Shortened Jackshaft

[YellowS4DHC]

The jackshaft in my twin cam has been shortened to about 1/3 the original length by a PO.

I've been told that that's a mod racers used to do, but I wonder - does it actually yield a tangible benefit, especially for relatively mild street engine (i.e., std compression, SE cams)? And are there any drawbacks, other than not allowing a return to a mechanical fuel pump?

Rick

[holywood3645]

You may have an issue if you ever need a mechanical petrol pump. Less mass to drive, +o.ooo1 hp
James

[rgh0]

Less mass but also less bearing and aerodynamic drag which may be more like 1 hp at high engine speeds for half the shaft and a large diameter cam bearing like this and when every hp costs you around $500 in engine build costs its cheap hp for a racer to just cut something off

Also one less bearing to feed oil to and less heat load on the oil so the oil pumping and cooling system is less stressed all of which is important for a racer but not really critical for a road car.

cheers
Rohan

[john.p.clegg]

and you "add lightness"

John

[YellowS4DHC]

Rohan, et al, thanks for the comments.

The car (a Fed Europa TC) was apparently set up for auto-crossing some years ago.

In addition to the shortened jackshaft and electric fuel pump, a "distant" PO replaced the original crappy intake and exhaust manifolds, added twin HS6 SUs, reset the intake cam timing for better mid range power (MOP at 105 deg instead of the normal 110), installed baffles in the oil pan, and most importantly, built the engine up on a 1600 block. Calculated displacement is about 1671 cc.

I bought this thing off of ebay as a whim - the guy I bought it from had no idea what was there, and neither did I until I pulled the head and pan to replace the water pump.

Oh, and I don't plan to ever return to a mechanical fuel pump, the nice block off plate is one less source for oil leakage.

Rick