The Technical Questions Thread

[drakkie]

I am not trying to start controversy or anything. I'm being serious here.

I would like to remind everyone that it is a good idea to research on your own what you have "learned" in this thread. There are a lot of half-truths being told...in particular by hightower99. I'm not picking on you hightower, I think it is cool that you want to know about cars. But lots of your info isn't 100% correct. Sorry if I offended anyone.

Basically that is the same as making notes in class and then dump it somewhere, because you remember anyway Lots of things discussed here are allready knowledge for me (most from 1st year of AE), so i tend not to interfere too much. I do read it, because there are sometimes interesting things.. It's true there are quite some mistakes made here, but I think it is corrected good enough most of the times anyway. We are almost all still studying some kind of engineering and we can't know everything

By the way, kingofthering, did you mean something like this in your post above ? If you want any specific information about it, I have good contacts with them, which may be of help.

Power Shift AMT (Plug-in Powershift Module for AMTs) | Technology & Benefits

[hightower99]

I am not trying to start controversy or anything. I'm being serious here.

I would like to remind everyone that it is a good idea to research on your own what you have "learned" in this thread. There are a lot of half-truths being told...in particular by hightower99. I'm not picking on you hightower, I think it is cool that you want to know about cars. But lots of your info isn't 100% correct. Sorry if I offended anyone.

I definately agree that everyone should do their own research and not blindly accept anything said (especially here) but there are many people here who can tell you what you want to know.

Also I don't mind if you think that some of my info is not 100% correct (that is a possibility) but I would ask that you point out what info you are talking about in a polite manner as sometimes I do only answer a specific aspect (normally the one I think is relevant) of the question that might be considered a "Half-Truth".

[drakkie]

I put online 42 MB of literature. The rar-file includes many aticles about automatic, sequential and DSG gearboxes, from various sources in the industry. Most articles are in German unfortunately, but plenty of English ones to make it worth the download. Enjoy.

http://stud.hro.nl/0783155/literatuur.rar

[kingofthering]

By the way, kingofthering, did you mean something like this in your post above ? If you want any specific information about it, I have good contacts with them, which may be of help.

Power Shift AMT (Plug-in Powershift Module for AMTs) | Technology & Benefits

No, I was actually thinking of something similar to the Audi R8 racecar, where there's a clutch and paddles.

But thanks.

[hightower99]

No, I was actually thinking of something similar to the Audi R8 racecar, where there's a clutch and paddles.

But thanks.

The R8 has a AMT (Automated Manual Transmission) which is shifted by paddles. It has a clutch because it gives the driver better control during stop/starts (for example in the pits). The clutch pedal is not used during gear changes during the race.

[Kitdy]

Ok, this is being brought up again. When is maximum acceleration reached? Peak torque or peak hp?

I have read that it occurs at peak torque, and hightower has said this to me as well, but if this is true, why are cars launched at redline (roughly where peak hp occurs)?

[hightower99]

Ok, this is being brought up again. When is maximum acceleration reached? Peak torque or peak hp?

I have read that it occurs at peak torque, and hightower has said this to me as well, but if this is true, why are cars launched at redline (roughly where peak hp occurs)?

Do a search and you will find a thread where all is explained but basically a car will have the best acceleration (i.e. 0-60mph in the least amount of time) if you keep the engine close to peak power. However the highest instantaneous rate of acceleration (i.e. m/s^2) occurs when the engine revs are at peak torque for any given gear.

Best performance has to do with power!

Read my signature!

[Kitdy]

Do a search and you will find a thread where all is explained but basically a car will have the best acceleration (i.e. 0-60mph in the least amount of time) if you keep the engine close to peak power. However the highest instantaneous rate of acceleration (i.e. m/s^2) occurs when the engine revs are at peak torque for any given gear.

Best performance has to do with power!

Read my signature!

Why then if peak torque rpm produces peak instantaneous acceleration does the car accelerate faster when launched at peak horsepower rpm?

[fpv_gtho]

Could be related to the revs dropping as the engine hooks up

[hightower99]

Why then if peak torque rpm produces peak instantaneous acceleration does the car accelerate faster when launched at peak horsepower rpm?

Thing is that most cars get a better launch when the launch RPM is close to peak Torque RPM.

