For the guys running 10:1 compression or higher, what kind of cranking pressures are you seeing on a gauge?

Tcl has like 50+ cells
:D

10.7:1 130psi cranking pressure. Will not run on 93. I'd say it has a pretty tight quench due to signs of Pistons slightly touching heads.

Quote Originally Posted by HaxbySpeed View Post
Yessir, 9.8 to 1, 89 octane

That's shocking --- and great to hear.

I have been in conversations regarding rebuilding my carb'd merc motors, which started off as 500 HP's. I have been told to stay under 9.5:1 with aluminum heads and less for steel. i have been known to run a boat hard from time to time and have been warned about not running enough "octane" vs compression.

Not to hijack the thread, but I would appreciate your feedback regarding how you are eliminating detonation at RPM, even with knock sensors, which will be going into the rebuild, at that high of compression. Lastly, I have been thinking about converting these motors to EFI's. I know there is only up-side to doing it, I just don't know if I will see enough upside for this boat.

Is there a different thinking on compression with a carb'd vs EFI set-up? My other boats have all been EFI's so the carb'd thing was new to me with this boat, which made it interesting (esp. to learn how the old school guys did it) -- LOL :-)


Thanks, in advance, for your feedback,

Joe

My engines are 10 to 1 and turn 6000 RPM - never any signs of knock. I am running aluminum heads. I am building a 598 and plan on being at least 10 to 1 NA - EFI. I will have knock sensors on that motor. But I will be running 93 octane. No unnecessary sharp edges in the combustion chamber and zero decked.

I too am interested in these 10:1 or higher N/A setups. I've never tried it, and never recommended it, basically because I was too scared too. I also was always told/taught, to stay conservative on compression in a marine engine. Mostly around here, the guys I boat with have larger boats, being on lake michigan. Wide open throttle run time, can be extreme. Some of the guys I know will run their boats at 6000RPM, for 20-30 miles without pulling back. The few guys I know who ran 10:1 or more on these engines, had a hard time keeping them together. Whether it was based on their tune, cam choice, or lots of other possible variables, I dont really know. Lots of marina's here dont have 93 octane. I always felt the power from more compression, wasnt worth the hassle of having to find 93 octane. Now, bolting a blower on that might net you 100-200HP more, to me is worth having to seek out 93 octane. But 20-30hp to do so, imo, isnt worth it. To me at least. 25hp a side in a 38 Cig or 42 Fountain, is probably 1-2mph top speed gain. I'd rather lose the 2mph, be able to run lesser octane, or simply have a little safety factor on those long wot runs. Just guessing on the power difference as well, maybe its 50hp, or 75hp??

That was my theory. However, it may be totally incorrect, and my ears are always open. Seems like there are some guys out there running high compression with good results.

Joe, they detonate long before 6000 rpm. I think 3000 is a lot hArder on them than 6000. 1 point in compression will net about 3% increase in power not much. But as you know compression supports a bigger cam and that opens up more power in the combination. I can get race gas for $6-7/ gallon if I do a 20% mix I end up with almost 96 octane. Runs about $80 more per tank. Not to bad. The octane in race gas does degrade but at a much slower rate than ethanol fuel. The extra cost per tank is well worth it in my opinion and it smells real nice when your pulling up at the local hot spots.

Lugging.

More compression can use more cam and thus peak power rpm will go up.

Too many freakin people prop for freakin stock rpm of 5200. Lug, lug, lug.

That's #1.

Engine. obviously cooling system flow and temp, head material, combustion chamber design, piston dome design, clearances, plus, and tune can all make a difference.

A properly designed driving helmet can help with aerodynamics and reduce the lugging effect.

Another reason why a fully programmable ignition curve is so beneficial. Being able to customize your curve around your torque curve as opposed to being locked at full advance allows you to run higher compression without the risk of detonation around the torque peak.