I have been using my GTECH to estimate my rear wheel hp. Now, I took the car to a salvage place and they weighed the car with me in it at 3580lbs. See mods below. I run 0-60 with the GTECH avg 5.2, and the quarter at about 104. But when I do a hp run through 2nd gear, I only register 209 rwhp. Assuming a 20-30% driveline loss that means anywhere from 260-300hp, but how can a car that weighs this much go this fast with this little power. I have 3.50 gears. I'm really confused cause I run almost equal and sometimes beat my friend with a slightly worked 99 SS but his comes up with 339rwhp and runs about 111mph. What can be wrong, what can I do to be sure? Thanks guys.

------------------
87 GTA 350 L98
K&N filter, beefed up TH-700r4, Accel Manifold, Superram, Fuel Press. Regulator 49.5psi, air foil, modified MAF, SLP cold air box, Accel 24lb Injectors, Ed Wright Fastchip custom chip w/160 thermostat, MSD 6AL and Blaster 3, B&M shift kit & megashifter, Moser Ford 9in Rear with Richmond 3.50 and locker, Edlebrock TES headers and Cat Back,Gutted cat, removed emissions, Hotchkis setup in rear, Poly suspension and Koni in the front, relocated MAT, bypassed TB coolant

 

From what I've heard those G-Tech's aren't all that accurate on 1/4 mph.they tend to read higher(mph.Best bet might be is to go to the stripe and see what kind of mph you get through the traps.and see how they compare to your G-Tech.just my $0.02



------------------
1991 1LE Z28 355cu 5SPD (ORIGINALLY 305 BEST E.T. 14.186@99.86mph)NO PASSES ON 355 YET.MINI RAM INTAKE,AFR 190 HEADS,10.23:1cr 24# SVO INJECTORS,BBK 52mmTB,AFPR
FULL ROLLER MOTOR,1.6 rockers MSD 6AL
AUTOMETER A/F GAGUE MARCH PULLEYS,HEDDMEN HEADERS 3IN EXH NO CATS FLOWMASTER,POWER TRAX NO-SLIP LOCKER,3:42 GEARS,LAKEWOOD LOWER CONTROL ARMS HOTCHIKS STRUTTOWER BRACE,COMPETTION ENG.SUBFRAME CONNECTORS
MALLORY TACH W/SHIFT LITE
BLACK, TINTED WINDOWS, TAIL LIGHT COVERS BIG BLOCK COWL INDUCTION HOOD,SHAVED DOOR HANDLES

67RS 327cu 92K ORIG MILES.NEWER GM HEADS HEADERS,650 HOLLEY CARB,CAM, 350 TURBO TRANS W/SHIFT KIT&COOLER 12 BOLT POSI 3.73'S MUTLILEAF SPRINGS
NEW VINYLROOF&CARPET, COWL HOOD,HIDE AWAY HEAD LITES,

 

I haven't gotten mine yet, but I've always wondered how they could calculate HP.

------------------
Brett

'91 Z28 Black
125K,L98,3.23,4L60.
Panhard,LCAs.
-------------------------
MN F-Body
Brett's House

 

You have to know the weight of your car to have it calculate HP for you. I'm sure there little gravity sensor in the little black box measures how hard your pulling, then does a simple calculation to figure out HP.

------------------
1991 Z28 305 5spd 1LE, TES headers, dual cats, 3 Chamber flowmaster, Comp cams .488/.495 212/218, 187 head castings with bigger valves, self-burned eproms.
All work done by myself, and proud of it!
SOON TO COME 327 Miniram (3/2002) PARTS NEEDED:Titanium valvetrain parts, stud girdles, fluidampnr, miniram, bulletproof bellhousing, MONEY(im in college).
Find a .bin on the internet and want it burned? I'll do it for you!

 

GTechs are MOSTLY accurate. As far as 1/4 mile times, they are usually w/in one tenth. As for speed, they are generally higher due to how it caculates speed vs. how its done at the track. The speed is pretty much accurate, but comparing it to speed given at the track, its apples to oranges.

The GTech used advanced acceleromters, the same ones used in Avionics, Supplemental Inflatable Restraint systems (airbags), Advanced Traction control systems, etc... So you know the technology is the "real deal". It uses advanced algorithyms to dertermine your speed and time based on data recieved from the sensors inside the GTech. Not to shabby for just plugging into your cigarette lighter. I own one and use it to compare mods when I can't get to the track.

