Here is my take on EGT's , use with some caution your mileage
may vary and the plugs are never wrong... I think I got the spelling
right on most of it but did write this over a couple days...
Using EGT to measure the fuel mixture seems to be a
carryover from aircraft and other fixed engine use. The
problem and/or difference compared to what most racers need
is the engines this is normally used on have some
differences. Most notably cam timing, you generally don't
see much overlap in a cam designed to run at 3000 rpm.
Second is they ran a spec fuel with known burn rates, and
the timing was fixed and held constant. Third and final is
they had data! They knew that and EGT of so much would
equate to an air fuel ratio of so much.. Plain and simple
they had data!
Now to use this information on a race engine is interesting
but only tells a part of the story. you really need to know
a few things about your engine to make EGT's a useful
number. First you really need to measure the air fuel ratio.
You can rough it in with reading spark plugs, but they only
tell part of the story. You can run a warm plug and be
slightly rich and look ok and you can be lean with a cold
plug and look ok to certain point. The only thing I might
add is a projected tip plug is a little more forgiving in
heat range than a standard gap, so if they fit in the engine
they can be helpful, and might even save your butt in a jet
guess that is too lean. The reason is they by design run a
little warmer at light loads due to being extended and by
the same token with high air flow rates and turbulence in
the chamber run a little cooler. They also put the spark a
little closer to where it needs to be in the center of the
chamber. This will not work on most supercharged engines,
heavy nitrous or nitromethane as there is always some
detonation due to cycle by cycle differences in combustion.
This will shatter the porcelain in the plug so you need to
get it out of the blast area.. But for a N/A engine, or with
low boost or a little nitrous they are fine and offer a good
compromise.
The real first piece of info you need is the air fuel ratio.
i use a Uego 5 wire sensor and corresponding amp to go with
it. This allows me to read air fuel ratio in real time that
vary from 10/1 to 20 or so to 1. Useful numbers range from
11/1 to 14/1 and do remember these do not really measure a
true air fuel ratio they measure excess oxygen in the
exhaust that can be inferred to an air fuel ratio so your
choice of gasoline will affect this some. Most notably
oxygenated fuels tend to want to run a little richer on the
fuel meter and the plugs may in fact look a tad richer but
not much if the heat range is correct. I set up air fuel
ratios for the following types of engines in these ranges.
N/A 13.0/1 to 13.8/1 remember lean is mean and it will
generally go faster right up to the point it melts... Also
very rich is not safe either as most engines have a rich
limit for detonation also. Yes you can throw a bunch of fuel
at it and blow it up. Nitrous engines run the same off the
bottle although generally like a little richer due to the
lower compression( if it has been dropped to run a big load
of juice). On the bottle I shoot for 12/1 to 12.5/1 and
sometimes leaner if i am really going for the big hp number
but it is not generally all that sensitive. Supercharged
about the same as nitrous until you get into big boost
numbers and need to try to cool the charge air down. The
heat comes from the compressor with roots style being the
least efficient 50% typical, turbos generally the best 75%
typical. An engine with a turbo and a good inter cooler will
usually be able to stand 13/1 at 20 or so pounds of boost if
the fuel and timing is correct but will not make much more
power than it did with an A/F ratio of 12.5/1 but will be
more
responsive.
So now we have a handle on the air fuel ratio the first
thing we needed to know. Now EGT might be useful. most
engines have been tested enough to have known timing
settings that are correct or very close with a given fuel.
So if you start there you will be fairly close.
Now for EGT. If you use a probe in every cylinder the first
things you learn are cylinder to cylinder fuel balance to
weed out poor intake designs and for those afflicted with
carbs you can balance the jetting some to even things out.
before you make yourself crazy with that though make sure
the timing is the same on every cylinder, mark the damper
off in a firing position for all cylinders and check each
one to rule out a problem with the ignition pick up. Also
make sure the valves are lashed correctly and do a leak down
test to make sure all the 'soldiers' are feeling well. If
that stuff all works out ok then start by stagger jetting
the crab to get them all the same. remember that if you add
some fuel to one corner you might have to take a little out
of another to keep the air fuel ratio the same. If you have
EFI and it is sequential then you can balance each cylinder
that way in short order as most will allow a cylinder trim
for fuel and ignition also so that really gets those things
fixed quickly. well now we have all the EGT's reading the
same but possibly to high or low. before we make any other
changes you have to consider the burn rate of the fuel and
if it is suitable for what you are trying to do. Most high
octane fuels will burn slowly until there is enough pressure
and heat to get things going, there is no sense running 116
octane fuel in a 10/1 compression engine unless it will see a
lot of boost or nitrous, so if it is a N/A engine with no
nitrous pick a suitable fuel to start. I personally don't
think you will go too far wrong with a N/A engine with the
following selections. 10/1 95 motor octane,12/1 106, 14/1
112,14+/1 114 to 118 motor octane. If you have nitrous and
it is a small amount the next fuel up from where you are
should be close. If you have a lot of nitrous or big boost
#'s my personal favorite is VP C-16 which is fine if you are running
the fuel classes as you can run what you want but for a
supercharged gas car I suspect the best you will do at
Bonneville is A-8C. This fuel has good octane #'s but i have
yet to get the EGT's anywhere close to what I consider
acceptable. Maybe they have a better fuel to try but I have
not found it. All the others are listed at ERC as being for
a N/A engine or the octane is not where it seems to be ok
for me. Now that we have a suitable fuel all that you have
to envision with the EGT is when it is high you have
probably started the burn too late so it is burning in the
pipe. If you advance the timing a little ( a couple of
degrees) then you should lower the EGT a bit. I shoot for a
target # of 1350 F under full load and if it creeps to 1375
or 1400 after a mile I don't even get concerned. By the same
token if the EGT is 1200 and the air fuel is ok I will pull
a couple of degrees of timing out to see if there is some
power to found there. If you are making changes in timing of
more that 3 degrees or so of known good #'s for the chamber
style you have then stop and rethink what might be going on.
