Stock ECU's and large changes to throttle body sizing

[Chris Wilson]

I have been asked on a Toyota Supra MKIV twin turbo forum whether a stock Toyota ecu will be happy if the owner fits a big Q45 throttle body and large volume plenum to a basically stock engine and a stock ecu. My feelings are there will then be a big disparity between throttle position sensor voltages and MAP sensor voltages that the stock ecu may be incapable of resolving. May I ask your thoughts even though this is not MOTEC related, although, as usual, I have often advised forum members of the superb support and tuner base fitting a MOTEC would bring Thanks.

[stevieturbo]

I think it would be best to beat him around the head with that 10x excessively large TB for the power he will be making.

I'd say it's bound to cause many issues unless he can re-tune the ecu.

[greenamex2]

Even if the sensors were identical (to be honest, they probably are but couldn't guarantee it), the fuel map would be totally wrong.

You could possibly correct the fuelling a bit with an adjustable fuel pressure regulator but at best it would be safe rather than correct.

In short, it will probably make the car worse unless you can remap the ECU.

[Scott@FP]

Aren't they mass flow in OEM form?

[AdamW]

Aren't they mass flow in OEM form?

I have never worked on a Stock supra, but I think JDM versions are SD and US versions are MAF.

In any case I'm not sure I agree with stevieturbo's comment, I cant see how you can have a throttle body that is "too big" on a plenum intake engine. On an ITB engine where the throttle forms part of the port and therefore will effect port velocity then yes, you could definitely go "too big" and effect VE, but I cant come up with any logic that would explain how a big throttle blade on a plenum intake is going to change air flow characteristics much.

The only real "tune" issue it might cause would be at small throttle openings (say at cruise) the bigger blade will be more "closed" and a smaller change in throttle position would have a bigger effect on airflow, so in my mind the biggest issues would be things like accel enrichment (if it is based on TP in the stock ECU) and possibly things like deccel fuel cut would initiate or terminate at the wrong TP.

[stevieturbo]

Aren't they mass flow in OEM form?

I have never worked on a Stock supra, but I think JDM versions are SD and US versions are MAF.

In any case I'm not sure I agree with stevieturbo's comment, I cant see how you can have a throttle body that is "too big" on a plenum intake engine.

Why fit a TB that can flow enough for say 1500hp, to an engine that will only make 500hp ?

At say 40% blade opening, it might flow all the air the engine can consume, rendering throttle resolution to crap as the latter portion of opening might make very little difference to engine output.
Also it then gives very poor driveability at the low end, because a very small blade angle change can result in a huge increase in airflow into the engine

There is just no sensible reason for some of the huge throttle blades some people use on their turbocharged builds

Even n/a you could lose resolution from going too big but it would be harder to go excessively big compared to a turb setup

[Chris Wilson]

Sorry for the very late reply, I lost my forum log in password and have only just been able to post again. Thanks for the info. Some 2JZ engines use an air flow metering unit, the Jap market cars pre the VVTi engine used a MAP sensor (SD).

I see Mazda used a dual stage TB on the FD model rotary engines, and I recall seeing others with a progressive mechanical arrangement of opening the butterfly, but I agree, a hige TB on a turbo engine just gives all or nothing from closed to say 1/3rd open, then not much change thereon. But a lot of folk seem to think a huge TB off a big N/A V8 is a must have modification on their turbo engines.

[stevieturbo]

Even if you look at the mechanical cam that opens the blade, most will be just that, a cam lob shape rather than simple circle so that it takes more pedal for smaller openings than it does near full throttle