------------------
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'87 Trans Am
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'97 Bonneville SSE

 

I agree with Bernard. And they are are very useful for quantifying mods and practicing lauches.

Personally, I wouldn't give a flying hoot about the calculated HP...it's "who is quickest" that really matters.

 

let me ask you this how can my car run comperable times to some ls1 cars that pulled arround 300 RWHP? the answer is average power. the l98 does not have good peak power numbers but it does make good average power. to be honest g-tech calculates the hp in much the same way that the dynojet does. G-tech measures acileration of a mass in a strait line where dynojet measures angular accileration of a big drum. They have a very fast sampling rate and they use an algorythum to calculate torque wich is used to calculate hp based on the speed that it beleves it is traveling. So you see the only reason the dynojet is slightly more acurate is that it is a more controled environment. Oh by the way horsepower is not everything torque is!!

------------------
90 IROC l98
last season best corected 13.62 @102
mods
full exhaust, AFPR, pulley, jet stage 1 chip, billet servo, 52mm throttle body, slp runners and some 1.6 RR's and a few other little things.
243 RWHP and 342 RWTQ with hot engine and 90 octain fuel

 

your low 14s, nothing more...

------------------
1990 G92 5spd LB9 - 3.42 80,000 miles

Accel Ignition/Wires (8.8mm), Advanced Timing, TB Bypass, Ported and Gasket Matched Plenum, Synthetic Motor Oil, 160 Thermo, TPIS Airfoil, K&N Air Filters, Centerforce I Pressure Plate and Clutch, Edelbrock TES Headers, Dual Cats, Edelbrock RPM Series Catback, Homemade Ram Air, Bosch +4 Platinum Plugs

14.4 @ 94mph w/2.0622 60ft (w/o headers and catback)

14.426 @ 96.19mph w/2.167 60ft(w/headers and catback)

Also newly modded w/o track times:

Holley AFPR, Underdrive Pulleys, KYB Shocks and Struts, Crane Ignition

 

90IROC how can he be running low 14's with a superram 350 wtih full exhaust when you are running low 14's with a 305 ported and matched plenum and intake and full exhaust. Personally to me that makes no sense what so ever.

 

darkhorse, its because ive seen built 350s run 14s. maybe wait, im smoking stuff, i dont think i noticed his susp stuff. im sorry, i would say mid13s. im sorry, iread incorrect.

 

The GTech measures acceleration during a run.

RoadForce = mass*accel (Newton's law)

RWTorque = RoadForce*(tire_radius)
RWhp = RWTorque*axle_rpm/5252

(RW = rear wheel)

EngTorque*trans_gear*rearend_ratio = RWTorque

EngHp = EngTorq*rpm/5252

If you step through the above you'll see that if you know the car mass (with you in it), and you have measurements of acceleration (from GTech), then you can figure out the rear wheel torque and hp, as well as the flywheel torque & hp. (note: I didn't include losses in the above; it gets messy)

To do the torque and power calcs for the engine, i.e at the flywheel, you need the tire radius, and you need to know the gearing in the trans and rear end.

Since you said power at the rear wheels, then the GTech doesn't need the gear ratios at all. It just needs the acceleration, and it needs to derive the revs of the rear axle (from the tire size).

What's missing from the above math, and probably from the GTech, is accurate knowledge of ALL the losses. You have an automatic trans, so the losses are probably close to 20% but there's also the loss due to air drag, and that's speed dependent.

Air drag is the one that's missing, IMO, from all of the preceding discussion.

So I'll take a stab at estimating it now...

With your mods, you probably have 300-320 fwhp (that's a pure guess btw). 20% losses in the driveline, *if* you were on a chassis dyno, would give you 240 hp at the rear wheels.

But the chassis dyno has *no* aerodynamic drag from the car moving at speed. In 2nd gear you are probably going close to 75 mph (guess) when the engine is around 5000 rpm (another guess for where peak power occurs), so the power loss is around:

AirDragPower = 0.5*Cd*row*Area*V^3

take

Cd = 0.35 (guess)
Area = 72*51*0.85 sq in = 2653 sq inches (72" wide, 51" tall, 85% filled)
= 1.711 sq meters (also a guess)
row = 1.2 kg/(m^3) air density (not a guess)
V = 75 mph = 110 ft/sec = 33.5 m/sec (guess)

AirDragPower = 0.5*1.2*1.711*(33.5^3)
= 38.7 kWatts

Now using conversion: 1 kWatt = 0.745 hp,

So AirDragPower = 29 hp

So if you measured 240 rwhp on the *chassis* dyno, you would measure 240-28.8 on a *roaddyno* (the GTech), or 211.1 rwhp which is close enough to the GTech's peak number of 207 rwhp that you reported.

