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HIGH exhaust temperatures (FE)
This may be way to technical to get a response on here, but I'm going to try here first anyway!
WHY are my exhaust temperatures "seemingly" higher than they USED to be. My OLD engine: 12.5 to 1 C.R. with a .030 below the deck piston height and solid roller cam AND very rich mixture with the center squirter "drag race" only carbs. Cranking compression was 240 psi. My NEW engine (re-build): 9.8 to 1 C.R. with ZERO deck height piston and "street" solid tappet cam. The piston position was raised specifically to introduce "squish" (some call it "quench", same thing) to the combustion chamber. This enhances and "makes better" the "burn" when the fuel ignites. New carbs are typical 600 cfm vacuum secondary Holleys. Cranking compression is 200 psi. My NEW engine seem's to be running considerably higher exhaust temperatures than the old setup. This remains true even if the carbs are "leaned out" with small jets or made to "run rich" with BIG jets. Question and speculation: Is it possible the "squish" "quench" thing has so enhanced the combustion chamber "burn" THAT is what accounts for the higher exhaust temps? |
Remember, if the ignition timing is retarded it can cause that problem. Elevated ex. temps indicate that the combustion process is continuing in the ex. not in the CC. If this is a new engine you may want to confirm the timing marks on the balancer and re-time it. Just a thought.
Steve |
Higher on all 8? You using the same intake manifold or a different one?
Were your thermocouples calibrated? |
Hot exaust
Retarded
Cheers, Bryan |
So when you were running rich the exhaust temp was lower than now when your running less rich ? Isn't that whats supposed to happen ?
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Good point about the timing, I forgot to mention that and it is a major player in exhaust temps.
I'm running 10 deg advanced base timing and about 30 total. Before, with the high compression, I ran about 6 or 8 base. Same harmonic balancer as before. When I was running rich with 12.5 to 1 exhaust temps were lower. Running rich or lean with the new 9.8 to 1 exhaust temps are higher, which just seems "weird". I think I will introduce more base timing to see what happens, good tip. Of course being careful not to advance TO much and the need to re-confirm the timing marks. Same intake as before. I used a hand held "Heat Gun" and recorded 900 deg's, but I'm not sure how reliable the gun is. No thermocouples to calibrate, just a melted tennis shoe BEFORE I finished insulating the fire wall. :D |
You didnt mention cam specs. But with much duration and "lower" compression, the flame velocity is lower, possibly allowing a lot of burning in the headers after the exh. values open.
Turn inital timing up to 18-20 deg; what happens; engine speeds up about a 1000 rpm; why? more time is created for more gas to be burned in the combustion chamber, creating more energy/rpm with less going into the exhaust creating less EGT cheers gn |
COMP CAM, as recommended by Gessford to match my rear end ratio and expected "cruise rpm". I see a number of people using the 282S Comp Cam, by the way.
Grind FB 282S 10, Lift 571 (Int&Exh) Duration, 236 (Int&Exh) 110 Lobe, but designed for and intalled at 106 Cam was "degreed" while in the engine to assure accuracy of specs. .022 clearance Int&Exh 2000-6000(+?) rpm range. Nice bottom end torque! 18-20 base advance sounds to high considering the TOTAL advance amount would then put in the 40's. I think I WILL try some radical base advance and if it works well I can recurve the mechanical advance for a better "total advance" number. |
Ernie, just another thought...wouldn't you get the most useful EGT reading under load at a given RPM, say 3000 RPM, where the total advance is all in? I'm not sure the idle EGT is a reliable comparison between two motors that are so radically different from each other. In airplanes, you take off full rich, and when you get to cruise altitude you start leaning to peak EGT, then richen it up about 100 degrees (to make sure you don't get near detonation). Seems like idle EGT is an interesting but not terribly useful reading, maybe?
Now just how ya gonna read the EGT under load? Maybe on a chassis dyno? |
I have a heat gun! Lets see, I could drill a hole in the firewall or just run really fast along side and "shoot it".
WHAT is an appropriate exhaust temp? One and only reading I took so far, suspect at that, indicated around 900 deg F. I THINK exhaust temp is a decent indicator of performance and could be yet another "tool" for "dialing in" the engine. In general, higher exhaust temp is a "good thing"? Within reason of course. |
From memory, the typical EGTs on aircraft were about 1150 degrees F at 75% rated power, but someone out there surely flies recips regularly and can chime in. I think idle EGT should be a lot lower (400-500 F?) simply because of the difference in volume of BTUs generated at idle v. load/cruise, as well as the timing, mixture, etc.
BTW, I would think that, all things equal, an engine that was more efficient (such as your 12.5:1 motor) would have lower EGTs than a less efficient motor (such as your 9.8:1 motor), as it would convert more of the BTUs from combustion process into mechanical energy, with less heat going to waste out the exhaust...similar to a turbo, converting waste exhaust heat into pressure+mechanical energy at the flywheel. |
Not trying to be a smart a, but is the heat gun accurate?
Bruce |
Not trying to be dum, but I'm not sure! :D
I just got it and I'm still "playing" with it. Off to the race track this weekend to compare readings with what the other guys are using. Raytek, Raynger ST (not certified above 300 celsius). Point and shoot, -25 to 950 Deg F. I bought for taking ROOM Temperatures for my work, not my "race car". :LOL: |
Some guys have previously mentioned that retarded engines will run hotter. Total base and centrifigul timing can max at 38* without detonation if carb isn't too lean, etc. You're running 30*, you won't loose anything by boosting it to 38*.
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It is not uncommon to run 38 to 40 degrees of total timing on these engines. The heat gun is of no use unless you have someone under the hood at wide open throttle measuring the temp about one inch down from the head. I assume you are taking a reading when the engine is at idle. The jets will not change this temp any since this is controlled by the idle circuit and air bleeds. You will have install thermo couplers about one inch down from the head and check at wide open throttle. The temp should be in the 1200 degree range to be on the safe side.
The other way to check this out and get the car tuned right would be to put on a chassi dyno and measure the air fuel ratio as well. The other engine may have had richer idle circuits which would have showed lower temps at a idle. Good luck, Keith |
Thanks for all the good feedback guys. The motor is running really good and with the recent change of jets to 62's it just about perfect all around.
I'll kick up that timing closer to 40 total and see how it goes! I'm looking into some chassis dyno time now that I'm "in the ball park" with it. |
Up-date, advancing the base timing to 14-15 degrees made a HUGE difference in overall "power and driveability". It also lowered the exhaust temp, but not a lot. Now THAT was a good tip!
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EGTs vary from car to car. I used to race a late model Mustang with an EGT gauge on it. On motor I would see 1350-1375* at the top end of the track. With the supercharger I would shoot for 1450*. My turbo Buick gets as hot as 1625*. A raytech is not a reliable source as it is only reading the surface temp of the headers.
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