5.7 Vortec CR
 

Have gotten myself into another project and am building an engine that I didn't expect to build (replacing a '72 LT1).

Have always been told that CR on a boat needs to be low 9's, however from what I'm reading, vortec heads are better off being run at a higher CR with a quench of <0.55 - 0.60.

I picked up a SUPER clean ZZ4 bottom end (GM forged crank, PMRs, Mahle hypereutectic pistons). No carbon or ring ridge, still some crosshatch. Currently at the machine shop, just had it honed; rings and bearings look good, but I'm replacing them anyway. Have a set of 906's that were just cleaned up that I'm upgrading the springs and exhaust valves (have been cut/drilled/tapped for threaded studs) and guides cut for clearance. Running a GM factory roller that is very close to the XM264 and 270HR; 0.474"/0.510", 208/221 @ 0.050, 112 LSA.

Pistons are sitting ~0.030 in the hole; to get the quench right, I need to run a 0.024 head gasket, which puts me at ~9.82. The next step up is 0.027", which puts me just outside the "magic" 0.055 quench and at 9.75:1.

Having done a search, I've seen several posts referencing CR's greater than 10:1 with these heads on pump (91) gas. Ignoring the whole, "fuel quality on the lake" discussion, am I reasonably safe here? I figured I would probably put a knock sensor on it just for giggles regardless.

If you are going to be using pump gas I would keep it at no more than 9.5 to 1. Thats what I recommend for cars using 93 octane pump gas. A boat is a ton more load and with 91 octane fuel I would personally stick to closer to 9 to 1.

Just my opinion but I like stuff to live for a while :)

Funny enough, I was going to send you an email tonight.

9.82,with okay quench vs 9.75 with no real quench.

All i did was say the same as you.

I believe vortecs had 9.4 stock. Ran great on 87.

they don’t need as much timing advance

Scott,

For my education.... What is "quench"...?

Thanks. Brad.

Quench Article

Was that a recommendation, or you thought I was too long winded? lol

Wally,

OK. That's what I thought. In model nitro engines, we always called it "squish", and that article makes a reference to it that way, too. It is a critical part of the combustion sequence in a 2-stroke glow ignition engine, as the velocity of the FA pulsing toward the glow coil plays a big part in ignition timing. I suspect it does with our larger, spark ignition, too, I just don't have a clue how like I do with glow nitro engines. And, just as that article goes into, we were constantly trying different things to maximize the effect of the "squish band". Interesting read.

Thanks. Brad.

Yep i remember playing with the "squish" on my snowmobile....had to get some thick solder strips and fold them over, then put it down the spark plug hole and rotate the motor over for piston to crush it. Pull it out and measure thickness, then adjust with the gaskets between the crankcase and cyl....fun times :D

Wally,

Many guys have done the same with our nitro 2-strokes. Personally, I always liked measuring everything with measuring tools, machining the head buttons for zero squish, then brass head shims to establish squish. This was the easiest way for me. But, then again, we have flat pistons and flat head buttons, typically with "hemispherical" bowls, or combustion chambers. I messed around with conical piston tops and matching angle squish bands, which increased the squish band area without compromising bowl diameter. This made measuring more difficult, though.

I always try and determine the similarities and differences in "speed secrets" between the two engine concepts. You'd be surprised how much they actually have in common. Physics is physics, after all. What we are burning, how we get it in and out and how we get it to burn are different, but the fluid dynamics don't change.

Thanks. Brad.

.07 is the difference.

Now think ‘okay’ quench vs not.

Now said another……
.07 difference is schit for difference but quench vs no quench is. :)