So the 207 GTech rwhp number does, in fact, map to a fwhp of around 300+, based on my math and IF the assumptions I made in all the above steps were legit.

BTW, I didn't munge with the math to get an agreement that close --- what you see in the above is my first shot at it. 300 to 320 fwhp seemed reasonable if you sometimes beat an LS1 Fcar, so I think all of the above is sensible.

FYI, HTH. - Ken


[This message has been edited by kdrolt (edited August 08, 2001).]

 

That last explanation seems very interesting, I totally forgot about the air drag, and a lot of things could factor into that that i wasn't thinking of. I guess drag and power loss are the biggest things that are unknown. It makes much more sense now. I have no idea about the math you did, I'll take your word for it, but it sounds right to me. Thanks very much.

------------------
87 GTA 350 L98
K&N filter, beefed up TH-700r4, Accel Manifold, Superram, Fuel Press. Regulator 49.5psi, air foil, modified MAF, SLP cold air box, Accel 24lb Injectors, Ed Wright Fastchip custom chip w/160 thermostat, MSD 6AL and Blaster 3, B&M shift kit & megashifter, Moser Ford 9in Rear with Richmond 3.50 and locker, Edlebrock TES headers and Cat Back,Gutted cat, removed emissions, Hotchkis setup in rear, Poly suspension and Koni in the front, relocated MAT, bypassed TB coolant

 

How can you run with or slightly quicker than a car that runs 7 mph faster than you in the quater. A g-tech will not give you extremly accurate performance numbers. I think they are best for comparison purposes only. To get a good estimation of engine horsepower you need to get on a dyno. Likewise a G-tech quarter mile time is not as accurate as a real strip.

------------------
82 z28 350cid, vortec heads, comp 262h cam, Holley 600cfm carb, 2in twice pipes, MSD ignition, turbo 350 trans, 3.73 posi, manly b&m megashifter

 

Actually guys think about it, a GTECH 1/4 time is much more accurate, cause it reads the real end trap speed, as where at the track they simply take an average of the last 100ft. As for how I stay with him, I don't know. He seems to run about 13.50 and I usually run about 13.80 but he runs 4-5 mph quicker. I guess it comes from shifting and tire spin. He usually spins more at the line, as where I have a shift kit. I lose a good amount of grip in second though, where he loses none. It might just be driving that is keeping us close, cause I know his car has more power than mine. I am just asking how can my car goes as fast as it usually does with only 209rwhp. I think the aerodynamic drag plays a big part though, especially since I have a hood that is not shut all the way and a spring that is not set right right now keeping one side a touch higher. To me, I think I should be reading about 250rwhp, but who knows. I guess the only real way to know would be to use a chassis dyno. Thanks.

------------------
87 GTA 350 L98
K&N filter, beefed up TH-700r4, Accel Manifold, Superram, Fuel Press. Regulator 49.5psi, air foil, modified MAF, SLP cold air box, Accel 24lb Injectors, Ed Wright Fastchip custom chip w/160 thermostat, MSD 6AL and Blaster 3, B&M shift kit & megashifter, Moser Ford 9in Rear with Richmond 3.50 and locker, Edlebrock TES headers and Cat Back,Gutted cat, removed emissions, Hotchkis setup in rear, Poly suspension and Koni in the front, relocated MAT, bypassed TB coolant

 

The track gives you an avg speed over the trap distance (100 ft), whereas the GTech updates the instantaneous speed at every sample. That doesn't mean the GTech doesn't have error in it btw; I won't post the math, but small errors can accumulate so the GTech isn't perfect.

What the GTech is very good for is measuring acceleration, and if you know the mass of the car+driver, then you do have an excellent way to measure power delivered to the road. So I think the 207 rwhp you got from the GTech is completely sensible.



Is he stock, or moddified? If he is stock, then he's got 305 to 320 fwhp, so I would expect the et and trap to be closer if you really have 300 fwhp as I suggested.



It sounds like he could run a better ET if he launched better, but his trap won't change much. You are probably a better driver so that keeps you close.

