dual plenum part two.....

[merlot566jka]

What would be considered small plenum vs large TB and vise versa?

Like a 3500 plenum and a small port 3100 TB? (52mm)
Or a 3100 small port plenum and a 65mm tb?

I had a 56mm on the 3500 plenum and it did pretty good. The 52 seems about the same maybe worse. I do seem to lack power in the high RPM compared to lower RPM, but that is probably because I still have the stock exhaust on. I should have better high RPM power than before because I went from small plenum, small long runner, to larger shorter runner and big plenum. The car is faster though. Just choking on the high RPM.

I will find out soon if it's choking because of the TB or exhaust. I'm getting a 62mm TB any day now. If it doesn't help in the upper RPM I know it's the exhaust (which will be fixed soon). And then I can test the TB's again.

I usually go by TB size by the amount of power the engine is making/capable of. 52mm is pretty limiting. A good idea is to look at other factory engines making the power yours will be making and see what TB size they have. But not all are a good example though. Some have huge TB's compared to the power they make. I haven't really seen any high power motors with tiny TB's on the other hand.

well according to what ive read, this is for overall power for the throttle body size vs the plenum size. i think they mean throttle size realitive to what should be on there for the size of the engine. for example, a 3500 has a 66mm tb with the stock plenum size, the throttle body should be ideal (if gm has made the best combo here). if you went to a larger plenum, you shouldnt increase throttle bore. if you went to a smaller plenum you should increase throttle bore. now i dont know how true this is, it seems worthy of testing. yet i have no way to test this kind of stuff. nor can i even fathom how to test this outside of using an engine dyno to record the results and compare. sounds like lab work to me.

in your case, not knowing completely your build, i would guess your lacking on the top end because of the lack of fuel. (and 200k miles, lets take a guess at what effect compression has) by looking at your sig, you have 3400 large port lim and 3500 upper....you now have the capability to ingest more air into the engine, but are you getting enough fuel now? it will show in your afr on a dyno whats happening to the power at the top end. now once you have enough fuel, then you can see whats going on on the top end as far as air. then you can go from there on the exhaust.
but this is all 'fixing' what you have done. not to say there is anything wrong, but its making everything work together. this can show what was thought would hapen when you combine things the way you have. but to predict the out come based on theory is a little out of my relm. i would like to say that i know what will happen, but i can only guess based on what i know.

looking at tb size is good for a comparison. but like youve said, its not always best. for example an individual throttle body engine...there could be 8 trumpets all 48mm and it only make 300 hp. vs an lt1 who uses a twin 48mm tb and makes the same hp. obviously things dont line up.

tb size should be a function of several things, desired engine speed, engine size, plenum size, and drivability concerns.

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[torq455]

my stock 3400 rounded off at 5500 rpm. or lacking top end. a tune UD pulley and the biggest--headers no converter. now my 3400 has top end. feels best to shift at 6000 rpm and is fastest down the track. it's not all about TB and plenum size. it's where's the air flow restriction/s is/are.

[IsaacHayes]

Stock 94 3100 with 3400 LIM and 3500 UIM. Stock 1.625" I.D. downpipe and exhaust and cat. Resonator removed, and muffler replaced with UltraFlo. Stock pipe is 2" O.D. press-bent. New O2 sensor, and timing chain. Intake ports in the head are clean as a whistle, so is the back of the valves. I seriously believe the exhaust is the key factor limiting me now. With the intakes I'm making more power everywhere, but the upper is being choked so it feels like it falls off. The compression is good I'm sure. I will get a datalog today if I can get traction (ice melting chemicals keep the roads wet and slimey) to see how bad fueling is off. There is no denying my exhaust system sucks. Hell a 2.0/2.2 OHC beretta has a better downpipe than my car (not double walled). And the exhaust manifolds we all know are a joke. The exhaust side is what's bottle necked on these/my engines. The heads and intakes flow pretty good.

What manifolds are you running? The 3500 short runners?

Torq455: Exactly. Exhaust on my motor and the manifolds in general are a piss-poor design when you compare them to the intakes/heads.

I wonder how much smaller and bigger a TB we are talking when changing plenum size. Perhaps it's just a few mm and the gain is just a few hp, nothing drastic. Hard to know without testing. One thing I know, is though I have gone to a bigger plenum, I'm not gonna make my TB smaller than stock!! 52mm seems at the limit of the stock HP anyways.

merlot, have you thought about doing a simple Ferrari/dodge viper style dual plenum/TB? You could still probably plumb in the LT1 TB. Take a look at those plenums for ideas on getting equal vacuum to accessories and such.

[bszopi]

http://www.theoldone.com/archive/int...old-design.htm

[torq455]

haha wow! summer of 1998!! where did you dig that one up Brad?

[merlot566jka]

What manifolds are you running? The 3500 short runners?

merlot, have you thought about doing a simple Ferrari/dodge viper style dual plenum/TB? You could still probably plumb in the LT1 TB. Take a look at those plenums for ideas on getting equal vacuum to accessories and such.

trueleo headers, had to port em out, but they are good now. headers go into a 2.5inch collector then to a 2in pipe, to a 2.5 in for the y pipe merge, and 2.5 all the way back.

i have, and wondered if it was more practical or not... too bad i have a transverse mount engine...i would have done it along time ago, they look so badass

[merlot566jka]

http://www.theoldone.com/archive/int...old-design.htm

read it a while back. along with your thread on a new intake...it was like 500 pages long. good info

[bszopi]

haha wow! summer of 1998!! where did you dig that one up Brad?

Its one of the best sources of technical info out there (the Endyn website), and Ben and I always like to refer to it. They have several little forum outtakes in their archive area that are good reference for people.