Quote:
Originally Posted by Jason@Nameless
We're also going to add one other thing to the new catless design (just finished the 125hz band pass resonator for the downpipe and switched over to the new header) - we're going to run a 1.75" bore flange and leave a slight anti-reversion step between the cylinder head port and the header. Any thoughts on this? Also contemplating some of the secondary collector options. Optimally we'd have a 14" long megaphone collector, but we've only got about 7-8" from the collector of the secondaries to the flange. Kindof on the fence in regard to how to proceed on this. We do have conical forms that we can use, but getting any sort of megaphone design back into that flange is going to make for a ridiculous transition. Hmmph. Thoughts? I was going to call in some favors with some of our friends in the morning over at Cone Engineering and Burns to see what their thoughts were. Might just be best to not even evaluate the megaphone option and go instead with the optimal straight collector size and bring it back down to 2.5" at the flange. Homework time.
Jason
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The anti-reversion thing is something I'm a bit unsure of. In big cammed OHV they may be useful but in our case it is the AVCS can do the same job. At low rpm/low intake inertia they can close intake earlier to lessen or eliminate charge getting pushed back out by rising piston. And at periods of 'bad' acoustic scavenging they can eliminate overlap so the exhaust wave can't transfer the intake.
So assuming we can control it this way with stock duration cams, all the step will probably do is create turbulence and some flow loss.
But I would rather test it anyways, because it could actually be used to scavenge at higher rpm if it can generate it's own acoustic pulse from the change in area. If you do test it, I would put the step at ~5" from the valve to start. I was thinking of this myself and had my welder practicing. I made the steps out of billet 304 on the lathe that was machined to smoothly accept the small inlet and large outlet. I could machine the transition between the two at any angle I had enough length for with a boring bar on the compound rest. They were then all press fit together and he lap-fused both sides. We wanted to test for flow (bench) and acoustics (on dyno) so we could identify where the gsins were coming from. But we never got good shop access and dropped our flow-bench project, too...