I agree.

Remember that on a chassis dyno the only losses are due to the drivetrain. On the track (or road), the losses are both drivetrain and aero. Since you did the run in 2nd gear, and since your peak power was made somewhere above 65 mph during that run, then there was significant aero power consumed. If you had done the run in 1st gear, then the drivetrain losses would still be present (and a they'd be a little higher due to 1st gear being more lossy than 2nd), but the aerodrag would have been a LOT less.

Power goes as V^3, so if the shift speeds were around 43 mph and 75 mph respectively (for 1-2 and 2-3), then the aerodrag power would be a factor of (43/75)^3 or 0.188. So using the 29 hp of aerodrag from my preceding example, the loss in 1st gear would be 0.188*29 = 5.5 hp. So the GTech would tell you that you peaked at 207 + (29-5.5) = 230.5 rwhp for a 1st gear run.

So you might see MUCH more power using the GTech in 1st gear than you would in second. It would also be even worse in third, because the aerodrag at 110 mph is MUCH higher than at 75 mph.

So you can either try the chassis dyno, or try three separate runs in 1st, 2nd, and 3rd gear using the GTech and tell us what you get.

I figure (in math not shown here) that you'll get 230 rwhp in 1st, 207 rwhp in 2nd, and 144 rwhp in 3rd and the differences (smaller rwhp with each gear) are all due to the increasing effect of aerodynamic drag. Meanwhile on a chassis dyno you'd get around 240 rwhp assuming that the engine was really making 300 fwhp, assuming 20% loss due to the MD8 700R4 trans. FYI. - Ken


[This message has been edited by kdrolt (edited August 09, 2001).]

 

I thought the GTECH was simply supposed to give you the highest recorded number acheieved?

------------------
87 GTA 350 L98
K&N filter, beefed up TH-700r4, Accel Manifold, Superram, Fuel Press. Regulator 49.5psi, air foil, modified MAF, SLP cold air box, Accel 24lb Injectors, Ed Wright Fastchip custom chip w/160 thermostat, MSD 6AL and Blaster 3, B&M shift kit & megashifter, Moser Ford 9in Rear with Richmond 3.50 and locker, Edlebrock TES headers and Cat Back,Gutted cat, removed emissions, Hotchkis setup in rear, Poly suspension and Koni in the front, relocated MAT, bypassed TB coolant

 

I don't know enough about them to answer that. Does the GTech take data right away (in 1st, then 2nd, then 3rd) and give you the max value, or does it wait after you push GO (which might miss 1st gear) ? I don't know.

What I do know is that aero drag and power both go to zero at 0 mph speed, and that they go as speed^3 (speed cubed), so the faster you go, the more aero loss there will be, and the smaller the number the GTech will indicate.

Check the owner's manual for the GTech and see what it says. - Ken

 

I understand what you are saying about all of this, and thank you, cause I forgot about a lot of these factors. I do agree with you. I am just trying to account for more of the loss along the way. My freinds SS is modified with y-pipe back exhaust, computer reprogram, air intake, throttle body, and a few other goodies I can't think of right now. Stock the car was 327 fwhp. I just find it very odd, with having the correct wieght in the GTECH, that this car would read 339 rwhp and mine only 209rwhp. I suppose it is possible, and I think the only way to be sure is to make sure everything in my car is working right, and compare on a chassis dyno. Afterall what really matters is who gets to the finish line first. The one thing I am curious about, and have seen many different opinions on is the drivetrain loss on these cars, obviously mine is going to be a little more give the larger Ford 9 inch rear. I have now seen everything from 10-30% I was told orginally that manuals were between 15-20 and autos 20-25? Any opinions? Thanks again.

------------------
87 GTA 350 L98
K&N filter, beefed up TH-700r4, Accel Manifold, Superram, Fuel Press. Regulator 49.5psi, air foil, modified MAF, SLP cold air box, Accel 24lb Injectors, Ed Wright Fastchip custom chip w/160 thermostat, MSD 6AL and Blaster 3, B&M shift kit & megashifter, Moser Ford 9in Rear with Richmond 3.50 and locker, Edlebrock TES headers and Cat Back,Gutted cat, removed emissions, Hotchkis setup in rear, Poly suspension and Koni in the front, relocated MAT, bypassed TB coolant

 

I understand completely your wanting to understand what's going on.

