Q's for engine gurus

[fangsport]

1. on a pushrod engine, how much shorter/longer can the valve stem be before it is going to cause problems with adjusting the clearances.

2. how much does O/S pistons affect C/R.e.g.85 x 78 (B x S) = 1770 at 9:1 C/R , what would 87 become ?

if i could find my decent calculator , i would try to work out the maths but would probably get it wrong anyway.

[solberg Fan]

You need to know your combustion area to use this but here it is anyway:
compression calculator

[fangsport]

what about the valve stem length?

[solberg Fan]

You know more than me!
I just stumbled accross that and thought id pass it on.

[fangsport]

a few mm in the bore equated to a few points i.e. STD 9.0:1 to 9.3:1, so i dosn't have that effect that i was hoping it wasn't going to do.

what valves do you have in your "K", and how much adjustment can be taken up if you fitted longer/shorter valves.

someone on here must have destroyed gear by being smarter than they thought they are, or am i going to vandal heaps of bits to work out a theory.

[MikeMan]

Why would you want a shorter valve stem? A shorter stem will mean less lift before coil bind on the valve springs.

Depending on the adjustment in the rocker arms you should be able to run between 3-5mm shorter if you have to, it should not affect the geometry to much.

On my 1600 Ford pushrod we ran Datsun 180B inlet valves and they are approx 1.5mm longer than standard and that was fine.

What engine and what are you trying at achieve?

[fangsport]

building up another 3T engine, and (being a cheap b'stard), found valves with same stem diameter, but a wee bit longer.
the aim is to fit larger diameter valves and seats to get better flow, but have talked to the head man today and my inlet theory will work , the exhaust side won't, but isn't necessary anyway in his opinion. too short and guides will get worn out quicker, too long and i would lose lift.

[RedMist]

Dont forget your also changing the accl curve of the valve by altering the geometry. It may be negligable, but remember you want the valve as quickly off the seat as possible while maintaining as low a spring rate as possible..... one of the hundreds of dichotomies of the 4 stroke.

[fangsport]

i was wondering when you would pipe up.

my theory is to use the biggest (sensibly) valve and to be able to retain single springs, as doubles create a valvetrain power loss. i don't build high-revvers anyway, prefer to be able to have a broad power curve than a peaky power anyday.

how can i measure seat pressure accurately (read cheaply)?