Performance Test done - SSI vs Stock SC

[tbitz]

Warren,

I don't think the analysis is "pointless".

My calculations simple state that if it takes the car 6.7sec for 0-60mph after the mods and 7.1sec before, then the AVERAGE acceleration must have increased by 6%, and torque is directly proportional to acceleration, thus AVERAGE torque increased by 6%.

Autobonrun stated the times were average over multiple runs (thus reducing the errors caused by unequal shifting). He also stated that he tried to keep all the conditions the same, time, temperature, location, etc...I can only assume he tried to do a fair compairson.

Is the analysis exact? No, but you make do with the data you have.


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Tony
'78 911SC

[Superman]

Yes, you do make do with the available data. But no, the 6% average acceleration increase assumption is not correct.

When data has known or estimable weaknesses, these can often be statistically mitigated. For example, if these tests involved one shift (from 1st to 2nd) and if that shift took 1 second each time, then you could adjust all figures down by one second. so the "before" would be 6.1 and the "after" would be 5.7. This would result in a 6.6% increase in acceleration, based on the old (6.1) time.

I'm not criticising the estimates already made here, just illustrating the kinds of adjustments that can be made to data in order to isolate the variable you are hoping to estimate. Other relevant questions in my opinion might involve whether the before and ofter 0-60 tests allow both engine configurations the same chance at showing off their power zones (that is, if the new setup makes way more power after 5500 rpm, and if 60 mph in 2nd gear is 5500, then the new setup did not get a chance to show its stuff). Also, does elapsed time really translate directly into horsepower? etc.

Okay, having said all that, I'd agree that the new exhaust seems to produce more power, and that 6% is in the ballpark in terms of the 'delta ET.' I'd consider this to be a conservative estimate.

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'83 SC

[autobonrun]

Originally posted by Superman:
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.

Superman, you called it exactly; the backfiring was caused by an exhaust leak . When I rechecked the bolts, the ones between the SSI and Dansk had backed off from 1/2 to 1 turn. This brings up an important lesson learned. First, I agree you should take the new SSI and exhaust to a machine shop and have the flanges decked. I know for certain one of my flanges on the new SSI was not exactly flat, but I thought the gasket would make it up. Second, retorque the bolts on the entire system after a few days. I think the instructions said to do that, but I didn't listen. The system now makes a slight burble during decel which sounds real good and I never touched the fuel mixture.

Thanks for the advice.

[Yargk]

Tony, pointless might be a bit far, however Warren has a valid point. During the zero to sixty run there are places where the acceleration is not increased at all, during the shift, and during the start (acceleration is determined by traction at that point). So sure you're giving average acceleration, but it's moot to convert that to torque because maximum torque at rpm isn't being always applied. I bet you would think it would be silly to calculate torque increases if the car was allowed to sit for 6.7 seconds before each run and that time was counted. Your average acceleration increase would be 3%. Is that valid? How about a 3 % increase in torque? Even if you subtracted the time it took to shift, you'd have to take out the time it takes to get in the same range of RPM after the start, that is used in second gear, otherwise you're getting the average torque from say idle to 6.5k and from 4 to 6k in the powerband. I think showing a .4 better 0-60 time is wonderfully helpful to those how wish to upgrade. But that measurement isn't satisfactory to be used to calculate torque increases. BTW it was quite silly to use d=(1/2)(a)(t)^2 for velocity. I don't mean to be rude. I'm just backing up Warren. I probably should have a bit more schooling before going off like that. I just started as a freshman at Berkeley majoring in physics.

Keith
79 930

[speeder]

I'm starting to be sorry that I brought up the HP/ 0-60 question. Plenty of dyno tests have been done on the SSI conversion on SC's showing something like 15 HP and similar torque gain in the middle (peak). I question the accuracy of anyone's self-tested 0-60 time anyways, but assuming that the times are 100% accurate how is this for scientific, Warren: 1983 911sc 0-60 7.0 sec. w/ 172HP. 1984 911 Carrera 0-60 6.1 sec. w/ 200HP (all factory figures). Same car, same trans gearing, same weight (approx.) That's more or less my data- math formula left me on the trailer, so to speak.

