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Old 05-13-2010, 01:45 PM   #16
George Bryce
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Default Re: Cranking compression question

Want more cranking compression? Set the lash at .050...dont run it like that...just try it on one hole...then put the lash back to where it is supposed to be and run it.
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Old 05-13-2010, 02:49 PM   #17
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Default Re: Cranking compression question

I can't believe how many of you ^^^^^ could be racing on 91-94 octane......
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Old 05-13-2010, 02:55 PM   #18
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Default Re: Cranking compression question

That is the compressin at 150 rpm at a higher RPM things change just a bit.
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Old 05-13-2010, 03:44 PM   #19
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Default Re: Cranking compression question

I've got a good friend and customer who builds lots of winning SBC dirt track engines. He always uses a software program to predict the cranking pressure, then verifies it after break-in. He says the software is always within 10psi and a very good "double check" that everything is correct. I don't know what software he's using, but I will find out if anyone's interested. According to him, he has the best luck with 230-240 psi on high compression combos (over 13:1), and shoots for that number when selecting and installing a camshaft. His experience is that engines over 240 get very picky on the tune-up, and under 230 he's leaving something on the table.

I know this is not a drag race application, and a broad torque range is required to pull the car out of the corner. But hopefully you can learn something from his approach and apply it to your application.
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Old 05-13-2010, 04:01 PM   #20
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Default Re: Cranking compression question

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Originally Posted by Paul Swartz View Post
I've got a good friend and customer who builds lots of winning SBC dirt track engines. He always uses a software program to predict the cranking pressure, then verifies it after break-in. He says the software is always within 10psi and a very good "double check" that everything is correct. I don't know what software he's using, but I will find out if anyone's interested. According to him, he has the best luck with 230-240 psi on high compression combos (over 13:1), and shoots for that number when selecting and installing a camshaft. His experience is that engines over 240 get very picky on the tune-up, and under 230 he's leaving something on the table.

I know this is not a drag race application, and a broad torque range is required to pull the car out of the corner. But hopefully you can learn something from his approach and apply it to your application.
What octane fuel is he running with 230-240 psi?


For those interested, here is a cranking compression calculator http://www.rbracing-rsr.com/comprAdvHD.htm
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Old 05-13-2010, 05:44 PM   #21
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Default Re: Cranking compression question

The old combo I ran for years, had 12-1 compression, and right at 225 psi cranking compression, and it ran really good, especially considering how relatively low budget it was. I had another motor later on that was around 15-1 compression, and with camshaft ~A~ had 260 psi cranking compression and ran pretty good. And before everyone says that is too high, camshaft ~B~ which was totally wrong for the combo, had well in excess of 300 psi cranking compression (the needle would sweep past 300 psi and finally come to rest pointing directly at the point where the hose attached.) The motor was LOUD as hell, and ran like shit.
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Old 05-13-2010, 07:44 PM   #22
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Default Re: Cranking compression question

You can pretty much hear it when it has the wrong cam. If it's too small making the cranking compression too high it sounds like it's beating itself to death.

I had a SBC on injected alcohol with 275 psi. 25 runs later it was hurt. Took it apart and all of the wristpin bosses on every piston were cracked.

I have had 2 friends with the wrong cams in their combos and I talked them into switching. They were both amazed at how much smoother the power was.
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Old 05-13-2010, 09:15 PM   #23
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Default Re: Cranking compression question

565" 12.3:1, cam dur @ .050 278 int 284 ex. 935 hp single carb

Cranking compression 185 psi +/-



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Old 05-13-2010, 09:55 PM   #24
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Default Re: Cranking compression question

Here is another calculator to determine cranking compression.

http://www.kb-silvolite.com/calc.php?action=comp
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Old 05-13-2010, 11:52 PM   #25
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Default Re: Cranking compression question

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Originally Posted by feetfirst View Post
What octane fuel is he running with 230-240 psi?


For those interested, here is a cranking compression calculator http://www.rbracing-rsr.com/comprAdvHD.htm
Sunoco Blue (112) for most applications or Sunoco HCR (114) when the engine is "on kill" or the conditions are demanding (long tracks or long races where heat can be extreme).

Thanks for the calculator link. I punched in a few familiar combinations, and this calculator seems to confirm what he's telling me.
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Old 05-14-2010, 12:02 AM   #26
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Default Re: Cranking compression question

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Originally Posted by Paul Swartz View Post
Sunoco Blue (112) for most applications or Sunoco HCR (114) when the engine is "on kill" or the conditions are demanding (long tracks or long races where heat can be extreme).

Thanks for the calculator link. I punched in a few familiar combinations, and this calculator seems to confirm what he's telling me.
I was going to try some 113, so its sounds like Im close. Have never ran race gas in my life so its a first.......
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Old 05-14-2010, 12:13 AM   #27
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Default Re: Cranking compression question

Hopefully all the numbers being said were tested the same way. Ifthe test was off for comparison sake, numbers can be off if the engine is cold, carb closed, not spinning it enough..etc. A good rule of thumb is for a race engine being run on race fuel, you will want 220-230psi. A pump fuel engine with alum heads will be fine at 185-200psi. A pump fuel engine with iron heads shouldnt exceed 175psi, but be more than 160psi. Again, this is all relative to the parts being used and IF the combination is right. The numbers can be misleading, as other parts affect it, not just cam timing, although that is the major player. You can have 200 psi in a stock engine with 11.1 and it will kill itself. If you have 185psi in a high compression engine, you are usually overcammed or just don't have the right induction setup. Don't just look at the number it gives you, the whole combination needs to be looked at. From keeping track over the years with my combinations, I know with what combination I need for timing events for a given profile. That is why I would need to know the entire combination to know if the cam timing is right on. The only problem with the formulas are that it doesn't take into consideration the entire combination. It mainly just goes by cam timing events...which again can be very misleading. Too low a number and the engine will be very lazy and bedown on power. Too high and it will run hot, be hard to start, and the tune up becomes very critical, as well as be very prone to detonating, which you will NOT hear. It also beats up the pins, and bearings..Also keep in mind, you don't want to "overcam" and engine to get numbers low just to say you can run pump fuel. If you think about it, cyl pressure does rise with rpm and heat, and by doing this, you actually build alot more cyl pressure later on in the rpm range. When you run longer duration and more lift, it allows more air to enter the cyl, which builds more cyl pressure. So there is more to it than just that...
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Old 05-14-2010, 03:42 AM   #28
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Default Re: Cranking compression question

exhaust closing event ( totally closed) with a look at the previous lift closing curve from about .250" lift ( before closed) is going to tell you how much blow down in crank degrees is going to hold back or contain any residual and thus create more compression on the gage.;
it is a realtive thing.
some people / builders want certain things, in order for the engine to run a certain way,
with HIS GROUP OF PARTS,
but another will want different comparative readings,
with his group of parts
when both make probably within one or 2 percent power of each other....
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Old 05-14-2010, 08:36 AM   #29
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Default Re: Cranking compression question

wow i almost feel like somethings wrong with mine 155 across the board (COLD).
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Old 05-14-2010, 09:10 AM   #30
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Default Re: Cranking compression question

I will repeat dont confuse cranking compression with whats going on at 8000 RPM with back pressure and blow threw and the ramming effect of th right plenum and runner lentghs with the right header lenghts and size.

That said a lot of guys use to high of octain for there engines but running pump gas is not a good idea just from the different formulations out of the same pump from fuel load to fuel load or town to town, this is one of the best reasons to run race gas, the tune up stays close if you run that close to start
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