Blown bbc too much crankace pressure
I have a 454 HO crate motor, fresh rebuild chrome moly rings with 24 thousand ring gap,10/71 with (2) HP 4150 carbs, comp cam blower cam, solid roller lifters, roller rocker, I have moroso evac system from the breathers run down into the headers. I am pushing my dip stick out and pushing oil. when I put my foot in it it will blow the breathers out of the valve covers. Compression is 95lbs per cylinder. cam is a custom blower cam 650 lift not sure duration don't have spec sheet now, 114 separation. 400 turbo, 456 gear. I am stumped I don't want to run it and break any rings or worse. I NEED HELP Why all the pressure. Engine runs great doesn't smoke was broken in properly, plugs look OK still a little lean with 90 jets in back and 86 in front.
In any event, upside down second rings will pump the crankcase nice and full of pressure.
Agree. The ring seal is not what you think it is. 95 PSI on a cranking test is way too low.
You want 150psi. Was the throttle open during the compression test? ya it matters.
Connect shop air the the cylinder and listen for air leak ( carbs, exhaust, valve covers)
bent,cooked valves, wrong size, failed, butted or upside down rings, rings bent on install Broken ring land(s),,, both) Wrong honing stones for chrome rings.
.024" ring end gap is too small for a blown BBC.
You probabily butted the rings and scuffed the rings and cylinder walls.
If the motor ever detonated or pri-ignited Lean carbs, crap gas or got hot even .080" ring end gap may not be enough.
If you are lucky and have not distorted the piston ring lands or lifted a ring land you may be able to just rehone re ring it.
Check the bearings and piston pins.
The .650" BBC roller is not big duration.
Check the valves for bent stems and VTP clearance.
possible leaking head gaskets.
Are these pistons stock 454 HO crate motor pistons?
Its best to do a leak down test at TDC. Is this an original bore block ?? How were the cylinders preped ?? Was it honed with a plate using a good hone VS a rinky dink one? What finish did they use on the cylinders?
No such thing as a chrom/moly rings and with any performance engine ring seal is key.
I assume the engine is a low compression 454 with around 7.5:1 cr.
So, with a blower cam (wide lsa) and a later intake closing point from a bigger than stock cam, I would say 95-110 psi with a compression gage is about right.
Furthermore, compression gages are not known to be accurate. I have 2 gages that are the same make and model and one reads 20 to 30 psi more than the other one. But, the difference between cylinders is what you need to look for, no so much the number.
Maybe the header evac system isn't working or the rings haven't set yet??
Tell us about the block prep, pistons, and clearances used.
First Thanks for the response.
Rings are all in correct, I assembled the motor, it has Manley stainless steel valves. Block was done at a very reputable shop. 95 lbs is what I have researched to be the correct compression reading with 114 lobe separation.The bore is stock 4.25, original forged aluminum pistons 8.75 compression. Sealed power E233K Moly rings. Harland sharp stud girdle. I took it to the strip and it ran bad too lean, I broke the #7 exhaust valve. I replaced the spring and I don't have a leak down tester but left the compression tester on each cylinder for at least 1/2 hour no loss of pressure.
Thanks For the help.
95 lbs is what I have researched to be the correct compression reading with 114 lobe separation.The bore is stock 4.25, original forged aluminum pistons 8.75 compression.
Don;t know that you have been reading. I had a 8:1 BBC with a bigger cam than you got (old nitrous motor) and it had a lot more than 95psi.
Your .650" lift BBC cam is 255in 265ex @.050" +/- 3deg. its not big
The ring seal is dead. You did more than cook a valve.
You scuffed the rings (butted) when it ran lean.
if the ring lands are not broken they are probabily distorted too. ( jammed rings)
95psi indicates a ****ed motor. (Parden my french) Much lower and it won;t even start.
If the ring seal was good the thing would not be huffing out the breathers.
The 454HO pistons are not suitable for hard supercharging (especially with a bad tune up and or crap gas) I was under the impression they were cast.
top ring end gap should be .032"
Get some JE or Venolia pistons if you want to lean on it that hard.
Flat top or forged dished.
If you want to lean on it hard, with boost using pump gas you want a low low compression ratio.
7:1 to 7.5:1 and big ring end gaps.
Engine compression testers have a pressure hold/release valve. The gauge will not drop unless you press the release valve.
Go over this link and you can see what I am talking. Putting boost pressure to the cylinders amplifies the poor block prep.
|All times are GMT -6. The time now is 08:35 PM.|
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
Copyright Hotrodders.com 1999 - 2012. All Rights Reserved.