Machining forged Pistons
Discussion
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
227bhp said:
If your valve seats are good and newly cut then to check for a good seal you use engineers blue. Too many people use the unbroken grey line from the lapping process as an indicator which is incorrect. What they don't realise is that the stuff is so thick it takes up any unevenness and makes it look like good contact has been made.
The lapping paste is thick but it breaks down fast and if there is eccentricity on the seat you don't get the same contact pressure all round. This means that provided you don't lap for too long and only use fine paste it does actually give you a very good idea of the contact.I don't like engineer's blue because it's messy and time consuming but an excellent way to check seat concentricity is with a 1 thou feeler gauge. Hold the valve head with a lapping sucker, quickly check the drag on the feeler gauge at 4 points round the seat with light pressure on the valve. You'll immediately spot any variances.
Pumaracing said:
227bhp said:
If your valve seats are good and newly cut then to check for a good seal you use engineers blue. Too many people use the unbroken grey line from the lapping process as an indicator which is incorrect. What they don't realise is that the stuff is so thick it takes up any unevenness and makes it look like good contact has been made.
The lapping paste is thick but it breaks down fast and if there is eccentricity on the seat you don't get the same contact pressure all round. This means that provided you don't lap for too long and only use fine paste it does actually give you a very good idea of the contact.I don't like engineer's blue because it's messy and time consuming but an excellent way to check seat concentricity is with a 1 thou feeler gauge. Hold the valve head with a lapping sucker, quickly check the drag on the feeler gauge at 4 points round the seat with light pressure on the valve. You'll immediately spot any variances.
Then of course you can throw in 'What happens to the seats once the head is torqued down and heated?' Will we get a better job if we lap when the head is hot? TBH, I've never tried the comparison as I think it's a step too far, but if you were doing just one for yourself and had the time....
I read something posted by one of the USs most respected engine builders/designers (Stan will know him) who mentioned you should never use grinding paste as it gets embedded in the seat, but I found it hard to believe.
mighty kitten said:
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
You can't find adjustable pulleys for that engine? I'm really surprised, exactly which one is it? Are they some sort of special fitting? Years ago I found two similar engines, one had electrohydraulic adjustable cam timing. I took the pulley from this, machined a hub for it which made it manually adjustable and used a pair of them on another engine.As you are no doubt aware, with more overlap you'll need a sympathetic MOT tester if it's used on the road, if you do plan on using more do a dry build to check how far the valve is from it's opposing brother too although with your lift and duration you should be ok.
I once stood by the side of a car on the dyno at over 8500rpm which I knew had 1mm clearance between ex and inlet valves at TDC. The dyno guy had asked me if it was ok at this RPM, I looked at the customer, the customer looked at me....
My brown eye nipped right up I can tell you, but it worked. It's the norm on racing motorbikes, but i'd never done it on a heavy car engine before.
Edited by 227bhp on Tuesday 4th August 13:09
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 .
227bhp said:
With respect, that depends on what tolerances you want to work to. You can lap with fine paste, clean off and observe a lovely unbroken grey line on both mating faces, apply some blue to one and it show gaps.
Then of course you can throw in 'What happens to the seats once the head is torqued down and heated?' Will we get a better job if we lap when the head is hot? TBH, I've never tried the comparison as I think it's a step too far, but if you were doing just one for yourself and had the time....
I agree entirely. I'm sure that like any measuring operation "perfect contact" is a matter of degree and that what looks perfect with grinding paste might still have imperfections at the tenth of a thousandth of an inch level.Then of course you can throw in 'What happens to the seats once the head is torqued down and heated?' Will we get a better job if we lap when the head is hot? TBH, I've never tried the comparison as I think it's a step too far, but if you were doing just one for yourself and had the time....
What it boils down to is what works in practice. Over 25 years of building race engines if I lightly lap each seat with fine paste, check with feeler gauges and make sure everything is spotlessly clean before assembly then we get no failure problems.
I also feel quite strongly that lapping can't cure a bad or eccentric seat, prolonged lapping just makes seats concave, coarse lapping paste is the devil's brew but a brief lap, 20 seconds or so, with fine paste improves seating over the "as-cut" condition.
mighty kitten said:
The rod has joined the " just a couple of lb more boost will be ok " rod on my wall of shame at home
That's the good thing about a properly set up turbo unit, it makes good power & torque before or without having to rev the tits off it.
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