Compression Numbers
Thread Starter
Safety Car
Joined: Dec 2007
Posts: 3,616
Likes: 5
From: Sandhills of North Carolina
Compression Numbers
Who can tell me if these are normal numbers?
The bottom end has 99K on it.
My procedure was as follows:
Prop open TB blades, all plugs removed, disconnect injectors then turn engine over 4 times on each cylinder and take the reading. I kept a full charge on the battery at all times. I started at #8 cyl and worked my way around to #7 cyl.
Bank 2
#8 190
6 190
4 205
2 190
Bank 1
#1 200
3 190
5 190
7 185
The bottom end has 99K on it.
My procedure was as follows:
Prop open TB blades, all plugs removed, disconnect injectors then turn engine over 4 times on each cylinder and take the reading. I kept a full charge on the battery at all times. I started at #8 cyl and worked my way around to #7 cyl.
Bank 2
#8 190
6 190
4 205
2 190
Bank 1
#1 200
3 190
5 190
7 185
Le Mans Master
Joined: Feb 2000
Posts: 9,715
Likes: 27
From: Kempner Texas
Yes, they look okay if you mean by "normal" that they don't indicate and sealing problem. If you mean by "normal" they're what a stock 96 LT1 pumps, I don't have a clue - never pumped one.
Of course I'm sure you already know that the actual number(s) depends on other variables like camshaft duration, overlap, etc.
Jake
Of course I'm sure you already know that the actual number(s) depends on other variables like camshaft duration, overlap, etc.
Jake
Thread Starter
Safety Car
Joined: Dec 2007
Posts: 3,616
Likes: 5
From: Sandhills of North Carolina
I was hoping some of you smart ones here could tell if they were about right with the cam I have. The numbers are in my sig. If more info is needed I can provide that. I think....
Le Mans Master
Joined: Feb 2000
Posts: 9,715
Likes: 27
From: Kempner Texas
Jake
Last edited by JAKE; Aug 24, 2010 at 11:33 PM.
Race Director
Joined: Feb 2004
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Safety Car
Joined: Aug 2008
Posts: 4,344
Likes: 48
From: San Marcos CA
Seems fine, Can't recall mine ever going over 180 or so (totally stock), but I didn't crank the thing to peak reading. A good way to do a poor man's leakdown test is to record the pressure achieved on the FIRST crank. usually a good (gasoline) engine is going to hit 90 or 100 psi on the first upswing of the piston on compression.
If any show lazy or weak, then do a real leakdown test based on the quick observations from the initial compression test.
If any show lazy or weak, then do a real leakdown test based on the quick observations from the initial compression test.
Le Mans Master
Joined: Feb 2000
Posts: 9,715
Likes: 27
From: Kempner Texas
Instructor
Joined: Jun 2002
Posts: 154
Likes: 3
From: San Diego CA
Someone correct me if I am wrong, but compression does not tell you anything about a cam. Your compression numbers come from the volume of air compressed between the piston and head. While the cam will influence the rate at which your combustion chamber fills and empties, it does not effect the actual compression ratio. The compression numbers will give some insight as to the condition of your rings and valves though. You can find the conversion from psi to ratio around here somewhere.
For what it is worth, I will post numbers I just got off of a used LT4 that has been sitting for several years in the garage.
Bank 1 2
#1 190 185
#3 205 205
#5 185 195
#7 190 200
You are looking for consistency. The 20 point swing on adjacent cylinders is probably pushing it on my engine.
For what it is worth, I will post numbers I just got off of a used LT4 that has been sitting for several years in the garage.
Bank 1 2
#1 190 185
#3 205 205
#5 185 195
#7 190 200
You are looking for consistency. The 20 point swing on adjacent cylinders is probably pushing it on my engine.
