mark kibort

True. integrating power is just much easier.
If you had a constant net force, (torque) then, acceleration would be constant. but, in this topic, force goes down with velocity, quite dramatically.

If you look at just power, and integrate power over a speed range, you get the comparitive answer with a lot less strokes. Using force or engine torque, you need to plug in the values, multiply with the gear ratios and correlate with speeds.

But yes again, there are more than one way to skin this cat. again, i use hp because of this common misconception of the role of torque and HP that Tommy mentioned. If you just look at HP as being equal to torque through the gears at any speed, 90% of the answers can be found for use in comparisons.

Mk

acceleration= power/(mass x velocity)
or
acceleration= f/m (but you need to know the force at any particular speed)

(equations corrected! oppps)

Normy

It's funny, what people think some times.

-I've read some of the posts on this thread and I was amazed at how many of them mirror my own perceptions. My S2 (aka: EURO) '85 with Ott X-pipe/928 Intl. Borla system gets the MAJOR attention when I want it to; the thing about this combo that amazes me the most is how quiet it can be when I'm dawdling around town at 1400 rpm, versus how loud it can be at WOT. Listen, when driving through traffic, if I encounter someone who drives stupid and deserves a quick "bwak"......

BWAA!

The sound level is amazing- it can't outdo the local Harley crew with their open pipes [hate to say it, but I think these open pipes ought to be illegal; these are too loud, and I'm a gear-head by the way!]

But if I underdrive the car, keeping it below about 1400 rpm..the car is nearly silent. Listen, people with 944's and 911's and a whole host of other cars will tell you how great their machines are. My S2 will go from idling through the neighorhood in 5th gear at 20 mph, making less noise than the pump that irrigates my lawn...to making noises that are almost vocal and LOUD as a horn in one second. And it can be more than a quarter mile away 13 seconds later-

"Bumm bumm bumm....hmmm.......BWA!"

-That was me in my 928 slowing so that I didn't rear-end someone in their silly Aerostar when they slowed excessively. The final BWA was actually followed by "Uhmmmm wuhm wuhm whum whm whm!" That Ott/Borla exhaust is loud enough to make you jump. And it fits the nervous nature of the S2 928.

Anyway...

N!

tommytomaso

Mark...i have to repect your racing history and what should be knowledge...but you my friend need a physics lesson!!!!your terms look good on paper but you keep throwing out ALL the BASICS!!!...at the expence of others!..but hay i wouldnt be the first to think i was right for years and years only to find out i was wrong..( can apply to others)....to bad everything i was taught,read and shown was all backwards....But then again iv driven many 928s and S4's (STOCK)and NONE seemed faster than my audi V8.(STOCK.maybe chipped)..and im not silly on this point by any means...your just not well informed my friend( all must of been in bad tune!!LOL) but then again its just an opinion from a silly AUDI driver that just happens to LOVE928s

Mongo

Borla exhaust....meh, that's for sensitive ears.... Try Dynomax Magnum Resonators.......JUST RESONATORS

By the way I'm stilll anxious to get my mits on a 16-valve as a daily driver

Kevin Michael

I'm not trying to get in on this but I have a 1997 BMW 740il that seemsFaster than my auto 87 s4. I have g-teched them both and the auto 928 s4 is a half sec. quicker to 60, and will flat out run the beemer at anything over that. Now according to avail. info. my 97 740il is a full sec quicker than your 93 AUDIv8. to 60. Let's bring in my 87 928s4 5SPD. It , on the same test and all things being equal will run circles around the auto928 s4 and especially the 740. (now that I got it running again) This car consistently delivers 5.3 to 60 and is a beast for aa mostly stock car. Your Audi vs. any 928s4 stock for stock , is a waste of good rubber. No offense meant ,just drive it for what it is like I drive my 740 for what it is- Kevin

GlenL

I think you mean

a = power/ (mass x velocity)

and

a = F/m

Makes perfect sense as F = power/velocity

You need to know the velocity at any instant in time.

