Louie928

Anthony,
If you were using a Dynapak dyno, it is likely that it wouldn't take the speed (axle RPM) if using 5th gear. Nothing against Dynapak, it's a fine unit. It has a speed limit of around 160 - 170 mph.

heinrich

My point wasn't anything like that ... it was that Keith started high, so the X added power, but the end result was a combination of already-high power and an X to boost that...

928ntslow

Louie, the dyno chart posted on the first page is titled "4th gear #1"...thats why I had mentioned it.

Louie928

Keith,
You are 100% correct. Why did I doubt??? I checked the dyno files and took another screen shot showing three runs. One with the 4th gear run at about 313hp, another in 5th gear that was made within 5 minutes of the 4th gear run. That 5th gear run showed about 310 hp. Both of those were with stock exhaust system. The last run was the 5th gear run we did with the X pipe on. Comparing 5th gear runs shows a solid 20 hp gain for the X pipe. I believe we used the 4th gear run (lower max hp) because it showed the least gain for the X pipe. Not exactly sure though. Things got hectic that day.

On this chart, the brown run is 5th gear with stock exhaust. The green run is 4th gear with stock exhaust. The pink run is 5th gear with the X pipe. Graph smoothing is set to "2" so peak numbers may be slightly different than the other graph with smoothing at 1 (no smoothing).
https://rennlist.com/forums/attachme...1&d=1219094393

Tom. M

Like everyone posted before..we can't take Dynojet numbers we are used to and compare them to what Keith got....best we can do is to say that the X provided a certain number horsepower over stock. Only way to get dynojet numbers is to run on a dynojet...which..up until recently, we have been trying to do. However, there has been a huge increase in the types of dyno's around...and people have been using them and converting...tough to do IMHO.

One of these days, we'll set up another dyno day up here in the PacNW (best place we've had in a while is the place near the Portland OR Int'l airport)....then Keith can get on there and we'll have a nearly direct comparison...

Otherwise..as Louie so eloquently points out...we'll get the "Amsoil cures cancer" argument...

RKD in OKC

Just a question. On my 944 Turbo we did 3rd gear dyno runs because the transmission ratio was closest to 1:1 for least drivetrain loss. Why aren't 928 dyno runs done the same?

Louie928

I don't understand your question, or the point. Do you mean why aren't all 928 dyno runs done in 5th which has 1:1 ratio, or why aren't all 928 dyno runs done in 3rd gear. I don't know why you'd do a 928 dyno run in 3rd, but you sure could do it in 3rd, or any other gear, dependent on the dyno limitation. Dyno runs on auto trans 928s are usually done in 3rd.

RKD in OKC

3rd is as close to 1:1 as the 944 Turbo transmission gets and the speeds are about the same in 3rd on both cars.

Louie928

Ok. I see. Dyno runs in 3rd in a 5 spd 928, although possible, wouldn't be advisable. On an inertia dyno, the 928 has enough torque (supposedly so would the 944 T) so that the dyno run would be over in very few seconds. Probably only 4 or 5. The 944T may not be spooled up fully by the time it was over. Other potential problems due to prolonged full throttle use may not show up. On a Dynapak where the ramp time can be set independent of transmission gear then it would be no problem. However, in the 928 transmission, 3rd gear is the center gear on the shaft. The shaft doesn't have a center bearing and 3rd gear flexes the shaft more than any other gear. Probably no problem with stock engines, but could be a major concern for higher torque engines. Also, you'd have more power loss in 3rd than using 4th or 5th on a 928 with a Dynapak. The 928 and 944T are different cars and no way to do a direct dyno comparison in a single gear.

You can do a dyno run similar to a drag strip run. Start in a low gear and shift up to the next gear at your shift point. Go through the gears to the top. Check time from start to the same top speed finish for each car. The dyno graph would be set up to show time on the X axis and speed on the Y axis. If comparing total performance output of engines in two cars, that is probably the best method. I don't know about other dynos, but mine (Dynocom) has an accessory that does exactly that. It has a drag strip tree that is connected to the dyno data acquisition unit. The tree works as they do on the strip. Timing starts on the green light and ends 1/4 mile later. You can enter vehicle parameters for weight, frontal area, Cd, and the software is supposed to calculate simulated 1/4 mile time and speed as well as outputting the raw performance numbers. Two dynos can be connected over the Internet so that two cars can have a virtual drag race. I don't have that accessory so can't vouch for how well it works.

RKD in OKC

I wasn't trying to compare the cars, just curious because all the Dynojet runs I've done or seen were done in 3rd gear. Makes sense if you are using the runs to tune to pick a gear that takes longer to get more resolution in your data.

When I was tuning my 944 Turbo on the streets doing 3rd gear runs with me and a copilot (600lbs of driver and passenger) we got the 2000rpm to 7000rpm 3rd gear run down from 14 seconds to 7. Was pretty pleased with it.

928ntslow

Hey Tom, didn't you strap your car down at Louie's at one point? What kind of numbers did you come up with? I would expect you had already dyno'd on another machine somewhere else. What was the difference in the numbers? I am sure one of the PacNW guys has been on Louie's and a Dynojet where we can find out what the difference in the numbers are.

Tom. M

Nope..not me. I think the only one who could likely have had done one on Louie's and on a dyno jet would be Jim H, then again, I'm not sure the cars he tested on Louie's were at the same stage engine wise.. That one day we were there I think only Lizard got his on the rollers... Would love to see what mine does on Louies dyno one of these days..I have to add my wide band O2 and get some custom Sharktuning done ..and put on that porkensioner too....but distracted by the track car at the moment.

BTW..I'm not trying to diss your numbers.....I just remember all the fiasco's we had in the past with this dyno and that dyno and people posting numbers all over the place...we went to great lengths during our dyno days to keep everyone on the same page...going as far as keeping the same dyno too..as there are variances between dynojets too (don't even want to go there)......hell...even Kibort was stirring up crap back then too by arguing for non corrected numbers because they represent true numbers... Funny..things don't change much round here LOL.

Louie928

Tom,
C'mon down when you get some time.
AFAIK, without going out to the shop and looking over all who've done dyno runs here, I can offer some apples to Amsoil type comparisons. Heinrich's S4 5 spd with X pipe and some intake mods did 300 rwhp at the Dynojet in Tacoma. My S4 5 spd with the same mods does 295 - 300 rwhp here. Different car, different dyno, but same mods, same output. My GT with some mods did 320 to 327 rwhp on several different Dynojets. Jim Hurtt's GT would do 325 rwhp with the similar mods on my dyno. I put headers on it and couldn't get over 320 rwhp from it. Stock '78/'79 928s do about 165 rwhp, '85/'86 about 240 - 255 unless it's Ken's, stock S4s about 260 - 275 but a lot have some problem with WOT switch, or flappy, not working. I think it measures pretty close to the norm. Dynojet specs are 6% variation between different Dynojets. I think mine falls within that range.

heinrich

Dazz right .... Louie is right .... dynos are calibrated and can be expected to deliver repeatable results and to meet empirical standards ... or otherwise we might as well take a seat-of-pants test instead and save the 80 bucks ....

928ntslow

Well then, I guess the only way we will be able to answer the question of accuracy is to dyno the same car on both machines.

Don't get me wrong, I am well aware that the actual hp and tq numbers don't mean crap. If a lighter 4 door with less power leaves me in a vapor cloud on the freeway, then thats the real truth.

BTW, after reading the 32v cam thread posted by Kibort, it reminded me that I dialed in the cams of the GTS a year or two back. This could account for the higher numbers on my car.