Chassis dyno graph

[furrystump]

Ok, ran the car on chassis dyno and here’s the graph. The small improvement was from drilling out the accel pump nozzle just a touch. The combo is

I think mine is a 78 360 block bored to .040. With Icon forged flat top pistons .010 in the hole. Scat h beam rods and Scat 4” cast crank. The heads are x heads ported by Dwayne Porter of Porter Racing Heads. Flow about 280 cfm intake. The cam is an Engle flat tappet intake .557 lift 245 degs at .050 lift. Exh is .551 lift with 261 degs at .050 lift. 112 degs lobe seperation. Intake is stock 68-69 340 manifold with the divider cut down and smoothed out a bit. Stock hipo 340 exh manifolds. Eddy 800 cfm AVS thunder series. Two fuel pumps. A big Carter mechanical and a Holley red pump back at the tank with Holley hydra mat inside. Mallory billet distributor and MSD 6al. Cope Racing trans 727 auto shift with a PTC 9.5 “ converter. 4:10 rear gear in an 8 3/4”. TTI 2.5” exhaust. Original 340 front torsion bars with QA1 R’s on the front and 1” over arched leafs on the rear with lame KYB shocks. Car is original 68 Barracuda 340-s Fastback, weighs 3445 lbs with me in it. Tires are f70-14 polyglas redlines. Best run is 1.71 60’ 11.83 at 116. Shifting at 6300 rpm.

It seems to me that it is too “smooth” for lack of a better word, like it’s hitting the limit of a part and saying “you shall not pass” at 6500 it started to touch the rev limiter.

 

[PRH]

Looks good to me.

Super Flow chassis dynos can be either load cell(acceleration), or inertia, or both.
Do you know what type of test that was?

What’s the flash stall of the converter?

What’s that...... 335-340hp?

Power hangs on really well.

 

[furrystump]

It’s a superflow, Dual eddy flow? My guess that is load cell. I don’t know the flash rpm. It’s got to be over 3400-3500 rpm. So the peak might be influenced by the flash rpm. Honestly I have tried to see what it just stalls at and no matter how much brake pressure I apply it will start to spin the tire I could put a bigger tire on it. I’m sure I could tighten up the rear brakes, but never bothered. 343 hp. I have zero experience with dyno graphs, The graph just looks straight no curve. Sorry, I have no idea in which mode it was done.

 

[furrystump]

Data.

 

[BigBlockMopar]

Stock intake & exhaust getting restrictive?

 

[PRH]

Looks to me like the power is hanging on great until the rev limiter kicks in.

 

[BigBlockMopar]

Ok. I would've expected a more parabolic HP-curve, instead of the almost 90° angle between 4500-5000.

 

[PRH]

My guess is, data was being recorded prior to the throttle being wide open.

I’m basically disregarding everything below 4500 rpm or so.

On an engine dyno, you get the motor stabilized at WOT prior to making a pull, so all the data is at WOT.

On the wheel style of chassis dyno, traction on the rollers can be an issue, so often times the bottom of the curve isn’t representative of the actual power output, since that part of the curve isn’t always at WOT.

With an eddy current absorber, provided traction wasn’t an issue, it should have been possible to have the engine stabilized at WOT, at the stall speed to the pull being started.
However........ if the roller/wheel speed was well below what would be correct for whatever that stall speed was....... when the pull was started, you’d see a huge spike at the beginning of the curve from the large amount of TQ multiplication that’s happening from the crank/trans input shaft speed differential.

It’s always important to understand that the dyno graphs between a chassis dyno vs an engine dyno can have vastly different shaped curves...... especially if there is a fairly high stall converter involved.

 

[dano]

Ask the operator what parameters were set. A lot of dyno's are not set to read until 3k or 3.5k. It could have been set to cap off at 6.5k. Did you discuss with the operator your engine capable RPM range?

 

[PRH]

at 6500 it started to touch the rev limiter.

 

[PRH]

If you had a 3500 stall, and had enough traction to be able to have the throttle wide open and the motor stabilized against the converter........ and did two pulls starting at different wheel speeds, it would change the shape of the curve.
A pull started off at 20mph or 60mph would yield different results, because even though both pulls were started with the motor loaded against the converter at 3500rpm....... the test starting at 20mph would have quite a bit more TQ multiplication at the beginning of the pull than the test started at 60mph, which would have hardly any TQ multiplication.

Chassis dyno results are much less “cut and dried” compared to engine dyno results imo.
The “car” adds a whole ‘nuther set of variables.

 

[PRH]

Rhett, this is how I test for stall speed.

If your car still shifts automatically and uses the kick down linkage, disconnect it for the test.

Drive slowly and let the trans shift into high gear.
Go along slowly at around 1000rpm........push the throttle to the floor and observe what rpm the tac needle jumps to and first stabilizes at.
That’s the flash stall speed for that converter behind your motor.

 

[furrystump]

Dwayne, I will test for the flash rpm. Thanks, It was really just a “get a bunch of cars together and throw them on the dyno” day. It is an auto shift trans, so the only real answer I have is each run started at about 60mph in drive. That is the speed that the trans would not downshift at. It was not noticeably spinning the tire.