Kenku

All right - due to a funny experiment I'm doing with port area, I'm ending up having to do a custom header... which isn't that much of a problem, it's pretty fun actually. But looking for some guidance for at least a rough starting point.

Building a peripheral port motor; roughly MFR size ports and timing. Theoretically this is for a dual-purpose open track and weekend toy 13B; probably not a daily driver, and even if it were I'm prepared to live with the powerband being biased upwards rather a bit.

What I'm looking at is the Racing Beat recommendations for custom construction; they're saying in the neighborhood of 10-18" length primaries, or 15-18" length for bridgeys. But then there's things like the SDJ header which are in the... been a while since I measured it, but I think it was around 22". And then GtoRX-7's mid-length one in the 50" range. And at least one SAE paper on MFR development where it looks like in the 15" range and the 50" range are both reasonable... well. It's not that I don't have the background to do the calculations myself, but there's so many answers that seem to work...

Any advice really? It's more or less looking like I may just have to do a dyno header and play with it, which is perfectly fine but thought I'd throw the question out here first since it would be nice to start in a length range that isn't completely ridiculous. Megaphone and expansion chamber (ala that other thread and ISC) affect things much too?

Lasse wankel

We dynoe'd a WR PP rallyross engine with 28" primaries and iDia 54 mm to 314 hp@9300 rpm on a Super Flow dyno a week ago. No fuel above the air horns from 5000-9800 rpm so i think that was a good combination with our MFR port times D shaped intake. 45 mm inner dia in housing and all exhaust porting later closing. I think with rectangular porting you get more trouble finding the right combination of length and dia that suit your engine. A dyno is a must if you shall sort things out. With shorter primaries i think you would have more peak hp but less torque. Keep on experimenting that's how i learned things over the years!

/Lasse