From: Tim Murphy [tmurphy@new.rr.com]
Sent: Saturday, January 19, 2002 5:14 PM
To: 928
Subject: [928] Re: Supercharger VS Stroker (was Devek Engine configs)

Correction: My HP/TQ at 6000rpm is 408/357
Sorry

> I thought I’d chime in here since I’m running a supercharger on an 88s4
> 5spd.
> For street use I believe the supercharger to be very reliable. The guy
> down in Atlanta has more than 20k miles on his with no problems. For track
> use, either a supercharged car or stroker car wouldn’t be very reliable
> without doing some further mods to remove heat, like big oil coolers and
> such.  As far as performance goes, the stroker has more torque (it’s all
> about torque) down low but falls off at 4k rpm where the supercharger
> keeps going. I would compare the dyno sheet of the 88s4 stroker from
> Devek’s website
> (http://www.devek.net/index.php?page=nfo_pro_per_stroker&cat=2) to my car
> but the White car’s dyno is actually closer to my results (Although the 88
> stroker has very impressive low end torque).
> Comparing my dyno sheet SAE hp/torque to the white car: 
> ………       Stroker VS Supercharger
> 2k rpm=   120/320 vs 103/274
> 2.5k rpm= 160/330 vs 140/295
> 3k rpm=   220/375 vs 196/342
> 3.5k rpm= 250/380 vs 245/367
> 4k rpm=   290/380 vs 287/376
> 4.5k rpm= 325/375 vs 345/402
> 5k rpm=   350/360 vs 371/390
> 5.5k rpm= 365/340 vs 392/375
> 6k rpm=   350/300 vs 408/401
> Peak TQ= 391@3200 vs 401@4500 rpm
> Peak HP= 365@5800 vs 410@5800 rpm
> 
> The white car has: GT cams, Devek headers, 4” exhaust, 30lb injectors, 6
> liter displ.
> My S4 has: cat bypass, RMB, Vortech@7lbs boost, 24lb injectors (now has
> 30’s) otherwise stock.
> 
> At the time of the dyno I had 24lb injectors and had to turn the fuel
> pressure way up to get the air-fuel ratio right above 4500 rpm. This
> caused me to be pretty rich below the 4500 rpm mark so my low end power
> has probably improved some with the new injectors. Next I’m going to
> install 2-1/2” dual exhaust because the stock pipes seem to be pretty
> restrictive.
> As far as drivability goes, you’d never know the SC was in there while
> just cruising around until you pressed your foot in it. It runs and drives
> like a normal 928 other than having a bit of a traction problem in 1st and
> 2nd gear <big grin>. Don’t misunderstand me here, I think what Devek has and
> provides for us is awesome!  Financially the stoker wasn’t an option nor
> did I want to tie up my car for the rebuild so I went the SC route. If I
> did have to do an engine teardown, I’d probably go the same way that
> Marcus did with the supercharger, lower compression and more boost. His
> car has the foundation to make some SERIOUS POWER!!
> Sorry this was so long but I hope you all find it interesting.
> 
> Tim Murphy
> 88s4 5sp supercharged burnout machine
> (could be a challenger for the fastest 928 but I’m to chicken to find out,
> 170 mph is good enough for me)



-----Original Message-----
From: mark k [mailto:mkibort@compuserve.com]
Sent: Monday, January 21, 2002 6:11 PM
To: 928
Subject: [928] Re: Superchargers and reliability

