Just as a noise comparison. My Paxton is EXTREMLY quiet. At idle and up till 2500rpm it’s totally silent. Especially since I installed a HKS Racing Bypass valve. Before HKS, it was making a lot of noise when the throttle closed. The 2 small bypass valves couldn’t vent all the air and the surging of the SC was very audible. I have to say it did sound wild and mean. Since the swap, there is a slight whistle above 2500prm and of course it’s louder at full boost, but as soon as I push the clutch is it goes silent. This HKS (1.5” dia) valve can vent 4 times more than my previous set up and at the same time it does this with nothing more than a slight sound of rushing air. I bet my car is either as quiet, or quieter than a TT setup. Not all SCs are loud and Paxton was always known to have one of the quietest SCs. This was until Paxton released the Novi system (gear driven) supercharges in to market.

 

I was planning on an innitial tune of 11psi. Can you set them huffers at that CFM to create that PSI?

I'm in to whatever you need for my numbers, be it 1/4 mile, dyno, 30-150 or something like that, but I drive. You can have whomever you want drive yours in in your state.

The car will be hand built down to every nut, bolt, and finish on the body and everything in it -

- so nobody else drives it.

Like I said, I will submit any info, and it can be verified if you want, but no secondary drivers for me.
I spent a year just painting this thing, let alone all this other work and time spent getting the best stuff at the best value to put together for a solid barn burner.

 

Thanks for all the response, I now have a clear idea that the TS, for me, with an auto & 2.20 rear end is the way to go.

Any grudge matches will be closely watched !

My main point concerned fuel consumption in the cruise, again thanks for good feedback.

 

My arguments about the merits/demerits of various boost methods relative to the 928 application are fairly well known and available with a simple Rennlist search – so I will not re-hash them here right now.

But – I am going to post two new topics I have not yet seen discussed in the last 4 years of supercharging and turbo charging conversations. ( I apologize in advance if you have discussed them…. then I have missed that thread).


I'm surprised sometimes how this community will debate the merits of 1 5/8” vs. 1 ¾” exhaust primaries and the size and length of the collectors at length – but act as if the same laws of fluid flow do not exist on the intake side of the engine.

Intake Manifold design involves the size of the intake runners in diameter, their shape, their length, and the size and location of the plenum and its entryway. It is an involved science whose goal is to generate (in a street-driven automobile) a long and enjoyable power band.

This stuff is not knew – from Vic Edelbrock to Smokey Yunick – you can grab dozens of books that describe how short intake runners benefit one end of the RPM spectrum as long runners benefit the other.

Like most automobile manufacturers – Porsche uses un-even length primaries in order to extend the power band as far as it can. If all the intake runners were the same length, the engine would have a very narrow power band in only one tight RPM range. This is unacceptable for most applications. So, they’ll design a pair of primaries that are tuned to pull best at 2,000 to 3,000 RPM, a pair that is designed to pull best from 3,000 to 4,000 RPM, and so on – to develop the drive characteristics that you desire.

Example: The 16v Porsche 928. Two of the intake runners are 38.6 cm long. Two are 37.8 cm long, two are 35.7 cm and two are 35.3 cm.

You do not have to take my word for it. Go out to your garage and pop the hood on your 16v 928. Look specifically at the intake runners for cylinders 1, 3, 6 and 8. Why are those 2 extra bends in there? Why aren’t those straight? Because it matters!

There is no doubt in my mind that a big open box manifold would be a lot less expensive to manufacture – so they must have had a very good reason to tune individual intake runners like this. And we know what that reason is.

Hours and hours went into dyno work and flow bench testing of the 928 engine by the fine engineers at Stuttgart (who are not known for slipshod work) and it would be arrogant of me to unbolt and discard their intake manifold with the exclamation that “I know better”. (Ask yourself why, if a single large intake plenum is a good idea, you never see it anywhere on any production vehicle.)

The 32v manifolds are no different – it’s just that the individual intake runners on the 16v motor are much easier to see from the outside in order to make my point. The 32v intake manifolds are beautifully tweaked and tuned inside – take the side covers off and see for yourself.

…and please do not say “but boosting overcomes all that”. The laws of physics, in this case fluid dynamics, are absolute and putting the intake under pressure does not “fix” things – it CHANGES things. Some problems fall to acceptable levels, new problems surface.

