Guess The Compression Ratio

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Thrashard340

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Hey everyone. I just bought another Duster. So far the only things I have been able to gather on the motor is it's a 360, 340 "X" heads, KB 107-8 (.060 oversize) pistons, Crower 31205 cam (290/298 duration, 507/522 lift), Mopar P4120094 head gasket (.024"-.028"). I don't know deck height, Crankshaft or connecting rod specs. I have a stack of receipts, but no short block machine info.

Based on the above info, can anyone guess approximate compression? I am trying to figure out what octane gas to use. I was trying to use the compression calculator on kb piston site and came up with an approximate guess of 10.5-11:1. Some knocking at WOT so I think gas is the cause.

Thanks - STAN
 
Based on typical 72 cc chamber volume and nominal 0.012" below the deck for the pistons the compression ratio calculates to 9.8:1.

It could be higher if the deck was machined or the heads have smaller chambers but the only way to know that for sure is to take it apart.
 
I'm with dgc. Probably below what the site says you should have. 9.8 or 10:1 max.
 
Here is another calculator I used with the fields that I populated (in blue) with the info dgc333 provided and part # searches on the web.

http://mopowerstyle.com/index.php?pid=8


Enter Bore size 4.06


Enter Stroke size 3.58


Enter Head Gasket Bore size 4.06


Compressed Gasket Thickness .028


Enter Combustion Chamber Volume in CC's 72


Enter Piston Dome Volume in CC's use "-" for Dished Pistons 5


Enter Piston Deck Clearance If ABOVE deck use "-" .012

11.06:1 Seems high. Did I do something wrong? Thanks in advance. STAN
 
I use this method for calculating static compression.

V1+V2+V3+V4 divided by the sum of V2+V3+V4

V1= bore x bore x stroke x .7854
V2= bore x bore x deck clearance x .7854
V3= bore x borex gasket thickness x .7854
V4= combustion chamber volume in ccs x .061

For you it would be;

V1= 4.06 x 4.06 x 3.58 x.7854
V1= 46.3475

V2= 4.06 x 4.06 x 0.012 x .7854
V2= 0.1554

V3= 4.06 x 4.06 x .024 x.7854
V3= 0.3107

V4= 72cc's x .061
V4= 4.392

The problem would look like this after you add it up: 51.2056/4.8581

Answer is 10.54025

This is for flat top pistons Stan, I didnt know for sure if you had 5cc dome or dish pistons so I just decieded to do it with flat tops instead, let me know and I can redo in a second for you.
 
BoredandStroked said:
I use this method for calculating static compression.

V1+V2+V3+V4 divided by the sum of V2+V3+V4

V1= bore x bore x stroke x .7854
V2= bore x bore x deck clearance x .7854
V3= bore x borex gasket thickness x .7854
V4= combustion chamber volume in ccs x .061

For you it would be;

V1= 4.06 x 4.06 x 3.58 x.7854
V1= 46.3475

V2= 4.06 x 4.06 x 0.012 x .7854
V2= 0.1554

V3= 4.06 x 4.06 x .024 x.7854
V3= 0.3107

V4= 72cc's x .061
V4= 4.392

The problem would look like this after you add it up: 51.2056/4.8581

Answer is 10.54025

This is for flat top pistons Stan, I didnt know for sure if you had 5cc dome or dish pistons so I just decieded to do it with flat tops instead, let me know and I can redo in a second for you.
Click to expand...

Clint, the specs on the pistons read out at +5cc

http://store.summitracing.com/partdetail.asp?part=UEM-KB107060-8&autoview=sku
 
Your head gasket bore size will be latger then your blocks bore size. Most Fel Pros /Mr Gaskets are about 4.180" bore
 
Thrashard340 said:
Here is another calculator I used with the fields that I populated (in blue) with the info dgc333 provided and part # searches on the web.

http://mopowerstyle.com/index.php?pid=8


Enter Bore size 4.06


Enter Stroke size 3.58


Enter Head Gasket Bore size 4.06


Compressed Gasket Thickness .028


Enter Combustion Chamber Volume in CC's 72


Enter Piston Dome Volume in CC's use "-" for Dished Pistons 5


Enter Piston Deck Clearance If ABOVE deck use "-" .012

11.06:1 Seems high. Did I do something wrong? Thanks in advance. STAN
Click to expand...

Yes the "5" cc's for the piston dome volume should be "-5" to accomodate the valve reliefs in the pistons. This changes the results to the 9.8 I had reported earlier. This value is only positive with domed pistons.
 
BoredandStroked said:
This is for flat top pistons Stan, I didnt know for sure if you had 5cc dome or dish pistons so I just decieded to do it with flat tops instead, let me know and I can redo in a second for you.
Click to expand...

The KB107 piston is a flat top with 5cc reliefs so you should calculate it as though there is a 5 cc dish. Also, most head gaskets have a bigger bore than the bore of the cylinder those two factors would get you the same answer as the calculators that come up with 9.8
 
Thanks everyone. Clint, great formula. dgc333, thanks for the clarification on the pistons. And Adam for contributing gasket info.

