It's not an F-body application but this is the only place I know to find non-biased, qualified information.

The naturally apirated version of my car has an intake manifold like a DOHC Cobra where it's got a "short' and a "long" intake path based on RPM and other factors, controlled by butterflies in the manifold.

I've seen arguments for and against removing this type of setup when adding a blower to a Cobra. Perhaps you have a friend who's blown a Cobra or work at a shop that has done such work.

Any opinions as to whether it would help low end power (or hurt top end)? I can get the manifold cheap and it's a bolt-on proposition except I have to rig a method of triggering the butterflies (not hard).

All insight is appreciated!

 

John,

I would think that adding a supercahrger, you wouldnt notice the slight power losses on the lower end if there were any, because the effiecieny of the engine is much greater. But maybe I am wrong.
Also the butterfly mechanism's may recieve undo stress from the boost applied to them, and I am curious if they even seal off the runners 100 percent. If this is not true, it would not be good if you were trying to push boost through the manifold, because you would have a leak possibly.

I would just get the manifold runners tuned for where you want your peak power with the boost, and you will be happy with that setup.

Good luck.

Hunter

 

If the turbo motors don't have the variable length intake runners from the factory but the NA cars do I would think there's probably a good reason for it. Variable length runners are cool for a street car, but most high output motors I've seen that originally had them have removed them (ZR1s, Cobras, Integra Type Rs). In addition I can't think of any forced induction motors with them from the factory, but there are several of NA motors that utilize the technology.

I would say keep your stocker unless there's some underlying factor here that I don't see.

-Chris

 

I've reached the limits of the factory turbos; this is in the ~ 425-450 crank hp area. They're small but spool instantly (stock it's rated at 320 torque @ 2500 rpm).

I'm about to add a pair of larger turbos that will take about 1000-1500 rpm more to spool. Granted, when the new turbos reach 5 psi it will yield more power than the stockers at 8-10 psi. What I was hoping to do was minimize the low end loss and then once things got happening (either WOT or over 3500 rpm, electrically activated) it would switch over to the short runners. Some people have said that the manifold, other than sheer airflow capacity, doesn't matter at all once it's forced induction.

The only turbo car I know that ever had a factory dual-length setup was the older Mitsubishis, in Japan only. This was called the "Cyclone" intake and it's a direct bolt on for 90-94 Eclipses. Again, some say it made no difference and others think it did. Usually the people that have that manifold have it because they bought a used engine from Japan.

That's why I'm looking for experiences with other engines. Nobody I know of on the import side has really got any experience with this type of setup at 400+ hp levels. It may be that it isn't worth it after all (see Chris's comment about the ZR1's) because the slight delay is made up for by sheer power on the top end.

I'm looking forward to the new haridryers though. I've already laid the foundation so once they're on, I'll go from 109-110 mph traps to 125+

 

My understandings are as follows....

1) For a turbo/sc engine you want longer runners and smaller plenum volumes to reduce turbo lag and increase intake charge inertia.... the trade off...... more runner area = more frictional loss.

2) "Tuned" runner intakes are difficult to build and most guys will tell you that they get especially complicated when designed for turbo/sc engines because all the pressure wave stuff that goes into the design of a "tuned" manifold, doesn't really act the same way it does in a n/a engine. Hence, you only see the tuned length "switching" manifolds in n/a OEM applications..... Cadillac, GM and some others that have been mentioned. The other manifold you see is the dual plenum in some of the more expensive n/a cars and I believe those are termed "expansion chamber types" with switchable crossovers.

I personally think it'd be more trouble than it could possibly be worth. The multiple "tuned" length problems mentioned, the frictional losses.... yada, ya.

I'd stick to the mainstream stuff and leave the variables for the guys who can spend months developing them. Maybe Chuck or someone with actual experience with this will chime in.

-Mindgame