One of these new problems is the increased impact of the intake pressure waves.

This is a key element in Horse Power production that all power train engineers know all too well. As the air rushes to your intake valve, it has a certain mass and of course, momentum. Suddenly that intake valve slams shut and a shock wave – called a pressure wave – is caused by the charge air crashing into the back of the closed valve and bounced backwards up towards the plenum.

That pressure wave, if not tuned out or taken advantage of, can literally prevent that cylinder from getting enough charged air the next time that valve opens.

Boosting does not overcome these pressure waves – it magnifies them.

This is why the tops of intake runners are tulip or bell-mouthed, and why each individual intake valve has its own runner, separate from the other intake runners – so the pressure wave created in valve A does not harm air flow to valve B, and so on. Even just the shape of the bell at the top of the runner has profound effects on how well the cylinder fills and at what RPM ranges.

The Porsche 928 further amplifies this problem because of their peculiar firing order, firing two cylinders on the same bank in succession. Look at the firing order that the 928 has; 1-3-7-2-6-5-4-8, all the cylinders are nicely staggered away from each other until we get to #6 and #5.

Here, we fire 2 cylinders back-to-back that are right next to each other. If you have an open tank manifold with no intake runners, it is more than probable that the pressure wave coming out of the head for cylinder number 5 will interfere with cylinder number 6 getting the correct charge.

The results will be some cylinders that run too rich at certain RPM’s and yet run too lean at a slightly different RPM.

The motor with no intake runners and a massive single plenum may appear to do very well at WOT but, in fact, even then some cylinders will be too lean while neighboring cylinders will be too rich. The O2 sensor installed in the collector will not show it as it is measuring combined gases at that point.

But the Twin-Screw works, you say. Yes, they do. They do start, run and drive.
A quick ride in Tony’s wonderful twin-screw 928 or one of the Jag-Eaton 928’s definitely shows that they do.

But, consider how deep Tony went into modern electronic engine management and that allows him to adjust and tune his injectors singly. The advent of modern electronic fuel systems has made it possible to tune out a lot of problems and return the car’s drive-ability. Tony is an exceptional home-mechanic and has talents (and patience) that not all 928 owners have.

Still, Tony’s best HP and torque numbers are not as high as Tim Murphy’s best HP and torque numbers – (meaning absolutely no disrespect to Tony and his awesome installation) – but I suspect that is because Tim’s centrifugal system is benefiting from Porsche-tuned intake runners and Tony’s twin-screw system is not.

I have noticed most of the Jag-Eaton twin screw’s are running 5 psi and doing well. However, they may find that at 8 psi they cannot fuel it or tune it correctly without going to all the trouble that Tony went to. An open-box intake plenum like theirs without any runners will behave wildly different at different pressure and CFM points.

Remember that most of us are taking our air-fuel measurements at the collector – and if the air/fuel ratio is right, we think we’re good. But, a lean cylinder (like number 6 in twin-screw) will be camouflaged out if the other cylinders on that bank are rich enough. If I recall, Tony installed 8 individual EGT sensors on his 928 to specifically measure this – I just wonder how many new owners are willing to go to these lengths.

Well anyway – these are two more reasons why I think that the centrifugal is a better choice for the 928 application. Not better for all applications, better for this application.

And that can easily change! If we had the willingness to cut a hole in our hoods – even THAT changes the application remarkably. If I cut a hole in my hood, then I could have different-length individual runners, and a nice fat intercooler and a Whipple on top of it and that would be BOSS. Of course, it’d be 18” high and I’d look like a fool, but there wouldn’t be anybody faster.

But – I do not want to carve a hole in my hood so I can raise my twin-screw up to fit in a proper intake manifold, so I think; once again, the centrifugal is a better choice for this application.

