JustJeff

iTrader: (13)

My car: 89 vert with a JDM S5 13B, N370 ECU, N/A harness with pins swapped, 3" Racing Beat REVII exhaust with presilencer, FCD, FD fuel pump, 800cc secondary injectors, custom TID.

I'm new to car performance and tuning. I'm to the point with my car that I need get some engine management and tools so that I know what my engine is doing.

My question is whether a wideband O2 sensor is redundant with an EGT gauge? It seems like both are giving you the same information in a different manner. I'm probably getting an LC-1 and a laptop to get my A/F info, but would it be a good idea to add an EGT for heads up feedback on what the engine is doing? Obviously I can add a gauge to my wideband, but I'm just wondering if an EGT gauge provides more or additional info that a wideband doesn't.

auricomXL

EGTs are effected by both spark advance angle and AFR. you should not expect to understand what your mixture is up to by the use of an EGT alone. definitely tune with the wideband. as far as normal driving goes, a wideband is less usefull, but it can help identify a problem that may come up. again, an EGT alone will not serve the function of the wideband due to the effects on EGT due to the sparks.

JustJeff

iTrader: (13)

Thanks for the reply, do you know of any quality links where I can read up on EGT?

auricomXL

i read some things about this online and in some literature (more trust-worthy). i can't remember the links but i can get you started on the basics:

how spark advance effects EGT:
when your spark advance is set optimally, peak cylinder pressure will happen when the engine is around 17-20 ATDC. if the spark is too retarded, there will be exsessive energy in the combustion chamber late in the power stroke. becasue the burning gas is not expanded sufficiently, you will have excessive pressure/temperature during the exhasut stroke. if you take the extreem case, you ignite the fule mixture VERY late, and you have the burn process extend well into the exhasut stroke. this is sort of like how some anti-boost-lag systems work. in this case you can see how retarding your advance to subsequently more retarted timing will result in increasing EGTs.

the reverse is a little more complicated, and i'm only 85% sure this is how it works...
when you have too much advance your peak pressure builds before the 17-20 degree ATDC sweet spot. this also results in increased EGTs. when the pressure peaks early in the power stroke, the volume of the chamber is still quite low. when you pressurize a gas in a defined volume it heats. if you pressurize the gas very slowly it heats very little, if the pressurization is quick, it heats more. when your advance is too high, the pressure rises to a peak when the volume you allow the gas to occupy is still small (early in the power stroke). this results in a higher temperature product of combustion. of course in the extreem case of this you get detonation. in this way you can see that if you increase advance beyond optimal, your EGTs will go up as well.

becasue of the two cases above, there should be a "sweet spot" in your spark advance where EGTs should bottom out. if you tune for this you will be timing your beginning of burn (spark event) to the right time when the pressure maximum matches the 17-20 degree range of maximum mechanical advantate the rotor has on the e-shaft.

how AFR effects EGT:
this is a little simpler...becasue of the way fuel is atomized, peak power is usually attained just rich of stoich. you may have heard the phrase "leaner is meaner" implying more power with a leaner mixture. this is said in reference to a leaner mixture than a VERY rich mixture (still richer than stoich though). chemically, an AFR around 13:1 should be your peak EGT since it produces the most thermal energy (and hence pressure). anything leaner or richer than this and you should see a decrease in EGTs since you are not providing enough propelant (lean condition) or not enough oxidizer (rich condition). hence your products have "left-overs" one way or another and were not able to fully do their thermal energy creation jobs. HOWEVER, depending on your fuel octane rating and your peak pressure it is likely that a mixture close to, or leaner than stoich will casue detonation. if this begins to occur, at first you will get mild detonations where only the last 10% or so of your charge explodes. mild detonation will result in EGTs being higher, so things get a little more complicated. in a very low compression motor, you should see EGTs max out at your peak-power mixture and get lower everywhere leaner or richer than that mixture. HOWEVER in any reasonable automotive engine, the lower temps that chemically should be measurable during lean burn conditions will often get masked by the higher thermal energy from detonation onset.

i hope that helps.

ultimatejay

iTrader: (1)

High EGT's can come from many diff. running scenarious and that's where the A/F meter comes into play. If you have a high EGT but your A/F meter says you are at a good mixture #, then you have a problem with your timing/ mixture, etc. Your goal is to get both meters to agree with on another. A good example is having a A/F that reads 13.2 with an EGT temp of 1530 at 9000 rpm's- N/A motor.

You can get by with one or the other but both meters is the best assurance and in my opinion a must if you want to learn how to tune seriously. So to answer your question, get both if you have the money. If you only have the funds for one get the A/F meter.

JustJeff

iTrader: (13)

excellent replies everyone, it's much appreciated and I learned something