BTW, see

http://www.corral.net/projects/subzero/gtech.html

"At this point you can release the switch and the weight will be stored for the run. From
here, proceed as though you were making a 0-60 or ¼ mile run (i.e. set the unit to .00
and press the switch, when the display reads “Go”, proceed to make your run). The G-Tech will display your rear wheel HP in real time as you make your run. After the ¼ mile is finished, your peak HP reading will be displayed"

So that suggests that it will show your peak rwhp in real time and then show the max value when you finish (probably triggered when the meter senses that you've hit the brakes, ie. deccelerate). And I think you had said you did your run in 2nd gear... So it would be worthwhile to watch the GTech display as you run thru 1st, 2nd and 3rd... of course you'll have to drive as the same time so this could be a problem.

You could also run in 1st and right after the 1-2 shift, you could hit the brakes. That would guarantee a 1st gear peak reading.



That's what I thought. He's probably an easy 360+ fwhp, so assuming the cars weigh the same, then he SHOULD beat you and his trap should be higher.

I also found on several web sites that the trap speed at tracks isn't measured consistently. Some tracks use the timing lights separated by 100' and some use 150'. Some tracks put the lights at -50' and +50' around the finish line, while others use -100' and the finish line itself. So track data traps can vary from track to track and most certainly vary as compared to the GTech (which trys to get the trap speed exactly as it thinks you are passing 1320' total distance.)

I also saw that a number of car owners (on the web) have compared the GTech data to the track data and found that the ET is usually (but consistently) longer for the GTech and that's because the GTech starts timing as soon as it senses acceleration; at the track the timer doesn't start until you cross the beam. So the GTech is measuring 1320 + a few feet, if you used it at the track. And the trap speeds of the GTech are always higher than any track result because of the way the track estimates the speed.

That said, the GTech is very repeatable and consistent as compared to track data. And I'll argue that it should also be good, if not great, for estimating power.



You'd have to know about ALL the mods the LS1 owner made. Remember that the LS1 isn't flow challenged at high rpms, while the TPI is (unless you do a few things to it). So the LS1 is easily able to make 400+ fwhp, so I could see it measuring 339 at the rear wheels (using the GTech).

The bottom line is that you are only a few mph and a few tenths off his pace on the track, so whatever he is making for power, you are only a little bit behind him.

If you were precise in your car wgt measurement, and aerodrag does what I know it should do, and if the swag numbers I used were reasonable, then 207 rwhp during a road/track run do equate to 300 hp at the flywheel. And 13.8 @ 105 mph or so with your car shows that it is making excellent power and probably consistent with 300 at the flywheel.

Btw with the several hp calculators on the web I found that a 3550 lb car running 13.8 puts out approx 330 to 340 fwhp (based on ET), and based on the trap speed the fwhp is around 320. And your car makes about what a stock later LS1 Fcar would make, and runs about the same on the track. So the loss game is assuming an appropriate percentage (20% which might be reasonable especially given the 9" Ford rear end) and accounting for the aerodrag.

Based on LS1 model Fcars, pure stock, at the track (tested by the Michigan State Police btw), the manual trans is a little quicker (assuming a good driver) than the automatic, so I'm comfortable with assuming 20% loss for the automatic and 15 to 17% for the stick shift car. The 9" Ford rear is probably a little bit more lossy than the stock Fcar rear end, so that might lower the numbers you are seeing a little.

The other problem to consider is the loss of the converter. There's very little data on the web to help deal with that more precisely.

And you're right -- at the track your car is deleivering performance on-par with a stock LS1 Fcar, so you have to believe that you've got 300+ at the flywheel.

I think your numbers have been reconciled; your friends car's numbers don't make as much sense unless he is a novice driver on the track (i.e. slower than he could be) and the GTech is telling the truth but the driver isn't matching up on the track. HTH. - Ken

 

Most tracks I've run at have about a ten foot area that they use to calculate speed. Thats 1/8mi though. I have also found the coefficient of drag to be between 0.37 and 0.38. In a book of corse. To bad it doesn't list the coefficient of lift. I'd like to know. I'ts supposed to be real good.

 

I think u should try doing a 0-60 when your doing your HP test if u can get grip in first. This is what i do and i get 246 hp when the car was auto and 3770 Ibs with driver.Now whether I beleve that or not is a different story but when your doing the test try for no wheel spin and do it like 10 times to get your number.

