Over frame manifolds / headers

[dal2.0litrefrogeye]

Hi all , any one gone from tubular headers taking the conventional route to the exhaust boxes, to over frame ones and noticed or actually measured any power/ tourque gains ,
Cheers for any ideas

[PeterCrespin]

I have over-the-frame headers on the D but the block only came back from sleeving the other day. They are standard parts so I assume Jag did their sums and they'll work OK with Webers and cams.

Mostly the benefit comes at a given rev point, chosen by designing the length and diameter of the primary header according to a proven formula. Over/inside frame has got very little to do with it. Not nothing, but not much. There is a back eddy swirl on the inside of the sharp bend at the port, which has the effect of reducing the usable cross section. This is reduced by having a gentler curve further from the valve, which is what over the frames do.

Other things being equal, the main thing is longer/narrower gives a lowish speed extractor effect that helps torque. Shorter/wider hurts low speed torque but can allow higher revs, thereby permitting higher hp, but of course if you never cane your engine you only experience the loss. The FIA approved lightweight E short design is clearly designed for max revs and not great for road use except hooning around Crystal Palace or Brighton prom. The factory manifolds are a good compromise but of course don't permit over-frame pipes. ,

[abowie]

Mostly the benefit comes at a given rev point, chosen by designing the length and diameter of the primary header according to a proven formula.

Which presumably means flat out for a race car.

[PeterCrespin]

Sort of, but you can literally ‘tune’ pipes like an instrument and even use them to ‘backfill’ a dip in the torque curve lower down.

Besides which, circuit racing is different from off road racing, which is different from trials and from from road use etc. My experience is with ikees but the principles remain. E.g. Tuning for one of the above involves raising flywheel mass, reducing port size snd dropping CR...