Abby Normal

</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica"> BTW, 951s (944 Turbos) have the same bottom end as the 944. So it might be safe to assume that a 928 motor can reliably take the extra power too.[/QB]</font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">That's incorrect, nearly all of the internal parts are different spec on a Turbo. It's built for boost and the compression ratio is different.

Lagavulin

</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">By Rich9928p
&gt;On a healthy engine, there is no need to do any of this.

True, but how do you know if your rod bearings are healthy?</font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">Rich, you are right as one really doesn't know the actual condition of their bearings unless physically checked.

However, with respect to the 928, I haven't heard anyone complain about their bottom end bearings when they took the engine down for repairs. Usually they remark that the bearings still look like new, another sign of the robustness of it's design.

I would say then it would be up to the discretion of the owner. If the car has chronically low oil pressure, or the rods initially knock upon cold startup, those signs are indicative of a problem which does indeed warrant attention.

In my case though, the results of the leak-down, compression test, and baseline dyno told us that my engine is in nearly perfect shape, thus made the assumption that the bearings are too since there is nothing indicating otherwise.

Carlos

Guys, rods don't break from cylinder pressure. They break right past top deas center from excess piston speed. It appears that a stock 928 can handle 10 or 11 lbs of boost w/ relative safety. As long as the mixture is right, I think the blower motor is less stressed than the stroker.

LT Texan

the rods can take it. I'd say the weak point is the cast piston. Will they take it? Works with a 310hp Euro S.

I'm not sure why we are discussing the high stress on a part designed to "take it"

Absolutely not the weakest link.

Fastest928

Folks,
A 5l supercharged engine making the same power as a 6.5l engine has a higher BMEP than the 6.5 l engine.

All things equal: bottom ends, head gaskets, cylinder wall thickness, piston materials, etc., increases in BMEP will reduce the life of an engine.

As an example, BMEP is directly related to engine bearing wear...there is a factor called the BMEP/main bearing ratio.

When I began designing for reliable power, I looked at BEMP as a guideline.

A 6.5l, 450 rwhp stroker has a BMEP of about 180 psi, a supercharged engine of 5l and the same hp, has a BMEP of 234. raise the power up to 525 rwhp, and the BMEP rockets to 273 psi. A stock GT engine...150 psi, a GTS, even less!

The fact is, the lower the BMEP the greater the reliability. Increases in BMEP also increases the engines sensitivity...sensitivity to preignition and detonation, oil temperatures and bearing wear, etc.

I am not saying that you cannot design a supercharged engine to be reliable....but, the engine must be designed for reliability! Stroker, Supercharged, hi revving, etc.

Marc
DEVEK

Marc
DEVEK

bcdavis

In every book I have ever read about supercharging, turbocharging, nitrous, etc, they almost always say that lower compression, and a very strong bottom end is *required* if you want long term reliability. Most of these books refer to American engines, but they always say that if you just bolt one on, it will not last long. That if you want reliability, you need to rebuild the engine to be as strong as possible. I know Porsche makes a strong and reliable engine, but I still think an engine rebuild is sound advice. I know there are quite a few supercharged 928's now, with stock engines. But they have not been supercharged for long. So time will tell if the engine will last for 10 more years, like the Calloway did, or if they will blow up. Another thing in favor of the stroker, is that if you slack off in your tuneups, or if something goes wrong with the fuel injection, your motor could be dead in miliseconds. Whereas a stroker will be less sensitive to fuel mixture irregularities.

PeteS

Since there's been some speculating about how much supercharging a stock 928 engine does or doesn't stress the bottom end, does anybody have any proof that it causes a problem? Since there are supercharged cars that have been around for at least a few years already, and John's Callaway turbo for 20 years, have there been any boost caused failures of the bottom end?

Lagavulin

</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica"> By bcdavis:
In every book I have ever read about supercharging, turbocharging, nitrous, etc, they almost always say that lower compression, and a very strong bottom end is *required* if you want long term reliability. Most of these books refer to American engines, but they always say that if you just bolt one on, it will not last long. That if you want reliability, you need to rebuild the engine to be as strong as possible. </font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">Which books are you referring to? I am curious, as I have read quite a few of them.

We are talking about street driven supercharged cars, and the two best books by far on the subject are by Corky and A. Graham Bell. And nowhere do they state it is ‘*required*’ to lower the compression nor beef up the bottom end as long as one works within the parameters of a given healthy engine: it’s stock compression ratio.

An engine’s compression ratio will determine how much boost one can safely run on an engine, which goes without saying, must be in verified good shape. And ‘safely run’ on a street engine always means to NEVER EVER run into detonation.

Here is the direct quote from ‘Supercharged! Design, Testing and Installation of Supercharger Systems’ by Corky Bell, 2001, Chapter 5, page 63:

“Certainly the supercharger will affect engine wear. Do you really expect to add power and not increase wear? No miracles here either. If you drive vigorously but with some respect for the equipment, you can expect about 90% of normal engine life.”

