I am calculating the static and dynamic compression ratio of my build and it would be nice if someone could have a look on it to make sure my calculations are right.

Here are the numbers:
cylinder head: 58cc
piston: +6cc (flat-top)
head gasket: 0.028 thickness and 4.000 bore
cylinder bore: 3.776 (305 on 0.040 oversize)
deck clearance: 0.025
stroke: 3.48

I got 9.59:1 static compression ratio from these numbers.

I am planning to go with the following cam: http://www.compcams.com/Company/CC/c...?csid=194&sb=2
The specification says that the intake valve closes 60 degrees after BDC.

This results in 7.94:1 dynamic compression ratio.

Could anyone confirm that these numbers are correct?
I would say they are reasonable and this thing could work.

 

Static is right. Didnt check the dynamic but that cam will work well on that motor at that static compression ratio.

 

Thanks for the confirmation.

By the way, your build is awesome. I have read your topic several times.

 

Flat top piston will have a negative piston volume. Some calculators will automaticaly correct for that otherwise be sure to check that the value is -6cc.
The head gasket diameter should be checked against the manufacturer's specs. Not sure about 305's but a typical 350 head gasket has a 4.100" diameter regardless of the actual block bore diameter. It may not add up to much but it's worth double checking.
7.94:1 is a desireable DCR for sure.

 

correct a flat top with valve reliefs have negative volume... some websites list it as +cc when it should be -cc.

 

I used this calculator: http://www.kb-silvolite.com/calc.php?action=comp

If I add the piston head volume as -6, I get CR around 11. It looks impossible for me with the above mentioned specifications. That's why I thought I should write +6 there.

 

That calculator should say Use negative for dome pistons. If you put in -6 for piston cc reliefs, its subtracting that volume from the equation so the numbers come out higher than whats correct.

Dynamic will be slightly off from ideal since it says use intake closing point at .050" lift + 15 deg....well for proper calculation, you need the seat timing events. Cam card should have it all.
Not all cam lobes have seat timing thats 15 deg more than what the .050" numbers indicate, infact i'd be surprised if any do. So use the .006" advertised number if they give that or whatever advertised number they use.

I like using this calculator/equation for DCR
http://www.empirenet.com/pkelley2/DynamicCR.html

 

I will check out DCR with the calculator you suggested.

 

Got the same (7.94:1) DCR as with the first calculator, so it seems to be right for me.

 

One more question on this topic. What about going with a Fel-Pro Q1094 head gasket? With its 0.015 thickness the static compression would bump up to 9.85:1 and the dynamic would be 8.15:1.

Is this still reasonable on a 305 SBC with iron cylinder heads and 95 octane fuel all the time?

I know that the higher compression means better performance, but I am not really sure if it will work or not.

 

It should be ok. Just need to try and get the best quench height you can get. 0.015" gaskets are very thin so just make sure you can seal everything well.

 

depending on your quench hight you can probably get away with low octane with 8.1 DCR, mine has a +-038 quench 8.3 DCR with aluminum heads and run 89 (usa mid grade)no problems i also run the 015 gaskets and they seem to hold pretty good

 

The 95 octane is not a problem as it is the lowest octane fuel I can get in my country.

With the .015 gasket and standard deck height I would have .040 quench height. The only difference is that I am planning to go with the factory iron cylinder heads instead of aluminum ones.

 

If it will seal well on factory deck, then .015 will work but I thought those were picky on surface finish? GM performance parts has a .028" gasket thats alittle thicker that could work but give abit less than optimal quench height.

You can try the .015 steel shims with copper gasket spray coating and try it.

 

My first calculations were done with those .028" GMPP gaskets you mentioned. I already ordered those but just started to think about the Fel-Pro ones to bump compression a bit more.

One more option would be to deck the block to .015" height and use the GMPP .028" gasket. It would result in a .043" quench height with 9.74:1 CR and 8.048:1 DCR. This way I could avoid using the very thin Fel-Pro gasket.

 

i think the octane ratio in Europe is rated on a different scale as the USA, your 95 might have less octane than our 87, i've sen that on the sone net site

 

After having a quick look at wikipedia I found this explanation:

According to a table that was on the wikipedia site it says that the "EuroSuper", "EuroPremium" or "Regular unleaded" fuel in Europe has an AKI number of 90-91.

If I understand it right our 95 octane fuel should be the same as the 90-91 in the USA.

 

isn't the "Euro premium" the 98?

 

It should be but the table on wikipedia mentions all of them in one row. The EuroSuper and the regular unleaded is what we call 95 octane as far as I know.

 

i remember when they stop making leaded gas in Europe they had the 98, the 95 and the "Super" which was the 95 (i think) with some additive to replace the lead for the older engines it's probably the one they referring to

 

So the plan is to have 9.8:1 CR with 8.1:1 DCR and running it with iron heads and European 95 octane fuel.

Suggestions and comments are welcome.

 

8.1 is a good number

 

What heads?

 

I am planning to use the 081 casting factory cylinder heads.

 

With an efficient cylinder head, one that has a healthy quench pad and an active combustion chamber, the tendency for detonation (due to high compression pressures) is reduced. This allows for higher SCRs and DCRs.
My experience with the Vortec head, which has such a combustion chamber, has allowed me to run a 10:1 SCR/8.1 DCR on 94 octane with little trouble. Total ignition can be reduced because of the efficient chamber and this further aids the resistance to detonation. (16 degrees lead with 34 degress total timing not including vacuum advance)
I'm not very familiar with the 081 castings although from what I've read, they don't fall into the efficient category. As such you will want to pay close attention to engine operating temps. A cold air intake of some sort will also help. These are the sort of things that should be considered with any performance application anyway.
Of course having 95 octane will add a greater degree of resistance as well.

 

I would mill the heads and use the thicker gasket, if one day the gaskets need to be replaced for some reson, you do not run in to the same situation again.

If I remember correct, 95, 98 and 100 RON European fuel is the same as 90, 93 and 95 AKI Usa.
98 and 100 RON are considered premium fuel in Europe.

 

A cold air intake system is definitely planned for my build. I am also going with a 180* thermostat and a high-volume water pump.

What would be the biggest SCR and DCR value that you would suggest with knowing now more details?

 

I would think that you're at the limit with 10:1 and 8.1:1.
By way of explanation, the problem I found in my application was that although the AVERAGE SCR worked out to 9.9:1, with the variances in the piston deck height (anywhere from .026 to .036 due to the stock crank/rods tolerance stackup) and the subsequent difference in SCR from 9.85 to 10.1:1 resulted in some cylinders being on the edge more than others.
After one summer of overheating (long story), an engine teardown revealed the rod bearings on the cylinders with the highest compression were obviously different in appearence. The crush had gone out of the bearings. I attribute that to the higher compression pressures inducing detonation in those cylinders and combined with the superheated engine oil, led to the demise of that particular engine.
That's my experience anyway.
The true test of sufficient SCR/DCR is a compression test. I've found a good target to strive for is at least 200 psi cranking pressure. Unfortunately, this comes after the engine has been assembled. There are calculators out there that will help determine what the potential cranking pressure may be based on several variables.