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Louie, 11 inches if intake tract sounds very short to me. That is about what we use for high spinning motorcycle engines. Looking at your power curve the optimum tuned (3rd harmonic) intake length should be about 15-16 inches. I know that is quite difficult to achieve under the hood but why are people so shy of modifying the hood?
Your air box looks very nice and you are all right that the air is entering the bellmouth from the side.
Everything you say is spot on, but what would Louie gain by having a 4-5" longer runner? Read what he said his goals were, his intake nailed it. Longer runners may have given him X more torque at Y RPM....would the extra power really be necessary? Not to Louie and it's his car, especially at the visual expense of changing the hood.
We can spend a lifetime chasing perfection based on models and theory, at the end of the day as long as the engine performs as desired, what does it matter how you get there?
If this were all an exact science, every engine in every car (foreign or domestic) would look the same.
As far as not wanting to change the hood, part of the challenge of these projects is staying withing certain perimeters, like not modifying the hood. To some people, that is "cheating" in a way.
Turbo Todd set out on day one to build the most powerful 928 he possible could without making external visual modifications (like keeping the stock wheels and not flaring the fenders).
Bottom line, some of us have no interest in changing the hood. You don't have to understand why, we just don't want to.
Originally Posted by Strosek Ultra
It seems there are two different schools regarding the design of intake system in the old vs. the new world. Over here I believe we are more familiar with individual intakes thinking of all the Weber carbs being installed on almost any high performance engine since the 50´s. Over there on V8-engines you are still using a single four-barrel carb with what we think a weird kind of intake manifold.
I can only assume you are referring to Nascar and muscle cars with the 4-barrel comment. That's due to regulations and nothing else. If Nascar teams were allowed to do anything they want, no doubt a very radical intake by comparison would pop up on those engines tomorrow.
As far as the "old school" muscle car world. A lot of what "they" do is for looks and nostalgic building. They know newer technologies exist to make more power, but at the end of the day that is not always ideal or the overall goal. If X-hp is "enough" for their goal, how they get there is irrelevant even if some other way would technically be more efficient.
I don't think it's an "over here vs over there" mentality either. People like Carol Shelby were installing individual Webers on engines back in the early 60's.
Also, I don't think you appreciate how well some of those 4-barrel intakes work. They may seem archaic to you, there is more technology into the design of some of those than they get credit for.
Everything you say is spot on, but what would Louie gain by having a 4-5" longer runner? Read what he said his goals were, his intake nailed it. Longer runners may have given him X more torque at Y RPM....would the extra power really be necessary? Not to Louie and it's his car, especially at the visual expense of changing the hood. We can spend a lifetime chasing perfection based on models and theory, at the end of the day as long as the engine performs as desired, what does it matter how you get there?
The benefit from longer runners in Ott's engine would be more average power over the power band and thus a faster car. If one wants a less peaky torque curve, the longer runners could be made with more taper, which would smooth out the peak.
As a counterpoint, at least simple simulation programs don't produce that large difference in average power with the runner length, as long as the total intake tract length is longer than say 9 inches and we use cams such as Devek B2. Why might this be? 9 inches or 15 inches of total intake runner length might both work about as well for the BDC-IVC tuning, given the cams. 15 inches might work better tuning the IVC-IVO period to give a high intake valve pressure at IVO, but with good headers and high-overlap cams one can already suck such a massive vacuum in the combustion chamber that IVC-IVO intake runner is relatively unimportant and the intake runner can be designed just considering BDC-IVC tuning.
Now, as a counterpoint to a counterpoint, with 15 inch intake track length, one might be able to move the IVC later and get more average power... I'm sure there's a counterpoint to the counterpoint of the counterpoint somewhere.
I personally hate altering the exterior of the car, especially if it can be avoided.
In terms of cooling, if you drive the high-powered 928 throughout the continental US, the front bumper unfortunately needs more holes in it. The cooling capacity doesn't work out otherwise, even if you put in all the heat exchangers on the earth in the engine bay. If you double or triple the air ingested by the engine thru those small holes and then double or triple the heat that needs to be shed into the remaining air flow, it's not going to work with the cooling air flow that is already marginal for the GTS. There's simply not enough air flow thru the stock S4 and later bumper cover, at least I haven't figured out a way to get there. One needs more air flow, and the options are opening up exit holes in the hood and/or increasing the cross-sectional area of the holes in the bumper. Inlet holes low in the bumper cover area is much less conspicuous, so that's my preference. Hood vents are totally in your face.
The hood modifications are something that I would really like to avoid. The good news is that one should be able to get relatively large cross-sectional area runners to fit under the 928 hood in a cross-ram configuration. I think the best configuration there is a cross-ram manifold with two inline-6 long plenums feeding four runners each and the remaining plenum bottom area used for a very large balance pipe. This means either ITB system with 8 throttles for large overlap cams or two throttle body system for cams with less overlap (at least on street). The large diameter balance pipe is needed to avoid undesirable pulse tuning effects at some rpms with the unequal firing order. That should fit under the hood, right?
