I posted something similar to this over in transmission but the topic moved away from what I was originally looking for. I appreciate the comments and help from people who responded in that post.
I got an email from a TC'er that suggested I do a slip test on my converter/trans using my chassis dyno data.
What I was told to do was to compare RPM and vehicle speed according to the chassis dyno results with real world mathematical "locked" versions.
I discovered a trend that indicated a 19-21% slip over the entire RPM range.
I was told to do a line pressure test, but I can't find a line pressure tester at a store, and my tranny shop told me the guys who work on the weekends are not well qualified enough to work on my car's potential problem.
I called the maker of my torque converter to ask about slip. He said he recalls cutting mine apart (I just sent it in to get checked out) and said my type of converter from his shop should slip at about 5%, maybe a tad more worst case.
I also have a gear vendors overdrive, which I suppose in theory could cause slip, even though it was not being :"used" on the dyno, but power was passing through it. Gear vendors says their units use 1/4hp per 100hp to operate, but that doesn't say anything about slip.
So here I am.. the dyno says I am doing 60mph @ 20% faster RPM than 60mph works out to be using my exact combo and even giving some leeway for converter slip, it still seems excessive.
!) Does anyone think this is a problem as a TCer emailed me and said so, or am I fretting over nothing?
2) Could this cause an unexpected loss in HP/TQ? My engine dyno runs were done in as real as possible operating conditions, including air filter, 88* room temp, my headers, my x pipe, my mufflers, my alternator, my water pump, my tail pipes, my engine wat temp (185*). I lost over 19% of my overall dyno hp, and more than 25% (25% loss of torque at highest pointt, but the curve is relatively flat).

Does anyone think that there could possibly be some sort of slip here that could be affecting power? or am I chasing ghosts?

matt

Marv, About a 20%-22% power loss on the Dynojet is about typical for a Turbo 400 equipped car. As far as trying to relate the attained speed with any degree of accuracy, that too is next to impossible. Any racing torque converter that i've watched run on a chassis dyno so far exibits a great deal of slippage and the speeds shown are about 15 MPH faster than the car actually runs at that RPM at the strip. My son's car showed speeds over 160 on the Dynojet but this owner at least had enough on the ball to know the car was only going about 140. Don't sweat inaccurate speed readings.

Harold,

Thanks for the reply.
But this is where it gets confusing, and this is where it derailed in the tranny forum. I am not talking about power loss. I am talking about vehicle speed and RPM according to the dyno and vehicle speed and RPM according to a pen and pencil, calculator, slide rule, abacus, whatever. 19-21% loss. The RPM and vehicle speed is way off. And I am trying to see if it matters or not.
Does it matter that this dyno run was done with a 12" recon converter that was a loaner to me while my actual one got checked out? I swear this this thing's stall speed was my idle, I had to footbrake the car at stoplights to keep it alive.
Or is this just something to still not worry about?
I was also under the impression that since I had my engine dyno performed with my air filter, 88* room air, my headers, my x, my mufflers, my tailpipes, my water pump, and my alternator, that I would see less than the standard 20-22%, which in my case was 19% and well over 25% for torque.
I am looking forward to a track run in a few weeks when I get the time, a wheel alignment, and move the other half of the car up 1/4" so it is atleast level but still scraping the headers..
I played around with my governor Jakeshoe style and got 6275 RPM when I want to shift at 6100-6200. I may switch to one spring heavier or just leave it alone, not sure yet.

Thanks.

Matt

The vehicle speed part of the Dynojet doesn't seem to be very accurate to real life speed for some unknown reason. My son also dynoed a truck he owns which at the time had a 3.07 geared single track rear end. It went over 150 MPH on the Dyno but in real life topped out at about 122 MPH. The Dynojet speed funtion is so far off as to be unusable. (The truck ran mid 12s @ 107 MPH with a 432 cu.in. B.B. @ 4080lbs. on the motor). (Dyno numbers were 376 H.P. and 535 lb. ft. torque).

I wonder why that is harold.. it should seem like the rollers should know how fast you are going.. but I have also seen dyno sheets from guys whose engines made so much power that the dynojet's RPM readings were off by hundreds of RPM, so there is no way it could calc reliable tq/hp..

The 20% slip thing had me going nuts. What also has me grumbling a bit is my boss's 03 c*bra went to dyno day for mustangs at the local chassis dyno and made just 25 less hp than me with just an exhaust and cold air.. new cars... gosh darn. I'll stay home for when he does the dyno with the high boost pulley.. <grumble>.