> Folks,
> I have gotten quite a few supportive emails...thanks, it is good to know
> there are quite a few folks out there who are able to think for
themselves.
> The 928 is the greatest car ever built and the community of owners is part
> of what drives our efforts......and make the risks worth the effort to go
> where no one has gone before.....
>
> Anyway, here is some further info for those looking a various upgrade
paths.
>
> Let me be perfectly clear...for any given power level on a given engine
> design (same type components...forged, coolers, etc), a NA engine will be
> more reliable than a supercharged engine regardless of the opinion or
owners
> pride.
>
> Why?
>
> Simple, to generate X crank hp, the engine must actually generate enough
> power to drive the supercharger such that the net result is x crank
> hp...pretty simple.
>
> So, if you want to generate 450 crank hp at 10 lbs boost, your parasitic
> loss will be approximately 50 hp or more (or whatever you want to
> believe)...so the engine is stressed to 500 hp...which is less reliable
than
> 450 hp of a similar NA engine
>
> Or you can look at it another way, you will need to generate a higher bmep
> in a supercharged engine to have equal output of a NA engine.  Unless of
> course, your engine "design"  is SO much more "efficient" than a similar
> designed and built NA engine.   I have yet to see that engine.
>
> This is a fact.
>
> The same is true with NA engines, a 450 crank hp engine will be less
> reliable and have less longevity than a 250 hp engine which used the same
> "stuff".     The 450 hp engine will wear components at a faster rate then
> the 250 hp engine.  A 500 hp engine will wear them quicker than a 450 hp
> engine.  This is a fact, not an opinion.
>
> There are "things" you can do to increase the reliability of high tq or hp
> engines, regardless of supercharged, turbocharged or NA, the factory did a
> few to the gts to keep the reliability at or close their design life
limit.
> Let me give a few examples.....additional and remote oil cooler = more
water
> capacity and greater cooling capacity of both the oil and
water....remember,
> engines are very in efficient and give off a lot of heat....or forged
> components.  The stronger the engine, the more heat generated and the more
> cooling capacity needed.
>
> The fact remains, the stronger the engine, the more it is stressed, the
less
> reliable the  engine will be compared to one that makes less power.
>
> Good luck.
> Marc
> DEVEK

Well said. So, this is one of the reasons that:

a: a lot of engine seem to be tuned by the factory at 14.7:1 air fuel
ratio, when most engines would COOK.  The S4 is running at only 60% or so
of capacity.

b: when you have a engine like my part euro, it was only 285rear wheel HP.
 this is like Mark anderson or Marc thomas running at half throttle.

c:  the supercharged engines that are used on the streets can be
dependable, because you are not using boost,or exceeding normal stresses
until you go beyond what that engine could produce naturally.  However,
Marc makes an interesting point. the stresses are more, because the
supercharger requires engergy from the engine as well.  (not to mention
higher intake air temps due to the force air induction)

bottomline, probably better to pull the heads and do some radical head
work and some cams, but if you want a quick fix, try NOS.

Mark Kibort



From: Jim Bailey [jim@928intl.com]
Sent: Tuesday, January 22, 2002 7:25 PM
To: 928
Subject: [928] Re: Superchargers and rel ..Very long

