Saturday Night Racer - compression question

havocracingsc29 LMR Rookie Offline slidin sideways in the dirt Posts: 52 ca	compression question 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
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XMODRACER LMR Senior Offline Dirt Late Models Rule! Posts: 461	Re: compression question Reply #1 - 03/03/10 at 6:49am 180 lbs @10.5-1 sounds about right . is that pic from victorville?
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Ensign3 LMR Senior Offline My heart says Latemodel, my wallet says Ministock. Posts: 297 Nipomo	Re: compression question Reply #2 - 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.
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sj_valley_dave LMR God Offline Love the smell of nitro in the air Posts: 1700	Re: compression question Reply #3 - 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..
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Andrew LMR God Offline 2006 SMS M.S. champion... more of a PB these days Posts: 1359 Camarillo	Re: compression question Reply #4 - 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.
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havocracingsc29 LMR Rookie Offline slidin sideways in the dirt Posts: 52 ca	Re: compression question Reply #5 - 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
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Dietzsch_Racing25

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Posts: 995

Re: compression question
Reply #6 - 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

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Ensign3 LMR Senior Offline My heart says Latemodel, my wallet says Ministock. Posts: 297 Nipomo	Re: compression question Reply #7 - 03/12/10 at 12:15pm I agree. I retarded my intake cam to pass compression at 180psi.
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rickshawII LMR God Offline The RickshawII Posts: 1166 china lake ca	Re: compression question Reply #8 - 03/17/10 at 11:15pm the two have nothing in common.
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Chad_E._Jeffreys LMR Senior Offline I love YaBB 1G - SP1! Posts: 258 California	Re: compression question Reply #9 - 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
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Dietzsch_Racing25

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Re: compression question
Reply #10 - 03/19/10 at 8:43am

Chad_E._Jeffreys wrote 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

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

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Andrew LMR God Offline 2006 SMS M.S. champion... more of a PB these days Posts: 1359 Camarillo	Re: compression question Reply #11 - 03/19/10 at 5:34pm Ventura has a whistler too, but they won't use the thing.
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Dietzsch_Racing25 LMR God Offline Posts: 995	Re: compression question Reply #12 - 03/22/10 at 9:22am Mike at VV used it on saturday.
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dirtmopar44 LMR Member Offline Posts: 212	Re: compression question Reply #13 - 03/25/10 at 5:11am Did everybody pass?
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Dietzsch_Racing25 LMR God Offline Posts: 995	Re: compression question Reply #14 - 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
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