Note that the cross-ram configuration that I believe is possible with fuel injection is much harder to pull off with Webers because Webers need to be mounted either horizontal or vertical, which really reduces installation flexibility. With Webers, it's probably unavoidable to go thru the hood.
Hi SU,
Of course you are theoretically correct. However, the effect of an intake runner isn't binary. That is no effect until resonance is achieved, then Bam nirvana. Any runner length improves performance right off idle RPM due to the mass of the column of moving air continuing to fill the cylinder as the intake valve closes. Even though the short 11" runner may not achieve resonance on some harmonic of the fundamental until 7500 RPM, I still get a nice benefit at 5000 and above and that is all I wanted. Another consideration is that if your resonance RPM is within the normal operating power range of the engine, off resonance RPM will probably produce a torque hole somewhere and I didn't want that. Keeping the resonance RPM above where the engine normally operates will give a flatter torque curve. I used 50mm TBs which are a bit small for 6.5L but also keeps the velocity up to maximise benefit with shorter runners. I also wanted to keep torque below 500 Lb Ft for driveline longevity. I did exceed the 500 lb ft a bit but so far, no problems. The engine is happy in traffic as long as the outside air temp is below 100F and very happy anytime the throttle is opened.
One odd thing is that when doing dyno tuning there is a horrendous very loud and sharp snapping sound above around 5000. At first, I was freaked out thinking it was some destructive thing happening. After a lot of analysis (mostly trying to get close to this monster) to determine what was actually happening. I realised it was an acoustic phenomenon as the intake was coming closer to resonance. Maybe the very fast air column ramming into the closed intake valve? I don't know for sure and it doesn't seem to harm anything. With a cover over the air horns you don't hear it at all.
That's about all I can add. While this may not be optimum in any one parameter, it does fit perfectly with the criteria I set out to achieve. I do have long 20" runners on my 3250cc Corvair flight engine for a torque peak at 2800 RPM. Yeah, I know too short.
Originally Posted by Strosek Ultra
Louie, 11 inches if intake tract sounds very short to me. That is about what we use for high spinning motorcycle engines. Looking at your power curve the optimum tuned (3rd harmonic) intake length should be about 15-16 inches. I know that is quite difficult to achieve under the hood but why are people so shy of modifying the hood?
Your air box looks very nice and you are all right that the air is entering the bellmouth from the side.
It seems there are two different schools regarding the design of intake system in the old vs. the new world. Over here I believe we are more familiar with individual intakes thinking of all the Weber carbs being installed on almost any high performance engine since the 50´s. Over there on V8-engines you are still using a single four-barrel carb with what we think a weird kind of intake manifold.
Åke
I am not really supposed to divulge this as it cost me time and money, but what the heck, I never planned on getting rich by selling ITB'ed 928s...
We dyno-ed the race engine with short trumpets similar to the ones in the picture first. And it drove ****. No other way to describe it. From memory it made about 330rwhp. We tried, and I mean we really tried to make something decent out of it. We were stuck at 330-ish...
Then accidentally we knocked one of the trumpets off the ITBs and it felt underneath the car. Getting it back, meant unstrapping the car.
This forced us into attaching the BMW produced long trumpets. By then we had given up - all the fancy lightweight high CR JE Pistons, Colin's cams, 968 valves, Flowed heads, 33lb/hr injectors were in vain. We had one shot at glory....
WHAM!
60rwhp and a ton of torque
after another 2hrs - 434rwhp and shed load of torque and a proper 7000rpm screamer, which makes mince meat out of most cars...
So yes, short trumpets on our NA cars are useless...
I know it is too much to ask for a configurable 928 32v intake, but how cool would it be being able to build/configure an intake which suits a 6.4L engine, as well as one all the way down to a 5.0L?
I know it is too much to ask for a configurable 928 32v intake, but how cool would it be being able to build/configure an intake which suits a 6.4L engine, as well as one all the way down to a 5.0L?
A guy could add what ever length bells or runners he wanted, we were only providing the flange and initial taper.
As it says on the webpage:
Adaptable: Our intake runners are designed to make your final intake system configuration a breeze. Mounting flange already provided for Individual Throttle Bodies (ITB's), or single or dual-plenum manifolds - whatever you like. And, because of the strength and location of our reinforcing fiber, you can have a large single plenum without concern that head movement during block expansion will damage it. Top flange already grooved to receive sealing O-rings (included).
Obviously for my build the hood can be altered. The whole car is already on roids as it is, a hood bulge won't look strange on this build and I am eager to see what performance gains can be made.
I know it is too much to ask for a configurable 928 32v intake, but how cool would it be being able to build/configure an intake which suits a 6.4L engine, as well as one all the way down to a 5.0L?
Fine tuning of the power curve can be done by having intake trumpets of different length. Easier fabrication is extensions that can be made in different lengths and installed between the throttle body and the trumpet. There are more ways to alter the intake duct length. The most advanced way is telescopic intake ducts that alter the length with the engine speed. Believe Ferrari is using it on some models.
Åke
05-24-2017, 12:51 PM