As I stated earlier, if one follows the above, one can double one’s horsepower, but expect a 10% loss in engine longevity. That's like getting a double scoop of ice cream and only paying the calorie price of the original scoop plus 10%, and the rest is calorie 'free'! I’ll take that ratio any day.

You make a very strong statement otherwise. PLEASE provide those books title, author, year of publication, and page number where it says that it is ‘*required*’ to lower the compression and beef up the bottom end for long term reliability on a street driven vehicle so that we too can read it, digest it, make a decision, and expand our knowledge accordingly.

</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica"> By Dan Perez:
I think we can conclude that engines are a complicated (and fun) subject. </font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">If you can’t already tell, I wholeheartedly agree, so let’s continue!

</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica"> By Dan Perez:
But what I wrote seems to be supported by the example in the graph on the link, that is:

Peak cylinder pressure (800psi) is achieved at 22 deg ATDC, and cylinder pressure drops dramatically with the piston movement down to 90 deg ATDC (about 100psi). Heck at 40 deg ATDC, it looks like cylinder pressure is already halved.

At 22 deg ATDC the piston has only moved downwards 3 or 4 mm. At 90 deg ATDC it has moved 44 or 45mm. </font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">According to Z’s first diesel link, you are correct with respect to a normally aspirated engine:

</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica"> From diesel link 1:
The timed spark ignites the mixture and cylinder pressure rises to a peak at roughly 15º after TDC. Because the combustion process takes time, combustion may or may not be complete by 15º after TDC depending on engine RPM, but for all practical purposes, we can say that the process of combustion is concluded early in the power stroke and that no more heating of the working fluid (the gases in the cylinder) occurs. This means the force acting on the piston top is highest at a time when the connecting rod has very little leverage on the crankshaft pin. As the crankshaft continues to rotate past TDC, the leverage the piston can exert increases, but the pressure on the piston top is dropping quickly. </font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">However, when we talk about supercharging an engine where we are cramming twice the air/fuel into the cylinder (..doubling the horsepower), the same principles apply to both the diesel and supercharged engine:

</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica"> From diesel link 1:
Once you envision when combustion occurs and the relationship between cylinder pressure and leverage on the crankshaft, it becomes obvious that if we could continue the burning process longer into the power stroke, additional cylinder pressure could be generated to push on the piston top as connecting rod-to-crankshaft angle improves for more leverage, and hence more torque. This is exactly what happens in a diesel. Because the fuel is injected into the cylinder after the intake valve is closed and the air is compressed, the length of the fuel injection pulse, called pulse width, can be extended well into the power stroke. ... but it is this continued injection of fuel that really makes the big torque numbers for diesels. </font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">With the supercharged engine, we are doing essentially the same thing. Although we do not have the continued injection of fuel as with the diesel, nor quite the dramatic results, we are initially packing more air/fuel into the cylinder which takes time to burn longer into the power stroke, thus continuing to produce torque well past 20 degrees ATDC.

Although two different types of engines, they both make lots of torque utilizing the same principle, which is pushing upon the crankshaft at an advantageous leverage angle.

Keep in mind too that this collaborating info is coming from two independent expert sources: Corky Bell and Gale Banks.

Z

There hasn't been one that I'm aware of. All the supercharged cars that I've heard of blowing up were from broken pistons due to detonation. Forged pistons are stronger than cast ones, but don't assume they're a requirement, or that they'll save you if there are problems with the installation or system. I'd much rather have cast pistons in a well done supercharger system than forged pistons in a poorly done system. Except for Marcus' car having forged pistons, all of the sucessfully supercharged cars I know of used the stock pistons without problems. On the other hand, that white GTS that was talked about previously had forged pistons and broke some before the owner even picked it up from the shop doing the installation. The 944 turbo has forged pistons, and there's no shortage of broken ones on cars that people have messed around with. Meanwhile there are cast pistons in boosted 928s that are still fine after years and tens of thousands of miles.

Even if cylinder pressure during combustion is higher than in a stroker motor, it doesn't necessarily mean that much more force on the bottom end. Don't forget that the 6.4 liter engine also has a larger bore. If the pressure in the cylinder was the same in both cases, that pressure has an 8% larger piston surface area to push on in the larger bore 6.4 liter engine than in the 5 liter engine. Cylinder pressure might be measured in pounds per square inch. More square inches of piston face area means more pounds of force downwards. That means more force being applied downwards against the rod, bearings, and crankshaft.

The supercharged 5 liter would have a higher BMEP than the 6.4 liter engine under boost only. But anytime they're at the same RPM, the 6.4 liter will have a faster average piston speed than the 5 liter. Before the engine even starts, while the starter is still turning the engine over, the 6.4 liter will already have higher piston speeds.

More and more auto manufacturers are going to forced induction, and more specifically supercharging. I haven't noticed any going to larger displacement stroker engines.

