SWCDuke

The classic cruciform-crank 90-degree American V8 is easily balanced to eliminate first and second order shaking forces and rocking couples, but yields uneven firing intervals on each bank.

A flat-crank 90-degree V8 has even 180 degree firing intervals on each bank, but has a residual second order horizontal shaking force that can only be balanced with auxiliary balance shafts rotating at double crankshaft speed, which are usually not included to due additional weight, cost, and internal engine friction.

Since two cylinders on each bank of the cruciform-crank configuration fire 90 degrees out of phase, the trailing cylinder of the pair blows down while the leading cylinder is on the upstroke of the exhaust cycle. It would seem that this would have some effect on engine performance and efficiency, but I have never been able to quantify it.

So my question for Tadge (or a GM Powertrain engineer of his choice) is what are the performance curve differences between a cruciform and flat crank (without balance shafts) V8, all other things equal other than necessary changes due to firing order differences, and how did you determine?

I am particularly interested in the differences on a production engine with typical streamlined exhaust manifolds and full street legal vehicle exhaust system.

Has GM Powertrain ever tested a production Corvette engine with a flat crank, identically configured other than necessary changes for the different firing order, and what were the results compared to the production cruciform crank configuration?

Thank-you!
Duke

jvp

Why do we even think that this will get submitted to Tadge for consideration when you're asking about things that aren't released and/or don't exist? Come on.

SWCDuke

I am asking whether any experimental flat crank engines have ever been tested and compared to an equivalently configured cruciform crank engine. There's no reason why results couldn't be discussed. I'm not asking if a flat crank V8 is being considered for production, just GM Powertrain's understanding of the performance differences.

It's also possible that sophisticated, professional modeling software could predict reasonable differences that exist. Why not let Tadge give it a shot?

Duke

Gary '09 C6

Maybe GM has done research on this and is willing to share it.