Mark Kibort ..."probably better to pull the heads and do some radical head
work and some cams,....." It is a very well known that the GT cams often
show wear and pitting from the higher lift lobes more radical cams would put
even higher stress on the lobes .  It is also interesting to note that
street miles and track miles are two very different things . The book ,
Project 928 on page 47 notes .." Then there was a total of seven
condensed-time endurance tests over 5,000 miles on the ultra difficult
Weisach loop which corresponded to normal road use of nearly 100,000 miles
each ." That calculates to a ratio of 20 to 1 , every track mile is
equivalent in Porsches opinion to 20 " normal miles " .  My point being that
in street driving of any engine you spend very little time at full throttle
(more if there is little power) but with any S-4 keep it at full throttle
for 15 seconds and you are going 100 mph. So normally aspirated ,
supercharged , or turbo charged you spend the vast majority of the time
idling around the street with little or no "stress" on the engine . Many
owners of 87-88 S-4s have had ignition failure of a coil or secondary stage
and continued to drive the car running on four cylinders to " make it home "
they knew something was wrong but it still would drive 60 mph .... This
would then overheat the cat converter the undercoating ignites and it burns
up the top of the engine . This was so common that in 89 Porsche added
temperature sensors on each cylinder head to monitor the exhaust and shut
down half the injection if the car was driven on four cylinders because
people would drive on only four cylinders . Which emphasizes how little
power is needed to drive "normally" . Mercedes now markets several models
with superchargers "compressor" but Mercedes of course is known for selling
unreliable cars ?? :) . The VW Corrado was supercharged for several years ,
one of our employees has 170,000 miles on his original engine, it did need a
new supercharger. Saab ,Subaru ,Porsche 930 have long used turbochargers to
make the engine more efficient . Nearly all semi tractors are turbocharged
they used to run the GMC 671 / 871 blowers now found on top fuel dragsters.
The forced induction makes the engine more efficient but most manufacturers
in the past have opted to just run a bigger engine . Porsche with the
initial design of the 928 engine had started with a 5 liter prototype but
largely due to the "energy crisis " downsized to 4.5 liters . The final
design using the 95 mm bore (same as a 3 liter 911) used a 78.9 mm stroke
 911 3 liter is only 70.4 mm , 2 liter 911 is 66mm stroke ) . This was by
far the longest stroke automobile engine ever used by Porsche with a ratio
of .83 bore to stroke . That is one reason why my 2.6 liter 911 race engine
would spin 8,500 rpm but now with the bigger crank 2.8 liter , I keep it
under 7,000 rpm . Porsche over the years bored to 97 mm to make a 4.7 liter
S engine , then to 100 mm for a five liter . The final step in factory
development was to increase stoke for the GTS to 85.9 MM which gives a bore
stoke ratio of 85.9 . The longer stroke required different much shorter
pistons which have proven to be rather poor at oil control with excessive
oil consumption being rather common for GTS engines a liter per 1000 miles
is "normal" . We have one ruined GTS engine (cracked block and cracked
crankshaft) because the customer drove several hours with out checking the
oil and nearly ran it dry. The additional stroke moving the piston down
farther (it can not move up with out hitting the head ) also added to the
windage problems and Porsche added windows at the base of each cylinder
 GTS block )to allow better movement of the air/oil mist under the piston
and added breathers on the drivers cam cover . The compression ratio of the
GTS was increased to 10.4 to 1 and is right on the raggedy edge of
detonation (moving the piston farther jams more into the same space ) . You
can only shorten the piston by so much and shorter rods give a undesirable
rod angle so they went with a higher compression ratio . As the crank shaft
rod journal scribes a larger and larger circle and the piston moves farther
down the bore it gets closer to the crankshaft often requires that part of
the counter weight be milled off to keep it from hitting the bottom of the
piston when it is all the way down . That also means that the side loading
on the piston becomes much greater as the rod angle gets bigger which is not
a good thing . Converting an up and down motion into a circular motion has
numerous challenges . The piston comes to a stop at the top and bottom of
the stroke but accelerates to very high speed in the middle . Increased
stoke by necessity means higher piston speed and increased piston travel ,
the rings and piston at 3,000 rpm are moving farther being exposed to
additional side loading (on a shorter piston) and seeing more compression
ratio as well as a bigger explosion from more air and more fuel . The Scat
enterprises stoker crankshaft is normally made with a 95 MM stroke which
would have a bore stroke ratio of .95 with the 100 mm piston or .91 when
bored to 104 mm  but with the increased stroke and limit of how far down you
can pull the piston it is very difficult to hold the compression under the
10.4 GTS level . If the heads need to be surfaced or have been surfaced the
volume of the combustion chamber is reduced now you are jamming more stuff
into a smaller space , the compression ratio can easily be too high to run
on pump gasoline . The change from the original 78.9 mm stroke to a 95 mm
stroke inside the same engine block is a huge change in the original design
and far beyond anything that Porsche ever envisioned for the engine . It
also results in a low rpm torque engine rather than a high rpm horsepower
engine as evidenced by the dyno charts . Marcus Hutchinson stopped by 928
International driving his supercharged 1985 car freshly rebuilt using 951
turbo pistons(forged low compression) s-4 heads and intake with intercooler
.... he also installed a 1990 automatic for better gearing for acceleration.
Should be interesting to see what it can do after it is broken in and final
adjustments made . Marcus has really done a fine job building his first 928
engine very nice attention to detail . Given the lower compression ratio
forged pistons , Pauter machine aftermarket connecting rods and intercooler
it should like a lot more boost. Horsepower / reliability there is no simple
answer suffice it to say if you can make 500 hp and use it on a race track
it probably has a life span measured in hours but it might run forever on
the street . The old comment " How do you make a Porsche engine faster ??
Take away some of the reliability " certainly seems to apply if you USE that
extra power .

Jim Bailey
928 International
Jim@928intl.com
www.928intl.com
(714)632-9288