Hemis and pushrods

[C M Smith <cmsmith@domain.elided>]

Hemi heads are not about good breathing per se, but about valve area. Hemi 
head allows the biggest valves to fit when only two are used. Bigger valves 
breathe better than smaller ones, duh. However, with multiple valves you 
can get better breathing without going hemi shape. When you dial in big 
bores and high compression you get multiple valve engines with pent roof heads.

Pushrods are fine for street engines. The current Corvette develops around 
60 bhp/litre which is respectable for any emissions legal street engine. 
Revs? Who needs 'em with 5.7 liters of pure grunt. The Corvette engine is 
by no means rev limited just because of the pushrods. Hydraulic lifters 
also have some limitations as do great big pistons restrained by a cast crank.

Remember our beloved Alfa 12 valve V6 has pushrods across the heads for 
exhaust valve actuation. Then there were those elegant crossflow heads on 
the BMW/Bristol engines with the cam high in the block and two pushrods for 
the exhaust valves, one from the cam and the other as for the Alfa V6 
across the head. That engine was what made the AC Ace popular enough to 
attract the attention needed to induce some crazy American to stuff a 
pushrod iron V8 in there. No, there are no real technical limitations to 
pushrods.

What really drives OHC technology is lengthening of service life for 
valves. A pushrod engine is much easier to service if the head has to come 
off which used to be a routine service before clean gasolines. Nowadays, 
pushrod engines can run 300,000 km with no head work required. In that 
light, a dohc engine or a crossflow sohc engine is much easier and 
economical to build using ohc design. The benefits of direct valve 
actuation make the emissions issues slightly easier to deal with due to 
more precise valve timing. Valve clearances are virtually set for life, and 
with the newer hydraulic lifters designed for ohc use, valve adjustment is 
a thing of the past for modern engines.

So, far from pushrod engines having the advantage of simplicity and ease of 
service, these virtues have been superceded by ohc design. For V engines 
however, the old virtues of compact design, short timing chains, low center 
of gravity and compact head design still breathe new life into those old 
pushrod V8 so loved by North American drivers. For pure unrestrained grunt 
the small block Chevy or an old Chrysler Hemi are hard to resist, just nail 
it and hang on.

Cost will eventually drive all pushrod engines out of the market place, 
even the mighty pushrod V8. Ironic eh? ( love that pun)

Cheers


Michael Smith
White 1991 164L
Original owner
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