Performance Test done - SSI vs Stock SC
 

I recently had new SSI exchangers with a Dansk exhaust installed on my 1979 SC. The Dansk is dual in dual out. I also had the injectors replaced at the same time along with the heater box valves and oil return tubes.

I've heard figures about the performance increase related to the SSI installation but wanted to see for myself. So first I purchased a GtechPro. I also bought dB Noise meter as the difference in sound was also a concern. I made audio recordings of the before and after sound which is interesting to listen to. I plan to digitize it in the future.

Here are the Stock (except my car started with a test pipe installed) results:

Time of day - dusk
Temp -82 degF

0-60 mph - 7.1 sec
1/4 mile time - 15.3 sec
1/4 mile speed - 97.2 mph

Noise figures at rear center of car, level with exhaust:

@1000 rpm and 3' - 87 dB
@1000 rpm and 10' - 76dB
@4000 rpm and 3' - 102dB
@4000 rpm and 10' - 97dB

With SSI and Dansk:

0-60 mph - 6.7 sec
Not enough nerve to take more multiple 1/4 mile runs. I'll save that test for the track.

Noise figures same location as above:
All are average figures.

@1000 rpm and 3' - 91 dB
@1000 rpm and 10' - 89 dB
@4000 rpm and 3' - 103 dB
@4000 rpm and 10' - 100 dB

Conclusion: 0-60 increase is about 0.4 seconds. For cars starting with Cat, the results should be even better. I also anticipate the 1/4 mile improvement to be noticeable due to the free breathing at higher rpms. The sound at idle is substantially louder and deeper. The noise level surprise was that the dB level did not drop off as much going from 3' to 10' feet as with the stock exhaust. During a trip to the store, it set off an alarm two cars over when I cranked it. The exhaust sounds great except for a slight drone from 2000-3000 rpm but you can stop this by down shifting. Above 3000 rpm and at idle, the exhaust sound is just what I expected, deep growl at idle and race car like at 5000 rpm and higher. I'll post pictures once they are available. Any thoughts as to why the sound level remains somewhat constant further from the car? Could it be the increased air velocity from the SSI/Dansk?

Thank you. Very nice.

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87 Carrera Cabriolet

It is pretty hard to compare early tuned exhaust to the untuned appendages hung on smogged, CIS cars!

Without the recordings and the capability to feed them to spectral analysis software knowing the specific engine rpms at various stages in the recording, it would be difficult to be specific! In general, the factory 'Banana' muffler acts like a very large volume crossover pipe connecting the two banks of the SSI system that would boost the lower frequency components of the exhaust note. Because of even fewer baffles than the factory muffler, the Dansk sport muffler is even more likely to boost those low-frequency components in the exhaust note.

The most commonly used 'A' scale for making noise measurements, denoted by a reading of 'xxx dBA,' is fairly heavily weighted towards the low frequency end of the audio spectrum, and low frequency (less than 1000 Hz) sounds are attenuated less over distance in air than higher frequencies.

It sounds like you ought to be pleased with the results of the upgrade!

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Warren Hall
1973 911S Targa
1992 Dodge Dakota 5.2 4X4 parts hauler

I wonder if it has to do with the lower frequencies of the lower tone. I think the lower frequncies of the deeper tones travel better than higher frequencies. I'm not a microwave techicen so I could be all wet. I have one too and I totally agree with the great overall sound.

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Mark
The Beast
mark@hargett.com

One error in my posting, the speed figures are average. The noise figures are maximum.

Considering that you lost almost half second 0-60 with 2900lb. car, must have gained 10-15 HP. Is that a good guess, anyone?

Thanks for sharing your experiences.

The drone/resonance at 2000-3000 rpm is that very annoying? I often cruise at speeds that in fifth gear lies around those rpms. Can the car still be used as a daily driver or will the noise drive you nuts?

Please make the sounds available as wavs or MP3s! I'm very interested in hearing them.

