5.7 Vortec CR
#1
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.
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.
#2
Registered
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
Just my opinion but I like stuff to live for a while
#4
Registered
9.82,with okay quench vs 9.75 with no real quench.
All i did was say the same as you.
All i did was say the same as you.
#6
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#7
Were doomed!
Charter Member
__________________
-Wally
Money can't buy happiness, but it can buy horsepower. And I've never seen a sad person hauling a$$!
-Wally
Money can't buy happiness, but it can buy horsepower. And I've never seen a sad person hauling a$$!
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wlrottge (02-27-2024)
#8
#9
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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.
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Wally (02-28-2024)
#10
Were doomed!
Charter Member
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.
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.
__________________
-Wally
Money can't buy happiness, but it can buy horsepower. And I've never seen a sad person hauling a$$!
-Wally
Money can't buy happiness, but it can buy horsepower. And I've never seen a sad person hauling a$$!