I think there is more to the motor. I will have to do some more research on the heads to find out true volume. It doesn't behave like 9.6:1 compression. I could be wrong though.
 
Those mopar head gaskets are for a bore of a maximum of 4.060 so I doubt there quite as big as the felrpros wich are 4.180 but still trying to guess the compression is basicly impossible unless the engines apart.My brothers 360 is at 10.75 with std. felpro gasket,kb 107s @ zero deck and .050 milled off the heads-and it behaves like it!-its a pretty nasty small block and is octane sensitive.
 
FWIW, my 360 with magnum heads is 10.6:1 and is not octane sensitive and will run just fine on 89. BUT, the closed chamber magnum heads make of a detonation resistant engine.

Also what is the total advance you are getting from the engine? If the distributor has not been recurved and you are running additional intial timing then you likely have too much advance.
 
I would say right at the10.2-10.5. Hard to tell without knowing the head cc. Just zero decking and kb's sure help though.
 
It sounds like you might check your ignition curve and advance settings. Too much advance = detonation. At what point in acceleration does the ping begin, and under WOT only, heavy load, any specific rpm range? All these things can be a good clue as to when or better yet where in the advance curve to retard the timing...or if course it could be purely mechanical, in which case this won't help much at all! But good luck!8)
 
I'll check the timing this weekend. Detonation is at WOT only, no particular RPM. No problems at cruise or part throttle acceleration. Too much advance? I'll also try to ID the distributor. Looks like a Mopar unit, but not sure at this point. What should timing be set at?

It pulls hard at WOT although I haven't had it there for any prolonged amount of time due to the knocking.
 
I added octane boost and the detonation went away. I checked the initial timing and it was set at 5 degrees advanced. Seems conservative. I figured it should be at 10. Total timing was about 28 degrees although I couldn't get a good reading here.

Seems to run great with the high octane stuff. I had to add almost 12 ounces (about 3/4 of the bottle) of 104 octane boost to less than 10 gallons of gas. I'm not sure what this brought the octane to, but it seemed to have did the trick. California gas...go figure.

There's a gas station about 10 miles from my house which sells racing gas...up to 100 unleaded up to 110 leaded. Looks like I'll be making trips to this gas station from now on for the 100.
 
The octane boost products only raise the octane a fraction of a point. That is it might raise 93 to 93.5. They are miss leading when the advertise it will raise the octane 5 points, they don't explain it means .5.
 
dgc333 said:
The octane boost products only raise the octane a fraction of a point. That is it might raise 93 to 93.5. They are miss leading when the advertise it will raise the octane 5 points, they don't explain it means .5.
Click to expand...

Thanks for the clarification. It's only a temporary fix. I'll take it to the station to get decent gas in there.
 
What do your plugs look like? It sounds to me more like you are a little lean on full throttle. Your timing is conservative. Which may be a good thing. You really dont want to detonate with those pistons. What is the cylinder cranking compression? My guess is it will be close to 180 psi. Try jetting up 4 sizes on the main jets, or down 2 rod sizes on an Edelbrock with the same jet.
 
moper said:
What do your plugs look like? It sounds to me more like you are a little lean on full throttle. Your timing is conservative. Which may be a good thing. You really dont want to detonate with those pistons. What is the cylinder cranking compression? My guess is it will be close to 180 psi. Try jetting up 4 sizes on the main jets, or down 2 rod sizes on an Edelbrock with the same jet.
Click to expand...

Thx Moper. Here are what the plugs look like. The pics came out a little lighter than actual, the brown is a little darker. I couldn't turn the flash off on the camera. Looks like I was a little premature on my previous post, pinging is still there at WOT only.

1st pic - cylinders, from left to right, 1, 3, 5, 7

2nd pic - cylinders from left to right, 2, 4, 6, 8

Does it look lean? I will get down to rejetting the carb next weekend.

Thanks - STAN

72duster062807 016 rs.JPG


72duster062807 017rs.jpg
 
There's an aweful lot of crud on those plugs for pump gas. A lot of oil on the rings too. Can yo urun a new plug or two and post what they look like after a few street miles on pump fuel? It may simply be a matter or changing the ignition curve. You could also have a phasing problem in the distributor. What type of ignition do you run?
 
moper said:
There's an aweful lot of crud on those plugs for pump gas. A lot of oil on the rings too. Can yo urun a new plug or two and post what they look like after a few street miles on pump fuel? It may simply be a matter or changing the ignition curve. You could also have a phasing problem in the distributor. What type of ignition do you run?
Click to expand...

Will do. I already swapped in new plugs when I pulled these ones. I matched the plug #, Autolite AP66. It seems to have temporarily cured the problem. I also put a little less than 5 gallons of 100 octane mixed with maybe 2-3 gallons of 91. The brown on the plugs look almost like rust. Is this possible? Car was not driven often and has maybe less than 5,000 miles.

Ignition looks like stock electronic ignition, Mopar distributor, black control box(when's the last time you've seen one of these?) ballast resistor, and Accel coil.
 
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