Manifold Length on Triple's

[byunique]

Has anyone done any experimentation on Manifold lengths with their triple carbs? I have the 6" Cannon with a set of triple 44 mikunis and 39mm venturis.

The problem my car has a huge flatspot up near 4500-5000rpm. Past that it pulls well up to the 7200rpm redline.

Prior to this venturi change, I had changed my cam from a 276 .515 lift cam to a 284/294 .515 lift split duration cam, looking for more power past 6500rpm. The cam didn't seem to add much power until I installed the 39mm venturis. After I did that it had enough breathing capacity to pull to the higher revs.

I spoke a little to Dave Rebello of Rebello Racing and he says for my setup, the manifold in combination of the carbs and 3" air horns is just too long. As a simple experiment, I removed the air horns to see if that change in effective manifold length would make a difference. It did make a difference, but it seemed the flat spot was smaller and closer to 4000rpm.

I am not sure which way to go. Is my manifold length inherently too long. I could try smaller venturis, perhaps 37mm that kind of defeats my purpose with going with larger venturis in the first place.

Anyone else have any experience with this?

Thanks in Advance,

Brandon:confused:

[Phred]

Brandon,

I've built a few engines with similar specs, and it seems your results are not uncommon. You are at that point where its hard to have the best of both worlds. Thats a pretty big cam for the street. They will tend to feel boggy till you get in the range where they are ment to run. You are amplifing this condition by Installing larger chokes (venturi). The larger venturi size will slow down the velocity of the incoming air. You need higher velocity. By removing your velocity stacks, ( they named them correctly) you created turbulance at the entrance of the carb, and while it may have seemed better, your horsepower at peak was less. I've seen up to 10 HP lost with stacks removed. So what can you do? Its not practical to shorten an intake manifold. Most all brands are about the same lenght. But you could put on the real short stacks that TWM offers.(find them on the Web) Another trick to try is to advance your cam a couple degrees. This will have the effect of moving the power band down slightly, making a bit more power down low. Making your flat spot a bit less bothersome. Also, Different venturis require different jetting. You might need to get one of those mixture sensors, or an EGT gauge to nail down the proper jetting. Getting the jetting right can make a world of difference, drivabilty wise. Its a tough problem, one which you might not be able to totally cure, but you should be able to make it better.

Good luck buddy.

Phred