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NORMAL COMPRESSION??? #841571
10/29/10 05:31 PM
10/29/10 05:31 PM
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Clinton Twp... north of Deetro...
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Labratt Offline OP
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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

Re: NORMAL COMPRESSION??? [Re: Labratt] #841572
10/29/10 05:51 PM
10/29/10 05:51 PM
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Rock Springs
Bob_Coomer Offline
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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


[color:"red"]65 Hemi Belvedere coming soon [/color]
[color:"#00FF00"]557" Indy engine 1.07 60ft 144mph in the 8th 2100 lbs package [/color]
Re: NORMAL COMPRESSION??? [Re: Labratt] #841573
10/29/10 05:56 PM
10/29/10 05:56 PM
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Shelby Twp. Mi
HardcoreB Offline
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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.

Re: NORMAL COMPRESSION??? [Re: Bob_Coomer] #841574
10/29/10 07:03 PM
10/29/10 07:03 PM
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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?


72 Dart 470 n/a BB stroker street car `THUMPER`...Check me out on FB Dominic Thumper for videos and lots of carb pics......760-900-3895.....
Re: NORMAL COMPRESSION??? [Re: Thumperdart] #841575
10/29/10 07:18 PM
10/29/10 07:18 PM
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Rock Springs
Bob_Coomer Offline
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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


[color:"red"]65 Hemi Belvedere coming soon [/color]
[color:"#00FF00"]557" Indy engine 1.07 60ft 144mph in the 8th 2100 lbs package [/color]
Re: NORMAL COMPRESSION??? [Re: Bob_Coomer] #841576
10/29/10 07:24 PM
10/29/10 07:24 PM
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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.


70 duster full chassis super pro 416 CNC Indybrock heads 727 w/brake

best so far 1.212 60 6.219 in 1/8 at 110.88 9.768 at 137.81 1/4
Re: NORMAL COMPRESSION??? [Re: Bob_Coomer] #841577
10/29/10 07:25 PM
10/29/10 07:25 PM
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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

Last edited by hookd; 10/29/10 08:53 PM.
Re: NORMAL COMPRESSION??? [Re: all spooled up] #841578
10/29/10 08:35 PM
10/29/10 08:35 PM
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Guess our junk`s wore out.


72 Dart 470 n/a BB stroker street car `THUMPER`...Check me out on FB Dominic Thumper for videos and lots of carb pics......760-900-3895.....
Re: NORMAL COMPRESSION??? [Re: all spooled up] #841579
10/29/10 08:58 PM
10/29/10 08:58 PM
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emarine01 Offline
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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?

Re: NORMAL COMPRESSION??? [Re: emarine01] #841580
10/29/10 09:02 PM
10/29/10 09:02 PM
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Canton, Ohio
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Sport440 Offline
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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

Re: NORMAL COMPRESSION??? [Re: Thumperdart] #841581
10/29/10 09:25 PM
10/29/10 09:25 PM
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BROOK PARK, OH
WILD BILL Offline
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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.

Re: NORMAL COMPRESSION??? [Re: Labratt] #841582
10/29/10 09:51 PM
10/29/10 09:51 PM
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Dodgem Offline
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look for it to be 160 to 165 cranking compression

Re: NORMAL COMPRESSION??? [Re: Dodgem] #841583
10/29/10 09:52 PM
10/29/10 09:52 PM
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or plug your numbers into this
http://www.rbracing-rsr.com/comprAdvHD.htm

Re: NORMAL COMPRESSION??? [Re: Dodgem] #841584
10/30/10 01:16 AM
10/30/10 01:16 AM
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Clinton Twp... north of Deetro...
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Labratt Offline OP
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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

Re: NORMAL COMPRESSION??? [Re: Labratt] #841585
10/30/10 01:37 AM
10/30/10 01:37 AM
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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

Re: NORMAL COMPRESSION??? [Re: Bob_Coomer] #841586
10/30/10 10:44 AM
10/30/10 10:44 AM
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polyspheric Offline
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large overlap cams bleeding compression

How exactly does this happen?


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Re: NORMAL COMPRESSION??? [Re: polyspheric] #841587
10/30/10 10:56 AM
10/30/10 10:56 AM
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Quote:

large overlap cams bleeding compression

How exactly does this happen?




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

Re: NORMAL COMPRESSION??? [Re: Labratt] #841588
10/30/10 11:07 AM
10/30/10 11:07 AM
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BSB67 Offline
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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

Re: NORMAL COMPRESSION??? [Re: BSB67] #841589
10/30/10 12:24 PM
10/30/10 12:24 PM
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Labratt Offline OP
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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?

Re: NORMAL COMPRESSION??? [Re: BSB67] #841590
10/30/10 12:24 PM
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polyspheric Offline
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...out the exhaust valve" ???????

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


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Re: NORMAL COMPRESSION??? [Re: polyspheric] #841591
10/30/10 12:36 PM
10/30/10 12:36 PM
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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.

Re: NORMAL COMPRESSION??? [Re: BSB67] #841592
10/30/10 05:09 PM
10/30/10 05:09 PM
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polyspheric Offline
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You're right, sorry, yes I understood your comment to mean "Whaaaa?".
Was referring to prior comment.


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Re: NORMAL COMPRESSION??? [Re: polyspheric] #841593
10/30/10 05:24 PM
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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.


70 duster full chassis super pro 416 CNC Indybrock heads 727 w/brake

best so far 1.212 60 6.219 in 1/8 at 110.88 9.768 at 137.81 1/4
Re: NORMAL COMPRESSION??? [Re: dusturbd340W5] #841594
10/30/10 05:34 PM
10/30/10 05:34 PM
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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.


1970 Challenger, all aluminum 528 Hemi, HDK suspension, Tremec 5 speed manual
Re: NORMAL COMPRESSION??? [Re: Labratt] #841595
10/30/10 05:44 PM
10/30/10 05:44 PM
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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

Re: NORMAL COMPRESSION??? [Re: roadhazard] #841596
10/30/10 07:16 PM
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I often just take the carb off! then no fuel gets squirted in.

Re: NORMAL COMPRESSION??? [Re: Dodgem] #841597
10/30/10 08:52 PM
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dusturbd340W5 Offline
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yep carb wide open or removed and all 8 plugs removed and a battery charger hokked up and running is how I do it.


70 duster full chassis super pro 416 CNC Indybrock heads 727 w/brake

best so far 1.212 60 6.219 in 1/8 at 110.88 9.768 at 137.81 1/4
Re: NORMAL COMPRESSION??? [Re: polyspheric] #841598
10/30/10 09:05 PM
10/30/10 09:05 PM
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602heavy Offline
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Quote:

large overlap cams bleeding compression

How exactly does this happen?




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

Re: NORMAL COMPRESSION??? [Re: roadhazard] #841599
10/30/10 09:49 PM
10/30/10 09:49 PM
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Clinton Twp... north of Deetro...
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Labratt Offline OP
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So Greg..my readings would have been higher if I opened the butterflies on the carb??

Re: NORMAL COMPRESSION??? [Re: Labratt] #841600
10/30/10 11:43 PM
10/30/10 11:43 PM
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polyspheric Offline
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Always.


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Re: NORMAL COMPRESSION??? [Re: polyspheric] #841601
10/30/10 11:51 PM
10/30/10 11:51 PM
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roadhazard Offline
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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

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