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Old 10-01-2010, 08:30 PM
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Compression Calculation

Can someone do the math for me here. I want to know what compression I get with a -14.5cc piston with a 58cc chamber head. I am getting conflicting information, then one source the compression is based on -.010 deck height. I don't know what deck height is used. I have a 68 302 block unbored. It looks to be in very good shape but we don't know that for sure til we go measuring. Me not knowing the deck height at worst case (undecked block) what should my compression be.

Also in thinking this build out do we want a zero deck block. What does this really truly accomplish.

The reason I asked about the piston/compression is because I got conflicting information from two sources, then again, we don't know the deck the other sources compression was at.

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Old 10-01-2010, 08:54 PM
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Zero deck, .041" thick gasket, 14.5cc dome = 12.89-1

.010" deck9down the bore), .041" gasket, 14.5cc dome = 12.44-1

zero deck, .041" gasket, 14.5cc dish = 8.65-1

.010" deck, .041" gasket, 14.5cc dish = 8.44-1

I did it both dish and dome because I was unsure which you meant by "-14.5cc piston", different calculators use + or - differently. You can easily see the difference that just .010" deck height makes becuase it is volume removed from the entire bore area of the cylinder, versus milling the head which only removes volume from the chanber area. Besides the compression ratio increase, zero decking allows you to set the quench at the idea distance(.035-.045") using a common .041" head gasket. Ideal quench reduces octane sensitivity and promotes good torque and complete mixture burn.

Calculator used from www.wallaceracing.com , on the Automotive Calculators page.
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Old 10-02-2010, 01:18 AM
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I see I should have specified Dish. Wow, this is pretty low( I think) compression. Do you think this volume will clear a 2.02 valve at say .512 lift.
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Old 10-02-2010, 01:54 AM
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Yes, compression is pretty low, dished pistons make for low compression on small short stroke motors.

Piston dish volume has nothing to do with piston to valve clearance, only the size of the valve relief eyebrow will determine that.

With a 2.02" valve in a SBF 302 it is common to have valve clearance problems radially with the 2.02" valve diameter with pistons that are not race pistons, as no stock 302 used a valve size bigger than 1.84", and most were smaller than that. If it is a dish with no eyebrow expect to have to cut some clearance into the piston, more due to valve diameter rather than cam size.
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Old 10-02-2010, 11:12 AM
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I believe these are considred a race or aftermarket piston. So how can I figure out for sure I will not have problems rather than waiting to for piston to valve clearance on assembly. I am mocking this build up. Has not happened yet, but I wanted to use parts I already had. Sounds like I may have screwed up putting 2.02's in the heads.
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Old 10-02-2010, 01:50 PM
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Quote:
Originally Posted by Carlos9
I believe these are considred a race or aftermarket piston. So how can I figure out for sure I will not have problems rather than waiting to for piston to valve clearance on assembly. I am mocking this build up. Has not happened yet, but I wanted to use parts I already had. Sounds like I may have screwed up putting 2.02's in the heads.
Do a trial assembly of one cylinder to check piston to valve clearance.

This is what seperates engine builders from guys who are just engine assemblers. The engine builder knows there will be areas that need clearance and plans to assemble and dissasemble various areas of the engine several times, to check clearances and find out where additional machining is needed and make the necessary corrections. Only way around that is just to build the exact same engine parts combo several times and learn where things need modification, then you are familiar with it and know what to change before hand.

Have you got a part # and brand for the piston??
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Old 10-02-2010, 09:47 PM
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It's an SRP piston = 138727
http://www.jepistons.com/Catalogs/SRP/Ford/dish.aspx

No this is the part where I come here and ask what's up. I know someone knows the answer which is what I'm trying to figure out. I am not an engine builder by no means. Would appreacite the help though. How much trouble would it be to have a 1.94 valve put back in the heads now that they've been ported and valves unshrouded etc. What does this change. I will be using parts I already have for the motor.. or at least I wanted to. ISKY cam for 351W. I believe it's 216 @.50 with .480 lift. I just want to know if all this is good and no problems with 2.02 valves. Will the 351w cam work in 302.
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Old 10-03-2010, 12:33 AM
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I should have clarified, 2.02" valves can have problems with stock 302 pistons, some stock replacement cast and hypers, and older TRW forgings, but most everything else will be fine.

Being an SRP piston, and listed as fitting most aftermarket inline valve heads like Canfield, Dart, Brodix, World Windsor - most of which are equipped with 2.02" valves would lead me to believe you will have no clearance problems at all. I would be about 98.5% sure of that, cam you are wanting to use is not that big at the point the piston and valve are closest together, which is in the 5-15 BTDC area during overlap. Duration is low enough that the valve is not far off the seat at that point.

Peak valve lift has nothing to do with valve clearance, the piston is far away from the valve, down the bore, at peak valve lift. It is big durations that give piston to valve clearance problems, more so with the fat lobe of roller cams.

I would really be very surprised if you even come close to .200" piston to valve clearance, i would expect it to be more than that easily. valve eyebrow in SRP pistons normally will clear a 2.08" valve with much bigger cams than that..

Trying to go back to 1.94" valves pretty much can't be done if the head has been cleaned up in the bowl for the 2.02" valve size. You possibly could have a smaller hard seat installed(but I'm not sure if it will even work) but it would result in a huge lip under the valve and absolutely murder flow.. If you felt you had to have 1.94" valves, about the only choice is sell the heads and start over with a second set.