This is what i got when i did my test
246*
235
228
246*
235
233
246*

BTW i took Gtech to the track and its 0.12 tenth of and 5 mph fast.

------------------
89 T-Top GTA 350 5-speed (soon to be 6-speed)
(Jet hot coated)1 3/4 SLP headers,flowmaster force 2,no cat's,SLP cold air,MSD 6al,jet chip stage 2,march pulleys,ported plenum,air foil,B&M ripper shifter,180 stat,alston SFC,spohn LCA relocation brackets,free mods.

G-Tech times
0-60 5.2s
E.T. 13.91@104.8MPH (2300ft)

 

So 13.8 @ 104 w/GTech is 14.0 @ 99 mph per track results, approximately.

I took the 14.0 sec and 3770 lbs and used the rwhp calc on http://www.gordon-glasgow.org/hpcalc.html
and it gave 278.5 rwhp

Assuming that the aero drag power math I did previously is applicable here, you lose approx 29 hp due to air drag in 2nd gear near 75 mph, so the power to the road (what the GTech would give you) would be 278.5-29 = 248.5 rwhp, and that's pretty much what you got.

Actually the air density at 2300' is less than at sea level so the air drag power will be a bit less than 29 hp @ 75 mph, but not by a lot.

In the same vein, I didn't correct for the 2300' altitude for any of the track data because the GTech doesn't do that, and you weren't trying to correct your track data for altitude either.

278 rwhp / 0.85 = 328 fwhp, using 15% loss for the manual trans. FYI. - Ken

 

well first I would like to thank everyone for their educated responses. However, it would seem that we are getting no where really fast in terms of accuracy. It would seem that there are a lot of factors that could yield a lower Hp reading on the GTECH and I am perfectly comfortable with this, but now we have a more important question, DRIVELINE POWER LOSS. Everyone has been putting up their estimates anywhere between 10-30%. The big question is, is there anyone out there who has had their car on a chassis dyno with just stock equipment. The only example I have is of the previously mentioned Crank and Chrome episode where they dynoed a 2000 stock vette for a baseline. We know this car had 345 fwhp and it came out 265rwhp. This is a 25.6% loss. But while this is the same transmission the rear end is different. And I am sure the overall quality of parts and labor has gotten better in the last 15 years. So my question now is who out there has had a chassis dyno, what were your results, and what was done to your car? Thanks very much for everyones help on this rather interesting, but hard to decided topic.

 

The right way to do this is to measure the engine on an engine dyno. Then run the same engine through the actual driveline on a chassis dyno. This will work for both a manual trans car, and an automatic trans car. The problem is that, with the mfgs as exceptions, few people would go to this trouble and finding the data on the web is even more challenging.

I can provide at least one example, though I don't know that it will make you happy.

See http://www.radix.net/~sdave/cobra-dyno.htm

where a chassis dyno test was run on a 5-spd 97 Ford Cobra. The car made 256.6 rwhp and 259.9 rw lbft torque, and 3rd gear was used.

THEN THE DYNO OPERATOR DID A COASTDOWN TEST to measure the driveline loss. It was not stated in the web site, but the correct way to do this is to leave the trans in the gear the dyno run was done in, as well as leaving the clutch pushed IN so the engine is disconnected from the trans/rear end. The test should start at the max rolling speed of the gear (and max rpm used in theat gear) and then the car is allowed to "coast" in that gear until the operator decides to end the test (when the roller speed, in mph, is lower than what you would ever use for the gear you are in).

Doing this will measure the coastdown drag vs time for the entire driveline in the gear chosen, and hence the frictional driveline loss in terms of hp may be MEASURED.

For that test, 3rd gear on a Ford (Tremec?) 5 spd manual, the max hp loss was 38.4 hp, which I will assume occured at the maximum speed of the coastdown run (say equivalent to 120 mph), and that the frictional driveline loss would have more-or-less been close to a linear decrease in driveline power as the coastdown speed decreased.

So the Cobra engine was really making approx 38.4 + 256.6 = 295 fw hp. NOTE: this assumes the rpm and roller speed on the dyno for peak rwhp was the same as the for the max loss of 38.4 hp). If I had the dyno curve and the coastdown curve I could do the precisely. For now we have to make due (unless you want to write to the Ford owner..):

38.4/295 = 0.1301 = 13.01 percent

Btw, the above isn't as good as testing the engine on an engine dyno (by itself) and then running the SAME engine in the car on a chassis dyno, back-to-back, but it's darn close.

