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Porsche 931 with 15 to 20 lbs boost - how much HP?
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Rick MacLaren  
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PostPosted: Sat Aug 04, 2001 1:42 am    Post subject: Reply with quote

Question for you English and Aussie guys who have these pumped up 931's:

I know for a fact that the Subaru WRX has a 225 HP rating at 6000 rpms at the flywheel, but only 168 HP rating at the wheels (Source: Turbo and High Tech Performance Magazine, August 2001). The WRX loses a LOT of power through all wheel drive. 0 to 60 in 6.3 secs.

The 1980 Porsche 931 (US Spec) has about 141 hp at 5100 rpms the rear wheels with 7 lbs boost (Dyno test by me July 2001).

If I pump up my boost to like 20 PSI, I've been told I can expect 200 HP at the rear wheels.

Question 1: Can I expect to beat a WRX with 15 lbs boost?
Question 2: Approximately how much HP with 15 lbs boost?
Question 3: Can I expect to beat a WRX with 20 lbs boost?
Question 4: How much HP with 20 lbs boost?
Question 5: What are the best conditions to race the WRX?

If any of you guys can help answer these questions it'd be greatly appreciated.

Thanks.

Rick
_________________
1980 Porsche 931 C6H12O6 injected 15 PSI
1979 Porsche 924

[ This Message was edited by: Rick MacLaren on 2001-08-04 04:28 ]
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-nick  
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PostPosted: Sat Aug 04, 2001 2:48 am    Post subject: Reply with quote

FYI - Dyno's -

Rick, i hate to break it you, but you can play around with a dyno and make it spit out any hp you want. the use of a dyno is to measure changes you make to the engine relative to a baseline.

now that you have a dyno chart, you can make some changes, take it back to the same dyno under the same conditions, and see if your changes made any net difference in hp or torque. that's all you're going to be able to get: x change gave me +y hp and +z torque. not, i changed x and now i have y hp and z torque.

i hope this helps, alot of people get confused on this point regarding dyno's.

but good luck in your quest against the wrx, i hear they're pretty quick.

-nick
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Rick MacLaren  
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PostPosted: Sat Aug 04, 2001 3:28 am    Post subject: Reply with quote

Nick, I've presented the dyno results for your convenience only . Thanks for the wisdom regarding dyno testing, but just like statistics can be said to "mean anything" (often a common guy's way of saying he won't believe a statistic even if it hits him in the face), I've learned after lots of training in experimentation and the use of control groups and validity testing that there are GOOD measures using stats and POOR measures using stats.* The issue with dynos, statistics, or any numerical measure is HOW it is derived and HOW it is interpreted. We USE dyno results in ALL auto specs from collector cars to antiques to new cars - I'm just going with the flow and using dyno data like thousands of informed automobile writers and journalists.

Besides, let's be realistic, I obviously can't buy a WRX, put one on a dyno, match conditions exactly and perform a controlled experiment with perfect counterbalancing. So, instead, I'm presenting all the numbers I can find to help out the reader/guesser.

I'd really like to know about the odds of winning, thanks. And if you don't want to use dyno results to base your guess, just look to heaven, say a mantra or two, and ballpark a wild-assed guess!

* Note on dynos: Even a RULER can be used badly...I can hold my eye at a certain angle and measure something 7 inches long (Heh heh ) and record a variance of 2 mm either way. That's just the nature of a measuring stick. And a dyno is just a measuring stick. I don't think it's mystical or some kind of super complex tool. It's device for generating a measure. Scientists used to say "Smoking is correlated with heart disease" and there were all kids of guys saying "that is just a statistic, it can mean anything". Still, smoking WAS related to cancer. Same issue with dynos. Holding the 'right' variables constant or similar is needed for a good comparison. Once done, a dyno and knowledge of vehicle weight and other factors can be good data on which to base a guess or opinion.
_________________
1980 Porsche 931 C6H12O6 injected 15 PSI
1979 Porsche 924

[ This Message was edited by: Rick MacLaren on 2001-08-04 04:05 ]
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larso  
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PostPosted: Sat Aug 04, 2001 4:04 am    Post subject: Reply with quote

Yes your tires and road surface play a big roll, you have to have the right tires on the right surface, and in teh right heat conditiosn (hot asphalt, loose gravelly asphalt...cold asphalt), you have to have tire pressures up to par with different conditions etc etc etc, dynos are just good for comparing base to blah blah.......like nick said
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Rick MacLaren  
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PostPosted: Sat Aug 04, 2001 4:07 am    Post subject: Reply with quote

Yeah, but Lars, forget the dyno if you don't want to think about dynos, just gimme your expert opinion: With 15 PSI or 20 PSI do you think a 931 can take a WRX?
_________________
1980 Porsche 931 C6H12O6 injected 15 PSI
1979 Porsche 924

[ This Message was edited by: Rick MacLaren on 2001-08-04 04:10 ]
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924RACR  



Joined: 29 Jul 2001
Posts: 8322
Location: Royal Oak, MI, USA

PostPosted: Sat Aug 04, 2001 4:15 am    Post subject: Reply with quote

OK, ya wankers... obviously we're going to have to settle it... I'll just have to take my (82) 931 down to the dragstrip with my coworker who's recently got a WRX (5-door US) and have it off... once I get the insurance agent to stop dicking me around and stabilize a rate... they've now raised it 60% from the initial quote... turns out they re-ran my license, and removed the incorrectly-added multi-car discount... really quite pissed at them, still have to call back and find out what their deal is...

Oh, yeah, cute little thing, my co-worker with the WRX is the same guy I face off against on track, as he runs a Saturn SC2 in ITA...

