Dyno Miata/SuperTrapp Test

[Mako Koiwai]

Thanks to Larry Chen's "Cheap" ($40 for 3 pulls) DynoDay at HPS (http://www.hiprospeed.com" onclick="window.open(this.href);return false;) we were able to test our Miata's exhaust system. We did a pull without any SuperTrapp Discs, with 14 discs and then 8 discs. Our Exhaust System consists of a Racing Beat Exhaust Header, a Hi-Flo CAT, Resonator and then a SuperTrapp muffler, which allows one to control the back pressure through the use of a variable amount of their Discs. We also have a Racing Beat air intake.

video clip: http://www.vimeo.com/6571752

[Mako Koiwai]

Before I arrived, another fellow with a SuperTrapp had been tested, and they recorded a loss of horsepower.

On our first pull, no discs, we saw 102.9 hp and 99.6 ft lbs of torque. Doing the SAE correction adds about 5 to 6 hp.

For the 2nd pull I added 14 discs, and we recorded 103.5 HP and 100.2 ft. lbs of torque.

I went down to 8 discs ... which is what SuperTrapp had recommended to me, for our Miata, to make maximum torque. Our HP dropped to 102.9, but our peak torque went up to 102.2 ft. lbs. A NICE INCREASE for our little Miata! I had immediately noticed that the car was much more tractable around town when I added the SuperTrapp arrangement. Before that the engine definitely felt flatter. For our fairly fast AX's lately, I had been using around 11 discs ... which is probably a good compromise between HP and Torque. For a tight course we could go to 8 discs.

A lot of people had advised us that the least amount of back pressure was always desirable. John Edwards was the first to say No ... engines require between three and six pounds of back pressure to be at their best. Typically CATS and Mufflers provide this back pressure. I guess our single Hi Flo CAT didn't quite provide enough back pressure. Of course tuning is always best, but we can't change our timing without going with a different/piggyback ecu.

The fellows running the dyno had never seen an increase in HP/Torque by adding back pressure.

[Mako Koiwai]

Dynapack dyno graphs ...

[Mako Koiwai]

Dynapack suggests that dividing the HP readings by about 0.85 will give one a rough approximation of crank HP with our Miata. So were at around 120 hp ... I'm guessing fairly typical for our car, which has over 100K miles on it. It would be interesting to get Michael H's, Tim Albin's and Ken Lord's Miata's on this dyno at the next Chen Dyno Day ... which he hopes to schedule next month.

[Mako Koiwai]

HPS emailed me the readouts ... SAE corrected ... Overall:

RED = Open Exhaust (hi flo CAT) Green = 14 Discs Blue = 8 Discs

[Mako Koiwai]

Pull 1 ... open exhaust:

[Mako Koiwai]

Pull 2 ... 14 Discs:

[Mako Koiwai]

Pull 3 ... 8 Discs:

[Will Kalman]

I wonder how much of that torque increase is due to resonant tuning as opposed to backpressure. I've yet to be convinced that back pressure itself is entirely responsible for this effect (note the careful wording there). It would be nice to install a backpressure sensor to see if that's what you actually did increase. Remember that a supertrapp ends with a "wall" that would reflect a wave back down the pipe and that the number of disks will directly influence the strength of that reflected pressure wave.

Personally, I'd take 4', 2' and 1' and 6" pieces of slip-fit exhaust piping which will allow you to try many pipe lengths (From zero to 7.5' lengths in 6" increments) and you can "binary tree" your way to the optimal pipe length (with or without muffler, etc). Your high-flow cat may be beneficial because it will allow more effective resonant tuning since those reverse pressure waves can make it back to the exhaust ports where they can make a difference.

OK, I suppose now it's Marshall's turn to chime in ;)

[Marshall Grice]

OK, I suppose now it's Marshall's turn to chime in ;)

i'm not really an engine guy...sorry.

[Mako Koiwai]

All I know is that John Edwards looked it up (the 3 to 6 lbs of back pressure) in one of his books ... and said to stop by anytime to check it out. As a reference book, it has to be used at the facility.

... of course most pure racing engines have straight through pipes ... but perhaps that's because it's typically all about HP and not Torque ? What about motorcycle (and go-kart) engines with those chambered exhausts. Aren't those all about resonance ?

[Jason Isley BS RX8]

Before I arrived, another fellow with a SuperTrapp had been tested, and they recorded a loss of horsepower.

On our first pull, no discs, we saw 102.9 hp and 99.6 ft lbs of torque. Doing the SAE correction adds about 5 to 6 hp.

For the 2nd pull I added 14 discs, and we recorded 103.5 HP and 100.2 ft. lbs of torque.

I went down to 8 discs ... which is what SuperTrapp had recommended to me, for our Miata, to make maximum torque. Our HP dropped to 102.9, but our peak torque went up to 102.2 ft. lbs. A NICE INCREASE for our little Miata! I had immediately noticed that the car was much more tractable around town when I added the SuperTrapp arrangement. Before that the engine definitely felt flatter. For our fairly fast AX's lately, I had been using around 11 discs ... which is probably a good compromise between HP and Torque. For a tight course we could go to 8 discs.