tommytomaso

<----is guessing he hasnt driven a well tuned 928 ...and by the way unless my audi has a chip..ill have to drive it to 156mph to find out..lol its faster then the 4.2 bmw 7s,(iL is the slowest of the lineup)i test drove them too...way nicer ride allround then my audi but $$$$$ and no all wheel drive. iv seen so little write ups on the 93 v8quattro...all most all write ups are of the 3.8 v8s...and thier slow!...but please dont counter me unless you drive one! Oooo..and if you do ,remember to put it in sport mode and at 60 slam the gas and drop one gear...you may find a surprise. QUOTE:I'm not trying to get in on this but I have a 1997 BMW 740il that seemsFaster than my auto 87 s4. I have g-teched them both and the auto 928 s4 is a half sec. quicker to 60, and will flat out run the beemer at anything over that. Now according to avail. info. my 97 740il is a full sec quicker than your 93 AUDIv8. to 60. Let's bring in my 87 928s4 5SPD. It , on the same test and all things being equal will run circles around the auto928 s4 and especially the 740. (now that I got it running again) This car consistently delivers 5.3 to 60 and is a beast for aa mostly stock car. Your Audi vs. any 928s4 stock for stock , is a waste of good rubber. No offense meant ,just drive it for what it is like I drive my 740 for what it is- Kevin

mark kibort

Tommy,

what im saying is the truth on paper and on the track. If i had a dollar for every guy that though his car "reved" faster or was quicker for alll the mods they did based on FEEL, id be a rich man. the dyno doesnt lie, nor does the 1/4mile timers, or the road course timers. if you change gearing and you are faster, you did the right thing. and what you did, was optimize the amount and time your engine spent at near max hp. After all, as I said, HP IS torque through the gears at any speed. what this means is that two cars with the same hp (or operating at the same HP level) at any speed, regardless of their engine torque, will accelerate exactly the same and with the same net torque to the rear wheels. (through the gears). (note: as long as the gear spacing and HP curve shape is proportionately the same) Now, if the hp curve is not the same shape, as in many comparisons with a viper vs a GT3RS, then the flatter HP curve will allow for more hp to be applied to the rear wheels over the same speed range, alowing for more torque to be developed to the rear wheels in those speed ranges too.

Its really not that complicated. if your Audi "feels" faster, thats one thing. now if it is quicker/faster, then there is something wrong with the S4s you have been racing or driving. the S4 has a flatter HP curve , shown by it having near 60 more ftlbs of torque and 20 more hp. its average HP to the wheels is going to be in the same range of advantages unless you have a close ratio gear box, which you dont.

on the physics side, there is not one person that can argue or prove what i have said is wrong. I deal with engineers on a day to day basis and have worked with these equations and practical applications for over 20 years, and so far, no one has debated these facts. But, ive had hundreds of intuitive racers and gear heads debate this stuff to no end, only to have most come back and say, "your right".

You and many other talk about "torque" and f=ma. however, what most forget is that its the torque (or force) at the rear wheels at any vehicle speed that determines the acceleration. So yes, it is f=ma, but its f=ma at the weels! a short cut to this is very simple...............HP. HP is equivilant to torque through the gears at any speed. Acceleration = power/(mass x velocity) . what this means, is that for any constant power, acceleration goes down proportionately with speed. It also means acceleration is directl proportionate and inextricably tied to power. If you could use f=ma catagorically, acceleration would be constant like on a rocket, but it isnt now is it!. Just go to the simulation page. plug in your torque values based on your torque curve. plug in the gear ratio numbers, rpm shift points and tire diameters. you can see the effects of gearing on a 1/4mile. the results will surprise you! gearing optimizes the hp you put to the wheels over a operational speed range period.

The best example i can think of, is the 550hp vs 550hp viper vs porsche GT3
the peak torque of the viper has 600ftlbs and the peak torque of the gt3Rs is 300ftlbs. yet, in all gears they can both put down the exact amount of torque at the rear wheels. if both cars are the same weight, gear ratios are the same spacing, and HP curves are the same shape, both cars will accelerate exactly the same at all speeds. In reality, the GT3RS has a more peaky HP curve, so the slight advantage goes to the viper, BUT, the GT3RS makes up for it by closer gear ratios, so the average HP put to the wheels over the speed range is the same, so same acceleration.

