"S" MFI velocity stacks

[Scott Clarke]

I'm converting my MFI system with mag stacks from E to S spec. I realized that the plastic E velocity stacks (the ones that fit within the air filter box) may be too small in diameter. Can someone verify this? Where can I get the ones I need? Are the ones for Webers interchangable?
Thanks,
-Scott

[rswannabe]

Scott,

Have you measured what the top and bottom opening diameters are? If so, I can go check my S velocity stack and get you the numbers for comparison.

Brooke

[Scott Clarke]

Brooke-
Thanks! I'll do that.

[Bobboloo]

The ones for Webers work. You can get a set of new chrome ones from PMO for about $100. The ones for 40IDA carbs are the same size as S stacks if memory serves.

[Wayne 962]

Yes, don't use those plastic ones - they can catch on fire (don't ask me how I know this). Only use the metal ones, and carry a Halon fire extinguisher at all times...

http://www.pelicanparts.com/catalog/shopcart/MISC/POR_MISC_BESTSL_pg20.htm

-Wayne

[Jim Richards]

Originally posted by Wayne at Pelican Parts
Yes, don't use those plastic ones - they can catch on fire (don't ask me how I know this). Only use the metal ones, and carry a Halon fire extinguisher at all times...

Huh? There's a helluva lot of 72-73 911's running around with plastic stacks. I've never heard of any problems associated with the stacks. Maybe you can share your experience, Wayne.

Scott, I bought a set of T throttle bodies and plastic stacks. I sent them to Eurometrix to be bored to S specs. They came back looking great. I can post pics after my wife returns from her trip. She has the digi camera with her. Here's a very useful thread to help you sort out things...

Bobby's MFI Induction Systems Thread

[Tim Walsh]

Jim,
He's talking about hte velocity stacks that sit on top of the 69-71 stacks. They came in 2 versions, plastic and metal. THe problem is the plastic ones catch on fire occationally if there is an intake backfire. Backfires aren't out of the question since those stacks use a cold start injector that sits above the entire stack, not down inside of it like ours.

[Jim Richards]

Thanks Tim! I was obviously confused, and I really don't know diddly about the earlier stacks & throttle bodies. Just the 72-73 setup. Sorry for the diversion, guys!

[Shuie]

Have your throttle bodies been bored out as well? Talk to Matt at Eurometrics or Harry Beiker at Beiker Engineering (closer to you in OR) about this. PM me if you want their contact info.

The plastic S stacks are the same as RS stacks. The RS stacks that I have taper from ~39mm at the throttle body side to about ~43mm at the top end. The throttle bodies should taper down ~2mm and match the intake ports.

[Scott Clarke]

Wayne-
Given your experience perhaps steel stacks are an item you should sell. I did a quick look, and Motormeister has them on Ebay! And they have a 99.6% positive feedback rating. And the auction is private, so no one else can see what a great deal you're getting!

The throttle bodies are at Eurometrix right now receiving the Matt Blast magic. I knew that I would have to enlarge the mag stacks, but the little velocity stacks were yet another little complication I overlooked when pondering this conversion.

[rswannabe]

Scott,

I'll measure my velocity stacks tomorrow and post the results for you.

As a side note I will vouch that the plastic ones can catch fire, and not only due to the stock cold start system. I have been running K&N filters on my 2.2S MFI system. The factory cold start (or "fire starter") system is disconnected and disabled. Recently when I pulled my K&N filter assembly to do some dyno testing with the stock airbox I noticed one of my velocity stacks was scorched and blistered and there was corresponding soot on the underside of the filter cover right above it. It was clear that there had been a small fire in there which was in no way attributable to the cold start system sinch it was not hooked up. I have to assume is was just a particularly rich backfire that started it.

Brooke

[Scott Clarke]

I wonder if either of these would do the trick.

http://www.cbperformance.com/catalog.asp?ProductID=512

Brooke-
Thanks for that!