[island911]

Originally posted by Yargk:
. . . But that measurement isn't satisfactory to be used to calculate torque increases. BTW it was quite silly to use d=(1/2)(a)(t)^2 for velocity. I don't mean to be rude. I'm just backing up Warren. I probably should have a bit more schooling before going off like that. I just started as a freshman at Berkeley majoring in physics.

Keith
79 930

Satisfactory for whom? The guy has the kahunas to put up some calcs, using the minimal data given for some first order correlation. He simplified the model/math to a linear curve fit.
Warren states: but 0-60 times are hardly uniformly accelerated objects Ya, so . . .From what I've seen of 0-60 V-T graphs, a comparison of linear curve fits is not *that* "silly." Yes there is non-linearity in the curve. But the before and after curves are of the same car, same driver, and are likely to share the same non-linearity. So for a first order comparison it is not *that* "silly." It’s not like anyone’s life is dependent on the accuracy, nor was anyone using 8 significant figures. . .when it comes to mathematical models of mechanical measurements you can always make it more complex; or you can do something useful.
Oh, BTW I'm just backing up Tony.

---

'81 Platinum Metalic SC COUPE

[tbitz]

Yark,

Who cares about "peak" torque and horsepower anyway? Are you driving your car around town at exactly 5500rpm all day? I think for a street machine indicating an average increase of torque from idle to 6000rpm is more usefull than saying the torque increased 30% at 5500rpm.


Sorry for being "silly" when I used d=(1/2)(a)(t)^2 for velocity. I am not "majoring in Physics at BERKLEY". It has only been 10yrs since I last took a dynamics class. I did see my mistake and posted a followup with arguably the correct equation indicating my first error. Some folks would have probably just edited the orginal post to hide their error. I am human, I make mistakes.






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Tony
'78 911SC

[Clark Griswald]

Well Tbitz, I hope you have learned your lesson.

[KTL]

Where the heck is rstoll when you need him?

Alright, i'll do it.

ALL YOUR AVERAGE ACCELERATION ARE BELONG TO US.

Anyway, I think the original intent of the post is verified. The SSI's appear to do more than just make more noise and make the underside of your Porsche shinier. Good enough for me!

All that data analysis jargon was giving me a seizure.




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Kevin
87 Carrera coupe

[emcon5]

I know for certain one of my flanges on the new SSI was not exactly flat, but I thought the gasket would make it up. [/B]

Am I the only one that thinks this is totally unacceptable for something this expensive?

Emcon5, no you're not. It surprised me too.

[autobonrun]

I agree with Emcon5 and Mikkel, the fact that this flange was not flat surprised me and was counter to the excellent workmanship on the rest of the unit. It was off around one bolt hole where the SSI flanges to the muffler. If I recall, about 3/32". I've heard of this issue on other SSI's. I seem to remember somewhere that the reason given was that the flange would go back flat once the bolt was tightened up. However this is not believable as the other flanges did not have this offset.

With respect to the HP issue, as mentioned earlier, my overriding concern was 0-60 and 1/4 mile times. However, you all have my interest up, so I'll run some HP tests within a week or so using the GtechPro. It calculates HP by multiplying instantaneous speed times acceleration times weight and displays the maximum attained. Since accuracy of weight is critical, I'll have my car weighed with a full tank before making the runs. I'll probably only go up as far as redline in 3rd (maybe 4th) since I don't want to push my luck with the law. Since speed and acceleration are used, this device measures HP at the wheels, which of course includes any drivetrain losses as well as the effect of Cd. HP will probably be much less than expected.

Two questions: Is the stock figure of 172HP quoted on the 79SC, engine HP or HP at the wheels?

Do dyno results take into account Cd? I assume the resistance is varied to simulate acceleration, but does this resistance also include air drag which does get factored into the GtechPro's figures. If not, the Gtech readings should be less than a Dyno.

[Early_S_Man]

172 is the SAE net figure, at the flywheel ... corresponding to the factory's 180 hp DIN rating.

No, neither engine nor chassis dyno figures take into account wind resistance, one reason that EPA mileage figures are artificially high.
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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 08-30-2001).]

