Hey guys, does anyone have or know where to find expected effects of altitude on HP? I am am measuring "Net" HP (includes rolling resistance and air resistance). I am not coming up with decent results from what I expected... Here's what I have:

Gtechpro/RR config: 4100lbs GTO; 2.0 degree pitch (default...working on better numbers for this). I use a second gear run up from around 2000rpm to redline for the measurement.

I get these measurements for HP and TQ:

HP: 221.7 HP @ 5408 RPM
TQ: 259.4 ft-lbs @ 2126 RPM

I have run the numbers on the aerodynamic drag (Drag Cf = .31), and figure at the top end I am using around 20HP to counteract that air resistance. This still stinks.

Rolling resistance and altitude are about all I can think of to get back to a sea level type number.

I realize that this is not a dyno...even those vary a great deal. Just a little depressing to see these numbers when your pretty sure they should be higher. I haven't been to the track yet, however, in trying to get a 1/4 run in (on what I think was fairly flat (hard to find a flat spot in Colorado)) I ran a 14.149@98.75 mph. Considering I had to come out of it for traffic at the 12 second point...I am pretty sure I would have been in the high 13's. I also broke the tires loose a lot going through second...so not an overall great run, but the times at this altitude look about right. Just the HP and TQ numbers are off.

I have attached jpg's of both the HP run and the 1/4 mile run.

Thought's anyone?

Sorry...I could only attach one file at a time...here is the 1/4 mile run... :rolleyes:

The software you see pictured is free off the Gtech site

http://www.gtechprosupport.com/support/pas...ss20install.exe (http://www.gtechprosupport.com/support/pass/2.0.0/GtechPass20install.exe)

If anyone is interested, I will post the two data files for these runs as well....

OK all...this is getting a little windy as I run around the net looking for info. Here's another thought from combining information on HP around the net.

Factory Delivered "SAE" HP 400HP (probably measured without exhaust and accessories being driven (alternator, Air conditioner (which I was not running above), and other stuff)

Estimated 5-10% loss for actuall install in Vehicle with all Accessories = 20 HP (on the low end)

Estimated Loss due to Drive line (this would reach our normal Dyno numbers?) is around 10-18% (?) = 38 HP

So the expected Dyno number would be around 342...although that's using the low end of the loss numbers above.

Now add rolling resistance (what I am looking for), Aerodrag: approx 30HP at 65/70mph and altitude losses (another number I am looking for and you get...

312HP - rolling resistance - altitude loss = < 312 HP...hence my questions <_< .

OK all...here we go again. I found altitude effects are at 3.5% / 1000ft.

So here's the calc at this point for Net HP in Denver CO.

400 SAE
-72 18% for Altitude (took this off the top as I expect this hit first)
-59 18% for drive line losses (combined accessories and driveline figures)
-30 Aero losses (not a percentage, but drag equations)
---------------------------------------------------------------------------------------
239 HP and still missing rolling drag.

The numbers above are making more sense...but did I just explain away my bad feelings over them? :huh: Please let me know your thoughts on what I am finding....shoot down my percentages and/or thought process if you see I have went astray. :type:

well your first initial numbers are off due to your weight. The GTO isn't 4100 LBS, its only 3789 LBs, a good amount of difference.

Secondly unless your running with the AC on, then your not loosing any power from the belts, and thus the only loss is due to the driveline (driveshaft/rear end)

Lastly, areodynamic drag increases the faster you go, you can't say Im using 20hp to fight the air. Its 0HP at 0 mph, but as you move up it takes more power to go faster, at 100MPH it can easily be 20-30.

OK...3789 is curb wieght...I was in the car...fuel (is that part of curb weight or not...couldn't find doco on it. I am hoping to hit the scales to nail this one down.) Since I didn't know...I started with the GVW and worked backwards for four passengers plus some cargo...that's where 4100 can from. 3789 will make the HP numbers go down some more.