 

Carl,
By your argument it seems that the centrifugal SC using the stock intake may be a more elegant application and there's a SUGGESTION that the "brute force" approach of the twin screw MAY cause problems.
Hypothetical example: cyllinder 1 is tuned for maximum efficiency at 3000 rpm. That suggests that it will be leaner at certain rpms and richer at others. At some rpms the pressure wave will be more efficiently handled than at others.
Has anyone graphed the individual cyllinder's afm at various rpms? It seems to me that temperature measurements should give this information. It would be interesting to see the tolerances accepted by the Porsche engineers.
Then there's the effect of increasing air flow volumn/velocity by adding, for instance, an X pipe. What will be the effect be on the pressure wave?
Doesn't all the careful dyno intake work done in Stuttgart become moot once the exhaust parameters are changed as well?

 

Very interesting. If the whole scenario changes with higher pressures, then either system needs tuning. The Porsche manifold was not designed with boost in mind.

I knew Tony was collecting a lot of data but I didn't know he was tuning per cylinder.

Tony, have you found issues with lean/rich cylinders that you have had to tune individually?

 

It seems to me, that all of this "new to the 928" adapted technology needs some more track record before ANY judgement can be made. Not just a few units here and there, but several over the course of a year or two. Then, tear a few engines down and see what the adverse (if any) affects are. I also think based on what Carl points out, there needs to be a definite measurement of activity, no matter the kind, happening in each runner, valve set and cylinder. Yes, pulling plugs will tell a lot, but this just tells us a result and the remedy(s) need to be addressed. I agree that throwing more "logs on the fire" isn't necessarily the best or most efficient way to make "heat"

In essence, the technology has been adapted, but now comes the lengthy time of fine tuning to see what is actually best and most efficient. Look at any proven technology out there...it took more time than most of us know to get to that point. Slapping an induction system or exhaust may/should provide more power, but HOW is it effective? This is of course where the fine tuning, many many hours of trial and error and a bit of cash are thrown into the process.

I think it is going to be a bit longer before anyone can truely answer the question of..."which is better?

 

I agree that the intake runner length is important on a naturally aspirated engine, on a supercharged engine it can be compensated with boost - when having enough boost the engine will get all the air it can suck pushed right in there. Your arguments make sense with a supercarger that does not give instant boost unlike twinscrews. Hearing the 'romp-romp' off idle acceleration of Jim_H's GT with it installed was intoxicating! At idle it had a respnse like a 1000cc Superbike!

With an efficient twinscrew supercharger there will be more than enough pressurised air for the combustion at any rpm.
If the intake runners are so important I am surprised that Ford, Mercedes-Benz, McLaren, Jaguar, Koenigsegg and others are use the same type of intake design as Andy Keel.

 

No, I'm NOT a boostard.

-I've always believed that the best way to make more power is to make the engine bigger. I don't have any plans for forced induction on my '85 for several reasons: It's too expensive, it wears the engine, my compression's way too high, I'm not a great driver/I'll get myself in trouble, etc. At the same time, turbochargers have always fascinated me. I love the way they come on in a wave- its like the power develops exponentially, and there is no connection between engine sound and the way the car pulls. It just feels like something is pushing you all of a sudden.

One of my previous cars, a 1985 VW Scirocco, had a turbocharger. Installed by the dealer and covered under the factory warrantee, this system was nothing but trouble. It knocked on 90 degree days, despite the fact that it only ran 6 lb of boost and the compression ratio was 8.5:1. It burned exhaust donuts up like they were paper [they were~], and the system needed constant tuning.

-The 2100 pound car accelerated like a bottle rocket due to its 140 hp and 170 ft/lb of torque....and it even got 28 mpg....one mpg more than it should have.

That instant torque from a turbocharged engine was uncommon back then, but it is typical now, since manufacturers have figured out how to build lag-free engines. A properly set up street turbo will accomplish anything you want to achieve, and will even increase your mileage if the car is driven the same way after the conversion as before. Greater efficiency due to the additional stage of compression causes this.

There's a guy on here who's built a perfect afermarket turbo 928, and another who's getting ready to start selling kits. Both have the right idea when it comes to forced induction as far as I am concerned.

As to superchargers: Carl, nice writeup regarding the harmonics of the intake manifold. But forced induction makes the light supercharging effect of gas momentum a relatively smaller effect, to the point where it doesn't matter anymore. I suspect that Tim Murphy/Lagavulin are seeing such high hp/torque numbers primarily because their blowers are so efficient, not because their systems still include the harmonic intake.
Twin screws are less efficient, but build boost in a way that overcomes one of the shortcomings of the 928's design: tall gearing. The instant boost wakes up low-rpm torque and makes this car perform as if it had far more power than it actually does. Centrifugals are more efficient...but have an advantage at very high power levels, since this car has an IRS rear suspension. This type of suspension is prone to axle hop in drag race conditions, and an engine that lays the power down a little more gradually might have an advantage.

