compression test on 69 L-46. What numbers should I get?

[Michael L.]

Thinking of doing a compression test on my 69 L-46 (advertised CR of 11:1 I believe) while I continue to work on getting it to run better. What kind of numbers should I expect to get?

Mike

[Duke W.]

Re: compression test on 69 L-46. What numbers should I get?

The nominal cranking compression pressures for all '69 engines are listed in the CSM, but consistency across the range is more important than the actual numbers. You will likely find that the L-46 spec is less than the base engine despite the base engine having a lower CR. For example, for '63 the 11.25:1 340/360 HP engines were speced at 150 psi while the 10.5:1 250/300 HP engines were 160. How can that be? I'll give someone else a chance to explain.

The engine should be warm, remove all spark plugs, block the throttle wide open, red wire disconnected from the coil or pull the plug on the amp if it's TI, and I suggest doing it three times to compare both across cylinders on a given test, and consistency across multiple tests.

Write the data down and post if you want some analysis.

Duke

[Mark E.]

Re: compression test on 69 L-46. What numbers should I get?

You will likely find that the L-46 spec is less than the base engine despite the base engine having a lower CR. For example, for '63 the 11.25:1 340/360 HP engines were speced at 150 psi while the 10.5:1 250/300 HP engines were 160. How can that be? I'll give someone else a chance to explain.

Duke

Cam overlap?

[Mark F.]

Re: compression test on 69 L-46. What numbers should I get?

... You will likely find that the L-46 spec is less than the base engine despite the base engine having a lower CR. For example, for '63 the 11.25:1 340/360 HP engines were speced at 150 psi while the 10.5:1 250/300 HP engines were 160. How can that be? I'll give someone else a chance to explain...Duke

"A BIG factor in cranking compression is the point that the inlet valve closes. For a given static compression ratio, the later the valve closes, the lower the cranking compression pressure. ...the '63 340/360 HP engines' nominal OEM specified cranking compression pressure is 150 psi versus 160 for the 250/300 HP engines despite the latter's 0.75 lower advertised compression ratio, and the reason is that the Duntov cam inlet valve closing is much later than the much shorter duration cam used in those engines that closes the inlet valve much earlier, which means less inlet stroke reversion at cranking speed than the Duntov cam."

did I get that right, Duke?
https://www.forums.ncrs.org/showthread.php?126655-Equation-for-Converting-Measured-Compression-Values-in-psig-to-Compression-Ratios

[Dennis D.]

Re: compression test on 69 L-46. What numbers should I get?

I did a compression on my 70 LT 1 last summer. Believe my 70 L46 was similar from what I remember.


20191006_142731.jpg

[Duke W.]

Re: compression test on 69 L-46. What numbers should I get?

Cam overlap?

Nope... but that's a common misconception. The valve event that DOES have a major effect on cranking compression pressure is explained in post #4 and is a quote from a post I made in a recent thread that touched on the same subject.

Duke

[Duke W.]

Re: compression test on 69 L-46. What numbers should I get?

"A BIG factor in cranking compression is the point that the inlet valve closes. For a given static compression ratio, the later the valve closes, the lower the cranking compression pressure. ...the '63 340/360 HP engines' nominal OEM specified cranking compression pressure is 150 psi versus 160 for the 250/300 HP engines despite the latter's 0.75 lower advertised compression ratio, and the reason is that the Duntov cam inlet valve closing is much later than the much shorter duration cam used in those engines that closes the inlet valve much earlier, which means less inlet stroke reversion at cranking speed than the Duntov cam."

did I get that right, Duke?
https://www.forums.ncrs.org/showthread.php?126655-Equation-for-Converting-Measured-Compression-Values-in-psig-to-Compression-Ratios

Yes, you quoted me correctly.

Duke