Altitude has some effect so you would expect to run a degree
or two more at bonneville than at sea level as the cylinder
pressure will be lower. If you have really big EGT #'s like
1700 on a N/A engine you do have a serious problem on your
hands. First thing i would question is the fuel, then the
cam profile, and last are the exhaust valves seating? if
you find with an iron headed SBC you have #'s like 44
degrees I suspect that you have lost the rabbit so to speak
somewhere. I might even start by checking TDB and the timing
marks to see if they are correct before procceding to melt
the engine. The temperature is not a problem for most of the
parts other than the exhaust valves and seats. As witnessed
by Mr. Turk last year when he overheated 3 of them and they
fell apart. he did go 228 on 5 out of 8 cylinders though
LOL. one last thing to keep in mind, every degree the charge air temp goes up
the EGT goes with it the same amount so if you dynoed with 50 degree air and you
go to Bonneville with 100 or so expect the extra 50 degrees and there is nothing
you can do about it other than running an inter cooler on a N/A engine which is
seldom practical as the pumping losses generally even out the temp loss for air
density.
Dave Dahlgren
Keith Turk wrote:
>
> Yeah I am interested in hearing it.... I know my story on EGT's.... it's an
> amazing deal when you see the gage spike past 1800 and don't give it a
> second thought....
>
> With the ERC A8C the fuel burned SO SLOW that it afterburned in the headers
> and just became part of the drill there at the World of Speed.... watch the
> Egt go to the Moon.... Hit the Nitrous and watch it Jump back down to
> 1300...( which is just a tick cool on my sensors ) and the Plugs showed a
> Very safe tune up ( thanks Dave )
>
> The VP C 16 we ran this weekend didn't have that as an issue... max EGT was
> 1475.... which is a tick hot but not unlivable.... ( I have had it up to
> 1575 on a long run and not touched the top of a piston ) I think we need to
> move up one notch on the VP scale fuel wise to help this out a bit...
>
> Best Egt's so far where with Torco 118 ... the Motor just seemed to like it.
> Egt was between 1325-1375 everytime we ran it....
>
> Timing was always at 38 degrees and we retarded it 2 degrees for every 50hp
> of nitrous added.... those were always constant...
>
> Keith
> ----- Original Message -----
> From: "Dave Dahlgren" <ddahlgren@snet.net>
> To: "Keith Turk" <kturk@ala.net>
> Cc: <ardunbill@webtv.net>; <land-speed@autox.team.net>
> Sent: Wednesday, April 04, 2001 8:15 AM
> Subject: Re: EGT
>
> >
> >
> > Keith Turk wrote:
> >
> >
> > The short story is EGT may tell you something about mixture
> > but less than a lambda sensor will, at least a good 5 or 6
> > wire one. EGT will tell a lot about ignition timing too.
> > More later if anyone is interested in what i have to say. I
> > have been burried at work here..
> > Dave
> >
> > > Amazed Dave hasn't jumped on this one.... Spark plugs are okay for
> tuning
> > > information but NOTHING like a real printout from a decent computer....
> I
> > > have seen several of these and understand the concept but with my budget
> the
> > > best I could do was a Dual EGT gage from Auto meter that monitors the
> > > highest and recalls both high reading's at the end of a run.... ( then I
> > > have to remember to clear them before the next one )
> > >
> > > it's a great tool for tuning not a data logger but it's been a wonderful
> > > toy...
> > >
> > > I put the probes on the 1 & 2 tubes cause that was what I thought made
> > > sense.... to be honest I don't think it makes much difference.... what
> is
> > > important is the differeneces when you run .... from your established
> > > baseline.... each one of these things is going to be just a tick
> different
> > > simply as a matter of where they are installed... so your baseline
> tuning
> > > should allow you to know what is too hot.....
> > >
> > > Dave made a great point a while back about keeping the Probe VERY close
> to
> > > the outlet to read REAL egt rather then a cooled down number that
> results
> > > from the headers cooling the charge...
> > >
> > > The Archives should have some great info on this subject... check out
> > > www.listquest.com under land speed racing....
> > >
> > > Keith
> > > ----- Original Message -----
> > > From: <ardunbill@webtv.net>
> > > To: <land-speed@autox.team.net>
> > > Sent: Wednesday, April 04, 2001 7:26 AM
> > > Subject: EGT
> > >
> > > > Hi Folks, last time I had a ride in a Beechcraft Bonanza I noticed the
> > > > owner had installed an EGT display which continuously monitored each
> of
> > > > the six cylinders.
> > > >
> > > > Just talking theoretically, each cylinder on a V-8 is bound to have a
> > > > different EGT, so for tuning purposes it would seem you need a
> computer
> > > > to record all cylinders during a run so that you
> > > > can make changes later accordingly. I gather people do that changing
> > > > nozzles for each cylinder with mechanical fuel injection, and with EFI
> I
> > > > suppose each cylinder can be individually managed with the computer.
> > > >
> > > > Tom already told us he changes bypass jets at Bonneville during a run
> if
> > > > his EGT so indicates. I imagine if the EGT is high it means a lean
> > > > condition and the engine wants more fuel. As John just pointed out,
> > > > there's not time to do anything but shift gears and steer at Maxton.
> > > >
> > > > Cheers from Ardun Bill
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