Shane

How long do you want to wait? Check on the lead time for both. Last I heard the stroker cranks took some time. Superchargers are readily available. Might payoff to set yourself a target date for your HP upgrade and see what system will fit in the time frame. Be a hell of a patient guy to drop $1500 deposit on a crank and wait six months to get it, then do the install, tuning, etc. Could be winter before a guy could realize the fun factor for his cash output. <img border="0" alt="[icon501]" title="" src="graemlins/icon501.gif" />

Fastest928

strokers = 180 bmep (forged rods, forged pistons, billet forged crank and nascar rod bearings)

Supercharged = 234 BMEP(stock rods, pistons, crank)

800 hp Nascar engine = 228 EMEP

All BMEP are at peak hp...

Current supercharged factory engines are designed form the start as such and carry 100,000 mile warranties.

Marc
DEVEK

Z

Since BMEP is based on the average cylinder pressure, it can mean that the greater BMEP engine has a higher cylinder pressure in the same period of time as the lower BMEP one, or it can have the same cylinder pressure for a longer period of time. As has already been stated several times, with several references, the supercharged engine produces power longer into the combustion cycle. Longer combustion pressure automatically means higher BMEP, even if the peak pressure in the cylinder is the same or even lower than in another engine.

I've seen BMEP referenced as a measure of an engine's effficiency many times more than any reference to it as far as engine longevity. A boosted engine will have higher BMEP because it's more efficient. I suppose that not seeing BMEP used as an indicator of an engine's reliability might just be an oversight on my part though.

48 horsepower Yugo = 80 BMEP.

Does that mean the Yugo is more reliable than the stroker?

Why aren't those same factories designing and engineering stroker motors with that kind of warranty instead?

Lagavulin's car is making about 578hp at the crank, or a little over 115hp/liter. That's less per liter than a stock Honda S2000.

Carlos

Regardless of opinions there are some absolutes here.

1. Longer stroke = more piston travel per revolution. This translates to increased wear any way you slice it. This increased wear occurs on EVERY revolution, whether at idle or full throttle.

2. The stroke alone does not increase the hp that dramaticly, it is the combination of cams, porting, etc along with the stroke that makes the big hp.

3. I don't believe that a stroker motor from Devek or anyone else is in the same league cost wise with a blower application. I could install or have installed, a well laid out blower kit right here in sleepy little Chattanooga (everybody doesn't live in California). Cost of transport alone for either the car or engine back & forth to the west coast for a stroker motor job costs as much as the installation of the blower.

5. The increased wear from the blower is nominal under ordinary driving. The additional torque results in a lighter foot on the pedal under nearly all drive conditions. The big cylinder pressure only occurs for only seconds. And anyone who wants to keep a liscense & a heartbeat wont be doing that too often.

6. If you burn some aluminum with a blower, you can replace an entire S4 motor for a fraction of the cost of a stroker.

7. Ford, GM, Caterpiller, Detroit Diesel, Jaguar and yes even Porsche have shown that manifold pressure is an easy & reliable way to increase power.

GoRideSno

Wow this thread is turning into a classic. Many fact based open discussion style posts with little or no flaming. Keep up the good work ALL.

Andy K

bcdavis

Ok. my last reply was on the old board, and it is missing.

Anyhow, Laughlin asked:

Here you go:



http://www.amazon.com/exec/obidos/tg/detail/-/0760308373/qid=1058290879/sr=8-1/ref=sr_8_1/102-4779348-9854539?v=glance&s=books&n=507846

So, like I said in my initial post, this book was not about 928 engines. I am sure they are much stronger than a typical American engine. But their point, which is the same as some others on here, is that you can boost a stock engine, but it may not last long. The main advice they had, was that if you were going to spend the money on a supercharger, and you want the engine to last, you ought to build a strong bottom end. In this book they say that a 4 bolt main, forged crank, and forged pistons, are essential to reliability.

This is my point as well.

Yes, you can boost a stock engine.
If everything is set up right, it will run fine.

My concern is that if you try to boost an s2, with a 20 year old engine, with many things that could go wrong with it's many systems, there is a decent chance of a problem. So if there is a problem, and the engine dies, you will then have to rebuild your engine, and it may have a rod through the block, or broken pistons, bent valves, and metal scoring and damage. So you will end up spending a ton of money to repair it. My thought is simply that it is *wiser* to take the engine out first, and rebuild it, with lower compression pistons, strong rods, etc. So if I was rebuilding an engine, I would probably just go ahead and use a stroker crank. And not even bother with the extra $8K on the supercharger. But if I did decide to toss one on top of that engine, I would at least know that my engine would probably be able to take the boost without being so sensitive to detonation. It just seems like a potential monetary disaster, to spend $8K on a supercharger, and then have it blow up your engine, and have to spend an additional $10,000+ to rebuild it. It just seems to make more sense to spend $10K on a stroker engine, and be done with it, and not do a supercharger at all. A stroker can use normal gasoline, is not sensitive to detonation, etc. It is just higher displacement. A bigger engine, with more torque. But almost as reliable as the stock engine. As far as superchargers being used on new cars, those new cars are designed to handle boost. I have no doubt that the rods, crank, and pistons, were designed to handle boost.