The above type of test can also be done on an automatic, but it's not going to be as accurate because the torque converter can't be decoupled from the test like a clutch can.



23.19%

Assuming 345 fwhp is a good number (perfectly tuned engine, we really don't KNOW if it really had 345 hp), and 265 hp was measured at rw, then the loss is 345-265 = 80 hp.

80/345 = 0.2319 or 23.19%

btw, if we had a timeslip for that Vette we could sanity check the fwhp, because running on an accurate track provides a way of indirectly backing out the power and torque.




Actually the 2000 Vette's 4L60E might be a little less lossy than your MD8 700R4, but I'll accept it as a wash. And I agree that the Ford 9" rear will probably more lossy than the IRS of the Vette.

I know that on several Impala SS cars (94-96, iron head LT1s, 260 fwhp, same 4L60E as used on the Vette, and essentially the same as your 700R4) run 215 to 220 rwhp on a chassis dyno. I have the curves buried on my desk, and they are on the web (HAIL web pages IMS). The driveline loss is therefore 40 to 45 hp.

40/260 = 15.4%
45/260 = 17.3%

The writers at GMHTP have also stated, in ref to F and Ycars with LS1s, that the automatic cars are often as quick as the manual cars... even when speed shifting the stick. So the driveline loss in the trans, ASSUMING THE TRANS IS IN GOOD SHAPE, is often close to the loss for the manual.

The above percentages (15 to 17%) for the Impala SS are very close to the measured data I gave for the Ford Cobra test. I know it's apples and oranges, but it the best I could do for now.

HTH. - Ken

 

Ok, here's another example:

http://www.speed-racers.com/dynographs/hptq2.html

The graph shows what I think is a modified LT1 from a Vette with what is probably a manual trans. The data was taken on a chassis dyno and the peak hp is approx 325 rwhp @ 120 mph rolling speed. Also note the coastdown curve, which is shown to be taken in either neutral (so the tranny gear loss is NOT included, but the rear end would be) or in the drive gear. The coastdown test didn't specify whether the clutch was engaged or not, but I think it was engaged and you'll see why in a moment.

The coastdown curve for the drive test gear is approx -140 hp at 120 mph, and it's not quite linear but it's close enough. By 30 mph it's reduced to -20 hp. The drive gear was probably 3rd or 4th gear (i can figure it out, but it's not important here).

So at 120 mph, the speed where the engine power peaks on a chassis dyno, the rwhp was 325 and the driveline loss was 140 hp (indicated as -140 on the graph, and ignoring the transient spike of -160), so the fwhp is approx 325+140 = 465 hp.

The driveline loss, as a percentage is therefore:

140/465 = 30%

30% is huge, so why so much?

Answer: the coastdown test was done with the trans left in the drive gear (the one used for the dyno power run), but the driver for the test didn't keep the clutch disconnected from the engine... so the engine frictional drag is adding to the loss measurement.
Since a manual trans, and gearbox are both designed to spin with only a little friction, then I'd say a significant part of the 30% is from the engine, the piston rings in particular dragging on the cylinder walls. So this is a good example of a dyno test pitfall.

If I mind any better examples, I''l post them. These are all good to understand if you ever plan on a dyno session, espec if you have a manual trans. - Ken

 

I would just like to comment on the above posts. First off I am sorry I was mistaken about the 25.6% I did not calculate it at the moment when I posted this, but much before and I had the wrong number stuck in my head, it certainly is 23.2%. However just a couple of notes. If you say that most 700 cars are only putting out a 17% driveline loss that would mean my car only had approx-250fwhp. My car seems to be fairly well tuned and appears to be running quite well. Stock the car was 225, so how can all of the below mods have only added 25hp. That's just crazy. Before you mentioned that you had calculated a number between 300-320 I believe, and that seems much more reasonable. Just curious. Thanks again, and thanks for taking the time to make these posts. Obviously you have had a lot of experience with testing so I am quite curious to figure it all out. Thanks again.

------------------
87 GTA 350 L98
K&N filter, beefed up TH-700r4, Accel Manifold, Superram, Fuel Press. Regulator 49.5psi, air foil, modified MAF, SLP cold air box, Accel 24lb Injectors, Ed Wright Fastchip custom chip w/160 thermostat, MSD 6AL and Blaster 3, B&M shift kit & megashifter, Moser Ford 9in Rear with Richmond 3.50 and locker, Edlebrock TES headers and Cat Back,Gutted cat, removed emissions, Hotchkis setup in rear, Poly suspension and Koni in the front, relocated MAT, bypassed TB coolant

 

You also have the Ford 9" rear end, and you probably fat tires that stick better, so they add "loss": the price you pay for better traction is increased rolling resistance.



