Discussion in 'The Garage' started by foolhardy, Jul 8, 2005.
is the compression ration on a 350 8.5:1 that seems kinda low
No not really...that's about right for a stock 350 with 76cc chambers and dished pistons
8.5:1 is about what you want, if you want burn regular 87 octane gasoline.
You can run upwards of 9.5:1 on 87 lotane but there is a method to the madness...
It can be done, BUT...I would rather just throw in at least a medium grade gasoline, and be done with it; instead of driving in a constant state of panic...having to listen to my engine the whole time I am driving, and worrying that it might start detonating at any time.
Word..... The second it gets hot with 9.5:1 comp and 87 octane it will start pingin.
which is preciesly why Vortec 5.7s come from the factory with 9.6:1 and run happily all day on 87 without a problem. its all in your dynamic compression ratio. static compression ratio doesnt really make any difference in octane requirements, providing you do your homework on the DCR
Not to mention fueling is better handled on the newer motors, and the combustion chamber design is much improved.
87 octane, 9.2:1 at 220* under load and not a ping in sight.
My only question to that is why do vehicles, which come with 5.7 Liter, 9.6:1 compression ratio Vortec engines (like Corvette or Camaro) have a recommendation in the owners manual to run only medium grade or better gasoline?
The camaros and vettes have never had the vortec motors. If you look they have even higher compression. You can happily run 12:1 static on 92 octane with the right cam/head combination so the static compression really means nothing.
Just picked a couple of years (info straight from GM's specs) with the 5.7L engine for the F-bodies:
1996= 10.4:1, 92 octane recommended, 87 acceptable.
2000= 10:1, 91 octane only
2002= 10:1 87 octane only.
All use cast aluminum heads which is only part of why they get 87 octane to run at 10:1.
As mentioned, no Vortecs in these cars.
After doing some research, I have found that 9.4:1 seems to be the compression ratio of a factory stock Vortec 5.7 Liter engine, and the recomeded fuel is 89 octane medium grade or higher. Corvetts ran the LS1 and LS6 5.7 Liter engines.
Where'd you find that info?
Out of gm's lit (all truck):
1996 5.7L 9.4:1, 87 octane recommended
2000 5.3L 9.5:1, 87 octane recommended
2004 5.3L 9.5:1, 87 octane recommended
Camaro's also used the LS1.
This is just one of a few places I found this info. http://www.marineworld.com/mastercraft_engines.htm
Straight from the horses mouth
I plan on buying an HT383 for my Crew Cab, which has Vortec heads, and I hope that 87 octane rating holds true for that engine as well. I wonder how a vortec engine would handle alltitudes of 3000 Ft. and higher with 87 octane? Here in the West Coast 3000 Ft. alltitudes and higher are common.
i don't think so........
There aint no way a 12:1 motor is gonna run CORRECTLY on pump gas, even with a cam that had excessive overlap to shred cylinder pressure at low rpm's
as soon as engine speed catches up with the overlap and starts building massive cylinder pressures comparable to a mild pro-stock engine that motor is gonna rattle like carton of BB's on pump gas.....
10:1 is the upper limit on 93, and that's pushing it dude
My 383 has 9.7:1 and I've used every trick in the book building it to prepare for a life on pump gas
i've built several street machine engines running 12:1, and all ran fine on pump gas.
where did you learn these tricks? what "tricks" have you tried?
and for those of you insisting that the Vortec 5.7 stock compression is 9.4, it is actually 9.6. my chambers cc'ed out at almost a perfect 64ccs all the way around. stock 4.00" bore, 3.48" stroke, .025" head gasket, and .025" down the hole at TDC. coupled with the just under 8.5cc valve reliefs in the pistons, gives you 9.62808:1 static CR.
I'm running zero deck with 18cc dish KB slugs and 64cc chambers.
Piston to head clearance is .035" with gasket to maximize quench and cancel any shock waves inside the chamber.
Hot ignition, plenty of cam and a fat A/F mixture to keep things cool.
i admire you for building pump gas engines that run with 12:1 CR, if i thought for a minute i could do the same i would....
It's not us insisting, it's GM.
And the measured difference between two motors out of a series of millions being .2CC I doubt is much worth arguing about.
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