I suggest you either go to a local drag strip and ask somebody to show you their notes which should include launch RPM (if they are anything more than total amateurs) or try drag racing yourself (at a proper track at the proper time and taking all neccessary precautions) and note which launch RPM gets the best ET/trap speed. It's very difficult to launch a car at peak HP RPM and not suffer traction problems off the line.

Quick example: When I do the odd drag race I find that I get the best ET when launching with 3500-4500RPM (depending on track and weather conditions aswell as car setup). If my Launch RPM is higher than that then all I get is either smoked tires or a worn clutch from having to slip it excessively.

[megotmea7]

Launching a car at redline is not the best idea if you want a fast start.

On lower torque cars it sometimes done(to the detriment of the driveline) to produce the fastest acceleration possible, this is because the rotating mass of the engine/flywheel stores energy as is spins, the faster it spins, the more energy stored. This energy is translated into forward motion when the clutch is engaged, traction permitting.

On higher torque cars, this is not always necessary as the motor produces enough torque to get the car moving quickly without having to take advantage of stored energy via the flywheel/rotating assembly.

Its a balancing act to get the car moving as quickly as possible but at the same time, not loading the tires to the point that they lose traction and just start spinning. Releasing stored energy from a high rpm start is just a way to make up for an engine lack of torque and take advantage of as much available grip as possible. or to keep a turbocharged motor 'on boost', but thats another subject.

[Kitdy]

Ok another one for you guys.

What is the relationship between boost pressure and horsepower increase roughly? Linear, or some sort of curve?

[jediali]

I would say its non-linenar becasue of all the variables involved (frictions, gas properties with changing conditions, compression ratio issues, ability for fuel to burn...)...but not massively.

[hightower99]

Ok another one for you guys.

What is the relationship between boost pressure and horsepower increase roughly? Linear, or some sort of curve?

Boost pressure alone will not boost power...

However if you injected more fuel as well as increase boost then you raise power.

FI is a way of increasing the displacement of an engine without changing its physical size.

For example a 2L engine that runs a turbo system that runs at 2:1 pressure ratio (meaning that if the turbo is fed with 14.7psi absolute pressure it would be putting out 29.4psi absolute pressure (14.7psi boost)) is basically acting like a 4L engine (2x2). Contrary to popular belief turbos do not provide a certain amount of boost, they create a pressure ratio that you can control and the final boost is a function of the absolute pressure that the turbo intakes multiplied by the pressure ratio.

So increasing boost does not normally lead to a linear increase in power... Depending on many variables the relation can be linear under some conditions but mostly boost : power ratios normally look more exponential... (i.e note that it can work both ways ie you could need to increase boost alot just to get alittle more power or if you increase boost slightly you could see a massive increase in power).

Again this is one of those questions that has several highly complicated answers and variables.

[culver]

OK, I have one for hightower99 (and others). This is not in any way meant to be an antagonistic question so please do not take it as such. I’ve thought about this my self.

I think we gear heads often think of the ability to rev high as one of the marks of a more sophisticated engine design. But is this always the case? I was actually thinking about this in context of Indy cars in the 70s.

Back in the 70s many Indy cars were powered by Drake-Offenhauser Turbo I4 motors.
Offenhauser Turbo Spec Sheet
Slowly the Cosworth DFV based turbo V8s replaced the Offys at the engine of choice in Indy car racing because the rules started restricting allowable boost pressures. Now the Offys used an integrated head and cylinder design and could withstand very high boost pressures. I don’t recall the peak pressures right now but I believe they were within 1/2 atmosphere of what F1 was running 10 years later. They were also producing over 1000 hp in the early 70s even before Porsche unleashed their monster Cam Am cars. However, the big I4s couldn’t rev very high. I think they were limited to something like less than 9000 RPM. Again, I may have my figures off a bit as this is from memory.

The Cosworth engine couldn’t accept nearly the boost pressure but it could rev quite a bit higher than the Offy. Initially the extra boost the Offys could withstand made up for the lower redline. As the boost got cut the extra revs of the Ford became an advantage. In the end the boost was cut so much that the Offy became uncompetitive.

If we think of the engine as an air pump you can get the same mass of air through the engine in many different ways. With the same displacement we could run high boost and low revs or low revs and high boost. The design of the Offy allowed a very high boost pressure but was inherently unsuited for high RPM work. The Cosworth motor was just the opposite.

Do we consider one way or the other technically better? Thoughts?