[Yargk]

Island 911: I don't understand what "kahunas" it takes to try to figure out an average torque figure from not just "minimal data," but wholly improper data for that application. For the record, I didn't argue about converting to linear acceleration equations, because he's going for average torque. I'm not just saying it's okay with me for a rough estimate. It should actually be correct for average torque if drag is negated, even though instinanious acceleration is varied, because that's not what's being calculated. I just have a problem with shifts and starts. I have no problem with using such equations for, let's say, a 3000-6000 rpm in thrid gear test.

Tony: I think the integrals of torque charts across the power band are better indicators than peak, so Average torque would be useful. However, I addressed that if measured from a 0-60 time, you'd get some weird average of torque delivered in first from whenever he dropped the clutch to his shift point and in second from the shift to the rpm that 60 mph would be. This is because the rpm sweeps wouldn't be the same in first and second gear.

Keith
79 930

[Clark Griswald]

Haruuumph!

[island911]

Yes Keith, I understand the problems you (and others) have with the model. And I remeber the pressures and expectations from school on using the "correct" equations. The point of which is to give you a feel of what's going on. But in the real world expectations change, and are driven by time and money. Assuming you make it thru and want to start working in engineering; you will find the error of the calculation you provide will be inversely related to the amount of time and/or money given to you. Simply put; high precision means high cost. . . .a quick no cost calculation means don't complain if it's "not good enough."

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'81 Platinum Metalic SC COUPE

[Yargk]

Island: I understand the whole cost and time versus precision. I forgot to mention that my point was not to spend the time on the calculations. Time=money, so it wasn't really a no cost calculation, was it? I retract this if Tony enjoys physics equations :-) That isn't that far out of the realm of possibility though, I find certain procedures satisfying upon completion. If they have to do with cars, all the better. However, if I post any, I do hope, and believe someone will grill me at every opportunity, to keep me on my toes.

The first chapter in my Physics book is actually about very rough estimates. I was asked to estimate how much rubber is put into the air in the US every year because worn tire tread mostly ends up as air pollution. I got a number around 1.4 x 10 ^ 8 Kg per year. Since I was only given that the average tread depth is about 1 cm and that the density of rubber is about 1200 kg/m^3 and had to assume the volume of tread used on average per tire per year and the amount of tires in the field, I think I was pretty close to the real value of 3x10^8.

While we're on the subject of equations:



Keith
79 930

[island911]

Originally posted by Yargk:
. . .I find certain procedures satisfying upon completion. If they have to do with cars, all the better. However, if I post any, I do hope, and believe someone will grill me at every opportunity, to keep me on my toes.

. . .asked to estimate how much rubber is put into the air in the US every year because worn tire tread. . .

Keith
79 930

Thanks for the clarification. . .so you’re a baiting pontificator? I say, as Monty Burn would say; Excellent.
(However, others on this board would say; There goes the neighborhood.)

On your rubber to air pollution calculations: You can’t just work on the premise that all of the tire rubber goes to air pollution; that would be silly. so your calculations are pointless. If you don't understand ... it is pointless to explain why!
Just a clue -- no rain, and no launches from a standstill leaving black-marks...

heh-heh-heh "There goes the neighborhood" I can hear the echos.

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'81 Platinum Metalic SC COUPE

[Yargk]

Why shouldn't I be happy upon completion of a problem involving the calculation of the horsepower needed to overcome drag on a 911 at 140 mph for example? The tire problem existed in physics world, you know, the one where silly things happen, and frictionless icerinks pop up way too often. Our friend with the SC lives in the real one I believe. Why would I be a pontificator if I want people to let me know of my mistakes? That makes no sense, if I was a pontificator, I'd go on with my dogma and expect others to swallow it. Thanks for all the sardonic posts.

Keith
79 930

[Bob Goding]

Early-S-Man---Warren, I would be interested to read your thoughts on the design theory of the Triad muffler. assuming that the first can is just an expansion chamber and the two mufflers are similar to regular glasspacks, it would seem that a similar setup would be fairly easy to fabricate. I have a cis 2.7 stock, with SS1,s and a buchered old 77 banana muffler.Shipping costs are a bit prohibitive to this part of the world!---Any one?
Thanks ----Bob