Question for those with engine fundamentals

[Bigglezworth]

The displacement on the C5 RS6, RS4, and R8 are all the same @ 4.2L. Same stroke and same bore for each. The RS6 makes different power due to forced induction while the RS4 and R8 make the same. My question is this: Why is the RS6 engine designed to rev 6500rpm while the others 8000 rpm? When you take the engine management out of the equation along with the forced induction, you are left with a long block that for some reason is designed to perform at lower RPM's. What factors are behind this? Cam duration? Vale spring rates? Certainly the main and rod bearings/bolts are designed to handle the higher revs so what is it? What I ask?? Just curious as it would certainly be nice to get the engine revving more in line with what it can do in the other high power Audi's.

Tim

[DuckWingDuck]

Good question, AMG uses a 6.3L engine for all their cars. Though I do believe the AMG 6.3 all rev the same....

[Mr Balsen]

If you let the RS6 Engine revs to 8000, the turbo boost will be so high that the top head will not survive ! ! !

Frederic

[DuckWingDuck]

Does the R8 not have turbos as well?

[Bigglezworth]

Does the R8 not have turbos as well?

R8 is NA and not FI.

If you let the RS6 Engine revs to 8000, the turbo boost will be so high that the top head will not survive ! ! !

Frederic

This is inaccurate. Boost is a result of atmospheric pressure. At sea level a turbo charged car will need to spin less than the same turbo charger at a higher altitude. Turbos are spinning many thousands RPM and are not affected by engine RPM. Blow-off valves and wastegates are in place on FI cars to dump additional pressure being pushed out from the engine specifically to address this. Don't forget that Formula 1 cars ran FI engines for a while and those engines turn double the RPM of anything on the street....

My NSX turns just over 8000RPM and it's FI.

[Bigglezworth]

If you let the RS6 Engine revs to 8000, the turbo boost will be so high that the top head will not survive ! ! !

Frederic

This is inaccurate. Boost is a result of atmospheric pressure. At sea level a turbo charged car will need to spin less than the same turbo charger at a higher altitude. Turbos are spinning many thousands RPM and are not affected by engine RPM. Don't forget that Formula 1 cars ran FI engines for a while and those engines turn double the RPM of anything on the street....

My NSX turns just over 8000RPM and it's FI.

[ben916]

This is inaccurate. Boost is a result of atmospheric pressure. At sea level a turbo charged car will need to spin less than the same turbo charger at a higher altitude. Turbos are spinning many thousands RPM and are not affected by engine RPM. Don't forget that Formula 1 cars ran FI engines for a while and those engines turn double the RPM of anything on the street....

My NSX turns just over 8000RPM and it's FI.

F1 cars run $hitty at 6000 rpm's, they need to run at 14k-19k. The engine is relatively small for the amount of power. Most importantly is the the stroke is short. From what I remember, the longer the stroke and high rpm's = boom...

The turbo has no knowledge of the altitude, it operates on the exhaust gases of the engine. The mass air sensor and computer determines the air/fuel mixture to the engine. The inlet air at lower altitude might be denser and thereby hotter but an intercooler helps...

Not a scientist or expert. There are others out there and on here that could describe better than I.

[Bigglezworth]

From what I remember, the longer the stroke and high rpm's = boom...

What you are referring to is piston velocity and yes longer stroke engines have a higher piston velocity than those with shorter strokes. Given that my question was specific to the same displacement engine with same bore and stroke this engine fundamental doesn't come into the equation. If we were talking about a stroker motor then clearly the rotational speed of the engine wouldn't be as high no.

The turbo has no knowledge of the altitude, it operates on the exhaust gases of the engine.

Accurate yes. But - a turbo does have knowledge of boost. Or more accurately the ECM does. Until such time the engine is receiving the prescribed amount of boost, the turbo will continue to spool up and build pressure. At higher altitudes that will take a little more RPM than at lower altitude. In the end though the turbo will still build the same amount of boost regardless of the RPM of the engine. Once the boost pressure is met the wastegate will bleed off exhaust pressure.

The inlet air at lower altitude might be denser and thereby hotter but an intercooler helps...

Intercooling is completed after the air is compressed and will assist engines at all altitudes.

Taking the discussion of boost out of the context of my question, I still am curious on why Audi has opted to run a lower RPM on the RS6 than the RS4.