351 cams can be used in early 289/302, distributor just has to have the plug wires rearranged to match the 351/302HO firing order. No big deal. Just be sure of the firing order, some companies sell all their SBF cams with the old 302 order, some sell them either way, and some only the newer 302HO/351 order. Any will work in all engines, just as long as plug wires are set up in the right order.

Last edited by ericnova72; 10-03-2010 at 12:40 AM.
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Old 10-03-2010, 12:38 AM
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Sorry Eric, I was typing while you were posting....

Block deck height for a stock, unaltered '68 302 cylinder block is 8.206". Stroke radius is 1.500". Rod length is 5.090". Compression height for the pistons you are talking about is 1.600". If you add together the stroke radius, rod length and compression height, you get a stack dimension of 8.190". Subtracting 8.190" from 8.206" will tell you that with the preceeding stack of parts in the block, the piston deck height will be 0.016", with a volume of 3.34 cc's, figuring a +0.030" overbore.

At +0.030", the bore will be 4.030" and the volume of the cylinder will be 627.07. The chambers will have 58 cc's. The pistons will have 14.5 cc's. A 0.040" gasket will have about 9 cc's. And then you have the 3.34 cc's from the piston deck height. If you add all the volumes up, 627.07, 58, 14.5, 9 and 3.34, you find 711.91 cc's. Now, if you drop out the cylinder volume and add the other 4 together.....58, 14.5, 9 and 3.34, you find that the mixture will be squeezed into 84.84 cc's with the piston at TDC. Dividing 711.91 by 84.84 yields a 8.39:1 static compression ratio.
Cutting the decks to zero would remove the 3.34 cc's of piston deck volume, so we would be dividing 708.57 by 81.5 and finding a 8.69:1 static compression ratio.

Now, you know how to figure static compression ratio for yourself. You need 5 figures if you're using a piston deck height that allows the piston to be below deck at TDC. You need 4 figures if you will zero deck the pistons to the block.

I think the 351 cam will work in the 302 block by changing the firing order of the motor to the 351 firing order. The cam is a little much for the low scr, so I might use a quick leak-down lifter such as a Crane 99380-16 or Rhoads 9008 to leak off a little duration under 3000 and make the cam work better with the low scr.
http://www.summitracing.com/parts/RHL-9008/

Last edited by techinspector1; 10-03-2010 at 12:52 AM.
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Old 10-05-2010, 03:30 PM
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Wow, thanks for the dicovery here. I would not have known compression was so low because the car ran good and strong. The cam was a 204-214 durations cam. Do you also think that was too much for this low compression motor. I am pretty surprised because there ain't much more I can do with the motor to increase compression with this current combo.
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Old 10-05-2010, 04:06 PM
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That 204/214 cam would be a good match to that low compression ratio, something bigger will make it sluggish at low rpm.
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Old 10-05-2010, 10:31 PM
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It ran very well, that why I was surprised to see compression so low. What do you think now if I went down another step on the cam.. or should I stay where I'm at. I was just thinking I could use parts I already had, but I also think I will return the cam I have (216 duration) for something else. Any thoughts on going down more on cam, and what would you recommend. That thing hit hard and everyone thought I had a big motor.. but it was a little 302 with that 204-214 duration cam.
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Old 10-05-2010, 10:40 PM
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Quote:
Originally Posted by Carlos9
It ran very well, that why I was surprised to see compression so low. What do you think now if I went down another step on the cam.. or should I stay where I'm at. I was just thinking I could use parts I already had, but I also think I will return the cam I have (216 duration) for something else. Any thoughts on going down more on cam, and what would you recommend. That thing hit hard and everyone thought I had a big motor.. but it was a little 302 with that 204-214 duration cam.
Good combinations can be elusive. If you have found one that works for you, why change?
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Old 10-16-2010, 07:00 PM
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Quote:
Originally Posted by ericnova72
That 204/214 cam would be a good match to that low compression ratio, something bigger will make it sluggish at low rpm.

I have a question on this one guys, now that we know exactly what we got for parts. I really thought the motor had more compression because it moved my torino around pretty good.

I wanted to know if I put that same cam back in, how much have I helped or hurt things by porting the heads and putting 2.02 valves in this 8.6 compression motor. As a reminder the heads were a 58cc 180 runner World Products, windsor jr head.

I ported and 2.02'd the heads to originally go on a 351 motor. That motor just got a new set of alum. heads so I am wondering what my original heads that were pulled off this same 302 would be now with some bowl work and larger intake valve. It's gotta help I would think.
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Old 10-17-2010, 01:38 AM
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Quote:
Originally Posted by Carlos9
I have a question on this one guys, now that we know exactly what we got for parts. I really thought the motor had more compression because it moved my torino around pretty good.

I wanted to know if I put that same cam back in, how much have I helped or hurt things by porting the heads and putting 2.02 valves in this 8.6 compression motor. As a reminder the heads were a 58cc 180 runner World Products, windsor jr head.

I ported and 2.02'd the heads to originally go on a 351 motor. That motor just got a new set of alum. heads so I am wondering what my original heads that were pulled off this same 302 would be now with some bowl work and larger intake valve. It's gotta help I would think.
You'll never know for sure without puttin' 'em on a flow bench.
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