68' 383 compression numbers

[Hard Charger]

what is the correct/acceptable  compression rate/reading on a 68 383HP from the factory?

[XH29N0G]

Others will know better.  Do you mean compression ratio or PSI from a compression test?  I am assuming the latter.  If so, I think it is important that all cylinders are close to the same numbers.  I think the ideal is 150 psi or so.  My 70 was at 130-135 psi and this was also OK.  I would assume something less than 100-120 would be an issue, but others will know.

[Cooter]

160 PSI and your into the 10.8:1 range...Don't ask me how I know.....You would not believe how much 5 PSI of cranking pressure can f*ck up your day.

[Hard Charger]

my engine is a factory build from 68, 80k miles  with compression readings at 130 for all but 1 cylinder which is at 120. there is a little oil burning from that bank but that could be the heads i suppose.
the first time the engine had been removed was a couple months ago. the original valve cover gaskets are still in use according to a Mopar Muscle article.

i am hoping the opinion here is that it is good and does not warrant a rebuild. i cannot afford it at the moment.

[XH29N0G]

Someone else will have to answer with more detail and probably correctly, but here are my  .

My experience with a 1970 383 magnum was about 130 to 135 ( recently - without opening the butterflies all the way - which I should have done).  I seem to remember higher numbers in the 1980's (more like 150).

The compression changed considerably with a cam change (which is probably due to different valve closing time on the compression stroke) and went up to about 150.

My guess is that you are looking at an old engine and unless it is burning oil, it will continue to work for a long time.  My guess is you have lost some compression and power, but may be OK.

[Mebsuta]

68 383, rebuilt in 1989, .030 with replacement flat-top pistons, same compression height as original.  They were available back then.  Blue Fel Pro gasket and 902 open chamber heads.  I get about 140-145 psi all around, cold.  Different heads or different cam with earlier intake closing would raise that a little, but it did not run any better.  My old Chilton book says 150 psi for 383 HP in 68-69.  I use factory pattern cam, which has relatively late intake closing.     

[Cooter]

Just FYI here, but take say 150 PSI and  Devide by 14.7 and you roughly have the compression ratio...(10.20408:1)

[Hard Charger]

great response thank you.

i am reading 130psi average with one cylinder at 120 & 108.

i am looking for a reason to not rip into the engine right now.

[c00nhunterjoe]

Cooter, you can't use the psi/14.7 trick. If it worked properly then my engine is a freak of nature. I can put out 190psi cranking pressure which divided by 14.7, would give would give me a 12.9:1 motor. And I ghrantee you that its not.

[XH29N0G]

Cooter, you can't use the psi/14.7 trick. If it worked properly then my engine is a freak of nature. I can put out 190psi cranking pressure which divided by 14.7, would give would give me a 12.9:1 motor. And I ghrantee you that its not.

I didn't understand this either and asked about the 14.7 trick a while ago.  What I am about to write is probably still not 100% correct.  The 14.7 trick does not assume that the gas should heat up as it is compressed (which will give a higher number).  If we had true 10:1 compression, the pressure would be much higher than 147 PSI because of the gas heating up as compressed.    The reason the usual compression test gives a number that is not through the roof is because the intake valves close a little later than the optimum for full compression and some air bleeds off.  I think this is also the reason why higher compression engines do better with longer duration cams and low compression engines do not.  Higher compression engines can deal with more air bleeding off.

I think the difference in PSI between different cylinders (more than 10-15%) is probably telling you something is up with them.  Having 130 PSI all around might still be fine with an old 383.

[Cooter]

Of course, it all depends on your cam specs. if aftermarket, but since we Assumed we were referring to a stock 383....

Mine worked out just fine with the "14.7 trick", as it won't start when hot @10.88:1 by the formula and it stands to reason.....I'm am not aware of any formula that will tell you to the ninth degree what compression your engine is as the reason I Stated ROUGHLY, but there are some like the one I offered that will tell you basically what you have.

[Hard Charger]

the engine ran beautifully.

the only issue was hard to start once and a while. i figured it was the ignition or old fuel line. everybody has a ritual on starting these old cars.

[1974dodgecharger]

Just FYI here, but take say 150 PSI and  Devide by 14.7 and you roughly have the compression ratio...(10.20408:1)

THANKS COOTER 

[heyoldguy]

my engine is a factory build from 68, 80k miles  with compression readings at 130 for all but 1 cylinder which is at 120. there is a little oil burning from that bank but that could be the heads i suppose.
the first time the engine had been removed was a couple months ago. the original valve cover gaskets are still in use according to a Mopar Muscle article.

i am hoping the opinion here is that it is good and does not warrant a rebuild. i cannot afford it at the moment.

Haven't you answered your own question? "i cannot afford it at the moment."

It gets you around, run it.

[Challenger340]

just trivia, but....

1st) The 14.7 "psi "ambient" air pressure only works at sea level elevation, as you move higher from sea level the air gets thinner.

2nd) Because the intake valve does not close until after BDC, when the piston has already started back UP, you only start building "compression" AFTER the intake valve closes, so you need to find the actual intake valve closing "point", to determine REAL Stroke as it relates to where compression starts building a "ratio".
The VOLUME contained at that point by the valve closing(cam event), is what gets compressed and determines "psi", not the full volume when the piston is all the way down at BDC.

The stock 1968 383 4 BBL Engine Cam has an intake valve closing point of about 58* ABDC, meaning with the stock length 383 connecting rod(6.358") angularity(determines speed away from BDC), means your actual "stroke" for compression purposes is 2.75" on an 8.5:1 static Compression ratio for the 135 psi Compression reading.
Assumes all 8 spark plugs removed for compression test, throttles plates wired open, 4 bumps only on the compression tester with the 1st bump 75% of the 4th and final bump.

[BSB67]

my engine is a factory build from 68, 80k miles  with compression readings at 130 for all but 1 cylinder which is at 120. there is a little oil burning from that bank but that could be the heads i suppose.
the first time the engine had been removed was a couple months ago. the original valve cover gaskets are still in use according to a Mopar Muscle article.

i am hoping the opinion here is that it is good and does not warrant a rebuild. i cannot afford it at the moment.

Haven't you answered your own question? "i cannot afford it at the moment."

It gets you around, run it.

Kind of what I was thinking.  If you don't have the money, and you turn the key and it goes, what is left to discuss?