Speeder,

Here is how I see the gain mathematically:

V = 1/2 * a * t^2
V= velocity
a= acceleration
t= time

this equation can be rearranged to be:


a = 2V / t^2


since in both runs the end velocity was the same (60mph) we can say:

a2 / a1 = t1^2 / t2^2

therfore

a2 / a1 = 7.1^2 / 6.7^2 = 1.12

This means the acceleration increased by an average of 12% through the power band. Acceleration is proportional to torque so you can say the torque increase by an average of 12%. Hp is torque * rpm, so again the Hp would have increased by an average of 12% thourgh the power band. If Hp had increased evenly throughout the power then you could say that given a stock 911 SC has 180Hp peak, the peak Hp increase by 180*0.12= 21.6Hp. This seems like alot, so I think what is happening is torque in the lower rpms is increasing more than 12% while in the higher rpms it is increasing less than 12%.

Autobonrun, did you richen your mixture after the SSI's were installed?

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Tony
'78 911SC

[This message has been edited by tbitz (edited 08-28-2001).]

[This message has been edited by tbitz (edited 08-28-2001).]

[This message has been edited by tbitz (edited 08-28-2001).]

According to Bruce Anderson, SSI on a 3.2 yeild about 17 hp peak based on dyno testing. And the improvement on a 3.0 is an unquantified "more".

So more probably is around 20ish hp peak improvement, consistant with tbitz calculations.

Tony,

Nice try with Newtonian physics lesson ... but 0-60 times are hardly uniformly accelerated objects!

A gain of 10-15 hp would be believable, but that would fall short of your calculations.

A 20 hp gain as discussed is starting into fantasyland ... the 2.7 CIS engine lost EXACTLY 10 hp from '74 to '75 with the reverse procedure you did. And, port sizes became a severe limit with the 3.0 and 3.2 engines, and it took 9.8:1 pistons for the RS 3.0 to get 230 hp. And, quite a bit wilder cams than the old 'S' AND 10.3:1 compression to get 255 hp at 7000 rpm out of the SC RS!
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Warren Hall
1973 911S Targa
1992 Dodge Dakota 5.2 4X4 parts hauler

[This message has been edited by Early_S_Man (edited 08-28-2001).]

Folks,

I got the equation wrong. The above equation is for distance not speed (ie: d = 1/2 * a * t^2).

The correct equation is:

v = a * t

Using the above and autobonruns numbers we get:

a2/a1 = 7.1/6.7 = 1.06

Which means the AVERAGE acceleration increased by 6%. This means the AVERAGE torque increase is 6%. If torque was flat over rpm then peak Hp would increase by 10.8Hp (180 * 0.06).

The above equation is for constant acceleration, as Warren pointed out. It is correct to say the AVERAGE torque increased by 6% over the run.


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Tony
'78 911SC

[This message has been edited by tbitz (edited 08-28-2001).]

Didn't Bruce Anderson perform this upgrade on many, many SCs, and report that hp gains were in the 15-20 heighborhood? He was not guessing. He's got a dyno.

I'll be making this same upgrade soon, along with 20/21 cams. Perhaps I should shop for an early ('78-'79) set of heads because of their larger ports. Are they larger intake ports, or larger exhaust ports?

Anyhow, I'm hoping for an extra 200 horsepower, but I'll settle for 30-40. And I may get it. You guys are just getting me excited.

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'83 SC

You can't expect reliable calculations of hp using 0-60 runs for data! It doesn't take a physicist to realize that!

If you want reliable results, the data must be in [b]ONE particular gear, no shifting involved, and over the same test terrain, i.e., the EXACT same stretch of road, same tempeature, humidity, wind, etc. I trust that you don't have the 'before' data that meets such criteria, so your calculations are pointless! Entertain yourself if you wish, but you can't calculate horsepower with useless data! Real simple fact.

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Warren Hall
1973 911S Targa
1992 Dodge Dakota 5.2 4X4 parts hauler

After reading a little more on sound propagation, I think one of the reasons that the sound travels further with the Dansk is that the exhaust tips now point straight back rather than down. Also, the lower frequencies should propagate further for a given dB.

