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  #16 (permalink)  
Old 07-23-2010, 06:16 AM
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Quote:
Originally Posted by F-BIRD'88


If you are going to play around with the motor and seek max timing put good gas in the tank. Busting a piston won't make your builder happy. Sunoco 94 is the best pump gas stuff sold in Canada for street motors.
ya it makes a difference.
Its got as good as we can get around here at 91 0ct. The builder knows thats what we have within a 100 mile radius

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  #17 (permalink)  
Old 07-23-2010, 07:58 AM
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Quote:
Originally Posted by sooper
Yes Cobalt. Primarily due to post 5 of this thread
What he's not getting is that the vacuum advance drops out under load. You can set an adjustable vacuum can to start losing timing at a high enough vacuum that there's virtually NO WAY for the engine to be over advanced under any significant load.

But this is just re-repeating what I've said already, so I will just let it go.

Good luck.
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Old 07-23-2010, 09:21 AM
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Quote:
Originally Posted by cobalt327
What he's not getting is that the vacuum advance drops out under load. You can set an adjustable vacuum can to start losing timing at a high enough vacuum that there's virtually NO WAY for the engine to be over advanced under any significant load.

But this is just re-repeating what I've said already, so I will just let it go.

Good luck.
And I agree Cobalt, but heres the thing. I think we can all agree that TDC with heads off is probably a pretty good TDC. This guy marks that on the BRAND NEW $130 damper,as well marks 36 degrees for me as a sort of permenant tape. Did a brand new damper slip from the get go?Do I set the rest of the engine some how to acommodate this 36 it dont like,or run what it does like. I hope you can understand how I struggle with the fact that I'm supposed to re-do TDC in the assumption that the builder had the best means possible and goobered it, or the new balancer slipped from the get go and then stayed put. I struggle with those. I'm going out now for a static comp test on this thing. I'm starting to disbelieve the 9.75:1
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  #19 (permalink)  
Old 07-23-2010, 09:43 AM
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Quote:
Originally Posted by sooper
And I agree Cobalt, but heres the thing. I think we can all agree that TDC with heads off is probably a pretty good TDC. This guy marks that on the BRAND NEW $130 damper,as well marks 36 degrees for me as a sort of permenant tape. Did a brand new damper slip from the get go?Do I set the rest of the engine some how to acommodate this 36 it dont like,or run what it does like. I hope you can understand how I struggle with the fact that I'm supposed to re-do TDC in the assumption that the builder had the best means possible and goobered it, or the new balancer slipped from the get go and then stayed put. I struggle with those. I'm going out now for a static comp test on this thing. I'm starting to disbelieve the 9.75:1
Oh, to clarify- I understand COMPLETELY your real concerns over the 40 degrees you saw to the 36 it should have been! I would have to know the "why" of it all, too.

I hesitate to mention this, but for a quick check, a piece of copper wire can be used to "feel" the point where the piston stops rising (cylinder #1 OR #6, whatever is easiest to work with). At that point, you will see if the marks line up within reasonable amount to TDC or not. Could you tell a 4 degree difference? No, you couldn't- at least not w/any real certainty. But if the slippage is significant- you will see it. If you've only reset the timing this one time and it was 4 degrees off, at least you've caught it early, and there has NOT been any damage done at 40 degrees BTDC using premium and w/your CR. If detonation would have been bad, you'd have heard it. So I hope you're not worried about that.