Maybe a 454 isn't a big enough big block anymore :D

Matt

Originally posted by Harold Sutton:
My son also dynoed a truck he owns which at the time had a 3.07 geared single track rear end. It went over 150 MPH on the Dyno but in real life topped out at about 122 MPH. Does it take into account aerodynamic drag? Trucks push alot of air...

Silver69, I was only making the observation that the Dynojets speed sensors are way off. My sons car runs 7200 RPM at the 1/4 mile at about 140 MPH but at the same RPM an the Dynojet shows a speed reading of about 160 MPH. The horsepower correction factor is also flawed and they now have their own which differs from the SAE J1349 somewhat which tells me the're trying to correct flaws they know exist in the machines. The speed funtion is junk and the newest Dynojet in town, Modern Performance Automotive, is the only one we've tried that didn't spin on the rollers.

Harold,

While we are on the subject, is there another reliable way I can test for slip of the overall drivetrain? (converter, trans, gear vendors)?
My engine builder insists something is wrong because of how much power I lost going from his engine dyno to a chassis dyno, but he only asserts this because of how he runs his engines on his engine dyno.
My converter shop said that my converter should slip about 5%, which seemed low to me but maybe I got what I paid for, who knows.
I would like to see if there is any other slip "eating" my power in my drivetrain. My speedo is essentially useless because I can be doing 35 or 40 at 2000rpm and you'd never be able to tell the difference with just the tach.

Matt

Ok how about this. Is the speedo properly calibrated? If it is, you should be able to drive the car at different speeds under different loads and see where the rpms of the motor are. Then figure out slipage semi accuratley. For example if your at 2500rpm in 1st gear. Then use the speedo and some math to figure out how fast the motor should be turning at the given speed. If the math says it should be at 2000rpm then there is a 20% slip somewhere. Obviously this depends on stall speed to, just want to make sure your above stall speed, you may have to use 4000rpm or something.

Later
Steve

Thanks Wanarace--

I didn't think of that.. the converter will be plus or minus hydraulically locked after my stall speed-- so wondering why I can go 35-42 mph at 2000rpm (or something like that) will be eliminated if I am on the highway doing 65mph according to my speedo. I can call my town police and ask if they have one of their radar devices set up (the gasp for a few seconds, but then tell you) to see if my speedo seems about right over stall speed (3400-3600).
I just ordered a new speedo gear sleeve, o-ring, and 45 tooth gear from gear vendors becasue mine was originally set up for 3.31's and the diameter of the shaft of the gear is a different size.. what I pain.. all I thought I had to do was change the speedo gear itself (10 min job) but nope.. first I tried drill pressing the speedo gear sleeve, but then I didn't go out far enough and a melted the speedo gear. I was off by just a few thousandths too.. what a pain.
So I called gear vendors, and said I want the right speedo. The last guy at gear vendors said I needed 44 teeth, this guy insisted 45 teeth-- I think he was wrong, because I kept saying 25.6" tall tire and he would go "so 26" (maybe he had to round to the closest thing).
They shipped a 45 tooth gear, new sleeve, had to find an o-ring myself (pain), after I told them I use a 3.90 rear gear and 25.6" tall tires, p315/35R17. the car looks alot worse with the back up about 3/4 of an inch, but I had to because I am oving the front up the same amount.. it looked cool low, but I couldn't drive it anywhere. One tube on my hookers is so flat I am amazed it is not plain rane ripped open.
if it rips, I may consider 1 7/8s or 1 3/4's, but there will be a big discussion before that.

Sorry for the insanely long posts.. its a dreary day outside today so I can't do anything other than wash clothes and type..

matt

I think the convertor guy and chassis/engine dyno guy are just full of crap..especially the dyno guy :D

We run a Dynojet 248 in our shop and have never had a problem with the speed calculations. You don't want to use the dyno to calibrate a converter or the mph/rpm on a car with a fluid coupling. The problem arises from the slip in the converter relating to the load at a particular speed. The rollers represent a constant load, while the car at the strip represents an ever increasing load. The 5400# of the rollers will typically raise the flash stall of a converter by 500+ rpm, and increase the apparent mph due to the reduced load at higher speeds. To demonstrate the loading vs torque effect take a car with nitrous and overlay the plots of speed vs rpm bwtween a nitrous pass and a n/a pass. The nitrous run will show a considerably higher slippage than the regular run, and a higher rpm for every speed.
With a manual transmission or locking converter you can easily test the mph/rpm at very high speeds. We tested a truck built for Nevada Open Road Challenge and were able to reach 191 mph on the dyno in 6th gear at a mere 3800 rpm. In reality the truck could never exceed 135-140 mph on the road due to wind resistance.

