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Dynamic compression and cranking pressure

  
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Dynamic compression and cranking pressure

 
thinman56 thinman56
User | Posts: 107 | Joined: 04/13
Posted: 01/23/14
07:20 PM

My engine didn't run as expected, so I ran a compression test and then took the top end off it and measured up all the variables to see what the compression ratio really was.  I used a few different compression calculators, including the Wallace Racing version.  They all averaged about 8.6:1 static and 7.3 or 7.4:1 dynamic.  I know, lousy.  But, the Wallace calculator also calculated cranking pressure at 140 psig or so, and my compression test averaged 160-165 in all cylinders but one, which was about 145.  My plan was to cut my heads and use a thinner gasket to get to a dynamic CR of about 8.2:1 as recommended by almost everyone here, but that'll probably raise my real cranking pressure up over 180 psi.  Will this be a problem with 91 octane street gas?  In case anyone's interested, here's the raw data:

3.75" stroke x 3.78" bore (0.060 over 326)
cyl head chamber volume = 67 cc
piston valve reliefs = 5.5 cc
deck height = .030"
Gasket thickness and bore = .049" x 4.25"
intake valve closes at 56 degrees ABDC

My plan was to cut 0.025" off the head and use the 0.027" gasket.  Can I get a sanity check before I go cutting things up?  Thanks...  

tuffnuff tuffnuff
Moderator | Posts: 2534 | Joined: 12/09
Posted: 01/23/14
08:00 PM

If your info is correct (double checked) then surfacing .025" from your heads is acceptable.,. since your squish band remains unaltered.
Your sanity is intact,,,, chuckles.

Smile  
When The Flag Drops.,.

tuffnuff

The Bull ***t Stops.,.
tuffnuff

P. Engineer, Engine Builder

shyrgfuh70 shyrgfuh70
User | Posts: 93 | Joined: 12/13
Posted: 01/23/14
09:24 PM

hi;

I understand your confusion, about your compression. One would think that math is math and the calculator should be accurate.

What I have seen is that the onlune calculators don't allow for enough variables such as overlap and area under the curve etc. This is one of the reasons that if one cam with an intake closing of 64 degrees creates an actual cylinder cranking pressure of fast 170, another cam witrh different overlap etc., but the same intake valve closing time might provide either more or less.

I think the fancy computer dyno's might allow for additional data like this so they can provide a more accurate cyl cranking pressure.

Your deck height, and therefore your quench area, is horrible because it is so large. My guess is that you might have aftermarket pistons. Imo, the ideal thing to do would be to get your compression from milling the block .025". I think the thinner head gasget is .031" so it will reduce your quench area a ***

As far as I know, aside from ignition timing, the actual cyl cranking pressure is the biggest determining factor for how much octane you need to run and no matter what, if you have 180 then you should theoretically be on the max you can run for 93 octane.

This being said, some here claim to have had cyl crank pressures of 210 on no detonation with iron heads.

I had 210 on a n/a iron head L72 427 engine and it pinged a bit on 93.

Gauges can also vary up to around 20 psi in readings.  

thinman56 thinman56
User | Posts: 107 | Joined: 04/13
Posted: 01/24/14
04:28 AM

I realize my main mistake was not decking the block while I had it at the machine shop.  The new Sealed Power cast pistons are 1.71" compression height, which theoretically leaves 0.040" deck height, I measured 0.030" to 0.034" with the one weak cylinder being at almost 0.040".  Quench with the stock gasket is awful at 0.080" or more, which I know now from other posts and Jim Hand's book effects combustion efficiency.  I'll get some of it back with the thin gasket.  I thought about stripping the whole thing down, hauling it back to the machine shop and decking the block 0.020", but at this point I've sunk more money into this motor than I expected, gotta draw the line somewhere.  I have it in the back of my mind to start saving and browse for a 389 over the next year or two and do it all again with all the tools I've gained here and elsewhere.

Thanks for the feedback.  

shyrgfuh70 shyrgfuh70
User | Posts: 93 | Joined: 12/13
Posted: 01/24/14
05:01 AM

LOL, Yeah, some aftermarket pistons are made with lower heights to compensate for blocks that might have been milled too much or something.

Don't forget, do not take more than a total of .028" off from stock, between the heads, gasket and block otherwise you might have intake fittment probs.

Why, why, why do you want a 389, good luck finding pistons. you might have to get some cast oem ones from egge machine in calif.

Just buy a 455 and be done with it, lol.  

thinman56 thinman56
User | Posts: 107 | Joined: 04/13
Posted: 01/24/14
07:20 AM

I like the idea of staying close to the vintage, hence the 389 instead of something newer.  I took the original 1964 motor, 2-speed tranny, shifter and console out and set it aside for a possible return to stock, because the car is otherwise a survivor.  Pistons are readily available from Sealed Power, Keith Black and others, so no problem there.  And, if I'm patient, I might bump into a mid-60's 421 core that I can afford.  So far, that ain't happened....  

shyrgfuh70 shyrgfuh70
User | Posts: 93 | Joined: 12/13
Posted: 01/24/14
07:45 AM

Ok, cool, i like the 389's too.  

thinman56 thinman56
User | Posts: 107 | Joined: 04/13
Posted: 01/24/14
08:04 AM

http://fme-cat.com/overlays/part-detail.aspx?brand=SP&PartNumber=288P%2030&pt=Piston&lu=1964%20PONTIAC%20GTO&vin=#.UuKArdIo7Gg  

shyrgfuh70 shyrgfuh70
User | Posts: 93 | Joined: 12/13
Posted: 01/24/14
09:13 AM

Hey, i just saw some glue on raised piston domes at summit, you might try those.