If it were me, I'd determine true TDC (w/the heads on the engine), then see where the timing marks are and go from there, as I'm sure you will do at some point.
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Old 07-23-2010, 11:17 AM
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Just a thought someone else might use when building an engine . When I had my engine down putting the camshaft in and had the timing marks on the timing gears lined up I put a line on the front of my flexplate to line up with the straight bottom edge of the block . Now I can always find true TDC for #1 cyl . Without anymore effort than removing the inspection cover and rolling the engine over . I was able to verify the marks on my balancer after I had installed it . It was true zero .
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Old 07-23-2010, 12:42 PM
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OK so,I've just spent the last while in the shop doin my thing. #6 was what I picked for accesability. I turned a rod that was snug for spark plug threads. When I slid it in I could use my depth mic to check what was sticking out. I'll post pic of plug with flat bar attatched to top for mic later but you get where I'm comming from. Then what I did before I even tried to roll the engine was to take about ten measurements off of this thing to see if it was a dependable source. Out of the aprox 10 times I may have had two go out by .002,the rest were closer. Me thinks this OK for piston height. OK so now onto TDC. I didnt count but I rotated it back and forth past the builders TDC mark maybe 6,7 times. Every single time without looking my biggest depth number was either splitting the builders TDC mark in two on the tab zero or maybe half his mark width one way or the other. Long story short ,Baby his TDC is on and aint moved. My comp is 175 pounds,within a pound all over.My timing at 1000 is now around 22, total is around 40 at 2100, and as the guy from edelbrock tech said "Give the thing what it wants and if theres no ping or starter lul leave your timing light in the drawer".

Its enough to drive a fella nuts getting all these mixed signals though.
36 only
36 or less
must be under 36
till it pings then back off
till its hard to start then back off
all in by 2500
all in by 2000
all in by 1000
all in at the turn of the key
run vac
dont run vac
run this much vac
Oh no, run this much
Stand on your head ,come back, go ahead..GEESUS
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  #22 (permalink)  
Old 07-23-2010, 01:29 PM
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The MAJOR "rule" is: Avoid detonation like the plague. If it's not detonating (and not all detonation will be heard) it will not be damaging itself.

BUT- just because you can run X of advance w/o detonation does NOT mean that's the maximum power timing setting. It may be something less or it may mean that detonation will prevent you from ever getting to the ideal total timing. Or it may mean better fuel will be required in order for the ideal timing to not cause detonation. This assumes an ideal quench and no chamber or head gasket aberrations that cause hot spots, obviously.

But if you need 40, so be it. The best pump gas will be needed, and brands vary as to the way your engine responds to the fuel additive package, so don't be afraid to try various brands to see if one is better than the others.

Keep a close eye on the plugs for peppering of the porcelain and any sounds of detonation, especially after a fill up. If you get a bad tank of fuel, this could be enough to cause detonation if you're right at the edge.

Anything you can do to give cooler outside ambient temp air to the carb will help.
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Old 07-24-2010, 06:57 AM
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Hold the frikin phone

175 psi on comp check

Isn't that a lot closer to broaching 12:1 comp than 9.75:1 comp or am I missing something???????????????
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  #24 (permalink)  
Old 07-24-2010, 09:01 AM
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I get 175 compression on two cylinders in my 350, with 487 heads, stock cam, and a confirmed 9.1 static compression ratio. The others vary down to 160 from there. Another example, my wife's Honda Civic (4 cyl V-Tech recommended min fuel octane rating is 86) runs static compression pressures just under 200 psi, and as an aside it has 150k miles on it LOL.

There are many variables to equating static compression pressure to compression ratio, and in my opinion it is not a reliable way to determine CR. For instance, two different cam durations in the otherwise same engine will affect static compression pressures. Accurate compression ratios are determined by volume measurements.
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  #25 (permalink)  
Old 07-24-2010, 09:07 AM
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Quote:
Originally Posted by sooper
175 psi on comp check