MadMarv, Most high stall torque converters slip more on the order of 8-10 percent no matter what the T.C. companies tell you. I think they don't want to tell the truth about slippage. That being said, the dynojets speed sensors are JUNK. A high powered car which is spinning on the rollers should read low if anything but just the opposite happens. I also know their power readings are slightly high hense their changing of the weather corrections factors from J1349. When we took my sons car the last time i noticed that the power corrected the wrong way and the operator didn't know what the actual spec. was so i printed out a copy of the standard correction factors and gave it to him. He was using some type correction sent to him by Dynojet Company and it differed from the SAE Standard. (SAE J1349-- 60 degrees, 29.92 barometeric pressure, and zero humidity constitutes a standard day).

MadMarv, Most high stall torque converters slip more on the order of 8-10 percent no matter what the T.C. companies tell you. I think they don't want to tell the truth about slippage. That being said, the dynojets speed sensors are JUNK. A high powered car which is spinning on the rollers should read low if anything but just the opposite happens. I also know their power readings are slightly high hense their changing of the weather corrections factors from J1349. When we took my sons car the last time i noticed that the power corrected the wrong way and the operator didn't know what the actual spec. was so i printed out a copy of the standard correction factors and gave it to him. He was using some type correction sent to him by Dynojet Company and it differed from the SAE Standard. (SAE J1349-- 60 degrees, 29.92 barometeric pressure, and zero humidity constitutes a standard day).

Sorry about the double post.

8-10% is very common in a good converter, that is true. But I have seen some over 12% too. In a street converter there are some tricks to allow you to have higher stall speed and retain efficiency. The tip is to go to a smaller diameter converter, and build it with a more-than-normal positive fin angle. This will give you the stall speed of a larger diameter converter, but with better efficiency since most street converters use a negative fin angle which is not condusive to efficiency. I have had many BTE converters done for guys doing this trick, and one popular deal is a 10" that stalls maybe to 3K. It is much better than a looser 11" with the same stall speed.

oneofakind67, You make good valid points about the differences of speed vs. RPM in an automatic transmission with a torque converter. The motor will overpower the converter at some point and N2O will make the converter appear looser.

I can fully understand 8-10% on the converter, but actuallty I was told to specifically ask for the 9.5" with the positive fin angle or whatever to keep it a little more efficent, my engine builder said all I needed was it loose down low to keep it from killing the trans or shifting or whatever he said.
But I can't understand the 20%. I didn't run the dyno with the converter I have now-- which is my 9.5" unit. I was using some junkyard loaner converter to get the car moving, but according to my engine builder the thing would read higher #'s because it the big standard converter is more efficent.
Maybe I ought to go back to the dyno.
I mean somewhere along the line I am losing 10%, unless I am reading the above posts wrong. Say my converter slips 10%.
Why am I getting 19-21% (RPM slip-- power loss is more like 25% across the board even though I engine dyno'd with air filter, 88* air, my headers, my xpipe, my mufflers, my exhaust, my alternator and my water pump-- I thought I'd lose less than 25% since most shops don't dyno like that)slip out of an engine that made 565hp engine, 440hp chassis. My boss's 03 cobra with $800 in mods made the same power I wanted to get sick. Its not a competetion, its just i spend so much time working on this car mechanically fixing this and tweaking that and then if I don't want to get laughed at I have to wash it before I drive it yada ya..
If it was off by 10%, I wouldn't have really said anything.
But 20%? I find that hard to believe.
I was told I need to get a line pressure test before continuing. Make sense to anyone?
I don't know if I said this, but I brought the car down to the trans shop on a saturday for a line pressure test and the owner told me all the technicians who work on saturdays aren't skilled enough to work on my car and I should come back during the week when the master builders are there.
I'm not using my speedo on the dyno, its all dyno chart vs. calculator math, if I was confusing anyone.

Matt

A high powered car which is spinning on the rollers should read low if anything but just the opposite happens.

Hafold,

Spinning on the rollers? Do you mean slipping? We've never had any problems with tires slipping on the rollers, even near the limits of the dyno. How does the Dynojet measure speed? How would you arrive at a different and more accurate speed measurement than the one displayed by the dyno? How do you know which is right and which is not? I'm sure I can get the Dynojet rep out to fix the problem once you identify it.

