your combo for max TQ and HP on the dyno ; or you had to do this at the track looking at max MPH. Correct?
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Joined: Oct 2004
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From: Pipe Creek TX
Originally Posted by SWCDuke
Compression ratio, A/F ratio, and timing affect EGT. Optimum spark timing to create peak torque/power at a given A/F will correspond to minimum EGT, but "lean" (near stoic.) mixtures need a little more timing than a rich "best power" mixture, which is typically near 12.5:1.
Engines just convert the heat of combustion of the fuel into work at the crankshaft. The lower the EGT the more of the fuel's energy is being used to create useful work. Higher CRs create greater thermal effciency by having greater EXPANSION ratio, so more work is done before the exhaust vavle opens, and all other things being equal, a higher CR should see lower EGT.
The low BSFCs, EGTs, and power you are seeing could be due to lean misfire. Lean misfire may not be bad enough to feel, but it will show up in the numbers.
You are right that the heads are the key to power. That's why modern engines have such high specfic output with relatively modest valve timing, which yields excellent torque bandwidth. Back in the old days, big cams were just a crutch for the lousy flowing heads and inlet manifolds. They made more top end power, but really killed the bottom half of the torque curve.
Duke
Engines just convert the heat of combustion of the fuel into work at the crankshaft. The lower the EGT the more of the fuel's energy is being used to create useful work. Higher CRs create greater thermal effciency by having greater EXPANSION ratio, so more work is done before the exhaust vavle opens, and all other things being equal, a higher CR should see lower EGT.
The low BSFCs, EGTs, and power you are seeing could be due to lean misfire. Lean misfire may not be bad enough to feel, but it will show up in the numbers.
You are right that the heads are the key to power. That's why modern engines have such high specfic output with relatively modest valve timing, which yields excellent torque bandwidth. Back in the old days, big cams were just a crutch for the lousy flowing heads and inlet manifolds. They made more top end power, but really killed the bottom half of the torque curve.
Duke
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Joined: Oct 2004
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From: Pipe Creek TX
Originally Posted by cardo0
Ok now i think i c what ur asking. The peak EGT curve doesn't match the A/F curve. And yes u can lean to a cooler EGT. Really simple idea that less fuel > less combustion > lower chamber temps. Piston aircraft are now leaning the fuel injected engines for greater fuel economy and cylinder/piston longivity (OK say engine lasts longer). Carb'd piston a/c are much more difficult to lean this way because most are updraft carbs with the intake charge already heated to vaporize the fuel (o what do ya call it) by passing through the oil sump.
Piston a/c have a mixture control to lean for low air density at hi altitudes while timing is always constant. The trick is to run rich for climb pwr and then pull back the mixture passing through peak EGT to run about 100*F lean peak for economy.
I know cars have water cooled mtrs but i though this might help. cardo0
Piston a/c have a mixture control to lean for low air density at hi altitudes while timing is always constant. The trick is to run rich for climb pwr and then pull back the mixture passing through peak EGT to run about 100*F lean peak for economy.
I know cars have water cooled mtrs but i though this might help.
cardo0You hit the nail on the head. This is my delema.
Other than swduke 's increase in compression , anybody have any ideas? Realize , I have this problem with supposily good heads ; it just doesn't make sense .
Burning Brakes
Joined: Feb 2004
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From: shawnee ks
sg4206. Make sure the egts are working properly. If you are running into a lean misfire you should be able to add just enuff fuel to compensate without dropping the reading that low.Thats how we tune our supercharged engines.1450 is right at max power and 1600 is truly lean to the point of too much heat.