strange combo

[Peter]

Hey guys,
My engine uses Jenvey 48 mm throttle bodies, but throttle response isnt the way i want it to be...
was thinking that the throttle bores are too big for the engine, so i would like to try to use some venturies
to gain response... might sound strange to use venturies with injection, but i think it could help???
So what size should i get? I was thinking of 40 mm venturies to put in the 48 mm bores..
the engine is a 2276 with rev limit at 7500 or something..

Thanks!

[GeirH]

What kind of EFI do you have?
You have to fill in the accellerate map correctly.
Don't think "to much air" is your problem, to little fuel is my bet.

[Peter]

its a megasquirt..
problem with the accel now is that when i push the pedal to accellerate, the pedal movement is too slow to trigger accel enrichment,  but in real life the engine want accel enrichment... difficult to explain... with small or slow pedal movements the engine sees big changes and the accel enrichment is not triggered

[John Maher]

Unless you're making close to 300bhp n/a, 48mm TBs are too big.
45s are good enough for 260bhp.
40s should see 200bhp.

When the TB is way too big you have a lot of airflow with very little throttle movement, especially just off idle.
At the moment you're probably supplying your engine's max airflow demand at approx 60% throttle position, making your accelerator pedal behave almost like an on/off switch.
Pushing the pedal upwards of 60% makes no difference to performance because the engine can't make use of what's available.

I've never used a TB venturi so don't know how it will respond.
Ideally you size TBs so they're capable of supplying the engine's peak airflow requirements but with no more in reserve than necessary.
This delivers best pedal graduation and response.

You could change leverage of the various parts in the connection between the pedal and TB to slow down movement at the butterfly and maybe use a stop to prevent throttle position straying beyond the throttle position % that achieves max power. Then reset open and closed settings for the TPS. Don't know if that's possible with megasquirt?
But it's a bodge 

A venturi should help but still doesn't overcome the fact the butterfly is too big.

I think people assume TB sizing isn't too important with EFI. It does make a difference.

[Karmann]

What you could try is to make up some kind of progressive throttle linkage. I had the same problem on mye engine. It was either idle or 3k rpm. Impossible to drive around town. Looked like I had filled kangaroo-fuel. I manufactured a device for the centre linkage made out of pieces from an Opel Omega throttlebody, which has progressive throttle, and some homemade pieces. Now 50% throttle at the pedal is about 25% open butterflies, the remaining 75% comes on when I press the last 50% on the pedal. The transition from idle to partially throttle is now much smoother and the car is very drivable.

[wph]

Are you using speed density or alpha-N ? You're definitely going to see tuning problems with SD,
go with alpha-N unless you're already . We are currently running 46mm throttle bodies on
a 2000cc engine with rather aggressive cam lobes. I can get the engine idle smoothly below 1000rpm
and the transition is almost like a stock fuel injected car. You need to set the ignition and fuel table
tuning points to match your torque and power curve as closely as possible (rpm wise) to get some tuneability.

Throttle linkage has to be "smooth" and preferably progressive, it helps you set the Y-axis of the
tuning tables on those "hard to tune areas". If you can get access to WBO2 use it with the VE Analyzer 
function of the datalogging software. It took only few 5 minute drive around sessions to get
a very driveable low rpm fuel maps. When basic mapping is flawless you can almost turn off the acceleration
enrichment, at least you will not need to use it so much.