WHP Conversion ? - Subaru WRX Forum
 
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#1 Old 07-22-2009, 06:56 AM
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WHP Conversion ?

Hi guys, is there a conversion factor (?) to convert wheel horse power to Crank Horse Power? I had a tune done and gained 65whp however I am trying to figure out what it gave me at the crank? Is it as simplae as just adding 65hp to my existing crank #'s? Thanks...
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#2 Old 07-22-2009, 09:10 AM
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add 20-22% for drivetrain gain/loss

Paul
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#3 Old 07-22-2009, 09:16 AM
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It's much more than that. Most dynos register whp 25-30% below quoted factory hp. If you agree with the factory numbers and want the same correction, you have to add the inverse of those. 33-43% 65whp would be 86-93 bhp.

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#4 Old 07-22-2009, 10:08 AM
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thanks bro.. i was hoping to have at least 300hp at the crank.. I still cant beleive I got 65whp more from the tune. But I got the dyno printout and was standing there while they were tunning it... My stock hp is rated at 224 so I guess I got at least 300hp now... Icould of got almost 70 out of it but it was running to rich so they dialed it back. I had to install a MBV as well and now have around 18-19psi.
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#5 Old 07-22-2009, 10:19 AM
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Typically, there are two main ways to look at drivetrain loss:

1. Fixed Loss = This basically means that no matter how much power is being made at the crank, the drivetrain will always absorb the same amount of power. In the WRXs case, it’s said to be about 62 hp.

2. Percentage Loss = This assumes that the ratio of crank horsepower vs. wheel horsepower remains unchanged. In the WRXs case, it’s said to be somewhere around 20-25%.

So, given that your car makes an additional 65 wheel horsepower over the stock 160 (another estimate), let’s say you make somewhere around 225 wheel horsepower.

By method 1, 225 whp + 62 hp = 287 chp.

By method 2, assuming a 25% loss, you might have 300 chp. Assuming you have a 20% loss, you might have 281 chp. Averaging the two, you might have 290 chp.

Based on all of these rough estimates, my best guess is that you have roughly somewhere just shy of 300 crank horsepower.

John
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#6 Old 07-22-2009, 11:38 AM
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Your math is wrong. If you say 20-25% losses, you have to add the INVERSE. 1/.8 and 1/.75 which are 125% and 133%

EXAMPLE:
Say a car has 20% losses. It has 250hp and dynos at 200whp. If you want to go back from 200whp to 250bhp, you have to multiply by 125% not 120%. 125% * 200 = 250. This is the inverse. 20% is .8. Inverse is 1/.8 which is 1.25 or 125%

I've seen the percentage answer hold up on two cars with different power levels. My 04 WRX stock quoted 227hp dynoed 155whp and the 300hp STi dyno'd 210whp on the same dyno. The correction is not consistent so a percentage makes more sense. Granted as you get into much higher numbers, the percentage loss falls off but it's better to use percentages at reasonable power levels.

Also, both of those numbers are 30% or more losses. Mustang dynos are considerably lower reading.

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#7 Old 07-22-2009, 12:44 PM
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Ok, when I had my rex dyno'd on a mustang it pushed 210 +/- whp on the stocker AND is nowhere close to the power the STi now has.... You are telling me that a "stage2" STi has 245 +/- to the wheels ? 35whp difference ? 30-35% is pushing it.

This is

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#8 Old 07-22-2009, 02:28 PM
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STi's benefit more from "stage 2" than a wrx will because the turbo has a higher flowrate. They're both dealing with the same crappy restriction but the STi can do more with it once it's removed. Stage 2 STi's on the mustang dyno's I'm used to are in the 260-280whp range where stock they're more like 210whp.

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#9 Old 07-22-2009, 02:55 PM
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^^ makes sense.. .thanks for clearing that up.

Paul
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#10 Old 07-22-2009, 08:43 PM
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Yeah, I'm 255 awhp on a mustang dyno stage 2 protune. I think fuel mods would've opened me up to higher numbers. I had stock injectors and fuel pump when I was tuned.
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#11 Old 07-22-2009, 10:23 PM
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Oh man, you're right. I failed that one. Please accept my humble apologies.

John
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