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Author Topic:   Intake Manifolds
Dirt Roller

Total posts: 19
posted April 09, 2002 02:56 PM
What effect does plenum height have on engine performance? The taller the plenum, the longer the runners = more bottom end torque or is it that it helps the top end more? I know I am showing my ignorance, but do not understand this plenum height thing.

Dirt Maniac

Total posts: 118
posted April 09, 2002 09:03 PM
I have been told that it's not the plenum height, but the plenum volume that matters. I believe more volume makes the engine think it has a bigger carb than it really does. I have done some research on this for restricted carb classes. Of course there is overkill. I believe more plenum and longer runners increases bottom end.

Dirt Forum Champ
Total posts: 564
posted April 11, 2002 08:35 PM UIN: 25419780
I agree with powercar...

Dirt Freak

Total posts: 310
posted April 11, 2002 10:10 PM
also the higher the plenum the straighter the runners therefore less turbulence in the fuel gettting to your cylinders

Dirt Freak

Total posts: 248
posted April 12, 2002 09:19 AM
I am not arguing with anyone, just asking. That said, wouldn't increasing the plenum volume help you more at higher rpm if it makes the carb seem bigger?

I'm very interested in this b/c I need good power at both ends. I've bought a Victor Jr for my 406, but I also got a spacer. I guess I'll find out what increasing the plenum height does.

Good topic, r19v.

Dirt Freak

Total posts: 353
posted April 12, 2002 12:38 PM
I think you are wise to put the spacer on. It will raise the plenum height and help straighten out the airflow into the cylinders. If you get a plastic or resin spacer it will also help insulate the carb from manifold heat.

I would suggest trying a 4 hole spacer. This will help keep your low rpm torque but also add height and volume to your plenum. If you get really ambitious try a 1" open spacer under a 1" 4 hole spacer under the carb. We run a 2" spacer with 4 holes tapered into 1 open hole and picked up several horse on the dyno.

If I had to pick a manifold I think the Super Victor would have been the way to go. It's design makes up for having to use spacers on the Victor jr. The victor JR is about 6 years behind in technology.

Dirt Roller

Total posts: 19
posted April 12, 2002 01:49 PM
If this post shows up twice, I apologize.

Thanks everyone for the replies.
The reason I asked the question is that I run a 406 with a Holley 4412 (500 cfm). 3250# stock car. The track is a 3/8 mile asphalt with wide sweeping turns and short straight-a-ways. Low rpm is around 5000, and high is around 6800 rpm. I have been told to use tall and short. I want something that works well in this rpm range and don't want to make a mistake.
Again thanks for all the replies.

Dirt Freak

Total posts: 310
posted April 12, 2002 10:29 PM
as a rule a dual plenum man is good on bottomm up to around 5500 or so an open is good from around 2500 to 7000 if it is the right manifold for the job they day the air gap works well I have no experience with them

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