Machining forged Pistons
Discussion
Over the years it's had 6 thou off the block and two light skims on the head so a little bit off the Pistons might be a good things as far as I could measure it was 8.9-1 before any machining to the surfaces and I have to run a little less ignition than friends cars with nearly identical spec to stay knock free
Thanks for the replies
Thanks for the replies
I haven't run the enem cams on stock Pistons But they offer a lot more clearance than the wossners on the left .
When the engine first got built the head and block had never been machined but over the years it's had 6thou off the block and two light skims from the head so my thinking is I need to take a little from the Pistons to go back to square one . A couple of mates cars with very similar spec run 1.5 deg more advance at peak boost than I can even though the wossners are 8.5-1 compared to 9.2 std .
10thou from the edge of the piston and a 20thou relief for the inlets should be ok . The only setting data for the cams is a lot higher at tdc than I set them at as it ran like s
t with virtually no vac at idle set at 1.5inlet 1.2 ex . I ended up with 1.0 mm on inlet and 0.6 off closing ex at tdc best compromise .
Haven't dynoed with the bb turbo but made between 450-480 with the last three similar sized Holset turbos .
When the engine first got built the head and block had never been machined but over the years it's had 6thou off the block and two light skims from the head so my thinking is I need to take a little from the Pistons to go back to square one . A couple of mates cars with very similar spec run 1.5 deg more advance at peak boost than I can even though the wossners are 8.5-1 compared to 9.2 std .
10thou from the edge of the piston and a 20thou relief for the inlets should be ok . The only setting data for the cams is a lot higher at tdc than I set them at as it ran like s
t with virtually no vac at idle set at 1.5inlet 1.2 ex . I ended up with 1.0 mm on inlet and 0.6 off closing ex at tdc best compromise . Haven't dynoed with the bb turbo but made between 450-480 with the last three similar sized Holset turbos .
227bhp said:
Set it to 1mm and have it correct. Less than that and you risk collision, more and it's not really a squish band any more.
The merits of tight squish clearance are easily found by using Google, it's no secret.
I'm not sure why you want to try and run similar ign advance to someone else's engine?
Regarding your cam timings, as you correctly state; it will be lumpy and have less vac on idle with more overlap, but have you considered the pros? For a 500bhp track car I can't see those slight disadvantages being much of a problem.
The extra power above 6k is fine but costs me at lower rpm so the best setting I've found means I never need to dip into 2nd gear to stay in boost which is a lot kinder to my gearbox . I'm going to put the reliefs in for the valves and do some measuring to see what the squish height is . The enem cams and going up a grade on the plugs has got rid of the knock I was seeing so I will try a little more once it's bedded in . The merits of tight squish clearance are easily found by using Google, it's no secret.
I'm not sure why you want to try and run similar ign advance to someone else's engine?
Regarding your cam timings, as you correctly state; it will be lumpy and have less vac on idle with more overlap, but have you considered the pros? For a 500bhp track car I can't see those slight disadvantages being much of a problem.
I was planning on having some Dyno time after the last td as the gearbox issues are now resolved but il try the settings from maptun with the Pistons machined . It just means some adjustments to the warm up and idle Map to stop it throwing lambda errors . Not having vernier pulleys make adjustments a bit long winded and the only bolt on ones come from the U.S. With very mixed reviews
The car is 1978 so emissions aren't an issue plus having my own test centre helps . I should really have stepped down to a 2876 or similar to have 350 with good response but running holsets making 450+ I knew I'd miss the warp speed . . Best bet is put the cams where I've set them before so I know they will clear and get a few pulls in to see where it's at . The Saab coil pack doesn't like going beyond 7200 and with no balance shafts I tend to put the limiter at 6800 as its the 2.3 rather than the better revving 204 .
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