How to get more flow from 993/964 plastic intake

[JoeMag]

How can I get more high rpm flow from above intake? Motor is 3.8 rsr p/c, 51.5 intake, 43?exh, in 964 heads with some porting on intake, 993 rsr cams. at this point torque starts to drop at 6k, max hp at 6.6k, and out to 7.7k hp has dropped ~25-30hp.

Would a larger throttle body help? I'm not sure what limiting factor is on these intakes at higher rpm.

[Fly911]

I made a IPD like diffuser in the air intake on my (previous) 993. Instead of the T-shaped plenum just below the throttle body, it was Y-shaped, dividing the flow to each side more efficiently. I used an aluminum sheet, and formed it to fit inside. Look at the shape of a typical IPD plenum, and you will see the difference. A bigger throttle body would be good too. You can bore out your existing one.

I also flipped the whole intake 180 degrees, so the TB and MAF sensor was pointing rearward, and cut the plastic between the TB plenum ad the resonance plenum, so I could rotate the TB plenum. In this way I could fit a cone filter and velocity stack to the MAF sensor body, and tuck everything under the RS spoiler that I had on my 993. I also made a box around the cone filter, to scoop cold air directly from the spoler.

I never dynoed the engine, but there must have been some gains, even if the internals were stock 3.6L. And it sounded awesome!

[Mixed76]

Is the inlet runner diameter 51.5mm, as you say the ports are?

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[Tippy]

Individual throttlebodies.

Anything else will be a choke point IMO

[JoeMag]

Fly911... Do you happen to have a pic of what you did that you could post?

[Mixed76]

There is another post on here somewhere where someone put an 80mm tb and got good results. No documentation though.

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[993Speedster]

If after asking nicely, the intake doesn't want to flow more air... I suggest using a few extra atmospheres worth of brute-force feeding

In all seriousness though, the 964 intakes flow well and based on your mod list, you might've outgrown them altogether. I think ITB's as Tippy mentioned is the only remaining choice. Everything else would be a compromise with your displacement and cam choice.

[JoeMag]

Yea, maybe... but I'm not looking for some step function in hp. I race in hp to wt, so just looking for something that may carry more torque higher in the rpm band to keep hp more flat rather than it going down.

[Ollies930]

Have you considered a 996 GT3 intake manifold? The runners are pretty big and you will probably need to have lowers custom machined to get the right taper from manifold to the heads intake ports. These manifolds were working on 3.6l engines running 8000rpm plus.

[sp_cs]

Originally Posted by Ollies930

Have you considered a 996 GT3 intake manifold? The runners are pretty big and you will probably need to have lowers custom machined to get the right taper from manifold to the heads intake ports. These manifolds were working on 3.6l engines running 8000rpm plus.

Agree with this approach, and something William Knight supports on anything 3.6 and above. On my 3.5, we went 993 intake but dyno tests found velocity stacks provided better peak figures with similar torque across the range.

[dannobee]

The 964 intake runners are significantly longer than the GT3 intake runners. And as we know, longer runners = lower peak torque RPM.

[clutch-monkey]

i have a larger throttle body on my 964, works well. no AFM, is aftermarket ECU. just waiting on final dyno tune for numbers to compare to the ITB's it had prior.

[Ollies930]

Dannobee,
don't forget that with custom made lower intakes, you can make them any length you want.

[JoeMag]

Clutch Monkey -- what size did you use and could you post some pics?

[dannobee]

Originally Posted by Ollies930

Dannobee,
don't forget that with custom made lower intakes, you can make them any length you want.

Exactly! It's been too long since I've had to calculate these, but if you want peak torque to occur at 6500, the intake tract (from valve to end of the stack) needs to be around 10.5". Theoretically, of course. There's a reason why the GT3 runners are so short.

[JoeMag]

yea... did not realize the 996 cup ones were so short. I was think'n maybe shorten mine, but the resonance cross pipe is just about hitting top of fan shroud. interesting how on cup one res cross pipe on same plane as throttle hole.

What is ID of 996 cup throttle?

Anyone know what the OD is of the cross pipe openings and the intake runner ends?

[clutch-monkey]

Originally Posted by JoeMag

Clutch Monkey -- what size did you use and could you post some pics?

only pic i have
it's still running in (fresh build) so will have dyno to compare it to in a few hundred km's. had PMO ITB's on before this.
will find the spec for the throttle body when i get it back on dyno.

[Peter M]

Sam,
Why the change from the ITB's back to the standard plenum?

[clutch-monkey]

Originally Posted by Peter M

Sam,
Why the change from the ITB's back to the standard plenum?

the PMO's were worn out, and i wasn't keen on replacing them, but jenveys are expensive and cannot justify it at the moment.
in the heat here, on track, the plenum is going to be safer also, no throttles to go out of sync and cause a lean cylinder etc.
engine builder said the hp will be the same, maybe more than before.
my other 3.6 has jenveys on it though but the engine is not as built.

[Tippy]

It'd be silly not to run ITB's on a 7700 RPM 3.8 if a racecar IMO!