Compression readings
Le Mans Master
Joined: Jan 2006
Posts: 8,319
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From: Wilmington DE, Drive it like you stole it, 68 327 4 speed coupe
Originally Posted by rcsron
What should be a normal compression reading on a small block .?
I got 125 lbs readings on 2 cylinders
I got 125 lbs readings on 2 cylinders
generally with 10.25 : you should see around 160 psi
call it 140-160 depending on you motor, and the varience should be less that 10 PSI across all 8
tim
Advanced
Joined: Jan 2006
Posts: 72
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From: Canton Georgia
I disagree on the fixed number for 10.25. It's dependent on static compression and intake valve closing point.
However, it should be consistant cylinder to cylinder. That will tell you if you have any compression issues.
However, it should be consistant cylinder to cylinder. That will tell you if you have any compression issues.
Le Mans Master
Joined: Jan 2006
Posts: 8,319
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From: Wilmington DE, Drive it like you stole it, 68 327 4 speed coupe
Originally Posted by VintageMuscle
I disagree on the fixed number for 10.25. It's dependent on static compression and intake valve closing point.
However, it should be consistant cylinder to cylinder. That will tell you if you have any compression issues.
However, it should be consistant cylinder to cylinder. That will tell you if you have any compression issues.
160 PSI is actually right out of the old school chiltons for the 350 motor with 10.25:1 CR. That is my source. Now of couse you'll see some varience across the cylenders, this is just a reference point, but it shouldnt be outside 10 psi or so. if you CR is lower, then you reading should be lower, again consistently across all cylenders, hence my point of 140-160.
now any mods to the engine from stock, heads, bore, stroke chages etc, with change this number some, but his questionwas about a stock motoer no?
tim
Big block usually see somewhere around 180 or so but dont quote me
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Joined: Jan 2006
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From: Canton Georgia
I believe we agree, but he just said small block.
He may have 8.5 to 1 compression, and I did not want to mislead him looking for 160psi while he only shows 125.
Stout pump gas motors are 180-200psi.
I don't remember checking an 8.5 static stock cam motor, but my guess would be that wouldn't top 140psi.
You are right....consistency is the key.
Not much you can do with low static compression except keep running a tiny cam to keep cylinder pressure up as high as you can.....a bigger cam in stock motor just lowers the cylinder pressure and more than likely will not perform as well...
He may have 8.5 to 1 compression, and I did not want to mislead him looking for 160psi while he only shows 125.
Stout pump gas motors are 180-200psi.
I don't remember checking an 8.5 static stock cam motor, but my guess would be that wouldn't top 140psi.
You are right....consistency is the key.
Not much you can do with low static compression except keep running a tiny cam to keep cylinder pressure up as high as you can.....a bigger cam in stock motor just lowers the cylinder pressure and more than likely will not perform as well...
Le Mans Master
Joined: Jan 2006
Posts: 8,319
Likes: 7
From: Wilmington DE, Drive it like you stole it, 68 327 4 speed coupe
Originally Posted by VintageMuscle
I believe we agree, but he just said small block.
He may have 8.5 to 1 compression, and I did not want to mislead him looking for 160psi while he only shows 125.
Stout pump gas motors are 180-200psi.
I don't remember checking an 8.5 static stock cam motor, but my guess would be that wouldn't top 140psi.
You are right....consistency is the key.
Not much you can do with low static compression except keep running a tiny cam to keep cylinder pressure up as high as you can.....a bigger cam in stock motor just lowers the cylinder pressure and more than likely will not perform as well...
He may have 8.5 to 1 compression, and I did not want to mislead him looking for 160psi while he only shows 125.
Stout pump gas motors are 180-200psi.
I don't remember checking an 8.5 static stock cam motor, but my guess would be that wouldn't top 140psi.
You are right....consistency is the key.
Not much you can do with low static compression except keep running a tiny cam to keep cylinder pressure up as high as you can.....a bigger cam in stock motor just lowers the cylinder pressure and more than likely will not perform as well...
the other thing to keep in mind with the cam is the right lobe sepration angle for the motor in question (valve overlap).
Le Mans Master
Joined: Oct 2005
Posts: 9,429
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From: Macon, GA
Absolutely correct to say that consistency is the key. Although those numbers sound very low to everyone, he may only have 130psi in all the other cylinders. Who knows what the condition of the motor is. We all need to know the other numbers he got in the test to figure out why he even did the test in the first place (performance? maintenence?). If these 2 cylinders are way out of whack, then there are some very easy tests he can do to figure out if he has a valvetrain or a ring problem.
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Burning Brakes
Joined: Jun 2005
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From: NorCal 71 Lt-1
with sperkins, we need more facts to make a good determination of the possible cause. are the offending cylinders adjacent to one another? how many miles on the motor? was a cylinder leakage test performed? what did the other good cylinders clock in @?
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Joined: Jan 2006
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From: Canton Georgia
While we are munchin on the popcorn, waiting for additional inputs, here is an article from CHP on the subject. From reading extensively on the net and magazines, you have to look at data very carefully because mistakes are made reporting data.
For example in this ARTICLE, there is an error in the data reported relative to static compression tests. The main article says a 355 with 8.75 static and a XE268 made 140psi and 9.75 static made 175psi. But in the pressure packed sidebar article, the chart shows a 355 with 9.0 static and a 224@.050 IVC at 60 (which is a XE268) made 180psi.
Point being the 9.75 static with an XE268 shows 175psi, which sounds right, and the 8.75 static with an XE268 showed 140 which also sounds right. But a 9.0 static with an XE268, according to the "pressure packed" chart says 180.
I also don't believe the numbers on one of the HT383 (in the same chart). The one with the 9.1 static with stock 196@.050 cam says 195psi....I can buy that, but the one with 9.1 static and an XR282 says 185psi....?? I doubt that one too.
My 355, 10.51static with an XR282 made 180-185.
I am wondering if calculated DCR is would closely suggest what cranking PSI is, for small and big blocks (assuming a good seal)?
Did you notice the exception....a stock LT1 made 130PSI....
For example in this ARTICLE, there is an error in the data reported relative to static compression tests. The main article says a 355 with 8.75 static and a XE268 made 140psi and 9.75 static made 175psi. But in the pressure packed sidebar article, the chart shows a 355 with 9.0 static and a 224@.050 IVC at 60 (which is a XE268) made 180psi.
Point being the 9.75 static with an XE268 shows 175psi, which sounds right, and the 8.75 static with an XE268 showed 140 which also sounds right. But a 9.0 static with an XE268, according to the "pressure packed" chart says 180.
I also don't believe the numbers on one of the HT383 (in the same chart). The one with the 9.1 static with stock 196@.050 cam says 195psi....I can buy that, but the one with 9.1 static and an XR282 says 185psi....?? I doubt that one too.
My 355, 10.51static with an XR282 made 180-185.
I am wondering if calculated DCR is would closely suggest what cranking PSI is, for small and big blocks (assuming a good seal)?
Did you notice the exception....a stock LT1 made 130PSI....
Burning Brakes
Joined: Aug 2005
Posts: 1,126
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From: Hudson Falls, N.Y. 76 Vette Modified L-48
OK, all good info...I was told years ago, depending on age and miles on the engine, as long as the compression is 5-10 PSI difference between ALL cylinders, you're ok...is that a true statement? I'd be curious what ALL the readings on this guys engine were... all 8...