I particially agree on the A/C...that is the Major device on the belts, but there are others...and I am not sure you run them when you put the engine on a engine dyno...but maybe your more experienced there and I am wrong. Given your other posts on this BBS...I'll go with you on this one.

I agree entirely...but the HP run I made didn't start at 0mph. It ends at a little over 70mph which is also pretty close to where I see the peak HP readings...this means I AM taking a loss to aero drag at that peak value....still don't know any estimates for rolling drag....

Thanks Nocturn...it was good to have someone try to add some sanity to my ramblings. If you all would like, I'll post actual weight of my Goat when I finally get it...might be at the track next week, I think they have a scale.

Please do, i would be curious to see what it turns out to be with gas and such.

Are you getting these numbers from a dyno or from a radar device?

But you will experiance a decent loss due to altitude that forced induction cars won't get.

I don't know anyone who would race with the AC on, on purpose anyways. Yes the AC does draw power as its another pulley to turn, but that in itself isn't a draw on the engines power untill it connects to the compressor. I don't know on the GTO but on most cars I'v driven you can feel the engine gain some power when you turn the AC off if your trying to pass someone.

So I mean if you DID have the AC on then you can take it into account, but the manufactorer doesn't,

Noturn...The device I am using to get the graphes and numbers is a Gtechpro RR. Neat little thing...if you can get all the numbers set right. Weight is a big one. I started out on the lower end of weight...but got really lousy HP numbers from the software. I figured I must have been incorrect about the weight so I increase it (and started calculating from the top end subtracting out what you might expect in the car (3 people, fuel, "stuff") until I reached 4100. I think I found a scale this morning I will drive over to (once everyone else in the city wakes up).

The device (I think) is really close on times and rpm's (it picks them up through your electrical system)...but the "Net HP and TQ" are really annoyingly low...on any car it runs in. Guy at work has a Mustang GT. His numbers were low also...until he made a 1/4 mile run on a down hill slope, then they beat mine slightly...but he 1/4 mile time was still slower than I was on a flat run...and I had to come out of mine around 11-12 seconds in as I came up on traffic...still ran a 14.1 at 5000ft altitude giving up the last two seconds of accerleration. I feel pretty good about that :) G's were up around .65 for launch...spun my tires WAY too much going in to second. Still...like the time.

OK...Jury is in...Nocturn you were closer to the weight. Scales say I am 3862 lbs with myself and my son in the car and an empty (or nearly so) fuel tank. Subtract our our 250lbs and you would end up with 3612...so the car is lighter than the curb weight listed in the specs. If you would like, I will post my next run from the gtech with this and the pitch corrected (gtech defaults to 2.0)...given my speed was off by 6-7 mph from gearing calculations, I suspect that I will get more accurate results with a 1.5 pitch angle.

If I make it to the track next wednesday I will post those as well...along with how the gtech compares with track times.

Just a thought on weight.

1. The scales you weighed on could be incorrect. If you are using one of those "trucker" scales where the big rigs weigh upwards to 40000 pounds, a 100 pound error in the scales is not that much. But at our weight of less than 3800 pounds, 100 is a larger difference. You might want to find another scale and get a weight, then take the average of the two.

2. I'm not sure how GM gets it's curb weight numbers. But if it is an empty car with full fluids, I would think a full tank of gas would make up that 100+ pounds of weight you seem to be showing short.

GM quotes 3725 pounds on both 04 and 05 GTO's, regardless of transmission. Now I'd be willing to bet that one actually weighs slightly more than the other but not by much. So I'd think a full tank (since you were almost empty) would push your 3612 number right up to that 3725. I may be wrong but doesn't gas weigh about 7 pounds per gallon? If you are 16 gallons low, 16 gal. @ 7 lbs. works out to 102 pounds. (I may be a little low on that 7 pounds per gallon too.) So 3612 plus 102 hits 3714. Just 11 pounds shy of GM's quote on curb weight.

If someone actually knows how GM calculates their curb weights, please share. :)

Very good point Orbit...The full tank would put the number pretty much right on the money. I think the weight may be 8 lbs/Gal...but that number was from my days running a diesel truck (Chevy 3500 Duramax...pulling a full load...a big difference to what I am wheeling around in now.).