Tim

[Mr Balsen]

Of course this is accurate, the boost pressure at 8000 will be much higher thant 6500 !!!

The exhaust flow will be much more important, therefore the turbo will turn much higher, creating much boost.

RS6 Engine is not ready to take that amount of extra boost and heat. Exhaust manifolds being the weak point...

In order to reach 8000, you will have to lower the compression ratio of the engine. But then the engine will not be powerfull as low rpm.

Frederic

[rs-mad]

Boost at redline ~6500rpm is at most 0.8bar with a free flowing exhaust only way to increase this is upgrading the wastegates the major issue with increasing rev limit in my opinion is the transmission.

[PeterJohn]

To protect the turbo from spinning too fast and/or building a pressure too high for the inlet.

You could install a wastegate, but why? The wastegate doesn't increase the power curve. It does the same thing the RS6 does with its ECU. If the ECU cuts out at 6500rpm you'll get the same effect as the wastegate opening at 6500rpm.

A turbo that spools up to 8500rpm will be a small one. You'll sacrifice low end grunt for high end power.

[Bigglezworth]

Boost at redline ~6500rpm is at most 0.8bar with a free flowing exhaust only way to increase this is upgrading the wastegates the major issue with increasing rev limit in my opinion is the transmission.

Well this thread wasn't suppose to be one about boost, but everyone seems to be stuck on it a bit. Boost is boost. You can increase the amount of boost that is applied to an engine by modifying the ECM code that controls the MAP sensor and any exhaust valving you have with your turbo. I have no knowledge of the Audi tranny to know if this is accurate. I would find it hard to believe the extra 1500rpm would comprimise the performance of the tranny. Possibly when you get the the tall drive gear for overall final output RPM, but not the lower.

Of course this is accurate, the boost pressure at 8000 will be much higher thant 6500 !!!

The exhaust flow will be much more important, therefore the turbo will turn much higher, creating much boost.

RS6 Engine is not ready to take that amount of extra boost and heat. Exhaust manifolds being the weak point...

In order to reach 8000, you will have to lower the compression ratio of the engine. But then the engine will not be powerfull as low rpm.

Frederic

I'm sorry Frederic, but this is entirely inaccurate. Compression of the engine has nothing to do with RPM of the engine nor turbos. As I've mentioned before boost is boost. What you are stating is that an engine only produces the maximum boost when it is at full engine RPM. Boost is set by the ECM code to a prescribed amount. The ECM controls items such as the MAP sensor that recognizes what pressure the manifold air pressure is receiving through the intake stream. As the engine speeds up, so does the speed of the turbocharger, and the pressure increases. When the pressure in the intake after the turbo reaches a predetermined level some of the exhaust will start to bypass the turbine via the wastegate valve, so the turbo won't reach a selfdestructive speed, and the engine won't get too much boost for it's own good. Depending on the size of the compressor on your turbo the build-up of boost could be quick or delayed. The Audi KKK turbos have a relatively small compressor which is why turbo lag on the RS6 is negligible. Boost builds quick. I have a MASSIVE compressor on my Buick and it doesn't start building boost until almost 4000rpm. Then it's solid on boost all the way through the remaining RPM range which was increased through internal engine modificiations with the cam, timing, and valve spring pressures.

To protect the turbo from spinning too fast and/or building a pressure too high for the inlet.

You could install a wastegate, but why? The wastegate doesn't increase the power curve. It does the same thing the RS6 does with its ECU. If the ECU cuts out at 6500rpm you'll get the same effect as the wastegate opening at 6500rpm.

A turbo that spools up to 8500rpm will be a small one. You'll sacrifice low end grunt for high end power.

All turbocharge cars have a wastegate. The photo below is that of the KKK turbos off an RS6. You can see the wastegates on the exhaust turbine. And both small and large turbos will rev 8500rpm. The difference in size is related to the amount of air the turbo can move under the prescribed boost setting. This is the downfall of many a hot rodder when they put more turbo on their car than the internal engine components are able to handle.




For those reading this thread that want a clearer read on fundamentals of turbo charging, you can read this link. It sums it up for pretty much everything and talks the areas a couple of you are getting caught on with turbos and rpm. http://www.wrc-cosworth.org/tech/tur...bocharged.html


I'd really like this thread to get back on track and talk about the engine fundamentals of the RS6 vs. the boost and ECM matters. If I bought an RS4 engine I could turbo charge the engine with either a single or dual set-up and control it with a computer. Something is clearly different in the set-up of the RS6 than the RS4 and I'm just trying to get a handle on what it is without having to phyically talk one of each apart and disect them. Tag you're it guys.

