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spuggy · 23-06-2020, 03:08 PM

Like Darren says, the "obvious" stuff - like lips and misalignments between manifolds etc are easy.

If you've ever taken the head/manifolds off something stone-age and low power (old Fords, Moggy 1000 etc), there's some very obvious things that can be laughably bad and don't help.

But then - and especially for half-way decently designed/made performance engines, like ours - it quite rapidly descends into the Black Arts and non-intuitive/opposite of what you expect effects - especially for a motor which won't run WOT 99.999% of the time.

For example; if removing a large casting cack or lip is good, then so would be polishing the ports, right?

Not necessarily; turbulence can result in better fuel atomization/mixing - and thus better power, economy, response - than mirror-polished.

Surely making the ports bigger adds power? Only if the ability of the port to flow gas in CFM is a bottleneck. If not, then by making it bigger, you're slowing gas flow. Which certainly provides less throttle response, and perhaps worse mixing. Like adding a huge carb/throttle body may make you slightly more power flat out (or not). But it won't run as well everywhere else.

Sure, if you can identify places where the factory was constrained by cost/manufacturing concerns (like only wanting to take 2-3 passes with a mill), you may be able to do better just by applying basic physics - like "gas doesn't like to go around corners".

With a well-designed/built motor, it's usually more complicated than "make it bigger" and "make it smoother". Otherwise, every muppet with a dremel and a bench vice would be Jerry Branch. Or Xtreme Cylinder Heads.

Without a flow-bench and some specific aims/goals, you're probably better off leaving that to the professionals. For a street motor with no obvious "duh" factor in sight, a good 3-angle valve job (the narrower the valve seat, the shorter the life between servicing - and also the earlier it opens/later it closes) and getting the motor right on factory specs (or better) is generally a wise use of time/energy.

23-06-2020, 03:08 PM	#5
spuggy Registered User Join Date: Apr 2016 Location: Farnborough Bike: M900sie Posts: 171	Like Darren says, the "obvious" stuff - like lips and misalignments between manifolds etc are easy. If you've ever taken the head/manifolds off something stone-age and low power (old Fords, Moggy 1000 etc), there's some very obvious things that can be laughably bad and don't help. But then - and especially for half-way decently designed/made performance engines, like ours - it quite rapidly descends into the Black Arts and non-intuitive/opposite of what you expect effects - especially for a motor which won't run WOT 99.999% of the time. For example; if removing a large casting cack or lip is good, then so would be polishing the ports, right? Not necessarily; turbulence can result in better fuel atomization/mixing - and thus better power, economy, response - than mirror-polished. Surely making the ports bigger adds power? Only if the ability of the port to flow gas in CFM is a bottleneck. If not, then by making it bigger, you're slowing gas flow. Which certainly provides less throttle response, and perhaps worse mixing. Like adding a huge carb/throttle body may make you slightly more power flat out (or not). But it won't run as well everywhere else. Sure, if you can identify places where the factory was constrained by cost/manufacturing concerns (like only wanting to take 2-3 passes with a mill), you may be able to do better just by applying basic physics - like "gas doesn't like to go around corners". With a well-designed/built motor, it's usually more complicated than "make it bigger" and "make it smoother". Otherwise, every muppet with a dremel and a bench vice would be Jerry Branch. Or Xtreme Cylinder Heads. Without a flow-bench and some specific aims/goals, you're probably better off leaving that to the professionals. For a street motor with no obvious "duh" factor in sight, a good 3-angle valve job (the narrower the valve seat, the shorter the life between servicing - and also the earlier it opens/later it closes) and getting the motor right on factory specs (or better) is generally a wise use of time/energy.