A lot of people had advised us that the least amount of back pressure was always desirable. John Edwards was the first to say No ... engines require between three and six pounds of back pressure to be at their best. Typically CATS and Mufflers provide this back pressure. I guess our single Hi Flo CAT didn't quite provide enough back pressure. Of course tuning is always best, but we can't change our timing without going with a different/piggyback ecu.

The fellows running the dyno had never seen an increase in HP/Torque by adding back pressure.

You will see variations of that magnitude from run to run with no changes. Three runs is a baseline. It is not sufficient enough to derive any useful data from when the car is changed each run.

[Mako Koiwai]

The Dynapack IS considered to be fairly accurate and repeatable. There are no tire variables ... we did the usual to keep the running cool. Yes, the changes are small ... but they do seem consist with what "we" were expecting. The other fellow with the SuperTrapp DID get lower readings ...

We can pick an "independent" representative at the next event to try the Miata out with and with out Discs to experience the too me very obvious butt dyno test.

[Jason Isley BS RX8]

The Dynapack IS considered to be fairly accurate and repeatable. There are no tire variables ... we did the usual to keep the running cool. Yes, the changes are small ... but they do seem consist with what "we" were expecting. The other fellow with the SuperTrapp DID get lower readings ...

We can pick an "independent" representative at the next event to try the Miata out with and with out Discs to experience the too me very obvious butt dyno test.

I spent more than two hours with my car on a Dynapak getting tuned. No less than three runs were done to get a baseline, and they were not the same. Every time a change was made multiple pulls were done, and variations in your range were recored from one run to another. I would only accept numbers with that small of a change from an engine dyno. One run does not get it done.

[Steve Ekstrand]

Tune the motor for the reduced back-pressure and you'll probably see gains. Though those gains might be higher in the rev range.

Is the LCA the best place for the remote reservoir? I see how it keeps from flexing the hose that way, and keeps it short, but it also adds a little weight to the suspension.


If you are able to keep the intake charge temps consistent, then often you'll see a small climb in power. As piston tops and combustion chambers increase in temp, you actually can make MORE power. What usually cancels this out is that the intake charge also gets warmer (and less dense) which reduces power. So, your efforts to keep running cool very well may have only affected the intake temps explaining the climb in power. Though as Jason points out, its probably just "in the noise" from a data standpoint.

[Steve Ekstrand]

Oh.....


And with Drag motors... You have to build several identical motors, because you'll wear out the motor on the dyno making baseline runs and then multiple runs on different changes... And that's 100's of single changes.... For hours upon hours upon hours upon hours.....

[Bob Beamesderfer]

FYI, engine torque is lb.-ft. not ft.-lb.

[Greg Peng]

It's the same thing.

[Bob Beamesderfer]

It's the same thing.

Not according to Road & Track's former tech editor.

[Jason Isley BS RX8]

It's the same thing.

Not according to Road & Track's former tech editor.

Oh, well if he says it. Everyone please make sure to consult your AP style guide before making any further posts on this forum.

[Bob Beamesderfer]

It's the same thing.

Not according to Road & Track's former tech editor.

Oh, well if he says it. Everyone please make sure to consult your AP style guide before making any further posts on this forum.

thhhppppppp! Go back to bed Isley. :mrt:

John Dinkel

John Dinkel began his automotive career in 1967 with Chrysler Corporation after earning two engineering degrees from the University of Michigan. Three years later he joined Car Life as Engineering Editor, followed by an almost 20-year career with Road & Track magazine, serving as Engineering Editor, Editor and Editor-in-Chief from 1972 to 1990.

John wrote the Road & Track Illustrated Automotive Dictionary; came out about 10 years ago.

[Marshall Grice]

It's the same thing.

Not according to Road & Track's former tech editor.

feet x pounds = pounds x feet.

[Mako Koiwai]

My WRX spend two hours on the Vishnu dyno ... it was a bit torturous to hear all of those hard pulls through the gears. At least with the Dynapack you just use whatever gear is closest to 1:1.

Cooling ... yeah ... I have that "special" extra leash that allows me to safely run with my hood cracked open. Not sure it would pass Tech. When I try it out in Practice, it's good to see how many people are Watching and yelling or gesturing to close the hood!

Besides the fan into the radiator opening, and of course having the hood open, we did spray off the intake manifold between runs.

I Vote for Honest Abe Kalman to do the comparison runs

[Bob Beamesderfer]

It's the same thing.

Not according to Road & Track's former tech editor.

feet x pounds = pounds x feet.

So everything says, however, the differentiation is that what's being measured isn't the same thing. Dinkel's been SAE chairman in addition to R&T editor, so no offense Marshall, I'll stick with his approach.

Turning or twisting force such as the force imparted on the drive line by the engine. Usually measured in lb-ft. It differs from work or power in that torque does not necessarily produce motion. Basically, the magnitude of a torque acting on a body is the product of the magnitude of a force and its force arm (perpendicular distance from the axis of rotation of the body to the line of action of the force). This product is called the moment of the torque about the axis or the torque.

[Michael Heinitz]

The red line is baseline and the yellow is after tuning the stock ECU.
1995 STS Miata tuned by Church. Notice that the 'ratio' is incorrectly listed as 4:30 which could make the figures read lower.
Top end power is good and climbs all the way to redline, but my midrange power is really lacking.

Mike