Too much compression for pump gas?

Posted By: Genuinejed

Too much compression for pump gas? - 02/19/10 03:57 AM

I'm swapping a set of Pro Comp victor knock offs on my 426 Wedge and I'm wondering if I'm going to have to run into problems with detonation. The motor is .070 over and has kb237s .025 in the hole. The heads are 72ccs and the cam is a Voodoo 60304. The compression calculator I used said 12.2 to one! Anyone have ideas or experience with a combo like this on pump? With the old 452s I never had any problems, but I'm a little concerned I might hurt the pistons.

Posted By: Cab_Burge

Re: Too much compression for pump gas? - 02/19/10 04:29 AM

What altitude is it where you drive it? What compression was the motor with the 452 heads and is it the same cam still?

Posted By: Thumperdart

Re: Too much compression for pump gas? - 02/19/10 05:05 AM

I`m not sure how totally accurate it is but cranking comp. seems to be what a lot of people gauge PART of it on and my 12.11 motor runs/drives fine on 91 pump junk. When I race I mix 110 leaded and hammer down.

Posted By: topbrent

Re: Too much compression for pump gas? - 02/19/10 05:21 AM

Quote:

I'm swapping a set of Pro Comp victor knock offs on my 426 Wedge and I'm wondering if I'm going to have to run into problems with detonation. The motor is .070 over and has kb237s .025 in the hole. The heads are 72ccs and the cam is a Voodoo 60304. The compression calculator I used said 12.2 to one! ...

Using the calculator on the KB website, and assuming that you will use a standard .039 head gasket, I come up with 10.727 compression ratio.

http://www.kb-silvolite.com/kb_car/performance.php?action=details&P_id=30

http://www.kb-silvolite.com/calc.php?action=comp

Posted By: Genuinejed

Re: Too much compression for pump gas? - 02/19/10 12:45 PM

Thanks for the link. When I enter the 72cc chamber KBs website says 11.5 to 1, the more accurate calculator says 12 to 1. I'm at 2120 feet above sea level. With the old heads and the same cam it was 10.47 to 1 using the calculator. I'm hoping the cam will lower the dynamic compression. I'll probably add some race gas when I go to the strip, I'm just hoping I don't have to run it all the time. Thanks for the help!

Posted By: Thumperdart

Re: Too much compression for pump gas? - 02/19/10 04:48 PM

Run a cranking comp. test and tell us what you`ve got...........

Posted By: BradH

Re: Too much compression for pump gas? - 02/19/10 04:59 PM

The online CR (static) calculator I like best is the one on the Diamond Pistons web site (diamondracing.net).

I don't know how big the cam is you're talking about using, but if you have 11.5 CR and anything smaller than about 275 @ .050", it'll probably ping on pump gas. I'm expecting to be at (or over?) the limit for 93 octane w/ around 11.0 and something in the 265-268 @ .050" range.

Posted By: moparniac

Re: Too much compression for pump gas? - 02/19/10 06:29 PM

67wedge runs a 268/272 at 11.4 to 1 on pump gas and i was 11.12 and ran pump 93 with a 248/254 and made my passes on pump 93 ......the higher the Intake closing ABDC cam timing event helps with this...

I checked my cranking pressure on one of those calcs(engine not done yet) and its saying 180psi and im gonna be right at 11 to 1 with a 269/269 bullet roller .... I was torn between the 11.5 /11.0 deal also...... I also had my heads milled back to the correct 75 cc rating after the cnc porting was done...

Posted By: Thumperdart

Re: Too much compression for pump gas? - 02/19/10 06:55 PM

My 12.11 comp. heap cranks at around 170 w/275-280 dur.@.050 and runs fine on 91 but, I ALWAYS mix in 110 when racing........do I need it? Not sure but gas is cheap and pistons/rings arent.

Posted By: gofish

Re: Too much compression for pump gas? - 02/19/10 07:05 PM

What is the quench area now? If the heads are a closed chamber type, that will be the problem.

Danny

Posted By: moparniac

Re: Too much compression for pump gas? - 02/20/10 04:26 PM

Quote:

My 12.11 comp. heap cranks at around 170 w/275-280 [Email]dur.@.050[/Email] and runs fine on 91 but, I ALWAYS mix in 110 when racing........do I need it? Not sure but gas is cheap and pistons/rings arent.

dom whats your dynamic compression ratio

Posted By: chryco

Re: Too much compression for pump gas? - 02/20/10 04:41 PM

I have a question . My 557"KB/RBRE Hemi is a 11.5 : 1 motor (static) and won`t run with 91 oct pump gas . Can the dynamic comp be higher than the static comp ????
Chryco

Attached picture 5818706-IMG_5771.jpg

Posted By: Thumperdart

Re: Too much compression for pump gas? - 02/20/10 05:54 PM

Quote:

Quote:

My 12.11 comp. heap cranks at around 170 w/275-280 [Email]dur.@.050[/Email] and runs fine on 91 but, I ALWAYS mix in 110 when racing........do I need it? Not sure but gas is cheap and pistons/rings arent.

dom whats your dynamic compression ratio

Don`t know how to figure that out......have always gauged it on cranking.

Posted By: moparniac

Re: Too much compression for pump gas? - 02/20/10 06:15 PM

Quote:

Quote:

Quote:

My 12.11 comp. heap cranks at around 170 w/275-280 [Email]dur.@.050[/Email] and runs fine on 91 but, I ALWAYS mix in 110 when racing........do I need it? Not sure but gas is cheap and pistons/rings arent.

dom whats your dynamic compression ratio

Don`t know how to figure that out......have always gauged it on cranking.

http://www.wallaceracing.com/dynamic-cr.php

Posted By: Thumperdart

Re: Too much compression for pump gas? - 02/20/10 06:58 PM

Unless I`m doing something wrong,it says my dynamic comp. is 7.59........damn that seems low.

Posted By: 602heavy

Re: Too much compression for pump gas? - 02/20/10 09:50 PM

Quote:

I have a question . My 557"KB/RBRE Hemi is a 11.5 : 1 motor (static) and won`t run with 91 oct pump gas . Can the dynamic comp be higher than the static comp ????
Chryco

Static will always be higher than dynamic , unless of course the intake valve shut @ BDC.

Posted By: 602heavy

Re: Too much compression for pump gas? - 02/20/10 09:58 PM

Quote:

My 12.11 comp. heap cranks at around 170 w/275-280 [Email]dur.@.050[/Email] and runs fine on 91 but, I ALWAYS mix in 110 when racing........do I need it? Not sure but gas is cheap and pistons/rings arent.

I lost a good 3/10ths down track using 116 race gas , the motor is 9.5:1 & i was using the 116 for nitrous , ran the motor NA on the 116.

If the motor does'nt warrant high octane fuel it will always slow down , you can add advance to the motor to compensate but the more advance you give the motor the more it will slow the piston down.

Posted By: Thumperdart

Re: Too much compression for pump gas? - 02/20/10 11:18 PM

Are you saying your static or dynamic was 9.5:1?

Posted By: chryco

Re: Too much compression for pump gas? - 02/20/10 11:54 PM

Quote:

Quote:

I have a question . My 557"KB/RBRE Hemi is a 11.5 : 1 motor (static) and won`t run with 91 oct pump gas . Can the dynamic comp be higher than the static comp ????
Chryco

Static will always be higher than dynamic , unless of course the intake valve shut @ BDC.

That`s what I thought. How would you get something from nothing ?

Posted By: 602heavy

Re: Too much compression for pump gas? - 02/21/10 12:54 AM

Quote:

Are you saying your static or dynamic was 9.5:1?

Static was 9.5:1 , i gave this as an example , obviously the motor does'nt need race gas @ that CR , but some guys use race gas for belt & braces...........higher octane requires the spark to be fired off earlier , as the mixture expands it trys to push the piston down the hole , the less spark lead to get optimum leverage ATDC the better , spark lead on my motor was around 40*

, since raised the CR for this reason.

Posted By: mopar dave

Re: Too much compression for pump gas? - 02/21/10 01:45 AM

my static will come in at 11.4, dynamic will be 9.85 with 207psi cranking. looks like pump gas may be out for me even though i will have .040" quench. will see.

Posted By: Maximus_Wedges

Re: Too much compression for pump gas? - 02/21/10 01:46 PM

Also double check valve clearence if you haven't already done so. The pro com heads I have are 2.25 intake and 1.88 exhaust. They hit the 426ci 4.25 bore block and my shalow valve pockets on my stock street wedge piston about the same time. I was trying to lift .570

Posted By: Dago Red

Re: Too much compression for pump gas? - 02/21/10 06:02 PM

My biggest problem with pump gas has never been detonation, but inconsistancy. My buggy only has around 8.5 but every so often it tosses out a really off pace run or picks up. I went to the 100 unleaded gas and it stopped. All fillin' station fuel is not created equal.

Posted By: Genuinejed

Re: Too much compression for pump gas? - 02/22/10 12:57 PM

Thanks for the input. The motor is stock 440 bore and I checked valve clearance on a 440 and it looks fine. The pistons have some pretty decent reliefs, and I plan on checking clearance there. The heads are off right now waitting on parts. I think quench is gonna be a strech at .025 in the hole, and a .039 head gasket. I was thinking about putting on a thick head gasket to drop the compression a tad since now would be the time. dynamic calculator says my CR should be 10.7 to one.