each of the two styles of throttle bodies should support a different kind of torque curve, on a 350 say, the smaller one should create better torque down low, but restrict higher hp, and vice versa on the larger tb, so, my question is, what rpm is each one suited for? like a cam or intake the tb should be working very well at a certain range, this should be considered like cam selection or anything else but i dont think ive ever seen it mentioned in this light.

anyone good with math know where the two tb's help to create power?

sorry editing my spelling turned into a double post

 

Not much effect on the torque curve unless its strangling the motor. It does, however, effect what kind of MAPs the engine will pull while cruising. I noticed on mine that the transition to PE is rather abrupt. Ill be gradually pushing the gas further and further and there wont be much of a difference in the power output and then, bam, it takes off. Turns out that under almost all but light acceleration and straight line cruise Im pulling around 90 kPa with the larger TBI at lower RPMs because it flows so much air. Basically its like a diesel at that point in that the AFRs are controlling the power output. In closed loop, normal acceleration. PE, fast acceleration. Surpisingly, the engine puts up with high MAP/lean AFRs just fine.

Basically, if you have a smaller motor that doesnt pull alot of air, the larger TBI will cause more issues then it solves. If you have a hot 350/383/400 and/or a higher stall TC, definatly step up to the larger 2" TBI.

 

I have similar finding to you, but PE is set at 25% in my Bins. As soon as I am on the gas, it is gone. 90 KPA comes in around 25-30 % TPS depending on the engine RPM.

My PE is very abrupt too.

My engine puts up with the lean A/F ratio to a certain extent as well. After it passes the threshold, it gets a little rough.

The PE on my old 312 was like a "hair trigger", in that it would litterally jump out at just a slight tap of the throttle as it entered PE.

 

kinda makes sense since you have a throttle blade that is moving faster, larger blade moving at the same rate covers more ground, faster throttle response and technically a shorter runner (in a sense, if you consider from the tb to the valve as the entire runner. anyone tried turning up the PE threshold to give you more definition?

 

i believe stock L03 TBI threshold is about 70% for PE. not sure what the trucks use in 7747 bins. many of us have lowered that. lowering will reduce gas mileage. i think i am it 50% currently and may go 40% and try that. putting the threshold a bit lower will put my onset of PE closer to the end of the AE event. in my logs i can see the AE time out and go lean just before the PE pulls the A/F back richer. that being with 70% PE TPS in .bin. at 50% there is a better transition of AE to PE but threre is still that time out and enlean evident.

 

in the AE cycle the motor is still in open loop right?

 

yes. AE is OL. but it will affect the BLM temporarily with enrichment.

 

while ive read quite a bit on tuning i have yet to do my own chips yet.
i notice with my car as i depress the accelerator you can feel the different steps the ecm goes through, like you can tell seat of your pants when you go into AE, PE, and such, with a good tune, can you smooth these different modes together, so that as you push the go pedal smoothly the car responds smoothly? i find it annoying that three inches of pedal travel yeild almost the same response but a quarter inch further and suddenly you're in PE.

also something just occured to me, with the newer p4 that fast and dimented have been playing with it was mentioned that there is a PE time delay option, is this accomplished by dragging out the AE cycle longer? still doesnt make sense to me how you delay pe without leaning out the motor

 

On this subject, I'll post numbers come next week. I am going on the dyno on saturday, the only change I made, was a 454 TB unit. So i will have numbers from last year vs this year, with no other changes. I'll see how it effects the power curve.

 

They basically have a transition from the closed loop AFR to the PE AFR based on the throttle opening rate, engine RPM, etc. Kinda annoying, so I disabled it.

 

Why would a carb say of 600 cfm produce bigger #'s as in HP/TQ than a TBI of 670 CFM(454/2.00 inch). we know the method of rating varies from carb to TBI (inch of HG). but it is believed by many that a carb will out perform a TBI on dyno with similar flow #'s. how can that be? if one see 95-100 MAP with TBI on WOT what possible advantage to go carb assuming air and fuel are totally adequate with TBI for that engine config?

 

carb is self tuning in a sense, tbi is not, carb has been around so alott of people know how to set them up whereas tbi isnt top on the list for an engine builders fueling system..also carbs spread the air out in the intake more through more ports(4 barrel) so higher velocity and lower turbulence inside the plenum. all educated guesses, what do the guru's say?

 

one other thing, i believe the only reason there are only two ports on a tbi and not four is because of the complication/expense in doing so, fuel lines and such, plus more injectors and wiring and crap. i think that while the same cfm can be used, more smaller ports means better velocity and fuel coverage so better atomization and more even flow into the plenum. the injector tower blocks a good 30% of the back airflow path rather completely, that hurts the flow rating as well.