cooltoys61

I need to know the safe limits for EGT about 1" out of the head on #2 & 7 cyl on 454 mag with procharger. I see everything from "1250 will melt pistons" to "1500-1600" is about happy for a turbo motor(scary). I'd say from consensus that 1300 is about tops and 1325 is getting HOT ! Who can give me something I can count on. Procharger tech(hee hee) says they don't use 'em ,They only use oxy ratio. I plan on reading plugs often just don't want to read them thru a couple of layers of Alum.

Reed Jensen

Exhaust gas temps are the product of several things... one is how much energy is being released through combustion... another is a product of compression... the higher the compression ratio.. the higher the exhaust temps... even with the same air fuel mixture...hence... the reading of the O2 as being more accurate.. also if the heads are aluminum and the pistons have special coatings... the temps can be higher without worry of melting pistons.. Detonation can cause melted pistons faster than a lean condition... but the detonation can be caused and exacerbated by a lean mixture... still following me? You can still have high exhaust temps without worry of melting a piston crown if the flow of oil to the underside of the piston is sufficient to cool the piston... that is why I prefer to run a thinner multi-viscosity oil that is 200 degrees... than a thicker oil that is 80 degrees... more oil flow from the bearings splashing up on the underside of the pistons removes heat better... so... if you have an oil cooler... good coatings on your pistons... you can raise the compression ratio... either through a tall piston or a blower and not worry... Any one else have an opinion?.... If you are running the engine on a dyno... you will see horsepower and exhaust temp rise as you get closer to a proper mixture ratio... then as you start to lean out... you will see exhaust temps rise and horsepower remain the same... any leaner and the power starts to drop off and the worry of damage begins... the reason for the EGT sensors in each cylinder is to see if you have one cylinder running dangerously lean before the others. Chevrolet small blocks tend to run hot on one cylinder... I can't remember if it's #5 or #7.... but they tend to burn valves on those cylinders... BBC's have similar issues.I should have mentioned that when jetting an engine ... always start out rich and then lean the mixture to max power... also.. combustion chamber temps and exhaust gas temp can differ depending on jetting...

Griff

I have always been told that 1400* is about the limit on most engines.

You're taking your readings on the right cylinders. 2 & 7 are generally the leanest and hottest as they are the farthest away when getting fuel.

cooltoys61

Thanks Griff.Let's see if this helps Reed. Assume a stock '90 454 mag,,which means 8.75 forged pistons with no special coatings. Timing set conservatively (31 max),93 octane,and 5 PSI from M3 with intercooler(does that matter??) Now I start rich and lean it down......What is the EGT I should shoot for??And what should be my max sustained temp(Assume again that my pyrometer is calibrated accurately)Thanks for any help

Reed Jensen

Ok Cooltoys... I just got off the phone with my dyno guy... he says.... which confirms what I had posted earlier.... lots of variables... he has seen engines run 1400 degrees and still be rich on the mixture... the high heat was due to high compression ratio... but with your static compression ratio and mild boost with an intercooler... you should be running 1350 or under for a long duration... then you will be safe..This number is safe as long as you aren't lean on the mixture.. 1400 would be too high... hope this helps.

rmbuilder

cooltoys61
It would be the opposite:
the lower the CR the higher the EGT
the higher the CR the lower the EGT

the leaner A/F Ratio the higher EGTs
the richer A/F Ratio the lower EGTs

This is discussed in length in this thread
http://www.eng-tips.com/viewthread.c...=71&sqid=90282

http://www.sdsefi.com/techegt.htm
Bob

cooltoys61

Now I'm more kowledgable and confused

cooltoys61

What about if I get my egt before blowing and under "normal" conditions and timing then shoot for the same egt after boost. Or should it be lower?higher?Thanks for all the help and links

Reed Jensen

Adjust jetting and timing for max power... check temps to avoid lean-out and melt down... the exhaust temps are supposed to be looked at as a safety measure... not a guidline.... in my opinion...You can get a better idea of the jetting by looking at the plugs. Some engines are happier running at hotter temps... doesn't necessarily mean that they are melting down.. that is the reason for good oil cooling and flow... If you have inconel valves, they can take the heat... if you are running stock stainless valves.... be careful of max heat and long high speed runs. The main reason engines that have been modified fail is because of a weak link in the chain... cheap exhaust valves... poor oil cooling and or circulation... you have to have oil to the undersides of the pistons to keep them from melting. You can't just "hop up" an engine and not expect failures. Car guys have been playing "bolt on" horsepower games with the same failures for years... What do you want out of your boat engine? 400 hrs without any problems?... leave it stock... if you want 400 hrs with a blower.. expect to pay for premium parts... Plain aluminum melts at 900 degrees... alloys up to 1200.... just because your exhaust temps are higher than the melting point of the pistons doesn't mean they are going to melt. But you have to be prepared to remove the extra heat from the pistons by proper cooling... the pistons transfer most of their heat to the oil .... If you have the new trick coatings on the pistons... they , for lack of a better word "deflect" the heat and don't allow the pistons to absorb it. The heat goes out the pipe and into the atmosphere. But it still has to pass over the exhaust valve... if you don't have the super alloy exhaust valves... you can expect them to tulip on a regular basis. Or you just have to jet so rich that the temps are low.... and expect less performance and more cost for fuel.

JimV

Another factor is camshaft duration. It's possible for a cam to over scavenge and cause the unburned charge to "burn in the pipe" erroneously elevating egt.