There's 3 different (that I know of) ways to read your HP/Tq, each with progressively lower numbers.
There's : Crank HP (measured on an engine dyno) Wheel HP (measured on a chassis dyno) Net HP (measured with a timing or accelerometer device)
Assume that crank HP is 'true'. Most feel that crank to wheel will drop about 15% (drivetrain losses). Any opinions on what wheel to net will drop?
I've just picked up a new G-Tech Comp Pro (www.gtechpro.com) and want to see if anyone's ever (for example) run a car on a chassis dyno, then used one of these for comparative readings.
In an ideal world, net HP and wheel HP would be the same. However, due to things like air resistance, your net HP will always be lower than your wheel HP. The problem is, the faster your going, the greater the deviation of the two.
GTechs are really not that great at estimating HP, since they rely only on acceleration and weight to "guess" your net HP. This means to get an accurate reading you need to get the weight exactly correct.
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12:49 PM
iluvmacs Member
Posts: 324 From: Monroeville, PA Registered: Feb 2002
If you do get the weight right, they can be very accurate. What you really use a G-tech for is when you're changing the setup of your car. You can use it (regardless of the input weight) to keep track of the changes you make.
15% is really low for drivetrain losses, even for manual transmissions. I think 25- 30% is much more probable.
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07:07 PM
Francis T Member
Posts: 6620 From: spotsylvania va. usa Registered: Oct 2003
I would suspect that finial drive ratio could throw curve into your type of device, whereas a car with 4.56 finial drive would seem to be making more HP than one with say, a 3.23 and that's not taking into acount the gearbox. Rear wheel is best, as it show what actualy avaliable where you need it. Now if you want the terms the engines use, they're 'ME' HP Mean Effective HP and Breake HP. I think one takes into account parrasitic drains on HP like water pumps, oil pumps, alt etc. Just thought I'd throw that in there. Anyway, you can still use your new toy to measure improvements, just keep tabs on how you do each test and consider things like ambient air temp to keep it accurate.
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07:21 PM
Francis T Member
Posts: 6620 From: spotsylvania va. usa Registered: Oct 2003
Originally posted by iluvmacs: 15% is really low for drivetrain losses, even for manual transmissions. I think 25- 30% is much more probable.
15's been shown to be almost bang on for a 'front wheel drive' (which is really what we are - we just drive backwards! ). 25-30 would be accurate for a front engine, rear drive.
Anyhow... all of the above are fine and dandy, but... still doesn't answer the original question!
Let's say you have 100HP at the crank. A chassis dyno (uncorrected) will say you have about 85HP at the wheels. What will the G-tech (and it's "net" HP) now say you have? 80? 62.5? (of course, I'm making all these numbers up)
There's got to be someone who's tried a back to back run of a chassis then G-tech run!
I know a guy just ran at the track a day or so ago with the same rig that I just got, so I can at least get some comparisons from that. Hopefully I can use all his data to come up with a sort of 'formula' for the comparison of the 3.
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10:54 PM
Apr 6th, 2004
watts Member
Posts: 3256 From: Coaldale, AB, Canada Registered: Aug 2001
Originally posted by watts: I know a guy just ran at the track a day or so ago with the same rig that I just got, so I can at least get some comparisons from that. Hopefully I can use all his data to come up with a sort of 'formula' for the comparison of the 3.
Oh well - so much for that idea this time around!
Wound it up, and never made it through the first pass - blew an axle.
See what happens next time!
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12:36 PM
Black-Azz-GT Member
Posts: 2326 From: Florida Keys Registered: Oct 2003
). 25-30 would be accurate for a front engine, rear drive.
