Sidepipe horsepower loss?

[tirod]

Well, there's something to be said for paying attention to details.

If this is a 383 stroker, a set of 195cc to 205 intake runner heads would be a better fit.

As for needing a bit of back pressure, some reading on cam theory suggests what is really going on might be related to too much overlap or duration. Engines don't need backpressure, we're trying to get them to do the opposite. That's what you pay for with $800 racing headers - the harmonic waves actually create a vacuum in the chamber that literally pulls the intake charge thru. If it's on a low profile cross over manifold, it will only starve one of the other four cylinders it's hooked up with. An open plenum will allow it to suck any other chamber with it's valves open on the opposite side. Things are that complicated. So the intake, cam, and headers are dynamically connected.

On the exhaust, too big is a guarantee of no velocity, and that's where aftermarket headers are generally weak. You need velocity to get the tuning, and if the exhaust pulse dumps into an effectively open chamber, then it can't develop a reversion wave to pull the intake charge in. A properly made set of smoothly flowing headers a size too small will almost always make more horsepower than some made too big.

How those headers are joined is another detail - look down them and see if the merges are tapered and have long tails in the center, or simply cut off and filled with a square blank. Further, are the merges in the 15 - 20 degree range, or steeper? Plus, after the merge, is the pipe the appropriate size? Again, a 2 1/2 collector sounds small, but it generally will make more hp than a 3" badly done. How long that collector is, plus the transition to the muffler is also important. And despite what I was told years ago by Hot Rod Magazine, glass packs properly fitted and sized do NOT mean a hp loss.

Frankly, I think this a a classic example of how the whole "blame the headers" camp of thinking has grown. I certainly do think many of them could be better made - and right now, if you didn't pay more than $400 for them, you aren't getting quality headers. You are just getting what you paid for, some tubes welded up. On the other hand, having made the decision to value engineer the headers, the decision making about other components of the motor and how they are assembled becomes a factor. As illustrated.

Motors can't just be screwed together, if it was that easy, then we'd all be getting figures like the high dollar crate motors 475 hp from a 383, guaranteed. Building hp from engines is a lot of choosing certain parts, and then knowing what other parts are immediately not usable, regardless of our affection to do so. That means the cam is on the table, and not considering it and reevaluating it's fitness a potential mistake. If it's got the wrong LCA or isn't timed optimally, throw it in the trash. It could be costing 60 hp, too. That's been proven more than once.

Engine building is about pumping - get the intake, heads, and cam working, then work on the exhaust to help. It's not wrong to look at it the other way, but if the back calculating shows the cam is all wrong, do you leave it? Don't box yourself in a corner.