Thom

Who needs a 3k rpm-wide powerband on a track car, especially with a close ratio gearbox such as the 968 NA's...

refresh951

It is all about trade-offs. I am willing to trade a bit of mid-range to make a bit more power up top. I have a 95mm stroker so I have a very healthy bottom end/peak torque. I actually dialed my tune back around peak torque because I worried about damage. I think I can push peak torque dynamic charging a bit farther up in rpm with shorter runners and be just fine. The dyno will show if I am right.

TonyG

My point was that you'd be lucky to have a 3k wide power band. Wider is better. And wider is faster unless you have a sequential gear box. Shifting takes up way too much time. But that's all besides the point.

The point is that the turbo engine is a N/A engine until it comes on boost. Thus you can tune the runners to bring up the VE of the engine off boost, to generate more power and thus more exhaust gas to spin the turbo sooner.

After it's on boost, the intake tuning with respect to resonance is irrelevant. At this point you're looking for flow equalization across the runners and perhaps the ability to taper the runner down to increase the speed of the air. And most importantly, that the runners that don't present a restriction (long or short). Thus the reason to flow the intake port with the intake runner to achieve an overall flow model.

TonyG

Thom

This is where you are getting subjective - wider is better for you, but some other who are comfortable with the gearshift may prefer a higher peak on a narrower rpm band.

The way I see it is that there is little to no point maximising ram effect off boost on a turbo engine of "large" capacity, as this prevents you from getting the most of what a turbo can bring to your engine, all the more so if you are running a 3L engine that is necessarily going to be fast enough off boost to avoid needing ram effect at lower rpm. This is of course true on a race engine where you will want to maximise peak power, but also true on a road engine where 3L still provide enough torque off boost with short runners to keep you below speed limits.

TonyG

It's not subjective. It's a fact. The only reason for close gear ratios are to keep an engine with a narrow power band in the rpm range where it makes power.

The narrower the power band, the closer the ratios.

And it's not a matter of being comfortable with shifting. It's that shifting takes time. Quite a bit actually. And it doesn't matter how fast you shift. Anytime you have to let off the gas to shift you slow down. If you ever race against cars with no-lift shift sequential gear boxes (assuming yourself don't have one...) you'll see just how much time you lose ever time you shift and let off the gas and they don't. It's not much... but it adds up real fast. This is the exact reason that people spend $35k for these gear boxes.

The wider the power band, the wider the ratio, the less shifting, the less time lost shifting, the lower the lap time.


First off, it's not about maximizing peak power. It's about usable power under the curve. Peak power is great... but peak power doesn't get you off the corner fast.

Secondly, for an intended operating rpm range, apples-to-apples, if you can effectively use a tuned intake to get the turbo to spool a few hundred rpms sooner, which either gets you a wider operating rpm range, OR you can then move up the turbine trim size, putting your turbine spool back where it was, and get more power.

As far as road engines go.... Nothing sucks more than an engine that makes huge power up top, but makes 120ft TQ at 3000rpms. That gets real old real fast.

TonyG

Thom

No, it is subjective. No one needs a 3k rpm-wide usable power band except people who can't/don't like/are too lazy to change gears.

Sure, and this is what I was saying 2 posts earlier.

The problem is that you make it sound all black and white, with nothing in between, which suggests to me you haven't actually played with this topic on the field. Porting and flowing the stock intake on a 3L engine is as useful rearranging the deck chairs on the Titanic. Well, I don't know... I thought the graph Shawn posted made the point well enough?

refresh951

There are good reasons to look at runner length. The SFR intake is a large plenum, short(er) runner intake and many a motor has run this with great success. The SFR intake has a 4.6 L plenum vs about 1L for the stock intake and the runners are significantly shorter (I will have the exact length soon).

If you look at the curve I posted, it is reasonable to look at shortening the runner length a bit and sacrifice some low end and get significant gains up top and the end result being a larger power band. In particular I think of Patrick, who has a dry sump and can run 7500 rpm but his motor is maxing out just past 6000 rpm. I am another case that merits a look because I have the extra stroke to support low end performance. It really depends on your setup and what you are trying to do. Also, it is pretty easy to prove out so we shall see.

67King

It is more the runner length than plenum volume. FWIW, here's one I did on a BMW 3.0L (naturally aspirated):


I know I owe you some info, I promise I'll get it soon.

BC

As king always says, its a system. Want more power up top? Then shorten the runners. But add compression and/or stroke to make up for it at the bottom-mid (torque at 1900 rpm is for towing a Porsche, not driving one)

When I was playing around with some Engine Sim software a few years ago, I notice that on the computer, shortening the primaries and making them a larger ID on a SC engine I was thinking of building made almost no difference from 1-3500rpm. But above that the larger and shorter I went (this is an exhaust manifold mind you) made huge differences.

The intake obviously was not as forgiving.

refresh951

So a bit more flow testing today. Flow tested the 2.7 NA head and got some very interesting results. Like to get some comments on the results.

Looks like the significantly wider port sides on the 2.7 head help the flow to make the turn without separating until fairly high lift. This allows the port to take advantage of flow around the full circumference of the bigger 48 mm intake valve. Once the flow separates, its advantages are somewhat mitigated. I was struck by the different sound of the flow through each of the heads. The 2.7 head is VERY quiet until the curve changes (laminar vs turbulent flow).

My thinking is to use 48 mm valves on the 951 head, widening the port on the sides while maintaining the "D shape". It seams clear that if you use a bigger valve you need a porting strategy that will keep the flow from separating until higher lift in order to take full advantage of the bigger valve. Once the flow separates, most of the flow goes out the back side. I think this is why my mild porting showed nice gains at high lift . By blending the lip between the valve seat and the back of the bowl, flow is not kicked back into the main flow path after the flow separates.

I am starting to see that you really must have two porting strategies, one for before the flow separates and one for after and the dynamics are very different. For example, just looking at peak flow could be very misleading as one could have a strategy to maximize high lift flow that causes flow to separate at very low lift therefore overall performance suffers. In fact, if you just looked at peak flow the 2.7 head's real advantage would basically be missed.

URG8RB8

Very interesting data Shawn, thanks for educating all of us!

Scott H

So apparently I wasn't the first to think about dumping the booster/MC and running a larger manifold using that newly gained space. Here's the engine bay of the Boss car:

refresh951

Looks like stock runner length with the sharp turns removed and a bit larger plenum. The sharp radii can cause the flow to separate which goes along with some of what I posted above. Pretty cool indeed.

333pg333

Nice work Shawn. So do you deduce that the more elongated or Oval shaped port on the 2.7 vs the round 2.5 is perhaps designed to keep the n/a motor closer to the turbo motor in the early to mid rpm stages...or how would the turbo motor respond to the Oval port shape?

Looking at the Boss car, perhaps they kept the longer runners due to the 2.5L capacity but with the 16v head and wanting to retain some early to mid range tq?

V2Rocket

Just noticed the BOSS belt routing...WHOA

Shawn - looks like your mild porting on the 951 head was some decent work. Nice linear flow increase with lift, but up top it is pretty significant. Looks to be between +10 and +15cfm at each of the data points after 0.3, which across 4 cylinders could prove to be a pretty substantial gain.