RB30 build, the importance of measuring!

[Stealth-Z]

It has taken me over six months to get the lower end of my RB30 built. Unfortunetly it has run into a slight snag. Now I am not a professional engine builder. My education on calculating compression ratio and machining tolorances has been accumulated over the last few days.

After assembling and measuring, my compression ratio comes out to around 7 to 1! Way to low for what I was expecting. This is using custom Wiseco pistons I got from this thread here.

http://forums.hybridz.org/showthread.php?t=112975&highlight=wiseco

Now let me emphasize

"THIS IS NOT AKWIKZ's FAULT NOR DO I HOLD HIM IN ILL REGUARD".

He was going by what Wiseco told him. Plus he has a set of these too and is in the same position as myself.

 

After installing the pistons, they were 0.022 inches in the bore. This measurement gave me a basis to start calculating my compression ratio. The bore measures 3.406 inches, the stroke 3.346 inches, head gasket diameter is about 3.45 inches, head gasket crushed is 0.047, and the RB26 head measures out at 64cc's

Using Wiseco's calculator http://www.wiseco.com/Calculators/HelpfulCalculators.xls Comp Ratio Calc For 4-Stroke Only

The ratio comes out to 6.783:1

 

Okay, so lets juggle some number by doing some decking of the block and shaving on the head.

In order to achieve a good quench of 0.040 inches, I can deck the block a total of 0.029 inches. This will raise the piston out of the bore by 0.007 inches. That changes the CR to 7.089:1 Still to low. Quench is good though.

 

Next comes the head. In order to get the CR up to around 8.5:1 requires the cylinder head to be around 48cc's. That is way to much shaving on the head for me! Plus taking almost 0.030 off the block is somewhat excessive to me as well.

 

Always measure and double check. Nothing worse than getting something together and it not perform like expected. Now it is back to the drawing board for pistons. A dome is definetly needed to get a higher CR.

These pistons would most likely work great for a drag racer running very high boost.

[rbs30]

My guess would be that you have yourself a set of pistons for the rb30et instead of pistons for a rb30e..when using a 26 head on a rb30,rb30et pistons will give you a cr of around 7.0 and rb30e pistons will give you a cr of around 8.3..