Please put this in front of any knowledgeable engineer or instructor, and try and find an error in the above.

thanks for the comment about my racing, much appreciated. I hope you take me up on the technical challenge and give this information credit. the only difference between torque and HP is that HP contains more information so you dont have to pull out the calculator!

Mk

mark kibort

yes, Ooops, a little dyslexic

and

Power= Force x velocity too!

thanks

MK

tommytomaso

iF WE ONLY LIVED IN COMPUTERS..lol. im gussing im confused by some of your posts ,saying how gearing or TQ dosnt mater..but unless your telling your computers how each motor is built..ie:how much mass internaly is being used to make power the figures will aways be off. and you must see that in the real world everytime you hear.""DAMM BUT THE COMPUTER SAID!!""computers are dumb..they only deal with the info they are given. but you are way versed in this topic i bet youv explained this alot.....damm your response should be a paste..lol and still your wrong on some points!!!... and Right on some........2 cars =weight=HP=speed but difer displacements or TQ will not
accelerate at the same rate..if your computer says so...its missing inputs , >>ALL THINGS EQUAL ARE NOT THE SAME...besides..how many times have you seen things on the track that didnt fit your computer layout?.......me i see it all the time in CAD design>>>but hay botom line your info is informitive on these principals ,even if thier missing a whole lot of the puzzle, but im guessing thiers not a message board large enough for all that info!!!!!!... keep up the good work....and try not to think your always the right one. = )

mark kibort

Hey,

I think you are beginning to understand. Sure, specs and calculations never exactly match reality, and your right, those calculations are only as good as the information input.
However, the things im talking about are true and i use same ratios, speed, gear spacing, same HP , to make a point. the real point is, is that engine torque tells only a fraction of the story of its acceleration potential. torque does matter, but you need to know the torque to the ground through the gears.

as i mentioned, all you need to know is HP and compare HP. (more accurately said, same HP to weight ratio and average HP used over the engine speed range ). torque from the engine can also be used, but you need to know vehicle speeds and gear ratios to get the actual net torque to the wheels.

comparing a 316hp/315torque S4 928, where most are dynoed at 265 to 275rwhhp and near that in torque, proves that the specs are not that far off.
Now, if your audi can produce these numbers to the ground, you would have a race. however, there are about 15-20% losses in a 4x4. that 285hp you quote is probably pretty close to right, and will net out 250 at the rears if you are lucky. this means, you probably wont be able to hang with a S4 after you are rolling. I contend that you probably have a big edge out of the hole!!!!

now, find the point where you think im wrong. yes, two different torque cars, as i have shown, can accelerate the same. 550hp for both, but one with 1/2 the engine torque, can put down the same exact torque to the wheels if they are both at rpms of near max HP and at the same speed. want racing proof?? look at the vipers vs GT3RS porsches in ALMS competition. they are dead even on acceleration, yet the announcers sometimes say at a slow turn, "watch the viper and all that torque blow by the porsche" and what happens, they are dead even off the turn, down the straight and at top speed. why??????? they both have the same HP!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! (or in some cases, HP to weight, and thats why that ratio is used to class racing!)

Again, HP is comparitively equivilant to torque through the gears at any vehicle speed.

MK

Kevin Michael

The 740il has a 4.4 v8 with 282hp and 296tq, not 4.2 there's no sdch animal.

AJK

Mark is correct!

Bill51sdr

Haven't we had this discussion a time or two, three, four... ad nauseum??? Just to keep it on topic, when i was first looking for a 928, I first drove an 86.5, follwed be an 85, and then an 84. The 84 was a slug compared to the other two. Can't say for sure if all contenders were running optimally, but it definitely influenced my decision to buy a 32 valver.

tommytomaso

Topics done killed over cooked finished...lol...jim picked his car...i dont think we could beat this horse anymore.....and kevin yes im corrected...4.4..i keep forgeting thier a little larger..its thier hp and tq ratings so close that made me think they were the same displacement. I drove a BMW 2002 740I sport...man that was a nice car..but too much$$$$... but id trade my Audi for one..(with traction control)