[Shuie]

Would these add any benefit?

http://www.cbperformance.com/catalog.asp?ProductID=511

[Scott Clarke]

I measured my "E" velocity stacks. They are 38mm inside diameter at the base, and are 50mm tall. Mounting hole spacing is 64mm center to center. As "S" stacks are 42mm at the top (from Bobby's thread), it is clear that the E velocity stacks won't work. Matt Blast said that the PMO velocity stacks are 46mm, and suggested that I could modify the mag stacks to that spec (same as '69 S). I wonder why Porsche changed the stacks so significantly between '69 and '70?

[jluetjen]

Has anyone ever tried those 6 inch trumpets on a set Webers? Is there even enough room? It would be interesting to try a back-to-back comparision of those with the normal stacks to see what they do to the torque curve. (Obviously I'm disregarding the marketing hype of up to a 15 HP increase).

[Early_S_Man]

John,

What many here may be ignoring [or may not be aware of] in the quest for max. port/stack diameter ... is the benefit of 'T' sized ports in the '67/'68 901/30 rallye engine that used stock 'T' heads with all other details being S-spec except the 46IDA carbs and manifolds ... the only cost was 10 hp DIN at peak, and mid-range torque was significantly improved over the 'S' engine.

Those tall stacks and 'S' cams may be a 'hot' ticket!!!

[Grady Clay]

John,

Way back when I did some dyno tests with the plates above the stacks and got both peak HP improvement and a broader torque. There was also clearly a distance effect. What light can you shed on this?

My perception is the plates added an “echo” for the intake pulse in addition to the one from the change in impedance of the end of the bell-mouth stack. It clearly worked with more effect using S and particularly C6 cams.

In today’s applications would it be worthwhile to have a solid aluminum or steel plate as the top of a K&N filter?

Jerry Woods has placed injectors in air filter lids with a taper above each stack (BA, 2nd Ed. p191.) I’ll bet that taper changes the resonant characteristics also.

Best,
Grady

[jluetjen]

Way back when I did some dyno tests with the plates above the stacks and got both peak HP improvement and a broader torque. There was also clearly a distance effect. What light can you shed on this?

My perception is the plates added an “echo” for the intake pulse in addition to the one from the change in impedance of the end of the bell-mouth stack. It clearly worked with more effect using S and particularly C6 cams.

You're conclusions is in line what I've seen. Whenever space permits -- it's preferable to have a flat surface opposite the trumpet bell. There's even a formula for how far is "ideal" based on somebodies work. I believe this is why Porsche had those interesting "rain shield" type of covers over the 917's intakes.

My question was more about using longer trumpets on top of the carbs. I've researched as much as I have been able to find so far about the accoustical tuning of intakes and believe that the longer stack would tune the intake to a lower frequency while also providing the opportunity for more harmonics. It would also provide more inertial at high RPM.

[Bobboloo]

My perception is the plates added an “echo” for the intake pulse in addition to the one from the change in impedance of the end of the bell-mouth stack.

Sounds like a venturi effect.

[Scott Clarke]

A follow up: I found the chart featured in this link: http://www.inglese.com/heightstable.htm[img]. Scroll to the bottom of the page and you will see a great set of dimensions for various Weber carb velocity stacks, and also an idea of the stacks made by Inglese Induction Systems. The photo shows their IDF stacks, which could be made to work on mag manifolds by cutting the velocity stack just below the retaining ridge and using retaining tabs to secure the stack to the top of the manifold. The tabs are available from Inglese. Notice that the 44 IDF stacks are almost exactly the right I.D. for S manifolds. Once the bottom portion of the stack is removed they are also nearly exactly the right height.
http://forums.pelicanparts.com/uploads4/3_IDF1107289606.jpg[/img]

Alternatively, the stacks sold by CB performance for 44, 45, and 48 IDFs should also work. They are probably about 44mm I.D. based on the info from Inglese's chart. Not nearly as nice, but cheap, and it seems like it would be no big deal to have the top of the manifold at 44mm instead of the stock 42mm.

(Edit 2/9/05) I ordered the stacks from CB performance. They are die cast alumimum, and are 45mm ID at the base. I'd say that they are quite a good value at $7 a piece, but they will require that I size my mag stacks to '69 spec.

-Scott