Any clearer yet?

N-

 

I hear this a lot. And unfortunately, it is only half right. While it IS TRUE that a boosted motor will over come restriction in the fluid lines (intake gaskets not aligned, rough welds on the inside of the pipes, sharp bends causing turbulence just above the valves) it is ALSO TRUE that the pressure wave coming back from a closed intake valve is more intense on a boosted motor - in direct relationship to the amount of boost.

You cannot overcome this shock wave by adding boost. The more boost you add, the stronger you make your own pressure wave. They match.

The pressure wave occurs because of the momentum of the air/fuel charge slams into the backside of the intake valve that suddenly shut. It flows back up the runner, and Powerplant Engineers try to turn it back around and have it hit the back of the valve at the precise moment the valve is opening.

Its interesting to note that the term used for this in the engineering textbooks is "natural supercharging".

The Powerplant Engineer turns the pressure wave back around by the size and shape of the bell at the opposite end (the plenum end) of the intake runner. The bell causes the pressure wave to cross sides and start back down.

If they can adjust the length of the intake runners just right - they achieve "natrual supercharging". The pressure wave rolls into the back of the valve just as it opens, and a larger than ambient charge enters the combustion chamber.

Note I did not say "larger than atmospheric" .... I said "larger than ambient" -
if there is 5 psi in the pipe.... and you tune the intake runner correctly (as Porsche has) you still benefit from the pressure wave helping you fill the cylinder over and above the boost alone. Unless, of course, you have removed the intake runners completely.

 

IMHO - I think the selection of boost for the 928 (whether it be Turbo, Twin Screw, Roots, or Centrifugal) comes down to one thing: sex appeal.

For whatever reason.... if an owner determines that turbos are "sexy", then, by God, he's going to go turbo and push through any and all problems he encounters along the way (hopefully before his wallet is empty).

If an owner feels that twin-screw is "sexy" - same thing. He's gonna twin screw and thats that.

The perceived sex-appeal drives the choice, creates single-mindedness, and seems to cause selective hearing. That's OK. Its human.

We all know that they can all be made to work. The only arguement is at what cost in a) time and b) money?

I just want the 928 ower considering boost to know what they are getting into beforehand in each camp.

Statements like "turbo is the way to go because it s the choice for all OEM manufacturers so that proves it" just chaffs my throttle body.

What a bunch of bunk. I feel bad for the rookies that hear stuff like that and take it as gospel.

Let me finally get this off my chest about that line of logic.

First: there is no relationship between the two - a retrofit to an existing car and a ground-up new design. Any car company can start with a blank sheet of paper and design an engine to build into a specific induction system - but that does not mean their decision (based on Marketing) is also good for our retro-fit project (based on Engineering). Geez.

We are talking about retro-fits here... an aftermarket application to an engine that was not originally designed for boost. That's what we need to concern ourselves with here - not what the OEM makers choose to build.

Second: the statement that "all manufacturers choose turbos" isn't even correct. Some do, some do not. Its a function of sex-appeal (like I said at the top of this) and what sells cars to the public.

I'm sure you would agree that GM is a big company and they can afford to put anything they want on any car in their line. Their choice for a boost unit for the Pontiac Bonneville? Twin screw. Worked very well.

I'm sure you would agree that Ford is a big company and they can have any boost unit they want. Their choice for the Ford Mustang in their Ford Motorsports division? A Centrifugal supercharger. Specifically, the Powerdyne Centrifugal supercharger (the one I sell in my SC kits). Thousands and thousands of these sold, (more than any other aftermarket SC kit in the world) and they run very well, thank you.

How about the DOD? Uncle Sam can afford anything. Their choice for the P-51 Mustang in WWII? Centrifugal blower. Saved the day. Won the war for air supremecy over Europe. Their choice for the M1-Abrams? Turbine boost.

