LT4 Cranking Compression-225 PSI!
Thread Starter
Melting Slicks
Joined: Jun 2000
Posts: 3,215
Likes: 6
From: Tampa Florida
LT4 Cranking Compression-225 PSI!
Did a cranking compression check on my original LT4 shortblock with CNC Heads (11.2 CR except for one at 10.4) and HOT cam...I was mildly surprised.
204 on the low CR cylinder
220 on 5 cylinders
224/225 on the remaining
Over 200 is pretty impressive for pump gas...heck my low perf truck 350 was only able to hit 165 and it run quite well.
Interesting...:)
204 on the low CR cylinder
220 on 5 cylinders
224/225 on the remaining
Over 200 is pretty impressive for pump gas...heck my low perf truck 350 was only able to hit 165 and it run quite well.
Interesting...:)
Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
Re: LT4 Cranking Compression-225 PSI! (No Go)
Those are good numbers; I'm running about 160-170psi with my 9.5:1 CR at 126k miles.
On that lower cylinder did you try squirting in a few drops of motor oil to see if it came up?
On that lower cylinder did you try squirting in a few drops of motor oil to see if it came up?
Thread Starter
Melting Slicks
Joined: Jun 2000
Posts: 3,215
Likes: 6
From: Tampa Florida
Re: LT4 Cranking Compression-225 PSI! (65Z01)
I didn't bother as I'm aware that #7's piston is down in the cylinder more than it should be (about .030) due to a bent rod or tolerance stack up hence the lower CR for that cylinder...besides those back cylinders are so tight to work with! :)
Shortblock mileage is 83,000 miles.
:)
Shortblock mileage is 83,000 miles.
:)
Safety Car
Joined: Dec 2000
Posts: 3,767
Likes: 5
From: Jacksonville Fl
Re: LT4 Cranking Compression-225 PSI! (No Go)
On my 10.4:1 383 cranking pressure was 225 on all 8 cylinders, all were .013 in the hole with flat tops and 73cc heads.
Static CR is not what makes the most difference...it is the cam profile. One with little overlap will have a higher crank pressure...as long as the cam degree timing is set right.
I've also built 12:1 motors with 200 PSI crank pressure...but they had serious race cams that wouldnt idle under 1200 RPM.
Static CR is not what makes the most difference...it is the cam profile. One with little overlap will have a higher crank pressure...as long as the cam degree timing is set right.
I've also built 12:1 motors with 200 PSI crank pressure...but they had serious race cams that wouldnt idle under 1200 RPM.
