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Vince, when you take the rocker out of the way to do this, then put the dial indicator on the outer edge of the lifter, outside of the inner piston, and you will avoid any issue with the inner piston moving on you. The inner piston will have movement due to preload that you cannot get around. (And you won't have to run the engine first). A couple of more tips:
Based on that 3 degree cam advance and the supposed Isky cam advertised duration, run with no modifying factors, then then ICA works out to 61 ABDC and the dynamic stroke is 2.19". Isky may have put a slightly slower closing ramp in those cams so the ICA could be a bit later and the stroke a bit shorter still; plus their advertised duration numbers are very probably not taken at .006" lift. (See discussion below). So you ought to be able to get pretty close to the .006" ICA number with your 'in-engine' method if you take good care and the chain slack does not enter into it much.
BTW, the Isky advertised duration numbers are based on very old standards. Isky was an 'early adopter' of trying to standardize duration specs, back into the 50's. When these cam profiles were designed in late 60's, the standards were still changing. So their advertised duration numbers on these old cam designs would have be taken at .008" or .010" lifter lift back in those days, which would make their (and OGTS's) advertised duration numbers shorter than if taken at .006".
Example: The OR-4H is supposed to be a 256* advertised duration, but when I profiled mine, it was 268* at .006" of lifter lift. The 256* duration occurred at .010" of lifter lift. I suspect Vincent's OR-66H cam will be similar and his 268* advertised duration is really more like 278*. That would drop the DCR computation by around 0.25 points. A 0.25 difference is not a game changer by itself, but the better the measurements, the more useable is the outcome.
So that is why I want to profile as many of the cams as possible, to get consistent data for this type of work. The seat-to-seat values are useless, the stock cam duration numbers for the US-sold engines vary from the Euro numbers, and the Isky numbers are to an old standard! This is a lot less problematic over in V8-land, as the advertised duration numbers are nowadays better standardized for the modern cams from the major cam mfr's.
- Keep some finger pressure on the lifter as it moves down the closing side of the lobe to make sure it 'follows' the lobe
- Make sure the lifter does not rotate. The retaining ring on that outer edge is not perfectly level all around and if the lifter rotates, that can move the dial indicator up or down. (The 'voice of experience' LOL) The best way is if you have an old lifter; take the plunger out and work from solid surfaces. (Or even do that on the present lifter....)
- Try to take readings in 2 degree crank angle increments to get adequate resolution. May be hard in-car....
Based on that 3 degree cam advance and the supposed Isky cam advertised duration, run with no modifying factors, then then ICA works out to 61 ABDC and the dynamic stroke is 2.19". Isky may have put a slightly slower closing ramp in those cams so the ICA could be a bit later and the stroke a bit shorter still; plus their advertised duration numbers are very probably not taken at .006" lift. (See discussion below). So you ought to be able to get pretty close to the .006" ICA number with your 'in-engine' method if you take good care and the chain slack does not enter into it much.
Glad you are getting good info out of this, Tom.Just another step in knowledge if you want to get deep into engine stuff... Bob's results on the size and shape of the chamber, the location of the plug, and the importance of keeping the burn process as short as possible, have all been validated over the years in multiple cases; those factors can all make a big difference in detonation resistance. Re-read that page linked above a few times; there is a lot of excellent info in there, and I just re-read it yesterday after 4-5 years and it makes more sense than ever. A lot of it seems to be 'casually mentioned in passing', but don't let that mislead you; it is all signficant info.SCR/DCR isn’t easy to wrap your head around but this thread has been very educational so far.
I like MR’s suggestions, I only wish I’d taken the duration readings @ .006” off base along with the .050” I don’t believe Isky goes by the .050” numbers.
BTW, the Isky advertised duration numbers are based on very old standards. Isky was an 'early adopter' of trying to standardize duration specs, back into the 50's. When these cam profiles were designed in late 60's, the standards were still changing. So their advertised duration numbers on these old cam designs would have be taken at .008" or .010" lifter lift back in those days, which would make their (and OGTS's) advertised duration numbers shorter than if taken at .006".
Example: The OR-4H is supposed to be a 256* advertised duration, but when I profiled mine, it was 268* at .006" of lifter lift. The 256* duration occurred at .010" of lifter lift. I suspect Vincent's OR-66H cam will be similar and his 268* advertised duration is really more like 278*. That would drop the DCR computation by around 0.25 points. A 0.25 difference is not a game changer by itself, but the better the measurements, the more useable is the outcome.
So that is why I want to profile as many of the cams as possible, to get consistent data for this type of work. The seat-to-seat values are useless, the stock cam duration numbers for the US-sold engines vary from the Euro numbers, and the Isky numbers are to an old standard! This is a lot less problematic over in V8-land, as the advertised duration numbers are nowadays better standardized for the modern cams from the major cam mfr's.