351W static compression STOCK...

[nvrstuk]

Anybody know what the stock piston dish volume is?

Trying to figure static compression on a stock '1970 block (9.48" deck ht)

TIA

 

[broncnaz]

Anymore as long as the lowest reading is within 10% of the highest your good that said. anything under 100psi and I would consider it being in need of a rebuild. most likely you should be in a 120-150 range for a good engine. but a lot depnds on the way you test it. it should be warm and all plugs removed then crank it 3 times for each cylinder with the throttle held wide open.

 

[Broncobowsher]

Lots of variables. For a '70 there was also a 1.5 point difference in compression ratios between a 2barrel and a 4barrel carb setup. Valve timing is a huge factor as well and that plays into timing chain wear as well. As the chain stretches the cam gets retarded reducing the cranking compression and low end power, but high rpm power tends to wake up.

The main thing you are checking is that the numbers are even. After you have checked the first 8, go back and check the first one again, just to see how much variation you have due to battery state of charge and oil tossed up into the rings as you tested the later holes. As for numbers, 120 I feel would be borderline low for that engine, but the top of the range I would expect closer to 165 and maybe even 180 PSI.

An engine that has been sitting for years can be completely fine and have horrible compression numbers. Sometimes they just need to be run a bit to free up all the rings.

 

[Quick & Dirty]

The 4 barrel engine had valve reliefs at -5.5 cc
The 2 barrel had a dish at -12cc

1.739" compression height, which should give .035" deck clearance on the early 9.48" block.

I think he's doing the math to calculate compression ratio, not doing an actual compression pressure check.

http://www.csgnetwork.com/compcalc.html

 

[nvrstuk]

Thank you Q&D...you are exactly right.

.035" quench sure seems tight for a stocker. Not doubting, just saying...

thanks again!

 

[broncnaz]

1970 351W compression for a 2v engine was listed as 9.5-1 for a 4v engine 11-1.

quench area is not really the same as deck clearance you also have to figure in a .039 or so thick gasket and the design of the heads. A lot of builders suggest running about a .040 quench height for best performance. So that .035 + .039 is way over the suggested and thats still not accounting for the heads. Most likely a 4v 351 had close to zero deck height.
You also have to remember that late 60's-very early 70's many factory engines were still performance engines. good comp ratios good power ect. So quench areas tended to be better than the later 70's low compression engines.

 

[nvrstuk]

I didn't do the correct math yesterday, just hit the "thank-you" button to Q&D after using the wrong piston ht.

But here's the math (with the correct piston ht):

9.48 deck ht


1.750 crank throw
5.956 rod length
1.739 Piston ht
_____

9.445 top of piston is .035" below deck.


I was using the wrong piston ht yesterday

.074 is more typical for that era's quench #'s. I was surprised with the .035" number I stated yesterday, but I didn't use the right numbers, my fault.

Thanks for the correction.

BTW, we checked my engine last year and it had 162#'s compression avg. We used two different compr. gauges, a Craftsman and a Snap-On. They were within 5#'s of each other across the board. Checked my buddies 393 at the same time and his was 165# across the board...