Boost vs Flow: Which to choose?

Someone posted a boost related topic on EvoM and I offered this response ...

Quote :

Running more boost lower at lower RPM actually produces a flatter airflow curve. So, if the turbo can flow, say, 39 lb/min (9.8 16G) and you run a flat 21 psi boost profile, you will slowly increase flow until it peaks. If you run 26 psi down low and taper the boost toward redline you can flow more air earlier and hold it longer.

So, for example:

21 @ 3500 and 20 @ 7000
25 lb/min @ 3500 and 36 lb/min @ 7000

26 @ 3500 and 20 @ 7000
30 lb/min @ 3500 and 36 lb/min @ 7000

Which makes more power through the powerband?

It really is something to think about. I've been using this method for a while and it's exactly why I choose to use the stock boost control system.

Basically, any turbo has a maximum RPM and flow rate. We know this. But many people don't understand how these relate to boost.

Too many people assume high boost = excess heat. At lower engine RPM the intake valves stay closed longer, allowing more boost to be built with less airflow. This basically means that you can run more boost with a lower turbine RPM and therefore no extra heat is generated.

Of course, if you run super high boost down low you will cause more heat to build toward redline, so this method should be used in moderation.

The basic thing to consider is that turbo system on the compressor/intake side is very linear. If the turbo spins at X RPM, it will flow Z lb/min.

The variables in the system are both engine related, and they are RPM and exhaust flow. Basically, the greater the engine RPM, the more air it takes to pressurize the system to a given PSI. The greater the exhaust flow, the greater the RPM of the turbo and the more air it can flow.

This means that there is a point of diminishing returns once the turbo reaches it's maximum flow rate. At that point, the increasing engine RPM can no longer spin the turbo faster so the flow rate becomes static, therefore the boost pressure cannot be sustained.

Just something to think about ...

So, given the two examples, your torque curve would be flatter using higher boost at lower RPM vice low boost at low RPM? If this is correct then this would be the "hot lick for a street (stop light to stop light) car. It would also seem to me to be a bit harder on the drive train as the twisting forces are higher lower down the rev range as horsepower is torque X RPM.

For me personally, I would prefer a powerband that did not have a narrow torque curve of 350 lbs ft and spike to 400+ WHP in the last 2K RPM of my powerband but gave me 350lbs ft of torque over a 2500 RPM range with a resulting more gradual rise to 400+ WHP.

Have I grasped the concept?

You are correct. With this method you'd have much more torque down low with basically no sacrifice.

Essentially, with a standard MBC you are leaving power on the table on the low end. This is simply because of their linear nature. If you want 25 psi on the low end, you have to deal with that same 25 psi on the high end. This does nothing but create excess heat after the midrange and that leads to knock, power loss, and a shortened turbo life.

Lemme work up some graphs real quick ...

Here is a graph to illustrate my point ...

Not to be slow on this (it's early and I'm working on my first cup of coffee), I would want boost to taper off to match / maintain maximum flow to reduce heat. In order to make higher horsepower I would need to start with a higher level of torque (boost) so once my boost curve flatend out RPM would take me to my horsepower peak?

This is sounding a whole lot like the way diesels make their power, albeit at much lower RPM's.

Yeah, diesels settle for huge torque down low because they will likely never max out the turbo with their limited revs.

I'm not sure what you mean by "match / maintain maximum flow to reduce heat." Maximum flow means maximum turbine RPM which means more heat. Sustaining maximum flow too long will definitely cause excess heat and knock.

You could also modify this technique a little and try to run as much boost as possible down low (28 or 30 psi) and taper down the boost even lower on the high end. This technique would offer huge torque from 3500 to 4500 but reduce top end HP. You would have to reduce flow up top to manage the heat.

TouringBubble wrote:

I'm not sure what you mean by "match / maintain maximum flow to reduce heat." Maximum flow means maximum turbine RPM which means more heat.

By maximum flow I meant there is only so much air that can flow through the cold side regardless of how fast you turn the engine over. There by you reach a point of diminishing returns by turning the engine any faster and only create more heat.

Some engines (VW 1.8T / 2.0T) the turbo (K03) is actually a restriction in the amount of air that the engine can ingest. By doing nothing more than replacing the K03 with the one from the S4/RS4 (K04) the amount of horsepower and torque increase disproportionally for the small difference in the size between the two, all else being equal.

