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Discussion Starter · #1 · (Edited)
The previous owner of my motor said the it has high compression. I don't know the exact numbers and realize that there is a good chance my question is unanswerable right now. I plan to get the compression numbers shortly. I am considering changing cams. Would I need to worry about the compression if I went from a 221/221 cam to a 212/212 or a 212/218 since the lower duration should raise the dynamic compression?

Also I have T&D roller rockers but do not know the ratio(previous owners doing). How can i find out this ratio?

EDIT: Compression test on all cylinders was within plus or minus 2 of 180. Thats on a cold motor. I donno if that helps with my question or not.
 

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2genGNdriver said:
The previous owner of my motor said the it has high compression. I don't know the exact numbers and realize that there is a good chance my question is unanswerable right now. I plan to get the compression numbers shortly. I am considering changing cams. Would I need to worry about the compression if I went from a 221/221 cam to a 212/212 or a 212/218 since the lower duration should raise the dynamic compression?
A *long* duration cam, postpones the closing point of the intake valve, and thus lets some of the incoming charge, bleed back into the manifold.
Using less duration, will trap more air, and effectively raise your CR, as you noted.

If it's already a high compression setup, why would you want to increase it even more?.


Measure from the center of the pivot to the center of where the push rod hits. Then measure from the center of the pivot to the center of the roller tip axle. Divide the second number by the first, and that's your ratio.
 

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Discussion Starter · #3 ·
Doc1of7 said:
A *long* duration cam, postpones the closing point of the intake valve, and thus lets some of the incoming charge, bleed back into the manifold.
Using less duration, will trap more air, and effectively raise your CR, as you noted.

If it's already a high compression setup, why would you want to increase it even more?.


Measure from the center of the pivot to the center of where the push rod hits. Then measure from the center of the pivot to the center of the roller tip axle. Divide the second number by the first, and that's your ratio.
I have a mild set-up for the most part(4.1, mild P&P heads, TE44 and such) but have a 221/221 .492/.492 cam in it right now. This set-up was done 5 yrs ago by someone other than me. I think the cam is too much for the rest of my components. Plus I keep having lifter issues and am looking to purchase a new cam rather than put new lifters on an old cam. Do you think going from a 221/221 to a 212/212 would increase the CR too much?
 

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sounds like you have around 9-1 cr which is similar to mine. I went from a 221 flat tappet to a 210 roller. My cranking compression went up a bit.

I have no problems with it. My other car has a stock cam and is around 9.2-1.

Other than wanting to blow the tires away with minimal throttle input, I have no problem with it either. The alky pump finally died after a number of years and I am currently running about 15# of boost with no problem. I used to run about 23-24# with a bit of alky and it was a handful at 40 mph when it downshifted to 2nd.

The first car is more responsive with the roller cam at a standstill but it was lotsa fun with the 221 cam at speed...had a very unique sound.
 

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Discussion Starter · #5 ·
Just a little update. I found out my T&D rockers 1.65 ratio. With the cam I had been running that would give me about .525 total lift.
 

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I don't know how we can even speculate compression ratio when the pistons are unknown, rod length is unknown, stroke of the crank ect.

With it being a 4.1 it especially could have a crazy compression ratio due to the lack of off the shelf pistons available in the correct bore size with the proper dish volume and pin height.

Aside from figuring out what parts you have, it would be interesting to find a shop with a 'puffer'. Those devices accurately check compression ratio AND cubic inch from the spark plug hole. Can you imagine bringing one of those to an organized Buick race and requiring the puffer test to meet class requirements? :eek!:
 

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It's fairly easy to estimate the compression ratio if one knows the static compression and the cam duration.
 

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Discussion Starter · #8 ·
I have stock rods and a stock crank. The pistons are custom JE forged pistons. I'll see if I can find a shop with a "puffer."
 

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2genGNdriver said:
I have stock rods and a stock crank. The pistons are custom JE forged pistons. I'll see if I can find a shop with a "puffer."
I think you should just compression test it. Then SW or Orlando can tell you if its ok. I expect they will have been there once or twice.

If you seriously want to find one of those things, the only one I know of is property of the local dirt track. I refuse to go to ET drags, but they probably have them too.
 
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