Smaller Throttle Body Beneficial at lower RPMs?

[nbleak21]

Originally Posted by wungun

Interesting idea...
Typically, a smaller diameter, longer intake tract is beneficial to low-end power, at the expense of high rpm torque/horsepower.
Exactly in rpm range you'll be cruising at.

I agree, but the OP isn't talking about changing his entire IM, just the TB, so I don't think that really applies, and if it does have some positive effect in that regard, I'd say it would be mitigated by the ECU compensating for the larger TPS reading at likely any given RPM.

[bestclimb]

The throttle plate restricts air at the TB. If you fit a smaller TB you will have more restriction there so you will open the throttle a little more for the same power. This will result in the same amount of air going in so the computer will send in the same amount of fuel.

The biggest change you may note is that it takes more "foot" so you will have to make your self push harder to go as fast as you did before. You may see an increase in mileage but it will be more of a driver behavior thing than from any magic happening at the throttle body.

A similar effect could be gotten by putting a heavier spring on the linkage.

[baldlobo]

First off for the record all chrysler, dodge and jeep products are MAP.

Second i've thought of this too; it might effect top hp by 2-3hp(the decrease in size is 8mm(0.32 inch)); it will increase vacuum and decrease throttle response.

but what the peanut gallery forgets is we're here for mpg increases not hp(most of the responses are from bench warmers).

i don't think a heavier spring will do anything other then help keep your throttle from jumping around on bumpy roads.(ps. i have one because wpg roads are crap)

[Varn]

If your car was carburated then you could improve metering by switching to a smaller venturi. But as others have said on a FI engine, little is gained.

I made up a 1<2 intake on my Yamaha Virago 700 instead of the twin carbs that it came with. Sure enough using one of the two carbs I got better mileage.

[RobertISaar]

Originally Posted by bestclimb

The throttle plate restricts air at the TB. If you fit a smaller TB you will have more restriction there so you will open the throttle a little more for the same power. This will result in the same amount of air going in so the computer will send in the same amount of fuel.

The biggest change you may note is that it takes more "foot" so you will have to make your self push harder to go as fast as you did before. You may see an increase in mileage but it will be more of a driver behavior thing than from any magic happening at the throttle body.

A similar effect could be gotten by putting a heavier spring on the linkage.

this guy gets it. with a smaller opening, you'll just be opening the throttle more to let the exact same amount of air into the engine for the same situation as a larger throttle.

however, i think in the end you'll end up with worse fuel economy. reason being that a LOT of stuff in the PCM is dependant on TPS readings. larger TPS readings mean you are closer to kicking into power enrichment mode, so you'll run into richer AFRs a lot faster. larger amounts of TPS will also delay shift points for the transmission, meaning you'll stay in lower gears for longer amounts of time or even get forced out of higher gears entirely.

being a MAP or MAF system won't make much difference here unless your PCM goes into a failure mode, in which case a MAF car determines airflow almost entirely from RPM and throttle position and a MAP car is from RPM, throttle position and a few other load adjustment parameters. if that were to happen, since you are going to flow less than what the PCM is calibrated for, expect significantly richer AFRs until the O2 sensor catches up.

[ChazInMT]

Keep in mind the basics here, you need a certain fuel to air ratio for your engine to run at the speed your going. This is going to require the same amount of air whether you use a big or small throttle body due to the displacement of the engine remaining constant. So, equal air volume either way means it doesn't matter how you choke off the flow, you're still choking off the flow the same amount.

Seems to me you run the risk of royally screwing things up by switching things out and you'd lose top end power which might come in handy once in a while.

My vote is don't spend the time & effort on something unlikely to achieve a gain and very likely to be a mess.

[Peter7307]

One temporary test would be to sleeve the original TB down to the intended replacement TB and try that.
Quick , cheap and relatively easy since all the rest of the ass'y remains unaltered.

If it works then all is well ; if not then remove the sleeve and you are back to the start to try another method.

Peter.