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Quote:
Originally Posted by Mark Henry View Post
Real RSR pistons require RSR heads or the engine will be a low CR pig.
My 79 3.0 has mahle RSR style pistons 10.5 CR on stock cc heads, we're making an educated guess but we think my pistons were made for a race team to make faux RSR engines for practice/back-up.
Interesting.. What i don't know is which specific cylinder heads were used for the build, except that they were matched to work.

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Originally Posted by dannobee View Post
One thing about compression tests is that the gauges vary. Actually, the Shraeder valve in the end of the hose causes the variation from one gauge to another because of varying spring rates. Using a tire valve stem Shraeder valve will really screw up the readings.

Aircraft engines use a leak down tester to measure "compression." i.e., 78/80. The first being the raw psi on the gauge at the cylinder and the 80 being the regulated input air pressure. Automotive, on the other hand, uses input regulated to 100 psi and measures leak down in a percentage of that, i.e., 95psi would be "5% leak down."

Look for the uniformity across all six, not the absolute number. In that case, yours looks fine.

Try another compression gauge or two and see if they read higher.

And yes, a big cam could lower your readings significantly and still be "good."
Good feedback, thanks! I'll ask the shop what the brand of their equipment is, it might shed some more light on the low compression numbers. Regardless, I hope to see leakdown numbers in a few days, once they get the car back on a lift..

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Old 02-23-2021, 07:51 AM
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That's a healthy engine! Interesting that the compression is this high with 20/21 cams..[/QUOTE]

20/21 cams have no overlap they're designed for cis/dme emission engines

Overlap is what give you low compression readings
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Old 02-23-2021, 03:05 PM
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Thought there was 7 degrees of overlap with 20/21s? I got basically the same compression numbers with the same cam on an SC
Old 02-23-2021, 03:48 PM
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Quote:
Originally Posted by Glenfield View Post
Thought there was 7 degrees of overlap with 20/21s? I got basically the same compression numbers with the same cam on an SC

Oh wow. That’s very good to know!


Sent from my iPhone
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Old 02-23-2021, 05:42 PM
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Quote:
Originally Posted by Mark Henry View Post
Real RSR pistons require RSR heads or the engine will be a low CR pig.
My 79 3.0 has mahle RSR style pistons 10.5 CR on stock cc heads, we're making an educated guess but we think my pistons were made for a race team to make faux RSR engines for practice/back-up.
My 98mm RSR Mahles measure 9.8:1 on late SC heads. You really need to CC the dome and combustion chamber and do the math.
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Old 02-24-2021, 06:48 AM
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Quote:
Originally Posted by Dan J View Post
That's a healthy engine! Interesting that the compression is this high with 20/21 cams..
20/21 cams have no overlap they're designed for cis/dme emission engines

Overlap is what give you low compression readings[/QUOTE]

________________________________

The card that came with the 20/21s when they were acquired 20+ years ago...



The current specs from Webcam's site, note the differences...



I was concerned that my compression test numbers were low, until I remembered the 20/21s. Sal Carceller provided me with this calculator to try and estimate a percentage of what a 3.2 motor with 20/21s would compression test out to vs stock.

Wallace Racing: Dynamic Compression Ratio Calculator
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Old 02-24-2021, 07:11 AM
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Quote:
Originally Posted by famoroso View Post
20/21 cams have no overlap they're designed for cis/dme emission engines

Overlap is what give you low compression readings
________________________________

The card that came with the 20/21s when they were acquired 20+ years ago...



The current specs from Webcam's site, note the differences...



I was concerned that my compression test numbers were low, until I remembered the 20/21s. Sal Carceller provided me with this calculator to try and estimate a percentage of what a 3.2 motor with 20/21s would compression test out to vs stock.

Wallace Racing: Dynamic Compression Ratio Calculator[/QUOTE]

Famoroso- great, relevant info, thanks!!
My lack of cam knowledge is kicking in here; how does the 20/21 cam compare to the 594/595?
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Old 02-24-2021, 02:57 PM
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594/595 is a lot more aggressive
Old 02-24-2021, 07:23 PM
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Old 02-24-2021, 07:24 PM
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Quote:
Originally Posted by emcon5 View Post
My 98mm RSR Mahles measure 9.8:1 on late SC heads. You really need to CC the dome and combustion chamber and do the math.
Yes I agree you really have to do the maths.

I see RSR thrown around a lot, "real RSR" 3.0 pistons have a matching smaller CC head, the pistons I have look like the RSR wedge dome head pistons, but were NIB 10.5:1 CR for stock heads.
Some claimed RSR pistons are just high CR with a regular dome.

I was pooping puppies till I did the maths and found out that my pistons were indeed made for stock head chambers.

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Old 02-25-2021, 07:54 AM
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Here is Mahle “RSR” 3.0 built on a SC case, with 935 heads @ 85cc and dome volume of 40.3cc: 10.6:1.
Old 02-25-2021, 08:29 AM
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Here is a Mahle “RSR” 3.4, built on a Carrera case with 935 heads @ 84.6cc. Dome volume is 37.5cc: 11.2:1.
Old 02-25-2021, 08:34 AM
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You cannot use those valve events in any calculator as they are taken @ .05 of lift

There is an additional 20deg of valve duration. Intake opens somewhere around 16 to 18 BTDC and closes around 60 to 62deg ABDC. Depending on what timing number you used

Ideally, you would need to take actual measurements at the valve with .006" lash


Quote:
Originally Posted by jakg104 View Post
________________________________

The card that came with the 20/21s when they were acquired 20+ years ago...



The current specs from Webcam's site, note the differences...


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Old 02-25-2021, 09:56 AM
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I'd be more interested in whether or not it had new head studs installed during the rebuild. That #5 leak could very well be from a popped stud.

And where was the compression test performed? You're going to get different readings in LA than you will in Denver.
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Old 02-25-2021, 10:19 AM
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These are mine.



According to my notes, the dome was 39.3cc, an my heads are 90 cc.

Andial advertised them as 98mm RSR.
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Old 02-25-2021, 11:27 AM
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As already pointed out, you need the true seat to seat duration events and no cam specs give this. We need to know exactly when the intake valve closes and hits the seat. Likely around 60-65 ABDC.

I think the compression numbers are a bit low even for that cam. I'd say you should be around 130PSI or so.

Just do a leak down to get the rest of the story. The leak down does not care about the cam.

The later the intake closes the lower the compression number will be.

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Old 02-25-2021, 11:45 AM
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