NORMAL COMPRESSION???

Posted By: Labratt

NORMAL COMPRESSION??? - 10/29/10 09:31 PM

Is there a "NORMAL" range when checking compression with a gauge? My 440 has a 10:25 ratio,using stock 84cc Edelbrocks. The cam is a solid with .600 lift and 262/266 @ .050 duration. Thanks in advance!

RandyB

Posted By: Bob_Coomer

Re: NORMAL COMPRESSION??? - 10/29/10 09:51 PM

There might be, but hard to say when you have large overlap cams bleeding compression out the exh valve.
When you test, make sure you run the test the same way. Like warm the engine up, and test with the carb at WOT. Make sure the battery doesnt run down so you have the same cranking power. This can effect the numbers, or if the engine cools etc.
Look at each cylinder and they should be with in 5% of all others.
I like my race engine in the 225-250 lbs + range.

I healthy street engine should approach 200 psi. But many factors come in play here

Posted By: HardcoreB

Re: NORMAL COMPRESSION??? - 10/29/10 09:56 PM

You'll probably fall in the area of 160-170 psi. You need to crack the throttle at least 1/4 open and crank away until the guage stops moving.

Posted By: Thumperdart

Re: NORMAL COMPRESSION??? - 10/29/10 11:03 PM

Mr. Coomer, are you serious 225-250+ cranking compression? I know lots of 8-sec. and faster cars and NONE that I have talked to crank in that range. Some are in the low 130-140 range. You racing a diesel?

Posted By: Bob_Coomer

Re: NORMAL COMPRESSION??? - 10/29/10 11:18 PM

Quote:

Mr. Coomer, are you serious 225-250+ cranking compression? I know lots of 8-sec. and faster cars and NONE that I have talked to crank in that range. Some are in the low 130-140 range. You racing a diesel?

Nope
most any car out of the factory is 175-180 psi. Why would you think a race engine should be lower? I know some is lost due to cam profiles, but thats why they make dome pistons and closed chamber heads for
I dont see how you can keep from it. Unless its a pump gas engine. I dont care how big the cam is either. Last wedge was at 16:1 and ran real, real good. But again I dont build pump gas engines.
Last one was a 493 with Eddy heads, and the 590 MP solid...heck it had 180-190 IIRC
If you build a engine with 13-14:1 compression expect it..
IDK what to say about the guys running 130 psi. I bet they would benefit in more compression or less cam

I be willing to lay money on it that Randys will approach 200 psi....
175+ anyways

Posted By: dusturbd340W5

Re: NORMAL COMPRESSION??? - 10/29/10 11:24 PM

my 14-1 small block stroker with a solid roller with 267/272 @ 50 only has 190# and the leakdown is less than 3% and it runs 6.32 at 109 in the 1/8 in a car that is 2975 lbs.

Posted By: all spooled up

Re: NORMAL COMPRESSION??? - 10/29/10 11:25 PM

my small block 230-235 psi cam is solid roller 650 lift duration at .020 in 296 exh 303
@ .050 263 270 in at 106

Posted By: Thumperdart

Re: NORMAL COMPRESSION??? - 10/30/10 12:35 AM

Guess our junk`s wore out.

Posted By: emarine01

Re: NORMAL COMPRESSION??? - 10/30/10 12:58 AM

Quote:

my small block 230-235 psi cam is solid roller 650 lift duration at .020 in 296 exh 303
@ .050 263 270 in at 106

Whats the compression ratio?

Posted By: Sport440

Re: NORMAL COMPRESSION??? - 10/30/10 01:02 AM

Quote:

Quote:

my small block 230-235 psi cam is solid roller 650 lift duration at .020 in 296 exh 303
@ .050 263 270 in at 106

Whats the compression ratio?

Based on the PSI though, I will guess, 13.5 or more

Posted By: WILD BILL

Re: NORMAL COMPRESSION??? - 10/30/10 01:25 AM

Quote:

Guess our junk`s wore out.

No,

Just points to how inaccurate a compression test is at telling you anything.

About the only thing you can learn from one is if you took one and wrote it down when the engine was built and then compared it to when you think you may have a problem.

I prefer a leak down test myself.

Posted By: Dodgem

Re: NORMAL COMPRESSION??? - 10/30/10 01:51 AM

look for it to be 160 to 165 cranking compression

Posted By: Dodgem

Re: NORMAL COMPRESSION??? - 10/30/10 01:52 AM

or plug your numbers into this
http://www.rbracing-rsr.com/comprAdvHD.htm

Posted By: Labratt

Re: NORMAL COMPRESSION??? - 10/30/10 05:16 AM

I just checked a couple of cylinders for S&G's! One was 175psi..the other was 173 or so! Didn't know about holding the carb open,though!

