newschool72

iTrader: (12)

I did a check of my cranking compression with the new cam in my LS3 (220-230 at .050, 629-629 on a 108+4) and came up with 218-220 psi. Is it just me or is that alot for 93 pump gas?
Anyone have an educated idea of what the max cranking compression is for pump gas and a good timing curve for a carb style intaked LS3?
My old school 406 with a static 11.4 compression and a solid flat tappet cam (252-260 at .050 on a 110) is at 170 psi and runs great on pump gas at 37* max advance. I know big duration cuts down on cranking compression but I need the short intake duration on the LS3 because I need a solid 16" of vacuum for my brakes. Am I stuck with a lazy timing curve or race gas with my mild street car?

Paul57

It is my understanding that the LS motors don't require nearly as much timing as the traditional SB Chevy and I believe that has a lot to do with the combustion chamber design. Some information I got from a very knowledgeable LS person: "You will make the best power with 24-26 degrees of timing at peak torque and 26-28 degees of timing at peak horsepower. Best MPG occurs around 35 degrees of timing at part throttle." I am sure every set-up is a little different so take it for what it is worth. Start your timing curve a little low and work your way up.

SSellers

Do some research on the Pat Kelly dynamic compression calculator. Don't worry about cranking pressure so much but keep your dynamic compression around 8.5 or below. EFI, aluminum heads, cooler T-stat, and tuning with knock sensors lets you go a little higher on 93 but with a carb I don't know.

Paul is right about timing. Old combustion chamber designs need more timing to get the flame going quicker to prop them up. The AFR heads on my 400 SBC only needed 30 degrees to make best power on the dyno.

Also you will not need 16" idle vac for your brakes unless you're only going to idle it around a parking lot doing panic stops. Once you're moving good and every time you decel your manifold vac will go up and charge the booster. I'm at 15" idle vac on my 400 SBC and only 6" on my 327 SBC. Both run 11" boosters with four wheel discs and have no problems.

newschool72

iTrader: (12)

My dynamic compression is at 10.11 !! I just took the car for a drive and ran into spark knock with the NO.6 pill in the MSD box. I am going to back it off to the NO.5 and so on till the knock goes away. My son rode shotgun and videoed it for me. The car ran very well and slammed into the 6500 rpm limiter without nosing over at all. I am going to raise the limiter to 6800 and see if it works out better. I will post the vid as soon as I can get it uploaded.

89gmcs15

iTrader: (7)

10.11 dcr?!? You sure you don't mean static? I'm at 8.7 dcr right now and I'm alittle worried its too high for 91. If you have the msd box make your own timing curve. Theyre way better and I'm pretty sure all those timing pills are too high. Also of your running the pills it should have a 5500 rpm limiter shouldn't it? Something I'd check in to.

newschool72

iTrader: (12)

Yea, its 10.11 according to Topbrent. My static is the stock 10.7. I think you are right about the 5500 limiter w the MSD pills. We took another vid with the NO.5 pill and I didnt hear any knock but the tach was on 5500 when the limiter kicked in. I have been setting the rev limiter with the Fast EZ EFI ,but the whole time the MSD box has been holding me to 5500. I need a laptop in a bad way.

89gmcs15

iTrader: (7)

i just got my laptop usb to serial cable done and am gunna check some timing lol. its a pain in the *** to get these things hooked to a laptop. whats your cam specs? that seems like its high....

newschool72

iTrader: (12)

Bullet Racing custom grind 629/629 - 220/230 at .050 on a 108+4

SSellers

What is the actual advertised duration? You can't go off the old 0.050" + 15 degree rule of thumb to get the intake closing point. I can tell you right now your dynamic isn't 10.1 because that wouldn't run with icewater in it and race gas.

I'm right around 8:1 with a 10.11:1 static and a 224/224 (276 advertised) on an old SBC cam that only has 0.536" lift. With your lift the cam has to have some slope to it so the advertised is likely larger than mine is.

newschool72

iTrader: (12)

At .006 the intake is 271 the exhaust is 281. You have to remember the LS motors run 1.7 rockers from the factory( LS7 is 1.8) so the lift #s are easier to get than on the old sbc.

SSellers

Yeah, I know about the rockers. Using your cam specs and an advertised intake duration of 272 I get 60 degrees as the intake closing point. This sets your dynamic compression to 8.825.

newschool72

iTrader: (12)

That is good to hear! 10.11 sounded extremely high and way out of range for a pump gas car. 8.8 sounds doable. The great thing is the car runs really good on 93 pump gas. Go to "Combo Decided" on this forum to see a 25-85 mph run that took 6 seconds. I think it is a solid low 12-hi 11 second car with drag radials. Not bad for 3.42 gears, 2200 rpm stall in a 2004r trans, AC, PS, PB w pro touring type suspension.
What times do you all think are possible based on the vid and the specs ?

rojs234

iTrader: (5)

I used the calculator on www.NOT2FAST.com. I guessed at a couple of your engine specs, and used your cam specs and came up with static compression of 10.67 and dynamic of 8.83...basically the same as * SSellers*.
He is right about not using the .050 valve closing point as the valve is still open .085. Hard to build compression with the valve open, LOL. The site I mentioned also has a calculator for cranking compression(psi). I didn't know your altitude (in meters),so I left it at sea level and it showed 223psi...close?
Ain't it fun? Ron

newschool72

iTrader: (12)

Extremely close on cranking PSI. My gauge showed 218 to 220. I just checked 2 cylinders, but you sound right to me, Im at around 700 ft. No need to guess at the specs, just look up a bone stock LS3 with a gmpp dual plane intake, the cam specs you know, and a good set of 1 7/8 long tubes with good street exhaust.

topbrent

iTrader: (1)

I used the Dynamic Compression calculator on Wallace Racing to arrive at that high dcr number. It differs from some of the other calculators out there like the KBSilvolite and some of the others mentioned. I should have run the numbers on some of the other calculators out there and/or rechecked my calculations to get a more accurate read on the actual DCR.

That 10.11 number certainly did seem high, but when I ran the numbers, I was basically looking to verify that the new cam has indeed increased the DCR by a significant amount, and documenting that change. Sorry if this caused any confusion.