Power to weight

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mheintzman
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Power to weight

Post by mheintzman » Sun Jun 02, 2013 12:32 pm

For you GT3 regulars, what is considered a competitive power to weight ratio?

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Jer
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Re: Power to weight

Post by Jer » Tue Jun 04, 2013 1:34 am

I think a competitive car would have to come in at less than 7.5 lb/hp. Of course, everyone lies about the power their cars make, with engine builders overstating and drivers wanting favorable classifications understating. Then there are different types of "horses", varying from the originating 'English draft horse" to the California PR department race engine horse. But I think I'm giving you a reasonable answer, leaving open only the question of how much "less than 7.5 lb/hp" you'd need!

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cyphert76
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Re: Power to weight

Post by cyphert76 » Tue Jun 04, 2013 12:59 pm

I agree, for GT3, a weight to power ratio of around 7.5 is what the top cars have.

jesracing
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Re: Power to weight

Post by jesracing » Tue Jun 18, 2013 4:08 pm

Sorry I have to ask what is likely a silly question. Are you talking hp at the crank or hp at the wheel?
Jim

mheintzman
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Re: Power to weight

Post by mheintzman » Tue Jun 18, 2013 4:26 pm

jesracing wrote:Sorry I have to ask what is likely a silly question. Are you talking hp at the crank or hp at the wheel?
Jim
I am under the impression the HP is at the crank so a 2060lb car would be in the ballpark with Approx 275HP (crank). Unfortunately, I won't get to a 7.5lb/HP quite yet plus I am also going to take the 2.5% weight penalty hit for IRS.

I will have the car out with the new motor at the Sears Point national in July to baseline the performance. If it hunts, I'll work harder on preparation and see if I can qualify for the runoffs in GT3.
Last edited by mheintzman on Wed Jun 19, 2013 12:09 pm, edited 1 time in total.

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Jer
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Re: Power to weight

Post by Jer » Wed Jun 19, 2013 2:15 am

Yes, I was using HP at the crank. If you are using a chassis dynamometer, the expected loss would be about 15-20%.

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jesracing
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Re: Power to weight

Post by jesracing » Fri Jun 21, 2013 3:53 pm

Hi Michael

I am in the same boat...2060 plus the penalty for IRS ( which I think is inappropriate if the car comes with IRS from the factory). When I built up my engine I asked Jesse to build an engine that would go at least 12 hours without touching it. Net net is we limited the cam lift but went full build on the compression ratio. Still working off a preped production crank. Bottom line is I get about 210 hp at the rear wheel with peak hp at 7800 rpm and the car is capable of about 2:30 at RA. Jesse estimates we are about 240 hp at the crank. So I am about 8.75 lbs/hp. Makes the 7.5 number suggested about right. Full preped tube frame GT3 are about 4-6 seconds a lap faster. I would like to think that the lap time difference is all car but I suspect the driver is about maxed out at the @ the 2:30 level in a 2100 pound car. It is a blast however to drive the car compared to the no throttle response GTL configuration.

Jim

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Jer
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Re: Power to weight

Post by Jer » Sat Jun 22, 2013 10:24 pm

210 hp at the rear wheels may translate to a little more than 240 hp at the crank, but you're right, it won't get you to the times the top cars are running. You still might get your share of good finishes, though, because that reliable motor may be there at the finish, while a check of race results will show more than a few fast cars in the DNF column!
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Rthuesen
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Re: Power to weight

Post by Rthuesen » Sun Oct 30, 2016 12:10 pm

so how does scca come up with the weight for a specific car in all the GT classes?

bavclt
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Re: Power to weight

Post by bavclt » Sun Oct 30, 2016 7:53 pm

One thing that I really like about the GT community and this forum is the honest information by the members. The knowledge pool is also somewhat remarkable.

From my experience building race engines for the last 30 years, it is still "amusing'' to see the difference between "Clydsdale" and "little tiny" horses!!
Sam Moore
GT3 Volvo

SCCA competitor since 1983

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