All:
To elaborate on what Dick has already mentioned, remember that
as the
piston approaches TDC (on the exhaust stroke) that the exhaust valve
will not be at maximum lift, because the valve will have already started
to close, and the intake valve will be beginning to open. So the margin
between the valves and piston is even larger than what Dick has
indicated.
At TDC on the compression stroke, both valves are closed so clearance
is not an issue.
If you have a big gnarly cam in the motor, then the valves open
further
and stay open longer, so piston to valve clearances go down.
Unfortunately,
the bigger the cam, the higher you need to raise the compression to
retain any
bottom end. So big cam motors are more at risk.
If you reshape the combustion chamber (using say, David Vizard's
recommendations) then you must shave still more off the head to make up
for
the metal you remove from the chamber. So in the end it really does
depend
on the modifications that you make.
I must agree with Dick on his conclusion that any *streetable*
motor
is unlikely to have enough lift and duration to pose a problem if the
chain were to break. But I am sure that with enough compression and
lift,
particularly with a domed piston of some sort, that it would be possible
to have a piston hit a valve. Particularly if the owner's definition of
"streetable" was in the 200+ HP (naturally aspirated) range.
Short stroke motors are more susceptible because the combustion
chamber will be shallower and the valves start out closer to the piston.
The GT6 motor, for example, will be more sensitive to this sort of thing
than is the TR6 with its longer stroke.
When hot rodding a motor, it is always a good idea to measure
piston to valve clearance (Which I didn't do on my motor. I ignore my
own advice, how twisted is that?) but the 2.5l in our TR6s has a lot of
margin to play with.
Cheers,
Vance
-----Original Message-----
From: owner-6pack@autox.team.net [mailto:owner-6pack@autox.team.net] On
Behalf Of Sally or Dick Taylor
Sent: February 02, 2006 4:48 PM
To: 6pack@autox.team.net
Subject: Valve/Piston Interference
<snip>
1) With the head upsidedown on the workbench, measure the distance from
the closed valve to the surface of the head. (On the later, stock head,
is will be around .625.)
2) Add to this the thickness of a used head gasket. (Approx. .030)
3) Add to the above the distance from the top of the TDC pistons to the
top of the block depth. (Approx. .015)
This adds up to around .670, which is how
much the valves can open without the risk of "interference".
Subtract from this the amount of lift from the camshaft lobes, along
with the rocker ratio. (on a typical stock cam this might total .290)
Add to this .290 the amount that may have been shaved from the head. If,
for example, the head is now at 10,5:1 it would change the .670 number
to about .500. This is now the maximum amount the valves can open
without intererence. This is way above what even the most developed
street engine would have.
<snip>
Dick
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