Anyhow, I do not post much on here but I need some help.
I am trying to figure out what compression I would get with my setup???
I have the following ready to be assembled, a B18c Block with B18a internals including an LS knife endge/micropolished crank, Eagle Rods and JE 10;5 Compression pistons. The head is a B16a, milled down 0.20.
So I am trying to figure out what compression would I have using a stock 4 layer head gasket and a 3 layer head gasket.
PLEASE HELP!!!!!

 

BUMP!!!! Please help

 

Ok, here we go. I did the math for ya, let me show you how it was done:

Compression ratio (CR) is just the volume of the cylinder with the piston at BDC compared to the volume at TDC. The formula for CR is:

CR= <U>D+PV+DC+G+CC</U>
........PV+DC+G+CC

Where:
D=Displacement of a single cylinder (cc)
PV=Piston volume, displacement of either a dome on a piston or a bowl. Dome would SUBTRACT from displacement, bowl would ADD displacement (cc)
DC=Deck clearance, volume between top of piston and top of block (cc)
G=Gasket volume, volume between top of block and bottom of head (cc)
CC=Combustion chamber vol, volume inside head with valves closed and spark plug installed (cc)

<U>Finding D</U>
Volume= Pi x (half the bore squared) x stroke
With your 81 mm bore and 89 mm stroke, displacement per cylinder is 458.62 cc

<U>Finding PV</U>
Went to JE's website. I couldn't find the 10.5:1 pistons, but based on their other ratio pistons, I would guess the 10.5:1 would be -4.5cc. It's negative because it takes away from displacement in the cylinder. However, JE pistons will tell you that piston dome is +4.5cc, because it adds to the piston volume, which <U>decreases</U> our volumes. A flat piston would displace 0cc and a bowl (low-compression) piston would displace positive cc's.

<U>Finding DC</U>
The optimal DC is 0.127mm. Converting this into volume is the same as our displacement calculation, but the stroke is 0.127mm. It came out to 654.43 cubic millimeters, or 0.65 cubic centimeters.

<U>Finding G</U>
The standard compressed gasket is 0.7366 mm thick. Again, the same volume formula and we get 3795.6978 cubic millimeters, or 3.80 cc.

<U>Finding CC</U>
The B16a head's chamber displaces 42.7 cc. Since you milled the head .020 inches, I converted that into milimeters then used the same volume equation making the milling the stroke and found milling the head took off 2.6 cc, making your head displace 40.10 cc.

Plug all that chit in, and you get 12.3:1 compression ratio.

Keep in mind the error and things I didn't compensate for. I didn't account for the head gasket bore (usually larger than block), which would add displacement. I used a common number for the deck clearance (will probably be more). I also didn't account for the piston-to-wall clearance, which would add volume to PV. All of these would mean a lower actual compression.

Good luck
Jake


Modified by wildcarditr102 at 2:12 PM 6/26/2006

 

Well according to my calculations made on http://www.c-speedracing.com , using stock GSR pistons your comp would be 10.91 using the 3 layer and stock pistons, I do not see how your setup can net 14:1+ with 10.5:1 pistons and only .020 milled off the head, not knowing what the piston dome displacement is makes it difficult, my 9:1 Wiseco's are -12cc and stock GSR P72's are 2.52cc per piston, you will need a 13+ cc piston to make a 14:1 compression ratio not factoring in the LS stuff in there which the site does not have provisions for. Using the LS crank has an effect on this but I do not think it is a considerable one, i could be wrong.

 

This time i did it for you, but nest time use the search feature. try this
http://www.zealautowerks.com/index.html

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by roma’s GSReg6 &raquo;</TD></TR><TR><TD CLASS="quote">This time i did it for you, but nest time use the search feature. try this
http://www.zealautowerks.com/index.html
</TD></TR></TABLE>

Very nice calculator to you

 

It does seem high for a B18c engine, but remember he is using a B18a crank. The B18a crank has a 89mm stroke compared to the B18c's 87.2mm stroke.

Keep in mind the error and things I didn't compensate for. I didn't account for the head gasket bore (usually larger than block), which would add displacement. I used a common number for the deck clearance (will probably be more). I also didn't account for the piston-to-wall clearance above the top ring, which would add volume to PV. All of these would mean a lower compression.

I would put my money on 12.5:1. If I did something wrong, let me know.

Very cool calculator above, by the way


Modified by wildcarditr102 at 2:21 PM 6/26/2006

 

Aahh, sorry guys - big mistake. I looked up the piston displacement wrong. The -9.3cc is for a 9:1 JE piston (and I read the +/- wrong), when JE says -9.3cc, they are talking about the dish, not less displacement (from more dome)-so their -9.3cc would add to our displacement. I can't find the JE 10.5:1 pistons anywhere, but the 12.5:1 is +6.7cc and the 11.5:1 is +5.6cc. We can just guess the 10.5:1 will be +4.5cc, just to see what happens.

Sorry guys, had a long day and a brain fart yesterday.

Plug that chit in, the formula gets us 12.3:1, not including the errors mentioned above. Think that'll produce about 11.5:1? Maybe 12:1?

**Fixed my first post**

 

Found another error. I didn't account for the shorter stroke and shorter rods having an effect on deck clearance (should have actually calculated it). I'm guessing it'll be MUCH less compression from a 1.8mm shorter stroke plus 3.9mm shorter rods.

Go with the calculators other people gave

 

Thanks for the HELP guys!!!
I am hopping the compression would be around 11-11.5:1
I am trying to run Nitrous with this motor as well, so that is why I wouldnt want my compression to be too high...
I do not know much about Nitrous at all as I always ran nothing but boost, but if I do net 11.5:1 compression, how many shots would I be safe with my setup????
Also, which size fuel injectors should I be using????
Remember, I have a fully built head with titanium everything, stage 3 Crower cams and stage 3 port and polish job, along with 70mm throttle body on a Victory X manifold (which I might switch to ITB). The head is a monster of its own!!!

 

Thanks, I think that would help a lot!!! are you russian any any chance???? Roma is a russian name so that is why I am asking


Modified by lv2exl8 at 5:27 AM 6/27/2006

 

BUMP

 

Nitrous works fairly well with high compression just dont go overboard. the cooling properties of N2O r great.