Compression in a sbc

midnight sun · April 05, 2013, 04:10:36 PM

Replaced my double hump heads on my 327 with a set of E street aluminum heads from Edelbrock. New heads have same 64cc combustion chamber, same intake runner size, and down from 64 cc exhaust runner size to 60cc. Also has bigger valves.

Before the switchover I had 150 psi compression in each hole give or take 5 psi. I am now down to 130 psi. Any ideas why??

enjenjo · April 05, 2013, 09:09:53 PM

If there was no cam change, the new heads have a bigger combustion chamber. If the cam was changed, that will affect the cranking compression.

midnight sun · April 06, 2013, 12:54:03 AM

Quote from: "enjenjo"If there was no cam change, the new heads have a bigger combustion chamber. If the cam was changed, that will affect the cranking compression.

The old iron heads had 64cc combustion chamber and the aluminum ones are advertised as having the same size combustion chamber, 64cc. Cam is the same.

GPster · April 06, 2013, 08:21:40 AM

Different manufacture/style of head gaskets? were the new head gaskets installed with the "old style" run engine to operating temperature and then re-torque head bolts? GPster

kb426 · April 06, 2013, 08:56:27 AM

Are you doing the test with the throttle wide open? has the engine set for an extended time period where the cylinders are dryer than before? Are you doing this at 2 different locations where the elevation is a couple of thousand feet different?

midnight sun · April 06, 2013, 09:06:28 AM

Quote from: "kb426"Are you doing the test with the throttle wide open? has the engine set for an extended time period where the cylinders are dryer than before? Are you doing this at 2 different locations where the elevation is a couple of thousand feet different?

Throttle and choke was wide open. Engine has set for about 4 months so you may have hit on something there. I plan on firing it up today so after it gets good and warmed up and lubed again Ill give it another shot. Thanks

midnight sun · April 06, 2013, 09:09:43 AM

Quote from: "GPster"Different manufacture/style of head gaskets? were the new head gaskets installed with the "old style" run engine to operating temperature and then re-torque head bolts? GPster

Head gaskets are definitely different. Old ones were standard gaskets. The new ones are for aluminum heads and I did notice they are a little thicker so that could be part of it. Will fire it up today and then re-torque heads and run another test. Thanks.

wayne petty · April 06, 2013, 10:43:23 AM

some of the early camel hump heads i think actually had smaller than 64CC chambers as i recall...

were the head casting numbers 461.. with screw in studs..? perhaps an X somewhere under the valve cover??? that would make them 461X heads ..

i realize that i am reaching..

factory head gaskets are steel shim in most cases...

there are still steel shim gaskets.. but ... err...

there are multi layer steel gaskets with a rubberized coating that work well .. but i always use hylomar on them... spread into a thin coating on either side.. let hang dry for 20 minutes.. before installation..

i use hylomar on every head gasket i install. i don't have time to do it a second time...

but as described... a warmed up cranking compression test is what's required.. only use oil if you find a cylinder thats significantly low... so you can see if its the RINGS that are not sealing..

were you able to run enough water thru the block before you took it apart to get rid of any sediment around the bottom of the cylinder walls .. so it does not contaminate the new heads... if not.. and you have not started the motor yet..

you might want to knock out the freeze plug by the oil filter and behind the starter to allow you to flush from the top and get the sludge build up out.. sorry.. this is just what i do .. i don't know if the others do this..

i have always wanted to find a hose attachment to wedge into the 1 5/8" freeze plug hole.. with a hose to allow it to drain cleanly into a container..

midnight sun · April 06, 2013, 03:47:58 PM

Quote from: "wayne petty"some of the early camel hump heads i think actually had smaller than 64CC chambers as i recall...

were the head casting numbers 461.. with screw in studs..? perhaps an X somewhere under the valve cover??? that would make them 461X heads ..

i realize that i am reaching..

factory head gaskets are steel shim in most cases...

there are still steel shim gaskets.. but ... err...

there are multi layer steel gaskets with a rubberized coating that work well .. but i always use hylomar on them... spread into a thin coating on either side.. let hang dry for 20 minutes.. before installation..

i use hylomar on every head gasket i install. i don't have time to do it a second time...

but as described... a warmed up cranking compression test is what's required.. only use oil if you find a cylinder thats significantly low... so you can see if its the RINGS that are not sealing..

were you able to run enough water thru the block before you took it apart to get rid of any sediment around the bottom of the cylinder walls .. so it does not contaminate the new heads... if not.. and you have not started the motor yet..

you might want to knock out the freeze plug by the oil filter and behind the starter to allow you to flush from the top and get the sludge build up out.. sorry.. this is just what i do .. i don't know if the others do this..

i have always wanted to find a hose attachment to wedge into the 1 5/8" freeze plug hole.. with a hose to allow it to drain cleanly into a container..

Thanks for the flush tip wayne. The heads were 462"s.

Crosley.In.AZ · April 06, 2013, 03:57:10 PM

Possible the cast iron heads were shaved a little bit a one time or other during a rebuild, reduce the cc volume in the chamber. Thinner head gaskets.

If the aluminum heads were not checked for cc displacement , who knows what size the chambers ended up as during manufacture process

chimp koose · April 06, 2013, 05:48:24 PM

The head gaskets under the old heads were likely the .015" steel shim ones. Most of the aluminum compatible stuff is around .042" . Felpro makes a steel shim gasket 1094??? that is coated for use on aluminum heads and is around .015-.021" I dont remember exactly. Those 462 heads could be as small as 57cc.Did you check one?

midnight sun · April 07, 2013, 12:31:20 AM

Quote from: "chimp koose"The head gaskets under the old heads were likely the .015" steel shim ones. Most of the aluminum compatible stuff is around .042" . Felpro makes a steel shim gasket 1094??? that is coated for use on aluminum heads and is around .015-.021" I dont remember exactly. Those 462 heads could be as small as 57cc.Did you check one?

Im starting to recall that the old head gaskets were the steel shim ones as you mentioned. The Fel Pro ones that I put on were .039". Havent checked the cc's on the old heads but they are under the work bench so I will do that just to satisfy my curiosity. Thanks for the input.

midnight sun · April 12, 2013, 07:46:21 PM

Quote from: "midnight sun"
Quote from: "GPster"Different manufacture/style of head gaskets? were the new head gaskets installed with the "old style" run engine to operating temperature and then re-torque head bolts? GPster

Head gaskets are definitely different. Old ones were standard gaskets. The new ones are for aluminum heads and I did notice they are a little thicker so that could be part of it. Will fire it up today and then re-torque heads and run another test. Thanks.

After re-torquing the heads and getting the engine nice and warm the compression now reads 150psi. Sounds and runs good. Had a small oil leak courtesy of some real crappy valve cover gaskets so went back to the old reliable cork and gave them a rub down with some vaseline before installing. Thanks to all for your input.