[fulstop]

Hi

Does anyone know how much it is safe to flycut valve reliefs in standard 2l twinspark pistons?

I want new cams without the added cost of new pistons.

Any help appreciated

 

[fatdarryl]

To start off, I don't know the answer to your question! But I am interested as to where you are going with cams and what other head work has been done / going to be done. The stock cams are actually pretty hot, I would have thought that you would get more out of gasflowing the head / opening the valve recesses than with hotter cams. Of course, all of that work would need to be done to make any effective use out of the new cams, but i would wager that head machining would give you the lions share of the gains


have you got Jim K's book? Its a must-read for TS owners

 

[fulstop]

I have Jim K's book and have found it very useful, although he skims over flycutting or using standard pistons.

I have already ported the head.

I will use aftermarket headers and I have dreams of an aftermarket computer.

but while its all apart I was hoping to make it really fun. and thought some hot cams might help.

I am having a bit of trouble deciding what direction to take regarding cams. There is a local grinder with a good reputation and quite (in my opinion) reasonable prices. I was thinking i would see how extreme i could go with standard pistons and get as close to Jim K's ultimate cam as i can.

Has anyone used some hoter cams with standard pistons? or is it not worth all the trouble?

 

[Alfar7]

Flycutting

You really do not need to change piston tops unless you mill the head for higher compression, or install larger valves (also unnecessary). Otherwise you are only reducing compression.

Simply adjust cam timing to acheive adequate piston to valve clearance. (retard intake until a minimum of .050 is reached, and advance exhaust until
.080 is reached) This minute LC change will not affect performance, and will enhance moderate RPM performance.
In general, more aggressive cams are usually timed this way(with wider LSAs) to correct timing of valve events.

The info below is from my installation information:

Initial timing and va lve piston check:
This is very important since when cams are advanced the piston clearance reduces dramatically.
And Lobe re=design can effectively advance the valve opening.
Set Intake and exhaust cams at initial LCs as indicated by provider`s set up instructions, and lash
settings.
By running the .050 plastic wire-tie between the Intake valve & piston, and .100 (or .080 ) on
the Exhaust side you can check if there is any interference and see that you have adequate
clearance between piston & valve. If by turning motor over by hand you feel some interferance
using a .050 thick wire-tie, (or .100 on Exhaust) but the motor continues to roll through, you are
at the limit! YOU MUST ROLL THE MOTOR OVER OVER BOTH FORWARDS &
BACKWARDS TO BE SURE THERE IS CLEARANCE AT THE LIMITS!! REMEMBER,
WHEN OFF THROTTLE THE CHAIN TENSION REVERSES!!
If it locks up, retard the cam(intake side, you would advance the exhaust cam a degree or more to
get more clearance) a degree (or a couple of degrees) and try again until the clearance is found.
I suggest as a test to find this limit, you advance the intake cam until you do, and permanently
mark that point on the cam cap. That will be the point where the cam must NOT be advanced
past. Then do the same with the exhaust only retarding it until the limit is found.
Timing initially is dependant on design of cam lobe. Aggressive large duration cams will be more
retarded.(intake) and advanced (exhaust) because of ramp & valve lift.
Tighten and instal lock nut on intake and bolt through sprocket and cam nut. Inspect carefully.
Roll the crankover by hand to verify timing.
When the intake cam is at the limit, or LC setting whichever is less advanced. You are there!