I am in the middle of making the runner holes siamese and opening up the throttle body holes. However, I was wondering if I could make the throttle body opening oval like a miniram? Will this be of any benefit or will it hurt me?

 

No need to make extra porting work for yourself, just make sure the TB openings are smaller then the plenum. I think I opened mine up to around 53 MM and have a 52 MM TB.

 

Porting the stock runners are not worth the effort IMO. They don't have much meat. Get some after market runners and port those, then yes I would do in fact believe they would provide horsepower gains. Just my .02

 

Thanks. I cannot buy bigger runners for my class. They have to remain stock but I can do some mild porting. Looks like i will just make the throttle body ports bigger and leave it at that.

Thank guys!

 

 

i am planning on getting the edlebrock tpi runners and baseplate manifold. How exactly are you supposed to port them out?

 

How closely do they examine for aftermarket stuff? AS&M runners look virtually identical to stock. I really doubt someone would notice the difference without close examination or measuring. Not that I'm advocating cheating or anything

 

 

Ive thought about it! Im actually chasing the club record for my class at one of the tracks we run, so if I get it I would feel bad knowing I cheated. Its just some local time trials but our club uses NCCC rules which dont allow much for my class (2E).

 

I agree that porting the stock runners has no advantage. I do advocate porting the plenum both behind the throttle body bores and the plenum to runner exit. In the area where the runners bolt to the plenum, you want to radius the exit on the plenum, not open up more than the runner.

Probably the largest area of restriction is the manifold itself. If you want to see more substantial gains, target this area.

FWIW - I saw a guy run 10's at the track with an 88 Firebird GTA. The car was forced induction but had stock runners, plenum and highly ported but stock manifold.

 

Cut them open. Or just buy AS&M since they are already larger.

An example of cut open to port:

 

Check this link... http://www.compcams.com/Community/Ar...?ID=1737510521 There you might find some difference between stock runners vs. siamesed (Yes, that engine is not stock 350, but still...)

 

I don't think opening the TB holes into an oval will help you. Here are some shots of my "new" 89 plenum after a recent porting the runners have been siamesed and the TB holes are now 58MM:

 

You mean like what I did?

 

Look at the pics of mega-porting on CorvettePlenum.com. They show maximum porting w/o cutting SLP runners open. Cutting open runners only makes sense if you don't want to use a short-runner intake.

The problem with Edelbrock Hi-Flo runners is their tubes aren't merged together to the extent that the SLPs are. So, siamesing potential is much more limited with the Edelbrocks. But, I do believe you can siamese them like the SLPs are in cast form. (Again, CorvettePlenum will show a pic of what I mean).

If you're asking how to port, use a carbide burr or a dremel. Flute the top as much as possible. Port match to the plenum and heads. You can even siamese the plenum as shown in a prior post of this thread.

The Hi-Flo runners may not be that different than ASM's btw. I've had other forum members measure ASM's, SLPs, and I had a set of Hi-Flo's. They all measure 1.65". But, it's hard to measure the uniformity of the tubes. If the ASM's trap 1/10 of a second faster, maybe they're slightly bigger and/or have better uniformity.

1/10 = 10hp which is hard to feel, but does win races. And, siamesing the Edelbrock runners like the stock SLPs might get most/all of that back. (On the SLPs it's supposed to improve hp ~8 over non-siamesed runners).

Siamesing the base is another option -- if you want to go down that road.

Regarding the original question: I agree add'l porting beyond the TB openings is a waste of time. Just make sure you grind down the humps behind the plenum openings.

 

That looks nice! Are you using a mono-blade TB?

The only other reason I can see to do this is you believe cross-suction between the middle opposing tubes interacts at high-speed. I've wondered what the result would be to insert a divider between the center holes (tubes 6&8 + 1&3). This idea came to mind after I read the CompCams TPI shootout linked in this thread. I wondered if this might be why the siamesed ASM runners seemed to hit a ceiling with higher rpms.

Since the ceiling got worse with higher rpms, I have to assume reversion waves are being disrupted somehow. Cross-interference is one possibility. The other could be inter-siamese-tube turbulence.

 

Yes I'm running a monoblade. Making 370rwhp at 6400rpm as that is as high as we went on the chasis dyno. Running a 700R4. Trying to give the heads all the air they want.

 

What is your setup?

 

L98 355 sbc. Dart Pro One 200cc heads. 228/228 cam with .576/.576 lift. 10.7:1 compression. Shorty headers with 3.5" Mufflex catback. First intake manifold, SLP runners ported factory plenum as shown in picture.

New motor in the making based on the good performance of the intake system. Dart SHP block with 370ci. New cam with 233/233 @.050 and .576/.576 lift. AFR 195 Competition heads with 11.2:1 compression. I will add exhaust termination boxes to the exhaust system.

Looking for over 400rwhp.

 

You may want to ask to confirm, but I looked back through the rules and there is nothing on changing to larger size runners. You cannot run a larger intake manifold in group 2, but I couldn't find anywhere that denied the use of larger or other than stock runners.