My thinking is match your turbo maximum airflow to the maximum engine RPM and then you would have maximum flow through the turbo at or near redline without the penalty of generating excess heat.

Or would it be better to get a turbo that will flow more than the engine and only boost to the desired horsepower & torque levels using electronics? This too me seems to be inefficent and wasteful.

JWRICH4 wrote:

My thinking is match your turbo maximum airflow to the maximum engine RPM and then you would have maximum flow through the turbo at or near redline without the penalty of generating excess heat.

Or would it be better to get a turbo that will flow more than the engine and only boost to the desired horsepower & torque levels using electronics? This too me seems to be inefficent and wasteful.

This seems to be very logical thinking. Although, some may opt for the bigger turbo if they plan on future upgrades to other components.

JW, I think we are talking about the same thing in different ways. You are correct, to get the most from the turbo you would push it to flow to its maximum potential ASAP and hold it there for the entire rev band while not attempting to push it beyond that point.

And yes, if you could get a turbo that would maximize its flow potential right at the highest engine RPM you wanted to run you would never pass 100% flow point and never be able to push the turbo too far. That is one way of approaching the situation and likely the best approach for a track car or similar application.

I think it's not a problem to run at 100% flow for a short period of time. That way you pull 100% potential from the unit for some period of time. If you had a bigger turbo, you would never reach 100% and that seems like a waste to me as well. But, this is more of a "do the best with what 'ya got" approach.

Aside: I always thought the 20v head on the 1.8t was the bottleneck. I know the K03 sucked (especially my tiny Beetle version), but the head was barely flowing enough to spin that tiny thing up by 2800 RPM so swapping in the K04 was still not a huge upgrade unless you paired it with head work. At that point, you may as well call up APR and get a StgIII.

I'm glad they went back to a 16v on the 2.0t. If they used twin scroll or variable geometry turbos, they might have a decent car ...

HMMMM APR!

I'm trying to work out how I will upgrade my Evo and the hows and wherefors of increased performance. It most assuredly makes power differently than my A4 so I have to back up and regroup from my initial assumptions.

Without a convincing argument otherwise, I plan on building the engine into and overbuilt anvil and then upgrade the output as I see fit. For me personally, right now, I want an engine that makes somewhere around 400+ WHP and mid to high 3's in torque. I would love to reverse that and have a torque monster but I have yet to read anything about a torque laden'd Evo. This should, given the plan, allow me to have OE reliability and good performance, if I need more "Oh Shit!" I've got the Busa.
The thing driving this train is to get assigned back to Europe once my wife is done with school. I don't want a peaky tempremental car over there, did that with my MR2, it was scary fast but it was no fun to live with as I was always working on it.

Even though RalliArt's European HQ is in Germany the Brits are the ones over there that are making big power with street Evo's. The drive is 8 hours from central Germany and the logi$tics of shipping large parts or the car for tuning back and forth is expen$$$ive. If I can make it bullet proof to start with I would be more likely to "exercise it fully" once we got there.

Txs TB for letting me pick at your brain.

Yeah, there isn't really a way to get that much torque with the 2.0 and not have even more HP. You'd actually have to de-tune the top end for that and it's wouldn't make much sense.

You can reach the numbers you are looking for with a FP Green turbo. But, if you want to over build it, you might want to look at something a little bigger like a GT30R. They are supposed to be very streetable and make 430+ on pump easy. Pair it with a nice twin scroll mani and you're set. Trae is the turbo guy though .. not me.

You could also go with a 2.2 or a 2.4 to get more of that torque you want, but that might be going a little far .. I don't know what you are comfortable doing since reliability is important. But, honestly, a 2.4l would spool a TS GT30R like the stock 16g and provide a super broad power band. You'd likely see 400+ tq @ 3500 and 450 hp @ 6800 or so.

Without meaning to get Joey all excited, I like stroking it (stop snickering back there).

I know it will lower my redline but it is a trade off that seems like a win - win in the end (STOP LAUGHING!).

Seriously, this whole thread has gotten me researching stroker kits, spent the better part of today doing just that. Much money but it just makes too much sense to get to where I want to go and still be civilized enough to be a daily driver. I've read the articles on the Mitsubishi UK FQ-400 and everyone talked about a really nice car but the engine was peaky, in the end each article said the FQ-360 was a better way to go on the street and ended up being quicker around a track.