604 Cranking Compression

[roundtrack]

What would be a good number for cranking compression on legal rebuilt 604 with less than 200 laps? Warmed up to operating temp, all plugs out and carb open. Thanks

[brad hibbard]

bad test-----do a leak down to evaluate engine condition but if a comp. gauge is all you have look for even ness between cyls.-----my gauge will read 180 (ish) but yours could read 160 or 200

also you will need to remove the distributor and install a oil primer to run constant oil pressure during the test or else you are just waisting your time

witout oil pressure the lifters will collapse causing a loss in lift and timing which will increase cranking compression #'s

Brad

[Matt49]

bad test-----do a leak down to evaluate engine condition but if a comp. gauge is all you have look for even ness between cyls.-----my gauge will read 180 (ish) but yours could read 160 or 200

also you will need to remove the distributor and install a oil primer to run constant oil pressure during the test or else you are just waisting your time

witout oil pressure the lifters will collapse causing a loss in lift and timing which will increase cranking compression #'s

Brad

While on the topic, could you outline the correct procedure and expected numbers for a leak down test on the 604?

[brad hibbard]

if possible I believe its best to start and run the engine for a few minutes just prior to doing a leak down----I then let the engine cool back down for a bit---no need to work on a hot engine but the little run will make sure that any debris is cleared from the cylinders and chambers.

I like to remove the headers and I always remove the rocker arms----removing the headers will let you actually see exhast valve condition should it be leaking (and they usually are)

I use 100PSI of regulated air so that the supply is constant

some say the piston should be at TDC but I usually just let it fly down to the bottom unless I am really looking for a problem----(you can tell plenty with it at the bottom and its much more consistent)

under 6% is just fine but if its in the 10 to 12 range I would spray some carb cleaner on top of the ex. valve and if its just hissing away right there it will probably run just fine----you can try tapping the valve tip a few time to see if its carbon or just not seating but a few extra percent of leakage past the ex. will not kill the power----its not going to heal and its gonna be an issue at some point but no need for immediate panic

if she's reading 15% and the ex valve is quiet you will have a power issue