Compression Ratio calculation.

HairbearTE · Post by **HairbearTE** » Mon Nov 17, 2008 7:28 pm

I'm attacking this from a slightly different angle as I want to know what chamber volume I'll need. I guess there's little programs that I could download to work this stuff out quickly so if anyone knows a link i'd be very grateful. In the meantime I know some of you do this kind of stuff in your sleep so..

If I have a 94.5mm bore, composite gaskets, 27cc dished pistons sitting .020 under the deck at TDC what chamber volume would I need to get a) cr 10.0-1 and b) cr 12.5-1?

The reason for all this hypothesis (is that the right word?) is that the new Real Steel stroker pistons fit the bill for my build in aspects apart from that somewhat cavernous 27mm dish. I can save a lot of money by using those pistons instead of getting customs made up but I don't want to do it at the expense of my CR. Words of wisdom much appreciated.

Wotland · Post by **Wotland** » Mon Nov 17, 2008 7:55 pm

For static CR :
http://www.csgnetwork.com/compcalc.html
For dynamic CR :
http://www.wallaceracing.com/dynamic-cr.php

HairbearTE · Post by **HairbearTE** » Mon Nov 17, 2008 11:29 pm

Nice! that's just what I needed. Many thanks.

HairbearTE · Post by **HairbearTE** » Mon Nov 17, 2008 11:36 pm

Hmm.. dosen't look good. 29cc chamber required for 10.0:1 or 15cc for 12.5:1

HairbearTE · Post by **HairbearTE** » Tue Nov 18, 2008 12:00 am

According to the second program my Dynamic Cr will be 8.63 with the staic Cr of 10.0:1 as calculated on the first program.

CastleMGBV8 · Post by **CastleMGBV8** » Wed Nov 19, 2008 8:39 am

HairbearTE wrote:According to the second program my Dynamic Cr will be 8.63 with the staic Cr of 10.0:1 as calculated on the first program.

Yes you have a problem, those pistons are probably meant for a blower motor.

I calculate that with comp gaskets you will have approx 8.65/1 and with tins 9.2/1

You could get a set of Keith Black Hypereutectic from Summit but then of course you would need to change the rods, can you confirm what stroke you are using. Flat tops would get you in the region of 12/1 with Rover 36cc. chamber heads and comp gaskets 12.5/1 with tins if 82mm crank.

Kevin.

topcatcustom · Post by **topcatcustom** » Wed Nov 19, 2008 10:23 am

Hairbear got some forged 4.0 pistons for a blown motor to get rid of ?!!?!?!

HairbearTE · Post by **HairbearTE** » Wed Nov 19, 2008 6:29 pm

Thanks for the offer but I want to find something to fit my rods (6.125 x .927pin, narrowed to fit rover). The Real steel ones are spot on for me if the dish in them wasn't sooo huge! The voices in my head say it would all work sweetly with a blower of some sort but I can scarcely afford to finish building the damn thing as it is! I'm sure running really tiny chambers to compensate for big dishes would have disadvantages somewhere too

kiwicar · Post by **kiwicar** » Wed Nov 19, 2008 7:52 pm

Big dished pistons and small chambers have four problems,
1/ difficult to the heat of combustion out of the engine as there is less area in the chamber to transfer the heat to the cooling system, all the heat ends up in the piston crown. You can get around this with coatings on the chambers and piston crowns but you really need to use an oil cooling from below to cool the piston.
2/ the combustion is happening further away from the spark plug so takes longer to happen meaning you need more advance on the timing than is ideal and you loose power and it limits the revs you can use efficiently.
3/ As a result of 2 you end up with more unburnt fuel in the exhaust and more exhaust in the fresh charge, this again further reduces the combustion efficiency and increases the likley hood of detonation reducing the amount of CR and timing you can use making it reluctant to pick up when you open the throttle.
4/ the deck area of the heads will be quite thin to get the chambers small enough, resulting in the head being liable to warp, also not good.
Mike

CastleMGBV8 · Post by **CastleMGBV8** » Thu Nov 20, 2008 12:42 am

Hairbear,

If you already have Chevy rods then one of the Chevy 305 pistons will work depending on required compression height. The long rods may be a problem if you already have them.

Keith black do a variety of designs. domes, flat with cut outs (chevy valves) and dished 12cc.

The size you need is standard 3.736" 94.89mm.

http://kb-silvolite.com/performance.php?action=comp

For prices do a search on Summit Racing site for Chevrolet 305 pistons.

Kevin.

topcatcustom · Post by **topcatcustom** » Thu Nov 20, 2008 8:20 am

Sorry hairbear I meant it like 'have you'