Header sizing 914-6 Options

[mb911]

So while my 914-6 oil tanks are being machined I going to start working through my header with heat exhanger design out of stainless.. I do not have non compete on these with m&k so that gives me design freedom.. However I will use similar design process as the m&k/ rarlyl8 design. So my question is 1.5" the most common size or allot of you using 1.625

[Mark Henry]

One thing I haven't seen discussed is the other half of the exhaust equation...what are you going to run for a muffler?

1 3/4" pipes on a stock banana? then you may as well stuck to a smaller tubes anyways.
Zero backpressure then you are likely very loud and may lose some bottom end torque.

Like everything in the high performance engine world there is no perfect combo, everything is a trade off. Too much of a good thing is usually bad for street drivability. Getting the drivability back for the street and you will lose the peek HP.
Hot street with driveability is a balancing act and with this type of engine I'm looking more at torque curve than peek HP.


One thing I was always seeing was peeps building full race tuned engines, then not being happy with the end result, because they only ever drove on the street.
Also here's a rub, in the 90's friend and I built same size 2.0 bug engines (78mm X 90.5). His had 044 big valve head's and mine had ported and polished stock valve heads, his had a w-120 cam and mine the slightly smaller w-110, his a 1 5/8" header and mine a 1 1/2", other than that I'd say everything else was equal. We drag raced (yes illegally) countless times, he never could catch me, I beat him off the line every time. He would be catching up to me, but he could never match my bottom end torque.

Now this is more experience with Type 1 vw performance engines and I build T4 engines with the same premise, but at 2.0 (78X 90.5mm) each cylinder has the same volume as a 3.0/6.*
I always noticed smaller (1 1/2") tube headers provided more bottom end grunt, where bigger tubes gain top end at the cost of the bottom end torque. As many know this is also true with cam selection, mild to wild what you gain with one you lose with the other. Same when added a bit of backpressure with the muffler compared to a megaphone straight out pipe stinger.

Since 95% of the engines I've ever build were for the street, I've always strived for the balance of an acceptable idle (a little loppy is OK as long as it will idle at 900rpm) with good bottom end torque and acceptable top end.

What I'm getting at is a bit of loss in exhaust flow may be worth it, you have to be truthful to yourself as to what are you using the car for.
Racing? you want the biggest baddest straight through loud pipes.
Quiet or semi quiet? with street drivability? Then the muffler is likely killing (at minimum effecting) any larger tube advantage anyways. Plus personally I'd be more interested it bottom end torque then the top end HP.

*Even with VW t1 big valves they're are still smaller valves then /6, etc. I know VW t1 references it's not a true apples to apples example but IMHO and just an FYI I've build a lot of T1&4, plus a few /6 motors, so it's close enough.