The scales were a truckers scale, but it's hard to find anything else. I am hoping the track has a scale when I hit on Wednesday (if the weather holds and I can run away from work early enough to get there).

Hey Orbit, I see from another post that you also used a Gtech. You wouldn't happen to remember your altitude when you did the <12 second runs would you? I think most of my problem is actually altitude and finding a really flat spot in Denver Colorado to do a run...hence my visit to the drag stip soon. Unfortunately our Drag strip here in Denver is 6000 ft (Bandimere). If my information/research above is correct...I am already losing 20% of my HP to Altitude. :(

I am hoping for a cool day to help help out on that note... :)

OK...I was drifting around the net and found a HP calculator that used weight and Trap speed for your 1/4 mile. I have a trap speed of 105 MPH from a run with the Gtech. Using a Drive Line loss of 15% (which seems to be a concensus from what I can find on the internet for the GTO) I got the following:

You Entered:
Vehicle Weight - 3800 (lbs) (should actually be on the low side for when I did my
run
Trap Speed - 105 (mph)
Drivetrain Loss - 15 (%)


Results:
Rear Wheel HP - 303.4
Crankshaft HP - 356.9
Power to Weight (rwhp) - 12.7 (lower is better)
Power to Weight (crank hp) - 10.8 (lower is better)

These numbers are more inline with what every one is reporting for dyno results especially when you take altitude into account and I was around 5500 ft when I ran the test. I don't think these numbers are "Net HP"...I suspect they have backed those out somehow, but the equation is not visable so I cannot say for sure. Next for me is Bandimere raceway (I hope) and results from there...

The site also had ET based calc, but stated that they believed is was not correct...

The site is http://www.smokemup.com/bb/index.php

That looks about right when you have that kind of altitude in the equation.

The Gtech is a useful tool for comparing before and after mods, but overall is not a very accurate device. There are too many variables for which it cannot accurately compensate. They are a lot of fun though.

M Night Blue,
I think you are correct. I believe the Gtech will be fairly accurate with ET's and Trap speeds (especially once I get the pitch corrected...which I can do if I make to the track this wednesday). However, I think the HP/TQ numbers it calculates are a bit questionable...hence the whole conversation above. Maybe it gets better with proper Pitch adjustments and weight inputs (which I was way off when I made the runs above as my weigh-in proved yesterday :))

Hey Orbit, I see from another post that you also used a Gtech. You wouldn't happen to remember your altitude when you did the <12 second runs would you?

I have the old GTech Pro. Unfortunately it doesn't have all the fancy stuff like the newest versions. As for altitude, I'm at approximately 500 ft. That's give or take 100 ft. either way. Heck the highest point in Illinois is just over 1000 feet. :D

The day I made my 12.96 run on the GTech was a cool, dry day. Perfect if you ask me for a good run. I'm sure altitude density of the air that day couldn't have been anything over 1000 ft. There is also the fact that although the stetch of road I made that run on seemed flat, it could have had a slight grade to it. But around here in Illinois that's a fat chance as the land is about as flat as a board.

Hope that helps. :)

I know when I get my dyno's in Denver they have a corretion factor of 1.24. which means we are only getting about 76% of the hp that we do at sealevel.. it only cost 55 bucks for 3 pulls in Denver. I have been there about 6 times. .its worth knowing. .and yeah I figure 18% drivetrain loss.

Cool...Thanks guys. The dyno correction factor is nice to hear...that's a bit higher than the 20% loss I was figuring (3.5% HP loss/1000ft) That might explain the figures I was getting when you put in aero drag and all. Still, I like the numbers from the trap speed times that I found above. :woot:

Orbit, Wish I was down there sometimes...it's a fat chance to find a flat spot here in Colorado...even the eastern plans are rolling and you really don't have flat spots...here in Denver (foot hills) you really don't have much of a chance. :rolleyes: