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Saturday Night Racer >> RACE TECH >> compression question
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Message started by havocracingsc29 on 03/02/10 at 11:17pm

Title: compression question
Post by havocracingsc29 on 03/02/10 at 11:17pm

how many lbs is 10.5to1? i see some tracks say 180lbs max and then some say 10.5to1. thanks

Title: Re: compression question
Post by XMODRACER on 03/03/10 at 6:49am

180 lbs @10.5-1 sounds about right . is that pic from victorville? ;)

Title: Re: compression question
Post by slidinbmw on 03/03/10 at 8:48am

My BMW mini stock is 10:1 and blows 210psi. It is dual overhead cam though so volumetric efficiency is very high. Also has stock cams. I think you can lose over 10% with aftermarket cams.

Title: Re: compression question
Post by sj_valley_dave on 03/03/10 at 10:32am

That question totally depends on your cam profile. How much overlap, etc...can't be answered with out more information..

Title: Re: compression question
Post by Andrew on 03/03/10 at 4:59pm

It also depends on rings. Total Seal rings hold a LOT more cranking compression. Cam profile and cam timing will affect it the most though.

Title: Re: compression question
Post by havocracingsc29 on 03/03/10 at 6:56pm

yes, that pic is from victorville, ok thanks guys, i was just wondering cuz i see bako hobby's rules say 180lbs max and then victorville says 10.5to1. i was just curious

Title: Re: compression question
Post by Dietzsch_Racing25 on 03/12/10 at 9:42am

Pumping compression is a measurement of how much pressure is developed during the compression stroke in the combustion chamber when the engine is turned over.

Compression ratio is calculated by comparing the combustion chamber volume (volume above the piston) at BDC vs TDC.

Pumping compression is dependant on several things including, as others mentioned: ring seal, how fast the engine is turning, how hot the engine is, compression ratio and cam timing. IMO the last two have the biggest influence. Also IMO the only thing that affects pumping compression (related to the cam) is when the intake valve closes. Closing it earlier (advancing the cam) increases pumping compression. Closing it later (retarding the cam) decreases it.

CD

Title: Re: compression question
Post by slidinbmw on 03/12/10 at 12:15pm

I agree. I retarded my intake cam to pass compression at 180psi.

Title: Re: compression question
Post by mighty16x on 03/17/10 at 11:15pm

the two have nothing in common.

Title: Re: compression question
Post by Chad_E._Jeffreys on 03/18/10 at 11:08pm

Bore and stroke.... now there's a rule!!!! Don't have to worry about conditions such as temp or humidity or even how fast the motor is turning. This I think is way to simple, they'll never buy!!! JMO Chad

Title: Re: compression question
Post by Dietzsch_Racing25 on 03/19/10 at 8:43am

Chad_E._Jeffreys wrote on 03/18/10 at 11:08pm:

Perris and Victorville went away from a pumping compression rule and now have a compression ratio rule for their street stocks (kind of bako hobby). VV has a "whistler" so it is an easy thing to check.

CD

Title: Re: compression question
Post by Andrew on 03/19/10 at 5:34pm

Ventura has a whistler too, but they won't use the thing.

Title: Re: compression question
Post by Dietzsch_Racing25 on 03/22/10 at 9:22am

Mike at VV used it on saturday.

Title: Re: compression question
Post by dirtmopar44 on 03/25/10 at 5:11am

Did everybody pass?

Title: Re: compression question
Post by Dietzsch_Racing25 on 03/26/10 at 9:54am

dirtmopar44 wrote on 03/25/10 at 5:11am:

Did everybody pass?

Yep. I don't know how it works but it pin pointed my ratio correctly.

CD