Post by Vindi (Russell) on May 6, 2021 20:10:13 GMT
Interesting reading up on Madjak's build, 13.8:1 compression and E85, wild cams, Honda intake, forged block and all sorts of custom work inside the head ... really an incredible spec, but he says it's about 200bhp. That's a much wilder spec than a couple of cars I know are running at 240-250bhp, although I guess it's a DIY build so isn't going to be perfect. I couldn't find anything about the specific gains from the exhaust, alhough he's clearly getting some good power and approaching it without following what everyone else is doing, which has to be a good thing
No engine needs back-pressure. They need proper scavenge pulse timing. Once you have that, avg backpressure differences have marginal effects.
By the way, Madjak's roughly 200hp was from his last dyno...220hp@the hubs. That compares favourably to 250hp@crank.
A-B testing exhaust diameters is difficult because the exhaust is such an integrated system. Use a 3" midpipe on a 2.3" exit header and you get a large reflected pressure wave at the step change. That affects scavenge timing and hp, likely in a negative way unless it happens at the right place/time. Test like that and you'll tend to see optimum exhaust diameters close to header exit dia.
I do agree that on nearly stock engines, playing with exhaust diameters does little more than increase drone.
The 4age in my kit car gained 16bhp when I changed it, that was the full manifold and exhaust rather than just manifold but that's as tested before and after, the day it was fitted. I guess that matches what you're saying about the whole system needing to match which makes sense.
The 250 bhp cars I'm talking about are both at the wheels, not the crank. Having checked, one was a bit more extreme than I'd remembered, the crank was a custom forged item to up the capacity to 1999cc. When asked, he's not been keen to disclose the cost!!
I don't know the technical terms ... back pressure vs scavenge pulse timing ... I do know that if you go too big, you start to lose power... and some engines are more sensitive to that than others. From what I can tell, the BP is one of those engines. I kept my 2.75" exhaust on my Mk2 RS after changing back from the turbo setup, it was way down on power and was massively improved by my swapping a 2.25" system on. That was circa 155bhp, so not exactly highly tuned
It's obviously a hugely complex subject, and one I don't know loads about but when AK Automotive, 949 Racing, Flyin Miata, Fujitsubo, Maxim Works, Racing Beat, Jackson Racing etc all use / recommend 2.25" pipes on these cars, I'm going to go with them.
Russell
le sigh