[eschaider]

I contacted Brian Wolfe to get an answer to the compression ratio question because it was an itch that needed scratching. His answers were interesting.

The blown version of the engine does in fact run 12.5:1 compression with C16 fuel. To avoid detonation they back the timing down to a "safe" number but, you really do have a 12.5:1 c/r in a Whipple blown Godzilla.

This approach is similar to what John Mihovitz started doing a number of years ago in Modmotors. John ran high compression (only about 10:1 at that time not 12.5:1) and pulled timing (or added timing depending on your point of view) until he found the engine's knock threshold. John got some fairly impressive power numbers out of the engine, 962 hp at 7000 rpm — from 281 inches. The engine definitely wasn't a wimp.

John used 18 psi of boost (2.2 BAR) intercooled and 10:1 c/r for an effective c/r of 22:1. Because of a late closing intake valve (for upper rpm performance) the dynamic compression was closer to 8.5 to 9 to 1. Making the effective c/r 18.5 to 19:1. John ran C16 fuel like Brian does.

The lower compression build model will be easier on engine parts, especially if the engine is street driven. High ambient air temps and high engine temps can encourage detonation.

It appears Brian Wolfe pursued a similar build model with his Godzilla. I need to give him a fairly significant attaboy for the success he has had with the build. However, it is important to recognize that while he used C16, which didn't hurt his performance, if you use any of the many varieties of pump gas available today, you will need to exercise a bit more caution because of the lower octane fuel's predisposition to detonate

My personal preference is still the 9:1 c/r solution particularly with a 93 octane grade of gas. Fuel becomes dramatically less expensive, readily available, and capable of quite stunning power levels. While the following statement is true of almost any supercharged engine, if you experience a whoops on a high c/r blower motor it is much more devastating than a whoops on a lower compression equivalent engine.

The trick (if that is the right word) to getting the high c/r engine to live and work is using just enough timing that the fuel air mixture has fully or nearly fully burned by that 7 to 10 degree window ATDC. Too much spark and the cylinder pressure rises too soon occurring "in front of" the target 7 to 10 degree window ATDC. Too little timing will have the max pressure point occurring after the target 7 to 10 degree window ATDC. Either way it will cost you power. One way to will cost you parts also. Experimentation will be necessary to find that optimum ignition timing for that 7 to 10 degree ATDC window.


Ed


p.s. The high compression and reduced timing window size changes and moves around as rpm and load change so a programable EFI system is necessary to "follow" that optimal timing window as it moves around throughout the rpm range.