Just wondering if anyone has made up some data points on how a change of resistance in the head temp circuit effects the mixture?

I know of some one long ago that had installed an additional head temp resistor in line. They wired a variable resistor (potentiometer) all the way up to the dash so they could add enrichment on the fly. They had no airfuel meter so it was all seat of the pants.

I am considering that a a possible fine tunning method on a D-jet 914. With an aftermarket cam, big bore pistons and non-stock compression ratio, I think there might be some room to add enrichment across the range.

what I am thinking is to add an airfuel ratio meter, or go by head or exhaust temp as a guide, and adjust the mixture via this method and watch the gauges to try to find an optimal setting.

Has anyone already tried this?

How does head temp reistance effect the mixture? is it uniform across the rpm and load range? or does it only effect certain running conditions?

Just begining the thought process on this, and dont want to re-invent the wheel if it has already been tried.

By running a parallel reisistance, one could also lean out the mixture.


I know some folks will tune the Pressure sensor (MPS) to change mixture, but I am thinking this is one more tunning tool maybe,and with a variable resistor wired to the dash, it can be changed on the fly, unlike MPS adjustments.

Brad Anders has done circuit analysis of the ECU boards. He had some comments about the CHT resistance and its effects. You may have to search around a bit to find the info: http://members.rennlist.com/pbanders

I don't think anyone really knows the exact effect of CHT resistance on mixture, probably because there are a ton of variables that go into determining the mixture on a running car.

In general, the higher the resistance the richer the mixture. The effect pretty much goes away completely under about 100 ohms, if I remember Brad's writing correctly.

The table about 2/3 of the way down this page (CLICK ME!) talks a bit about the CHT's effects.

Aha, here (CLICK ME!) is his very detailed discussion of what the CHT circuit does inside the ECU!

--DD

The MPS won't like the cam overlap. Maybe LJet or Mega Squirt but not DJet. Do a search. It's been discussed many times here.

thank you, this data is starting to help me figure it out.

Thaks again!

my understanding (but go read all of the brad anderson stuff)

the CHT is designed to only enrich while the motor is cold for the first 10 or so minutes...

yes you can "trick" it into thinking the motor is always cold so that it effectively "always" has the "choke" turned on....

its probably across the board
but its a gross enrichment and not really good for the fine tuning of an engine management

You'd be much better off tuning the MPS and doing things the right way
the MPS adjustment allows you to tune multiple settings... ie: you can tune WOT with the MPS and also tune partial load (2 settings, unlike a megasquirt with 2 million settings)

the other part of your problem is the cam
the Djet system operates on vacuum
a big cam has high overlap
during the overlap periods there is a pulsation feeding back into the plenum
it screws up your vacuum signal

you really need to use a mild cam with DJet
the raby djet cam is supposed to be the best beyond all others.

I have an Elgin cam made for FI D-jet, so the cam overlap is not an issue for running D-jet. It runs much smoother than a friends car with simular engine with a raby cam. so I think the cam is not an issue. could be this engine needs tunning becuase of the mps drifting from factory settings (it is near 40 years old afterall) or other factors.

friend with raby FI cam has tried adjusting the MPS with so-so results, (still bogs down on start from a stop, and has a flat spot.) One other thing is he cant adjust the mps on the fly. which makes tuning more time consuming.

from what I read on the Anders site if I add resistance I may richen over the entire range, but may over richen at cold conditions if I used a fixed value resistor. that might be a problem, I dont know.

cool then you cam sounds like you don't have a problem.

you really need a wide-band sensor

I've tuned a couple of MPS' with my innovate
its easy...

you prep the MPS by taking the puddy out
you find an open bit of highway
you do WOT throttle runs to 60 or even 70mph
you pull over... adjust... do another run

then you tune the same way for cruising

its really easy to do
but without the wide band you are only able to guess by the seat of the pants

a wide band will tell you exactly what is really going on.

yes a Air / fuel meter is probably the way to go (more money!!!)

Have you any experience in tunning via a head temp or exhaust temp reading? i know that exhaust temps have been used for tuning, probably before Air / fuel meters became popular.

I really don't
I know exhaust probes we're the way to go for years
and still are a great way to go

but rich is good with a 914 motor
so tuning to the absolute limit is not necessary on these

all of tuning I've done has been with the innovates
a hardwired gauge type... and a hand held type

most of my experience has been tuning carbs
and like I said, a couple of MPS's