My point... starting with a blank sheet of paper, a good powerplant engineer can design around any boost unit they desire. But their decision making algorythm does not match ours - and we should not accept their choice as also automatically being good for us.

 

What an elegant post, and I am so impressed by the opinions, comments, and collected wisdom and experimental data made by all the participants.
It is true that wave resonance are define by length and tube diameter. And truly any discussion regarding modifying exhausts should take into account intake manifold modifications. whenever an engine modification increases the average area of the airflow paths into and out of an engine, there is a chance the velocity of the flow will decrease. Most of the time the factor of velocity decreases is very small compared to the area increase, so flow is generally increased. Intake flow is define by exhaust velocity in the normally aspirated engines. Specific Port Flow(cubic meter/sec) = Flow Velocity(m/s) X Average Path Area. Indeed the beauty of the work done by the Porsche engineers in regard to the 928 are only applicable to normally aspirated engine. In aspirated engines there is a vacuum on the intake side ,as the driving force is the positive exhaust pressure. In the boosted engines you overcome this with positive pressure, and any deficits are overcome by brute force as opposed to efficiency of design. Because my typing is so slow, this is a restatement of both Ander's and Norm's post. And I had missed the last two posts by Carl that are great!
This post came at a good time for me as I was vacilalting as to which car to boost, now I have no qualms and I'm with Miles... "I now have a clear idea that the TS, for me, with an auto & 2.20 rear end is the way to go."

 

While all of Carl's points are perfectly legitimate, I think the engineer in him is disgusted that it's the "prettiest" (twin screw) installation that's getting the most positive response.
Dave Roberts' marketing and customer service skills have diverted all the attention from the centrifugal setup even with no dyno results.
I'm not suggesting that one setup is better than the other, but Carl's correct that "sex appeal" (human nature) wins over intellectual rigor (science) every time.

 

You can say that but it's not why I chose a TS. I got a lot of input from some very knowledgable 928 people but I won't put them on the spot by naming them. I chose what ultimately, to me, would be the easiest to maintain and work around. I also wanted the SC that matched my driving habits. I think saying looks is what the deciding factor for most of us is not giving the boostards enough credit.


I'm sure there are a lot of arguments back and forth on this. What chaps my hide is that besides the 3 posts the TC guys keep on the front page they make sure and throw the same argument into every SC thread and any other thread they can invade. At least that is the way it looks to me.

 

I'm speaking only as a potential, and hopefully, future buyer of a SC kit.

I don't know much about boosting or engineering. But I think the "sex appeal" factor is very relevant. In fact it might be most relevant factor after the pricing.

I've been thinking along time about putting an SC on my car. The numbers between the two systems are relatively close. The variances seem to be installation to installation rather than system to system. Both system are put out some good numbers. I don't plan on going to the track or drag races. I'd like a little more umph to keep the import guys out here off my butt. Personally, I'm leaning towards the Murph kit. Why? Well, I like my powder coated Porsche intake. And basically, I think the TS intake is ugly. Even the shiny one with the stickers on it that DR did. When I look at a TS install I think, "dang, he may as well have put a chevy engine in there." Does this sound silly? Maybe, but you've got guys on this board that wouldn't buy a 928 because it had side moldings on it.

I find it interesting that guys go ga ga over the shiny TS set up from DR. Ernest brings up a good point about 928 Specialists marketing and selling the TS kit. The fact that one of the Big 3 has picked a kit to sell lends a level of validity and credit to that kit. I don't mean this as any kind of an insult to Andy K, but my impressions of his operation were that he was working out of his garage. I think he's done a tremendous job, but I would have had reservations about trying to install one of his kits. I would have been worried about how long he might be in business. What kind of support there would be, etc. 928 Specialists have alleviated those concerns. A very smart move on Andy K and DRs part.

Now, I do think that Carl has gone to great effort to elevated 928 Motorsports to the level of the Big 3. A smart move on his part.

It is apparent that both systems work well. Most of us wouldn't be able to tell the differnce between the two unless we were on the track. You guys can argue TS, CF, turbo, pressures, the bends of pipes, efficiency, waves, or whatever. For me, what will get me to 400rwhp, is easy to install with good support, no permanent changes, pricing, and looks good will get my business.

What each owner wants generally seems to be what he argues is the best.