high rpm power mod
 

This will be a trial an error mod similar to those intake manifold spacers.I was looking at an old engine and decided that the lower intake manifold can be installed between the normal manifold setup with the stock hardware.I contacted a local machine shop that will fill in or remove the extra injector bosses.The shop owner asked about installing nos injector nozzles or extra injectors,but I really plan on just getting the bosses cut off and filled in.In thoery I should get a better breathing engine along with a better powerband.I see one issue in relocating the egr system or just get rid of it all together.This mod is better suited for a supercharged engine for bigger gains,I feel about a 15whp gain is doable and is very inexpensive since I already have the parts.Any thoughts

Looking forward to seeing the results. Sounds like a good project. Are you going to cut out the hood for clearance?

Theory indicates that longer intake runners should provide more Hp "up top". It will be interesting to see the impact on the low end.
-Bill

This seems backwards to me.

Longer runners increase intake velocity for the upper end.
Shorter runners increase charge density for low end.

That was noticed this morning about hood clearance,but maybe a sti scoop will cover things,also I can leave the egr in place.I will drop off the lower manifolds friday.About bottom end torque,my car is fwd and losing some torque will help with traction.Just tapping the gas spins the tires so I feel that by making our runners longer we can get more power.Stock runners on the svx is somewhat short to give good torque and long enough to make a little power up top.Now I may be going down a dead end road but it won't cost much and should spark a few new ideas on the forum.If this works like I feel it should,any stock sc svx should easy see another 20-30 whp in the upper rpm where power seem to fall off.My little idea came from reading about honda civic skunk racing manifold making 15-25hp over a stock manifold.

Agreed. Commonly, longer runners are for torque and short runners for high rpm use.

Yes wrong way around.
 

You shorten the inlet tract to raise the speed that the torque is produced.

Harvey.

After reading more about intake manifold design,I think we have room to play with since overall runner length won't exceed 7 or 8 inches.One site tested a 18inch runner vs a 12,an a 6inch but 18 produced some killer torque,also the svx has a very large plenum so I think some gains are there to be made.Correct me if I am wrong but by adding the lower manifolds to the existing setup I will increase the overall intake plenum volume as well.I should have the machine work done by next week so cross your fingers.

I also agree that this seems bit backwards.

I was always under the impression that a longer runner provided more torque and a shorter one provided more hp. in the high end. That's why when people put in something like an outlaw spacer they notice an increase in tq. but not much else. That's what happened on my probe anyway, and a few other cars that I've noticed.


There are a few OEM manifolds that have the best of both worlds. One off hand is BMW. I forget what engine it is, but it has a variable length intake tract. Long runners for good low end torque and then it opens or spins or something and the runners become shorter for great high revving power.



That's what I always thought, anyway...

If you really wanted, couldn't you just mill down some of the manifold, or something like that, and then do some porting? That seems like it would help on the top end.

A slight correction -- my earlier comment had to do with the "ram air" approach that MOPAR applied back in the 70s;)

There's a good write up on intake manifold design here:

http://www.team-integra.net/sections...?ArticleID=466

-Bill

Superbikes like the R1 use this.

I think we have established that the original comment seems backwards. :p

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edit: Ah yeh beat meh. I'm not sure why it took 7 minutes to post this.

"Ram air" (beyond a cool sticker for your Pontiac) goes way beyond the inlet to the air filter. The effect itself happens in the intake runners, as the charge pressure bounces up and down the intake runners between the intake valves and the plenum and throttle body. There is plenty of good reading available on it. Overlook the bits about inlet scoops being stuck on cars, and you'll find some pretty wild pressure wave science. The link SVXRide posted is very good.

Intake runners are tuned! I don't think they're often designed primarily out of convenience or aesthetics.

It would be spectacular to see some math behind a manifold modification, and the border of insane to see a reprogramming of the IRIS valve to accommodate it!

There are plenty of cars running variable manifolds now; the original Catera had a 3-stage manifold. It worked a lot like our two-stage one.

Another thing that would come into play regarding resonant frequencies is when the cams open. If you make the runners shorter or fatter, or change the velocity of the air coming in at a given rpm, you're going to have the frequencies bouncing around at different speeds than they were before, and this changes things as well.

Getting the IRIS valve to accomodate these changes would actually be one of the easiest things. You would just have to do dyno runs with and without it, before and after, blah blah blah, and just match the plots up and decide at what RPM to have it move. Then all you'd need is like an MSD window switch to activate it.

This kind of thing was popular with the Mazda KL series motors for a while. Since it also has a variable resonance intake system (VRIS) people were always swapping on J-spec manifolds, or porting their own, and then just playing with the activation of the window switchs to make the most of their new power. Worked really well, actually.

The only problem is that we don't have another manifold that we can just slap on. It's either a custom one-off piece, or just alot of porting on the stock one. But, I don't think that the IM is a restriction for MOST people anyway, so it's not like much of this will be done anyway.

It looks externally rather large, but in fact is pretty darn small inside. Most of the space is taken up via the IRIS crossover and by the thick cross-sectional aluminum casting. The true plenum volume is less than 1/3rd the size it needs to be for a true performance engine, IMO.