Thanks fella's!

RandyB

Posted By: Sport440

Re: NORMAL COMPRESSION??? - 10/30/10 05:37 AM

Thats more like it. On one of your other threads you registered 185 PSI witch I felt was high for your combo. IMO with around 10.25 Comp and that 262* cam your 170-175 PSI is in the normal range for the combo. Maybe even a little high.

But, FWIW, the Calulater that Dodgem posted was Happy about by 20# PSI for my Combo. But with your engine specs, it was almost right on.

mike

Posted By: polyspheric

Re: NORMAL COMPRESSION??? - 10/30/10 02:44 PM

large overlap cams bleeding compression

How exactly does this happen?

Posted By: BSB67

Re: NORMAL COMPRESSION??? - 10/30/10 02:56 PM

Quote:

large overlap cams bleeding compression

How exactly does this happen?

...out the exhaust valve" ???????

Posted By: BSB67

Re: NORMAL COMPRESSION??? - 10/30/10 03:07 PM

Quote:

I just checked a couple of cylinders for S&G's! One was 175psi..the other was 173 or so! Didn't know about holding the carb open,though! Thanks fella's! RandyB

If your doing it for S&G, and you like data, pick a cylinder, get a good consistant technique and baseline numbers, then change your intake lash plus 0.004" and minus 0.004". And then of course report back

Posted By: Labratt

Re: NORMAL COMPRESSION??? - 10/30/10 04:24 PM

Russ..I might just try that in the near-future! If I do,I'll be sure to post the results! BTW...how important is it to have the butterflies partially open,as Hardcore suggests?

Posted By: polyspheric

Re: NORMAL COMPRESSION??? - 10/30/10 04:24 PM

...out the exhaust valve" ???????

Which cam is that where the exhaust valve is open during the compression stroke?

Posted By: BSB67

Re: NORMAL COMPRESSION??? - 10/30/10 04:36 PM

Quote:

...out the exhaust valve" ???????

Which cam is that where the exhaust valve is open during the compression stroke?

I think you failed to understand my post. Obvously none, that's why there are the question marks. I'm quoting the other poster.

Posted By: polyspheric

Re: NORMAL COMPRESSION??? - 10/30/10 09:09 PM

You're right, sorry, yes I understood your comment to mean "Whaaaa?".
Was referring to prior comment.

Posted By: dusturbd340W5

Re: NORMAL COMPRESSION??? - 10/30/10 09:24 PM

But, FWIW, the Calulater that Dodgem posted was Happy about by 20# PSI for my Combo. But with your engine specs, it was almost right on. mike

it's not even close for my motor and as stated my leak down is less than 3% so I know it is sealed up well I also know someone that there roundy round motor is well over 600hp and only 165 cranking compression.

Posted By: ProSport

Re: NORMAL COMPRESSION??? - 10/30/10 09:34 PM

I run a big overlap pump gas cam, I get 150 cranking compression.

I put one gallon of 110 race gas in the tank when I race but don't need to. I've used lawn mower gas to drive to a local car show before, and still goes 9.80's.
If I go E85 this winter, it will get a better cam for performance.

Posted By: roadhazard

Re: NORMAL COMPRESSION??? - 10/30/10 09:44 PM

Quote:

BTW...how important is it to have the butterflies partially open,as Hardcore suggests?

LABRATT, I feel it's very important. With the throttle closed can cause a restriction and produce lower than normal cranking compression readings

My personal preferance is to set the carb @ WOT when testing. Usually I'll hold it open with a wire or use the return spring to hold it FWIW.

Don't forget to return the throttle closed before you start the motor again

Posted By: Dodgem

Re: NORMAL COMPRESSION??? - 10/30/10 11:16 PM

I often just take the carb off! then no fuel gets squirted in.

Posted By: dusturbd340W5

Re: NORMAL COMPRESSION??? - 10/31/10 12:52 AM

yep carb wide open or removed and all 8 plugs removed and a battery charger hokked up and running is how I do it.

Posted By: 602heavy

Re: NORMAL COMPRESSION??? - 10/31/10 01:05 AM

Quote:

large overlap cams bleeding compression

How exactly does this happen?

My thoughts exactly , overlap has no bearing on compression whatsoever.

Posted By: Labratt

Re: NORMAL COMPRESSION??? - 10/31/10 01:49 AM

So Greg..my readings would have been higher if I opened the butterflies on the carb??

Posted By: polyspheric

Re: NORMAL COMPRESSION??? - 10/31/10 03:43 AM

Always.

Posted By: roadhazard

Re: NORMAL COMPRESSION??? - 10/31/10 03:51 AM

Can't say with certainty but I also forgot to post as stated above "Remove ALL Spark Plugs"

p.s. got your PM & I'll get back with you on that

Greg