I just want to know if this is something I should look into or not. I have seen other dyno runs with stick cars on the exact same dyno I used and their slip numbers were almost spot-on dead accurate.
I assume the dynojet has to measure vehicle speed accurately.. I don't see how it can't.
But a 20% RPM slip seems excessive, and I am not sure what to do. I was told line pressure test, #1. I'm running my old 3400-3600 stall precision industries unit again, the car wasn't dyno'd using that converter.
I know converters screw around with dyno's alot, but you would expect to see, if I were to redyno my vehicle with the current converter and still see 20% RPM/Speed difference should I be concerned? Think there is a chance it will come out to be 10% and I will go home happy?
Should I do a line pressure test first?
I really doubt my converter is screwing up 20%, so something must be going on behind it.
Or not? Does it matter? Any experts care to chime in? I am pulling my hair out. I know I keep repeating the same dribble but the frustration of the car I should put it up on blocks and wait 30 years before starting it again..
And anyone ever notice show winners don't seem to have inspection stickers? grumble

Matt

Matt,just go to the track and stop chasing these "number ghosts" and make sure you run with real tires too-none of this street tire crap either :D
Pay no attention to the dyno slips..they are what they are,you have the numbers-move on.
What you need to do is focus on your 60 ft.
What was your best 60 ft. ever? A 1.85?
You need to get into the high 1.6X/low 1.7X range,and you will be in the 11's..way into the 11's

No cage Chris...

So there is no point in stickys.

I won't gut and weld DOM tubing into a numbers matching LS5 so I can stick some real slicks on there. The last engine combo had guts for 11's with some 28 or 29" slicks.
But since I can't cage it, I want all out mph..
In a few years when I can afford another car, I'll get one I can cut up..
But obviously I'm a little ticked cus I spent a couple thousand for a cam swap and it if I get a 2mph increase well..
I had 11's painted all over me last year, but its pointless when you can't run the safety equipment. A set of 28*11.5 ET streets would put that car right into the low 1.7 or maybe 1.6 range without alot of practice...
So if I want MPH, I want to know where my power went.

Matt

Matt,just run the car,and see what it does..my old 442 W30 clone ran an 11.96/113 w/3.31 gears last year at LVD,and no got kicked off the track..it only did it once,as the other runs where 12.03-12.10 zone..1.66-1.70 60 ft.'s too..was running with 26" ET drags.
Now I understand your MPH deal smile.gif but regardless,just go and have fun graemlins/thumbsup.gif
Maybe you will be suprised by your MPH (in a good way)

Since your power loss of 19% is almost spot-on for what everyone else sees on engine dyno-versus-chassis dyno numbers, I would say your speed slippage is right in line with everyone else. If you had an extra, unaccounted for 10% or so in speed slippage, I would say that your power loss would also be an extra 10%, or 30% total for engine-versus-chassis. And that is not what you have measured. I wouldn't worry.

I'm looking forward to the track soon. Seeing as I'd trailer to LVD (I think the car gets gallons per mile), I might put the 26*9.5 ET Streets on instead, especially with my old rear springs installed, and run them with 18psi, (I ran 16psi last year, but I think I am going to run on a full gas tank this year). That would put my car at about 4130lbs with me I think. I don't have a DD program but "calculators" (yeah I know just go to the track already :D .. trust me, I'm working my way there!) say anywhere from 111 to 120 mph.. lol. If there is a nice saturday its LVD, if there is a nice wed and everything is ok by then (ride height, wheel alignment, etc) then NED, but I have NEVER been able to get a sub 2.0 60' there.. and I have been there 20x as often as LVD.
I have a few quirks to work out on the car, (ride height adjustment, wheel alignment, safety inspection), but I expect to get down to the track between the 13-22nd.
I am putting mph at 115-119, 115 I would be pretty disappointed though.
I am looking forward to going.

As for the power loss/rpm loss being the same, while it doesn't seem like it matters, it sort of makes sense.
I am raising the car to make it safer for the headers on the road, even though it will make the car look worse.

Thanks for all the positive statements everyone.

Matt

oneofakind67, What i'm refering to is the car reacts differently to sudden application of power on the Dynojets rollers than at the track which i understand because the instant center is changed and the rear end rises instead of squating under load. The only visable tire problem occured at Street Car Fall Nationals at Springfield,Mo. The pull started at about 3500 RPM and the motor overpowered the E.T. Streets and a visable trail of blue smoke came off the top and bottom of the passenger side tire when the reading exceeded 700 lb.ft. of RWTQ. I then noticed the car wasn't exactly straight on the rollers. On the next pull my son suggested they take it above the converter stall speed and it came back with a reading of 636 H.P. and about 685 ft. lb. of torque. The N20 was done at too low a speed and went straight to the rev. limiter with a mid 840 H.P. reading with much blue smoke from both tires. They then went to a 5000 RPM starting point on the final pull and got 888 H.P. but seems like only 715 Torque or thereabouts. You are totally correct about the torque converter slipping more with the nitrous engaged and this car only gains a couple of tenths when the juice is on (175 hit). Drives rite thru the converter. All other Dynojet pulls have come back at about 646 H.P. and 700 lb. ft. of tq.