Took the 931 for a spool down the highway last night, very cool... my friend driving chase (in case of breakdown) had the windows down, and said he could hear the turbo spool 5 cars back at 75mph...

Can't wait to crank the boost and slap in an intercooler...

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Vaughan Scott
Webmeister
'79 924 #77 ITB racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype
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Rick MacLaren  
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PostPosted: Sat Aug 04, 2001 4:25 am    Post subject: Reply with quote

Spank that puppy, Vaughan. We gotta end this WRX "I gots so much horsepower in my tin can" bullshit right now.

How much boost will you run?
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-nick  
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PostPosted: Sat Aug 04, 2001 5:27 am    Post subject: Reply with quote

hee hee hee-
how about 0-60 times, that's my personal favorite method of measure. most stopwatches count seconds the same

engine dynos and chassis dynos are _very_ different animals. the only way to get an idea of how your engine compares is to pull it out and put it on an engine dyno. which, of course is entirely unrealisitc!

i take chassis figures with a grain of salt, they can easily be off by 10% or more. although if you show me a dyno of your 924 making 300hp off a dyno, i'll definately give you some respect! 10% low on 300 is still a whole lotta power.

FYI- check out this link for a good explanation of dynos:
http://www.pumaracing.co.uk/power3.htm

i think Vaughan has the right idea. however, his insurance might not be so bad if he would stop racing wrx's on the street

regards,
-nick
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Rick MacLaren  
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PostPosted: Sat Aug 04, 2001 5:50 am    Post subject: Reply with quote

The article link you gave, Nick, provided no data on 4 wheel or all wheel drive...so I'll inform you: Typically you can expect 25% to 30% parasitic loss on all wheel drive vehicles. Hence, 168 HP ATRW for the WRX. Not bad, but decidedly NOT 227 HP ATRW.

Now, Nick, you've spent two posts now and a lot of words instructing all of us mere mortals on dynos and times and proper measures and chassis characteristics...I've read your links...and nothing was said about the real issue...a WRX all wheel drive versus a 931 at 15 to 20 lbs boost.

Um...do you think when the oxygen gets thin and you come down from atop your throne you might try taking a stab at answering the question sometime soon? Just take a guess: Do you think under the boost characteristics specified in the first post that a 931 can take a WRX?
_________________
1980 Porsche 931 C6H12O6 injected 15 PSI
1979 Porsche 924

[ This Message was edited by: Rick MacLaren on 2001-08-04 05:58 ]
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larso  
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PostPosted: Sat Aug 04, 2001 5:55 am    Post subject: Reply with quote

OK rick, what do you mean, TAKE?

At a stop light? or what?
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larso  
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PostPosted: Sat Aug 04, 2001 5:58 am    Post subject: Reply with quote

Oh and RICK, even though the WRX loses lots of power in the AWD...they get good traction...so in order for you to beat one, you have to master your 931 first, because the WRX won't be spending time spinning his tires since its AWD, do u get spin in your 931? be careful, spinning tires is an easy way to lose a race.

SO under the heading "best conditions to beat a wrx", I would say don't race a WRX unless you are on black hot pavement, any gravel on a back road and the wrx will gain an advantage.

How much does a WRX weigh?
168HP on how many pounds, how much torque does it have.

168HP dyno rear wheel HP? or 168 road real wheel HP????

[ This Message was edited by: larso on 2001-08-04 06:05 ]
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Rick MacLaren  
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PostPosted: Sat Aug 04, 2001 6:02 am    Post subject: Reply with quote

Good point Lars. I'm interested in 0-60 and top end. I have Z rated Bridgestones and pretty good traction. I seldom if ever spin my tires.

Yeah...I'd like to know if, with 15 to 20 lbs boost, I can take one off the line. Also, if I'm on the highway, I'm guessing I can blow it away on top end. Whaddya think? Is it worth trying?
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larso  
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PostPosted: Sat Aug 04, 2001 6:09 am    Post subject: Reply with quote

well what I would do is get a GTECH and find out your REAL 0-60 time, they aren't gonna be accurate for your real HP and all that crap, but they will tell you your 0-60 time pretty good i think.
I think gtechs cost $200US or what is it?

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Rick MacLaren  
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PostPosted: Sat Aug 04, 2001 6:28 am    Post subject: Reply with quote

Lars - Good questions again.

1. Yes, I won't be on gravel or ice. I'm not giving away an advantage to a WRX.

2. The WRX is a little over 3000 lbs.
The WRX has 165.5 foot pounds of torque at peak output.
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924RACR  



Joined: 29 Jul 2001
Posts: 8322
Location: Royal Oak, MI, USA

PostPosted: Sat Aug 04, 2001 7:07 am    Post subject: Reply with quote

Much good news... the car is on the road (officially now, as I've managed to find a much more compliant insurance agent who doesn't ask so many silly questions... yes, of course I put in for a personalized plate! I finally have a car that's worth it!

Anyone want to guess what it says? Or will say, once it arrives? (fortunately after the insurance guy takes his pics of the car )

As for racing on the street... very funny, but no need for that. #1, we both have competition licenses. #2 his racecar is way faster than mine, sad to say. #3 We can both go to Mid-O or Gingerman for a Driver's Ed event and compare lap times (now how's that for meaningful!) #4 Milan Dragway. Need I say more?

I'm sure I can con him into heading out there to make a few passes w/ me...

Oh, yeah, I'm still running stock boost, haven't completed any plumbing mods to raise it up, haven't got a boost gauge yet, haven't put in an intercooler yet, and haven't even dealt with a few regular maintenance issues associated with getting it back on the road, like PM's on the trans, wheel bearings, brakes (still waiting for new hoses, but got new pads), etc.

_________________
Vaughan Scott
Webmeister
'79 924 #77 ITB racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype
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