Asked this same question on thridgen and didn't get too much feedback.

Been awhile since I messed with TPI. My last TPI I ported the plenum and siamesed the runner pair exits with good results. Later I saw some have siamesed the throttle body holes and I wondered how that affected the tpi. Now That I own another TPI I am thinking of giving this a try. Not the whole area I'd leave the triangle on the bottom and I may weld up the egr holes.

 

I don't understand how you would siamese the twin throttlebody bores when you have two butterfly. Thee is a monoblade throttlebody out there. Is that what you are referring to? Are you running a 52 or 58mm throttlebody now?

 

I put a 58mm throttle body on my 383 and siamesed the front of the plenum only because it made it easier to get the tooling in to widen the outer sides of the plenum. I don't think it makes any difference airflow wise, tpi's runners are the airflow hindrance, not the tb.

 

I did that on all the TPI/LTx plenums I did only to make it easier for the tool. Theres no real "gain" to be had outside of that.

 

Oh, I get it. The plenum devider is removed. Little slow here.

 

my plenum machined
[IMG][/IMG]

 

Good point should make it eaiser to rework. I was kinda wondering about the air flow, but it didn't seem like it would really affect it.

My main thought it is adds a lil plenum volume.

 

I've done a bunch of porting and a handful of TPI sets and I have yet to see any differences with the TB bores opened in the plenum.

TPI works in pulses, or waves,. That begins at the backside of the TB blade and the 'wave' then travels deeper into the plenum with each intake valve opening as it creates a surging effect that creates this wave. The flow is fairly slow at the TB blade, so the flow is easy to achieve thru that area. The magic is in the runners where the velocity changes due to the single, smaller passage that causes the air flow to accelerate toward the intake valve.

If anybody remembers a few yrs back the hot ticket for TPi was the spacer block to move the TB another 1" away from the plenum. This added distance, which equates to another wave or pulse. Those were all tubed, with a tunnel for the air to travel thru before getting to the plenum opening. They were an added 1" of a tunnel for the flow to travel thru. Not Siamese at all. That DID create more low end power due to the higher vacuum that pulled harder against the TB blade sucking (trying) to get air...so it accelerated the flow when it DID get some air and that would stuff a tiny bit more air-charge inside the combustion chamber as the intake valve closed. Result was a bump in low end. The downside (always is one) was that the top end that already struggled for a breath, now dropped from 5500 where it starved down to 4500 with the spacer. The additional wave/pulse was inefficient at higher speed. Good at low speed, bad at higher speeds.

I have often wanted to experiment with baffles and shapes inside the plenum to assist flow into each runner...like velocity stacks on carbs...or the ducts on the front of a jet engine. The shapes cause air to flow into the port faster and more efficiently than a plain, opening with no directional shape in front of the 'hole that its entering. Siamese runner openings do something to aid the flow shape as it enters the runner tube...the more like a velocity stack with the curled or rounded edges...the faster air will enter.

If anything, you might bevel the tb/plenum edges to be certain there is no ridge to interfere with the flow, but the bridge between the ports matters little since the flow is so slow right there...I seriously doubt that cutting the plenum will hurt anything...just don't think it'll help.

Note:

plenum vol was mentioned, and I do know that the plenums on GM F body cars was cleaner, therefore larger inside then the Chevy Corvette plenum...for why I do not know or understand but if you set them side by side the GM version is cleaner inside with a bigger "box" for the air to travel thru. I siamesed one and installed and there was a noticeable bump across the board in power. I attribute that to the runner openings AND the size of the plenum insides. Damn hard to get tools deep inside the plenum to clean up the ridges and bolt bumps...the gm casting still has some stuff inside, just less pronounced and less ugly.

 

Good info thx Leesvet. Still got my 85 TA tpi I ported, so guess I'll clean the paint off it siamese the front and use that plenum.

 

Hmmm. I just put an F Body plenum on my car along with a remanned OEM 52mm ported TB and keep thinking the SOTP feels more power. I only changed my plenum cuz I was polishing the F Body one a little at a time.

Nice post BTW Lee.

 

Thanks !
IIRC there was the difference of the 2nd egr hole each side on Y body TPI cars and the F cars are a single egr port as far as I know.. Mine was. No issues bolting the runners to the new plenum. Looking in thru the TB ports, you can see that the humps are smaller, the over-all "stuff" that protrudes out into the airstream appear to be smaller or less in the F body plenum...The one I had, I had to look close to make certain that it would even fit ! Over all, it looked like it was smaller on the outside, yet roomier or more open inside, if that makes any sense...not real sure why.

There was enough difference that I thought it must have been for the baby V8 tpi. It was an obvious difference.

All I did was cut the runner ports, beveled those edges, gasket matched ea port and knocked down whatever bumps and casting seams I could get to...and bolted it up.

Like I said before, to me, it sure seemed like an increase across the entire range...smoother, good revving and the low end stayed. I was worried that it was just SOTP placebo effect but now I am fairly sure this plenum, with the mods DID something !
May have been from tossing one EGR port..? naw...had to be my porting !

Seriously, I like this one better. I'm gonna hog out the Y plenum and polish it and set it on the shelf ready to go in case of a need somewhere.

Makes me curious, I wonder if there are differences in the F body intake base? any that could be untilized with porting and matched pieces maybe? I look for projects like this just to find something and build a nice piece and do it cheap. That's the fun of TPI projects....almost anything that you do will be an improvement, so the risk of screw up BIG are smaller than the odds of winning the state lottery.