Hi folks- another chapter begins in the MG Midget MZR project- I'm finishing the rust repair and getting ready to build suspension/fit engine, etc.
I'm beginning planning the exhaust manifold- planning a 4/2/1. I used this program:
http://mezporting.com/exhaust_length.html
Found a cam card for a Crower stage 2 (62552, 276 In/268 Ex) camshaft set. Exhaust open 46 deg BBDC, Exhaust close 6 ATDC, RPM 6500, 500cc/Cyl. Going for mid/high RPM. Advertise peak hp at 7800. Nice.
Plugged in the numbers and this program is showing 1.71 inside diameter primaries. Seems huge to me- am I off? Is there a better program?
My 2300 411 engine does 192HP at 7000. The engine builder for it specified 1.62" tubes for primary. So, your numbers look about right.
Lengths? Primary and collector.
Primary it says 15" valve to merge for 4 cylinders. Secondary 12" @2.256 Diam, "tailpipe" 23.61 @2.309
Without knowing the flow numbers I'm guessing but that sounds about right. If you've got one of those engine analyzer programs play around with those numbers because you might find a wider torque band with a slightly smaller ID and longer primary.
If it's for road racing I've always found trading peak power for a flatter wider power band improves lap times and gives you places to pass that more power over a more narrow band can't.
The only times I've ever found the reverse to be true are when racing at very high speed constantly. ( oval track ) or if you have a gearbox with the ability to quickly change gear ratios. And a Quick change rear end.
When I used my Seinz and Halibrand I could bring 3&4closer together and then match that by changing the final drive ratio to make 5th drop down close to 4th that gave me better acceleration from tighter corners and while it may have cost me top speed that was only reached after crossing the start finish line.
Thank you Frenchy- all I've got is a redneck's hot rod and intuition. I don't need it but I like the sound and feel of a high revving motor. I'm doing it with the '06 Miata 2.0 MZR/Duratec and 6 speed. So - I'm thinking - using the original manifold- cut the stubs at 1/2"- they are 1.5 OD right now- and use them as CV stacks while building a 1.75 OD primary header around them.