If this was my daily driver, I don't think I would install the exhaust just for the additional power. The looks and sound are great but it is louder. I don't think I would like to hear this everyday. Since I drive the car less than 2000 miles a year, it's a perfect change from my 740 BMW. The droning sound is not that bad at higher speeds (55 mph+)even if I let it get down below 3000 rpm. In traffic at low speeds, I tend to try to keep the car over 3000 rpm. I don't want to entice an officer to inspect.

As far as the performance, I think that 0.4 seconds or so improvement is reasonable. I never expected the 0-60 times to increase that significantly. Again, I anticipate that the 1/4 mile test should really improve. The gTechPro does perform a HP calculation, but again, I need to be on a track to run it. Although I didn't run a pre-installation HP test, the HP figures on the current system should be reliable if they remain consistent over three or four runs. When I ran the 0-60 tests, I did try to pick conditions as similar as possible (temperature, time of day, etc.) and ran them at the same location. My figures are obviously not exact but met my needs by giving me more than just a gut feel that improvements occurred.

[This message has been edited by autobonrun (edited 08-28-2001).]

Yes I did richen it. The original settings left the system so lean, it would not even idle with the SSI's. It had to be richened quite a bit. Strangely enough, the car appears to be getting better gas mileage now. Before the installation, I was getting 16 mpg around town. I'll know for certain once I fill back up. As little as I drive, I'm waiting for fuel prices to fall from the current $1.80 per gallon price for regular before I refill. On some things I'm still cheap.

By the way, is it normal to have some backfire on decel or does it mean my air/fuel mix is still off? It is not popping, more of a burble.

You may not be far off given the fact that I started without a Cat. For those starting with a true stock SC, a gain of 15-17 HP may be correct. I'll know when I make the HP measurement runs. It's good when theoritical comes close to actual, recognizing that both include some assumptions that keep the results from being exact.

I continue to doubt whether a lean mixture alone can cause exhaust backfiring. It can cause intake backfiring, for sure.

I said here a few times that my mechanic friends insist that deceleration backfiring in the exhaust means a leak in the exhaust system. At one of the joints, of course. I am not ready to doubt this, though at least some one has felt that a lean mixture did cause this kind of backfire.

Did the exhaust parts go to a machine shop for 'decking' to ensure they are flat and true? It has been my experience that they seldom are. Most need this machining, even brand new headers.

At least, if you do decide they need to be removed, you'll be glad they are the early style instead of the set you replaced.



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'83 SC

Easy there Warren. What's you definition of "reliable data" +/- .003%? Granted more precise data than supplied by autobonrun exists. But this is a nice sanity check from someone who's not trying to sell us something. Besides if word gets out that variances can't be managed then the gig is up for Statistician and engineers everywhere. http://www.pelicanparts.com/ultimate/smile.gif

<HR><font color="#CCC4A8" face="Arial Black"><strong>'81 Platinum Metalic SC COUPE</strong></font></p>

You could measure 0-60 times down to the nanosecond, and it would still be usesless data! But, if you don't undertand ... it is pointless to explain why!

Just a clue -- no shifts , and no launches from a standstill ...


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Warren Hall
1973 911S Targa
1992 Dodge Dakota 5.2 4X4 parts hauler

Two comments and yes, I am a statistician, of sorts. First, the most important part of a statistican study is the data collection. If you wait until you have collected the date before you hire a statistical consultant, you have made a mistake. So, Warren is correct in that if you want the most reliable results, you would consider starting with data that contains no shifting, for example. If possible.

Second, (and this is the other side of the story) while some researchers have the luxury of studying simple behaviors using reliable measurements (dyno results, load tests, etc) those of us in government and/or behavioral sciences MUST make due with data that at best resembles the real measures you are trying to predict. So, you do the best with what you've got.

Unless you're Superman, of course.

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'83 SC