I'd have to scroll waaaay up and reread what I wrote, and then apply the math again. I don't want to do that (too early, not enuf coffee, need to get to work)..

I think the matter is reconciled. Your timeslips are consistent with 300+ hp at the flywheel, and probably higher than that. I also did a swag on the aero loss (which the GTech doesn't correct for) and that came out spot on, that is that GTech rwhp + aero hp loss + estimated typical driveline loss = 300+ fwhp. I also did the aero math for one other Fcar and that was also close enough for back-of-the-envelope engineering estimates.

Your engine might be putting out more like 320+ but there's no way to be sure because we can't precisely estimate your driveline losses unless we do back-to-back tests on a (1) engine dyno and then (2) a chassis dyno.

But getting on a chassis dyno should give you around 237+ hp at the rear wheels, so maybe you should just go to a chassis dyno session, if you think it'll be worth the money to satisfy your curiousity.

But I don't think you'll learn anything new that hasn't been discussed in the above. Your engine makes 300+ fwhp, the driveline eats 15 to 20% (typically) of that, you lose some to aerodynamics, and the rest is used to power the car where the rubber meets the road. And your timeslip(s) are consistent with the power that your car should be making, and they pretty much agree with the GTech numbers (after aero and driveline correction).

I don't think there are any huge mysteries here, except for precisely identifying the drivetrain loss. If the 700R4 tranny is in good shape, it's probably 17% for the driveline with a stock rear end. If the trans innards are worn, it might be 20+%. With the 9" Ford rear end, both numbers could be several percent higher.

FYI, HTH. - Ken

 

i would like to know how anyone of thses guys are running 14s im running 15.6 and 7.1 0-60 with a gta l98 and i have alot of mods maybe they are right but if so why is mine so slow.

 

The only answer I can give is tuning, and maybe there's a problem with mismatching components, can't say without knowing details.

As for flywheel horsepower, why is that the real question? In my opinion, flywheel horsepower is incidental data, and of little real value. Rear wheel horsepower tells you how much go juice you've got. The only value I could potentially see in fwhp is if you were attempting to somehow reduce your drivetrain loss, and to date I've never seen anyone attempting to do that. So, rwhp is matters a whole lot more, and track times are what matter those most, cause all the horsepower in the world is worthless if you can't get it to the ground.

And as for GTech, I'll double check this when I get home tonight, but I believe the manual specifically says Horsepower calculations are only performed up to 60mph. So your peak horsepower will be the peak calculated at or below 60mph. They state this is done specifcally because at that speed aero drag starts becoming a much more significant unknown variable in the equation. So if your car hits peak horsepower in 2nd gear above 60mph, Gtech won't be accurately reporting peak horsepower. That's the inherent weakness of the GTech when it comes to power calculations. At high speeds you have aero drag fudging the results, and at low speeds most people will have traction issues fudging the results. Not to mention there's also the need to accurately input the weight of the vehicle as driven.

 

I have a question, and I hope this doesn't put a dent in a good discussion, but how does G-Tech know when you've reached the end of a quarter mile?

 

The foundation of the G-Tech is an accelerometer. That little thing senses acceleration. The electronics track this acceleration very precisely. From acceleration you can derive speed. From speed you can derive distance. It's all physics and math. Add in weight and you can derive power output.

Velocity = acceleration * time
Distance = velocity * time
Force = mass * acceleration


These calculations are performed hundreds of times a second (well, probably just a hundred times a second) to accurately track all critical data. It's got it's limitations (most of which have been debated in this thread), but it's got it's uses too.

They're working on a new G-Tech comp series that's more advanced. Uses 4 accelerometers to track pitch, yaw, longitudinal, and lateral acceleration for more accuracy and easier use. With DSP it will be able to sense engine speed through the cigarette lighter adapter itself allowing you to get peak torque as well as peak horsepower, AND the RPM those peaks were at. It has a small LCD display, and can interface with a PC to download data to. It's also going to be significantly more money, but it's a cool little gadget nonetheless, and will still have it's uses.