Actually, I've never read Yaw's theory on this... I'm speaking from my own experience and from the research I've done on this topic.

I think this whole argument is stupid... Think about it... where does a bridgeport make its peak power? At very high rpm. What do long primaries do? Lower the rpm needed for peak power. What you end up with is a bridgeport that develops peak power at a more usuable rpm.

We arent saying "if you go bridge, use short headers"... What I've been trying to explain since the beginning of this thread is that an engine running short headers will require more RPM to develop peak hp. On a bridgeport engine, this is BAD!

I stand by my original point that its very unlikely that a bridgeport engine running short primaries would LOOSE power in the low end when switching to long primaries.

 

Well, that is exactly what happened. I do wish I had more lowend. But theres no mistake, the lowend is VERY boggie compared to before. Before, it would pull reasonably ok around 3000rpm+ with less topend. WIth these long pipes, it is worthless below 4000rpm. In first gear this feels like a turbo spooling almost as you cross 4000rpm. IT has a deep sound and as you pass 4000rpm and 4500rpm, it turns into a high scream all the way past 8500rpm. And it does rev harder then before beyond 7000rpm now, "very worthwhile mod to the exhuast I concluded". Much easier to get beyond 100mph quick, I'm sure the quarter mile is much improved. Its not so much peaky, but all the power comes on after 4000rpm now. Before, it would fall off quicker at high rpm. IT now wants to rev as high as you dare.

My explaination to long tubes is related to the overlap in the exhuast. WIth short headers, the harmonic pulse is designed for near stock overlap and the speed of sound is what determines how fast the pulse arives back at the port if its still open it will hurt flow. But with it already closed, it will reflect and push the exhuast faster. With larger overlap, the port is still open when the pulse arives and keeps the gases near the port. So a longer header is needed to get the harmonic pulse to arive slightly later so the port will be closed.

Have you ever seen the road racing headers racing beat offers. They are un collected and the buyer determines where they collect down stream by the extent of the exhuast porting.

 

The quote with BP and PP primary header pipe lengths thats being used as proof that high rpm needs long pipes is being used incorrectly. The quote actually proves the point that tuning for higher rpm requires shorter primaries. Notice the PP primary tube length is shorter than the BP. A PP usually has a higher rpm hp peak than a BP due to more overlap, port timing, port volume, etc, and therefore benefits from a slightly shorter primary pipe length.
The fact that racingbeat sells a header to us with fairly short primaries for stock port engines is due to price and ease of packaging, not for optimal length. In their own literatue racingbeat says the optimum primary length for a stock port motor is longer than the car. Every other length is a compromise.

 

peejay is correct. I've had long and short systems on a couple of different RX-7's. The long system always made more power on the low and high end of the USABLE powerband. A short system will ultimately make more top end power but if the peak is far above the usable power range, you'll never feel it. The ong system is designed to give the greatest average usable power throughout the rpm range.

Good to see youy sticking to your gun's peejay! It doesn't matter how much arguing gets done, it doesn't change the fact that YOU are correct.

FWIW: The Racing Beat long primary system makes more power EVERYWHERE over the short primary. This includes the top and bottom end.

 

Pj,rotarygod and all the guys who agree with them are correct.

I`am using a short primary header on my Rx7.The torque isnt much @ low rpm,but up top it really,really comes alive.(My car is in my sig)

The one i`am currently using is very short,it collects just before the driver seat.(i`ll have to measure it to get the precise lenght)

I spent many hours on designing it,and the result was excellent.The collector is a 2.5" and it gradually tapers down to a 2.25" Exhaust.

*Little off topic*
Have any of you ever heard the guys with the dual DCOE carb setup complain about having now low end ?
Its because the manifold is so short..short manifolds are for topend.