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I think that there are two strategies to create the same streetable torque curve. The first is to have short intake runners and a smaller turbo hotside. The second is to have long intake runners (more generally, an intake tuned to low rpms) and a larger turbo hotside.
The first has become the dominant solution with fully variable valve timing in production cars made by car factories. The variable valve timing and variable boost control together are so versatile tuning tools that the priority is now just to get the turbo spooling early.
Without the variable valve timing, though, the second method is about as good as the first one. The ability to use a variable geometry resonance intake strengthens the case in favor of the second method. As we all know, S4 has no variable valve timing but the stock manifold has the flappy.
One could make the argument that the second strategy leads to high exhaust back pressure at high rpms, because the boost has to be ramped up above 6000 rpm. However, what people making that argument fail to realize is that the first strategy also leads to a high exhaust back pressure, just for a different reason: the small hot side.
The pump gas fuel adds the consideration about charge temperatures. The second strategy requires more intercooling and benefits more from intercooling because the high boost charge gets temporarily very hot and then gets cooled twice: first by intercooler and then by the low-pressure manifold anti-tuning.
One more factor that points towards the second strategy is that the compressor high efficiency island on the compressor map wants higher boost at higher mass flow rates:
While the compressor wants to make boost, these young ladies want to go tubing:
I think that there are two strategies to create the same streetable torque curve. The first is to have short intake runners and a smaller turbo hotside. The second is to have long intake runners (more generally, an intake tuned to low rpms) and a larger turbo hotside.
I have observed the exact opposite on my 3L 944 turbo, and using a GTX3576R 0.82 like you, fwiw.
I have observed the exact opposite on my 3L 944 turbo, and using a GTX3576R 0.82 like you, fwiw.
The exact opposite of there being two strategies to achieve a given broad torque curve or something else? That people build/tune their pump-gas street cars like race-gas race cars for a high and narrow power band and then can't take advantage of the turbo because of hitting the knock limit?
For a similar torque curve, short runners work well with large hotsides while long runners work well with small hotsides.
I think that we have either miscommunication issue or a genuine disagreement. Let me try to clarify.
I’m trying to build an engine that has a flat torque curve in the 3000-8000 rpm range. I think that there are practically two strategies to do that: short runners, small turbo or long runners, big turbo.
What people usually do is combine a small turbo with long runners and get 2500-5500 rpm monster torque on race gas or combine a large turbo with short runners and get 4500-8000 rpm monster power again on race gas. In my opinion, these methods make more torque or more power than what I’m planning, but only in a narrower power band. And, in my opinion, they require high octane race fuel or very low compression that makes the car not fun to drive outside the power band.
Is the above consistent with what you are experiencing / observing?
No disagreement. You make theoretical speculations that I also made before trying things for myself and out of the 4 configurations considered here my experience is that the two you are considering will get you neither monster torque nor monster power. Just my engine though. For now your engine has long runners and large hotsides, but the long runners will prevent the turbos to flow healthily way beyond 6000 rpm, and I mean without wasting energy in heat. I will be happy to be proven wrong though!
By the way, it’s about the last days for anyone to see the car in Northern Kentucky at John’s shop. There’s going to be maybe five more days of torture for the car on the dyno there and then the car will head back to MA again. Dead or alive, depending what five more days on and off 8000 rpm does to the car.
On the question where we are on the compressor map
Here are some estimates where we are on the compressor map, at 650-750 (93-octane pump gas) rwhp and 6000-6100 rpm, depending on the experiment:
We have a smaller compressor cover than what the base compressor map indicates and therefore the x-axis shifts by about 10%. The location on the map is estimated from all three map parameters: The measured pressure ratio, mass air flow, and turbocharger speed.
Next question is what is the best direction to move from here. To stay on the high efficiency parts of the compressor map, I believe we should follow our plan and move northeast, that is, ramp up the boost with additional rpm beyond 6000 rpm. With a short-runner intake we’d be moving east, and get to lower compressor efficiencies quickly.
I think that there are two strategies to create the same streetable torque curve. The first is to have short intake runners and a smaller turbo hotside. The second is to have long intake runners (more generally, an intake tuned to low rpms) and a larger turbo hotside.
The first has become the dominant solution with fully variable valve timing in production cars made by car factories. The variable valve timing and variable boost control together are so versatile tuning tools that the priority is now just to get the turbo spooling early.
Without the variable valve timing, though, the second method is about as good as the first one. The ability to use a variable geometry resonance intake strengthens the case in favor of the second method. As we all know, S4 has no variable valve timing but the stock manifold has the flappy.
One could make the argument that the second strategy leads to high exhaust back pressure at high rpms, because the boost has to be ramped up above 6000 rpm. However, what people making that argument fail to realize is that the first strategy also leads to a high exhaust back pressure, just for a different reason: the small hot side.
The pump gas fuel adds the consideration about charge temperatures. The second strategy requires more intercooling and benefits more from intercooling because the high boost charge gets temporarily very hot and then gets cooled twice: first by intercooler and then by the low-pressure manifold anti-tuning.
One more factor that points towards the second strategy is that the compressor high efficiency island on the compressor map wants higher boost at higher mass flow rates:
While the compressor wants to make boost, these young ladies want to go tubing:
Just starting to look at this thread. Started on the last page, of course, since the things at the beginning may/may not still be viable.
My suggestion:
Do what the ladies want....
Happy wife, happy life.
Right now, it’s set up to stay open above certain load threshold and rpm threshold all the way to the fuel cutoff rpm.
That's promising. Let's see how closing it past a certain rpm may widen the [peak torque rpm ; peak power rpm] window and hopefully reduce the torque drop, which seems quite dramatic now.
08-18-2020, 09:46 AM
The strategy
