Cranking compression?

[88bluestar]

Factory stock, 6200 max rpm ,pump gas
350 .040 over 9.1 compression ratio
-Scat crank 3.480 stroke
-Scat rods 5.7
-Light weight JE flat top pistons gas ported
-low friction rings

Cranking compression is 100-110 psi on most cylinders cold(70 degrees), could it be re-ringed and honed to fix the low cranking compression?

[Tom68]

Leak down test first.

[DCal]

Factory stock, 6200 max rpm ,pump gas
350 .040 over 9.1 compression ratio
-Scat crank 3.480 stroke
-Scat rods 5.7
-Light weight JE flat top pistons gas ported
-low friction rings

Cranking compression is 100-110 psi on most cylinders cold(70 degrees), could it be re-ringed and honed to fix the low cranking compression?

That's pretty darn anemic. My old Vega had 145 and its not the gas ports, I have an engine on the street with them and it cranks 185.

[PackardV8]

Without the cam specs, none here can offer an opinon.

[hoodeng]

Packard you are right, so is Tom, leak down first. If the leak down is good, that thing has the wrong cam timing for whatever reason in it no matter what the grind.

[David Redszus]

Given your engine specs and inlet pressure of 14.7 psi, your cranking compression pressure should be:

cam IVC 109deg BTC = 190psia
cam IVC 114deg BTC = 201psia
cam IVC 119deg BTC = 211psia

Do leak down first, wet and dry.

[88bluestar]

current cam is comp cams special grind.
.451 lift
.020 270/274
.050 244/248
lsa 106 102 cl

open close
int 20 ex 44
54 14

Was told the engine has close to 30 races on the short block. How much cylinder psi would go up at say 200 degrees engine temp?

[Schurkey]

Faulty pressure gauge?

I connect my compression testers to "shop air" now and then, to see if the gauge on the compression tester agrees with the gauge on the air compressor regulator.

All my compression testers have an "Industrial Interchange" (Milton "M" or equivalent) quick-coupler in the hose.

Compression_Tester_Tester_01.jpg

Compression_Tester_Tester_02.jpg

[mag2555]

You know deep down that your going to need to tare it down just to know truly how well the short block was put together.
Even if the cylinder pressure issue is resolved easy, all I can say is that I would not put my foot to the metal and feel safe about it until I did inspect it.

30 runs made with the engine mis-tuned can make for issue’s more like 150 passes have been put on the motor .

I can’t see cylinder pressures as low as 110 with that cam even considering that the motor is not running to have the gas ports load those light tension top rings like they should be.

[BillK]

Was told the engine has close to 30 races on the short block.

What kind of races ? 30 races on a dirt track with warm up and qualifying laps and it is overdue for a freshen up. 30 drag strip passes and it should just be getting broken in. Big difference.

How are you doing your compression test ? Wide open throttle ? Well charged battery ?

And like Schurkey said you might want to double check your gauge against another one.

[rebelrouser]

As mentioned, do a cylinder leak test, much more accurate. On cranking compression, a lot of things will affect the readings you get. Cranking speed, must have good battery and starter. All textbooks say all plugs should be removed, why? It increases cranking speed. Throttle should be open, less air in engine less compression recorded. Last how many pulses are you recording? Most textbooks say 4 pulses before you take the reading. I taught automotive for 26 years, used to do a demonstration for students and we did cranking compression on a new car, doing all the variables, the readings varied as much as 40 lbs The other thing is how close are all the cylinders? Should be within 10% One trick is do a compression test throttle open and then one throttle closed, if you do not get more compression throttle open, you got a problem in the intake track, like flat camshaft or carbon buildup on the intake valves. And last but not least a running compression test on one cylinder at a time. You will see really quick by watching the pressure reading at idle and how it goes up when you hit the throttle, some of how dynamic compression works in a running engine. Running compression picks up weak valve springs and loose valve guides. Modern shops use a pressure transducer to display compression on an oscilloscope. Somebody good with one can pick out a loose timing chain and cam phaser problems.

And you have low static compression and a fairly big camshaft, the more duration the less cranking compression. I use an engine program that calculates a cranking compression when I am putting together a combination, on pump gas I shoot for 140o to 150 lbs, cranking compression is an indicator dynamic compression. A very important factor on how an engine will run, and what kind of fuel it will need.

[stealth]

You know deep down that your going to need to tare it down just to know truly how well the short block was put together.
Even if the cylinder pressure issue is resolved easy, all I can say is that I would not put my foot to the metal and feel safe about it until I did inspect it.

30 runs made with the engine mis-tuned can make for issue’s more like 150 passes have been put on the motor .

I can’t see cylinder pressures as low as 110 with that cam even considering that the motor is not running to have the gas ports load those light tension top rings like they should be.

Provided the gauge reads correctly.... This is solid advice... I know it hurts, but it's the truth...

[Stan Weiss]

Check to make sure your cam phasing has not changed.

Stan

ab-cgp-88bluestar.gif

[prairiehotrodder]

i recently pulled a stock TBI engine out of a 90's chevy suburban that was rebuilt. My intention was to sell it as a good runner. I had never driven it myself but i had seen it run. I did a compression test on a stand in front of the potential buyer and it was around 100-110 psi. Neither of us were impressed. I started tearing down the engine. First thing i did was remove the timing cover. The timing chain had been installed 1 tooth off. It was a newer double roller chain and wasn't wore out, it had been installed that way. I corrected the chain and immediately did another test and got 140 - 150 psi. He bought the engine and it has been working great ever since.
Brian

[Tom Walker]

If leakdown is consistently bad, what prairiehotrodder suggested is what I would inspect next.