Tim

[Mr Balsen]

I disagree with you but anyway...

I don't get the prupose of your thread. The RS4 engine is revving up to 8000 because Audi decided it was the right rpm for 420hp. Audi has done 4.2l V8 for years now. It is a combination of marketing / industrial / technical / financial decisions ! They also decided that the RS6 had to have 450hp and they could get that amount of power at 6500 rpm. Why in hell would you want a turbo engine to reach peak power at 8000 if we are talking 450hp ??? 450hp for a 4.2l twinturbo engine at 8000 rpm is absolutely ridiculous...

If you want your engine to reach peak power from 6500 to 8000 rpm, you will have to change pretty much everything. And will end up with a lot more horsepower. Knowing the Gumpert Apollo engine (from MTM) reached more than 900hp at less than 7000rpm, you can guess for a 1000rpm more...

Formula 1 Turbo engine at 1hp for each 0.1liter at over 15000 rpm....

Frederic

[Bigglezworth]

I don't get the prupose of your thread.

Sorry to confuse the matters as that was not intended. This thread went off topic when discussion about turbocharging was brought into the discussion. What I am looking to get a stranglehold on is what makes the internal makings of the RS6 engine differnt than that of the RS4. What you are stating is that Audi merely programmed each to operate differently to market a car. They were fine with the claim to 450hp so they said we can do that at 6500rpm. All likely true. Just not what I was trying to better understand.

Being an owner of many turbocharged cars, and a power hungry car enthusiast, I always look to methods to increase opportunities for more power/acceleration out of my cars. My Buick is a 4.1 Stage II which is comparable in displacement to the Audi and puts out amost twice as much power with a single turbo. It this with a tremendous amount of $$ spent on internal engine workings. Prior to delving down any of the paths regularily crossed by the 'bolt-on performance parts' owners, I wanted anyone with any insight on the Audi internals to provide some insight on the differences. I am not above modifying an Audi engine to beef up it's internals more than the stock offerings and stuff it all into the RS6 in place of the stock. Something to pass time if you will.

Anyway, this all said I have NO desire to tread on anyones beliefs about their Audi and in no way mean to offend anyone. I plan on owning my RS6 for a very long time and discussion boards like this one are a great place to exhange information and share car talk.

Regards,
Tim

[DuckWingDuck]

tim, certainly don't want to speak for anyone else but for me this entire conversation (off-topic notwithstanding) has been extremely informative

[Mr Balsen]

but you can increase the power of your engine without changing the internals of your engine ! ! !

It is not by using the internal of the RS4 V8 in your RS6 engine that you will increase the power output. Nor by increase the peak rpm. Again, the Gumper Apollo, which has been the most powerfull RS6 based engine topped more than 900hp, but not over 7000 rpm.

You can be power hungry but not rev hungry...

Frederic

[Bigglezworth]

but you can increase the power of your engine without changing the internals of your engine ! ! !

It is not by using the internal of the RS4 V8 in your RS6 engine that you will increase the power output. Nor by increase the peak rpm. Again, the Gumper Apollo, which has been the most powerfull RS6 based engine topped more than 900hp, but not over 7000 rpm.

You can be power hungry but not rev hungry...

Frederic

That is agreed. Just think what that additional 1500 rpm does for your acceleration times though. Even top speed would be increased (albeit very few places in the world anyone could put it to any use). When I run any of my cars at our local roadcourse it is advantageous to get through the lower gears and accelerate in as quick a manner as possible. With an additional 20% in RPM I would get that much more distance out of each gear in an increasing velocity and with an increasing amount of power (assuming all the engine dynamics are supportive of the increased air flow of course). Running my turbo Buick at 450hp with a 6250rpm redline certainly didn't get me ahead in the race. BUT, running my turbo Buick at 450hp and allowing the engine to rev to 7800rpm allowed me to change the gearing in the car that in turn helped me accelerate quicker which then helped me get ahead in the race.

Cheers
Tim

[Jani]

Main reason is the transmission. That can not handle more than 6500 rpm, that's why the engine is designed for this rpm as well.