EGT and MPG

[88turboii]

so i just got an EGT gauge and have been observing.. it runs pretty hot at cruise around 1600F. timing is about 25 deg at 2.5krpm to 30 deg at 3krpm

so is there a relationship between EGT and best gas mileage? or can you not detect a sweet spot with EGT?

[RICE RACING]

Originally Posted by 88turboii

so i just got an EGT gauge and have been observing.. it runs pretty hot at cruise around 1600F. timing is about 25 deg at 2.5krpm to 30 deg at 3krpm

so is there a relationship between EGT and best gas mileage? or can you not detect a sweet spot with EGT?

My RICESP runs following:

Light cruise 780 to 840 deg C (in backwards English IMPERIAL scum units this is 1544 deg F

This is with an AFR of 14.7:1

There is a direct relationship between AFR and EGT (and fuel usage and BSFC < economy to power) I set my RX7 to between 15 and 14.7:1 up to 100kpa absolute (0kpa gauge pressure). This returns the lowest amount of fuel usage in my case.

IF you set the AFR from lean or stoich to slightly rich (excess fuel) you have far far worse BSCF but you have much lower EGT, to give you an example if you set the AFR to 13:1 you can drop the EGT around 100deg C.

For me on inclines where load goes to around 25kpa (gauge pressure) I can easily run stoich, but power suffers and also EGT goes much much higher (simple mass flow here, and not enough fuel cooling the motor & turbo) and the EGT will go to your kind of levels or even higher to 950degC, so what I do is progressive set the AFR rich to around 13.0:1 and keep the EGT constant at my target 780 to 840 degC or lower..... this while not giving MAXIMUM economy (lowest BSFC) it does give highest power and lowest stress on the turbine and engine... with very acceptable fuel economy

If you start dumping fuel at a higher rate you will watch the EGT fall drastically, but you will also watch your fuel gauge drop in a similar fashion

[RETed]

Assuming you're talking vacuum ranges and not in boost?

If so, just keep leaning it out till you hit "lean surge".
Then just add a tad more fuel just to get rid of the lean surge.
This'll depend on how much of the lean surge you can tolerate.
Most people get annoyed with any hint of lean surge.

At the lower RPM ranges, EGT isn't much of a concern.
EGT's start to get really hot once you get north of 4k RPM's.


-Ted

[TitaniumTT]

To really get the full picture, you need to log Lambda, not this old school afr's

The timing seems like it can be advanced a bit witch will lower your EGT's as well. 1600* is nothing to worry about though.

[NoDOHC]

I see this as two issues that you are addressing.

WOT lamda and WOT ignition timing is not the same as cruising lambda and cruising timing.

TTT, I am trying to use lambda, just for you.

I run lambda of 1.1 under cruise and 52 degrees BTDC at -45 kPA gauge (cruising vacuum). I get no lean surging at this lambda. I can safely touch the exhaust after cruising for 100 miles at 2,500 rpm (obviously it cools down between me stopping, opening the hood, getting out and feeling the headers). If I have been climbing a long hill (-20 kPA) for several miles, I can't safely touch the exhaust headers.

I run lambda of 0.9 under WOT and (after talking to RETed) about 25 degrees BTDC at 2,500 rpm.

I have actually found that lean surge kicks in about 17.4:1 (oops, I meant lambda of 1.18) and that I actually get better instantaneous fuel mileage at lambda of 1.1.

If you have a 0.72 Overdrive (and you are in 5th) and 550 cc/min injectors, instantaneous fuel economy is closely approximated by:

90/injector energize duration in ms

This is if you are only using the primary injectors (staged setup).

If I run 1.18 lambda, I see injector duration of 2.7 ms, at -37 kPA (averaged over several miles of flat road, steady state driving) = 33 mpg instantaneous.
If I run 1.1 lambda, I see injector duration of 2.5 ms at -45 kPA (averaged over several miles of flat road, steady state driving) = 36 mpg instantaneous.
If I run 1.0 lambda, I see injector duration of 2.7 ms at -48 kPA (averaged over several miles of flat road, steady state driving) = 33 mpg instantaneous.
If I run 0.9 lambda, I see injector duration of 3.0 ms at -51 kPA (averaged over several miles of flat road, steady state driving) = 30 mpg instantaneous.