Isn't that a lot closer to broaching 12:1 comp than 9.75:1 comp or am I missing something???????????????
Remember too, that the 175 psi reading does not come from just one cylinder compression cycle- typically, I use three or four "hits" on the gage per cylinder, per reading. Same number per cylinder, though.
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Old 07-24-2010, 09:33 AM
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Quote:
Originally Posted by cobalt327
Remember too, that the 175 psi reading does not come from just one cylinder compression cycle- typically, I use three or four "hits" on the gage per cylinder, per reading. Same number per cylinder, though.
I think our opinions differ here Cobalt. Although it may take many rotations to get up to true compression. Compression numbers cant be stacked. That is to say I dont think that means my true static compression is like 140 and woulda shown 140 accept by rolling three more times I added 10 pounds per roll. I'm hoping thats not what your trying to say. My compressor only makes 120 psi. If I run it all day it doesnt make 10,000 psi by days end. Did I misunderstand you.
However I got it through my head now bout the CR and the static comp. Catches a fella when he sees them numbers though
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  #27 (permalink)  
Old 07-24-2010, 09:41 AM
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I am saying it takes 3 or 4 "strokes" to get a usable reading. It doesn't happen w/just one compression cycle.

Dividing the compression reading by 14.7 is not going to be representative of the true CR that the engine sees, or there are a lot of 9:1 CR engines that have 10:1 plus CR instead. I suppose if the cylinder could be sealed completely and the valve timing were removed from the equation and the gage were accurate enough, a closer approximation could be made.

I seem to remember somewhere that 14.7 would need to be reduced from the initial compression reading before the rest of the division by atmospheric pressure is made, but I'm foggy on the details or if this is even true, Somewhere I did read it, in any event. Garbage in, garbage out possibly.

Last edited by cobalt327; 07-24-2010 at 09:52 AM.
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Old 07-24-2010, 10:56 AM
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Quote:
Originally Posted by cobalt327
I am saying it takes 3 or 4 "strokes" to get a usable reading. It doesn't happen w/just one compression cycle..
Thats kinda what I thought/hoped you was saying Chum

Quote:
Originally Posted by cobalt327
Dividing the compression reading by 14.7 is not going to be representative of the true CR that the engine sees, or there are a lot of 9:1 CR engines that have 10:1 plus CR instead. I suppose if the cylinder could be sealed completely and the valve timing were removed from the equation and the gage were accurate enough, a closer approximation could be made...
I get that now

Quote:
Originally Posted by cobalt327
I seem to remember somewhere that 14.7 would need to be reduced from the initial compression reading before the rest of the division by atmospheric pressure is made, but I'm foggy on the details or if this is even true, Somewhere I did read it, in any event. Garbage in, garbage out possibly.
I THINK thats negated in so much as all gauges (like the one for comp test) are dead weight zeroed to compensate for the 14.7. I could be missing something though
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  #29 (permalink)  
Old 07-24-2010, 11:09 AM
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I just saw this, in another thread:
Quote:
Originally Posted by miles
Has anybody out there put a 871 on a 455 olds. apparently I jump before I did my homework. Here is what I got, 1972 455 olds with J heads remanfactured engine.Casting no 396021F, I run a compression test and it come out to 180 psi which the people at BDS told me is a 11.24.1 compression ratio...
This would seem to agree w/what I remembered, as the 11.24 is derived by subtracting 14.7 from the 180 psi reading the OP had given BDS.

And, no- I didn't read this thread before I posted above. I'm rather anal about citing sources, and this would have been no exception.

Last edited by cobalt327; 07-24-2010 at 11:19 AM. Reason: Remove g.
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Old 07-24-2010, 11:37 AM
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Quote:
Originally Posted by cobalt327
I just saw this, in another thread:This would seem to agree w/what I remembered, as the 11.24 is derived by subtracting 14.7 from the 180 psi reading the OP had given BDS.

And, no- I didn't read this thread before I posted above. I'm rather anal about citing sources, and this would have been no exception.
Do I understand right then that my ratio at 175 pounds tested is 10.9-1?

I know I should be able to understand and when I hesr it I'll go "Ah he11 ya" but for the life of me it just wont hit me why you take an atmosphere(14.7) right off the top?But holy frig, that still leaves me with a stiff comp ratio. Maybe thats why no one can believe how strong my 9.75-1 engine runs?
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