dmoffitt

i'd love to hear from any 2.8L street-driven owners, what you've run into problems, suggestions, changes, things to avoid, stuff recommended...

like, what size injectors are people running?

boost levels?

which turbos are being used?

fuel pump? gas mileage (dare i ask lol)?

stuff like that...

also, big one -- ENGINE MANAGEMENT - what are people doing to (properly) control air-fuel? is a Digital MAF + chips suffecient?

thanks in advance <img border="0" alt="[cheers]" title="" src="graemlins/beerchug.gif" />

David Floyd

I can give a little info on this, have about 3000 miles on my new all bore SFR 2.8.

Garrett TO4E, #8 turbine, 60 trim

15 psi so far

Large Bosch fuel pump from Lindsey Racing

Gas mileage (LOL)

MAF at the moment, going to the Guru Racing Link-1 EFI for total control

72lb injectors going in with the Link

Once all of this is dialed in I will give my opinion on performance and such

Bill

I just installed 65 lb injectors on my 2.5 and I might as well have installed a V8 with dual quads.

I think the phrase is "Power or Economy" pick one.

Chris Cervelli

I have almost 60,000 miles on my 2.8 engine. 208,000 on the car. I built it about 4 years ago and the head has never been off.

It is a stroker 2.8. 100.5mm bore and 87.9mm stroke. I can't imagine why you'd want a big bore 2.8 for a street driven car. The torque comes from stroke.

I run a K27/8. It is pretty lame on the top end, but spools up fast with a 2.8. I run only .95 bar boost because it does not get race fuel since it is a daily driver.

I run 550 cc/min injectors. It gets about 21mpg city and 30mpg hiway. With stock injectors the mileage was much worse because of the high fuel pressure required for a safe top end. Despite rumours to the contrary ballast resistors for low impedence injectors are not necessary. At least with the ones I use.

Fuel pump is stock. Despite rumours to the contrary the stock fuel pump is fine for 400rwhp or so.

I run an Autothority/Technodyne MFS kit with the above mentioned injectors. Drivability and cold running is as good as stock. This allows you to tune the part throttle/low load area very lean for good mileage. Getting the full load fueling right is the easy part.

Chris Cervelli
Technodyne Inc.

David Floyd

</font><blockquote><font size="1" face="Verdana,Tahoma,Arial,Helvetica,Geneva">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">I can't imagine why you'd want a big bore 2.8 for a street driven car. </font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">Why Not ?

</font><blockquote><font size="1" face="Verdana,Tahoma,Arial,Helvetica,Geneva">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva"> The torque comes from stroke </font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">Does bore not do anything?

What are the HP and Torque numbers for your car?

Just looking for your point of view, as a well respected builder.

Chris Cervelli

As a very general rule engines that are thought of as 'torque' engines have a long stroke and small bore. These are referred to as 'undersquare' meaning the stroke is larger than the bore. 'Horsepower' engines tend to have a large bore and short stroke. They are called 'oversquare' All Porsche engines are oversquare.

A great exception to this rule is the new BMW M3 engine which is a little undersquare and yet revs to 8000 and makes over 100 hp/liter.

Anyway, in the normal non-BMW case, the small bore limits valve size and therefore top end breathing. The long stroke imposes some mechanical limitations on high revs and thus further makes the engine more suitable for low rev 'torque' type operation.

Now keep in mind that the 2.8 engine is still very 'oversquare' (bore bigger than stroke) so I am talking only in relative terms here.

Here is what I expect to see from a stroker 2.8 compared to a normal 2.5 (all other things being equal):

A very large increase in low end torque and a very small increase in top end power. This borne out on the dyno as my 2.8 makes 320rwhp and 375 ft/lbs at only .95 bar. You would also expect to see the peak power and torque occur at a lower rpm.

For the all bore (106mm??) 2.8 I would expect to see a roughly 10% increase up to say 5000 rpm or so, where the percent of increase would dwindle unless the cylinder head's breathing ability was improved. I have not done a 2.8 this way, and don't intend to, because I feel if you can't use factory type pistons, you can't guarantee 100,000 mile durability. (I could be wrong here, I have heard a few sleeving success stories lately)

If we are talking about street cars only, it is obvious that the area in which the 951 needs the most help is low end torque. The stroke 2.8 is going to acheive this much more effectively than the bore 2.8. Plus, you can use all factory type parts are be assured of the durability.

For an all out race engine, the bore 2.8 has more potential. The larger bore allows more room for big valves and should improve breathing automatically. But if you are going this far, why not make a 106mm bore engine with the long stroke? That would be around 3.15 liters I guess.

I have built a ton of stroke 2.8 and found them to be really great for street cars and mild race cars. They have two problems that seem to recur over and over:

They are a lot easier to blow up than a 2.5. The extra displacement puts you that much closer to the fueling limitations with all the stock engine controls. Go lean with a 2.8 and they melt quick. To get a 2.5 to melt you have to work at it.

They are not necessarily better race engines. If you keep all your other stuff and just switch from a 2.5 to 2.8, it is unlikely that you'll go faster. The extra low end torque can be a liability in a race car, since traction should be in short supply. Also your peak power occurs earlier, which means you shift earlier etc.

Now if you put the right turbo on a 2.8 you have a nice race engine. It has some grunt even off boost, so if you get caught down there you are not dead in the water. With the right turbo you can easily make 400 hp at 6500 rpm, which should run very, very well. The torque band will always be wider, so you can be a little lazier with the shifting.

Of course there is the right way and the wrong way to do a stroked 2.8. The right way:

Turbo block bored to 100.5mm
New Mahle pistons for 8:1 CR and 100.5mm bore. These pistons have the pin moved to accomodate the extra stroke. Otherwise the piston would stick out of the bore by about 4.5mm at TDC.
Carrillo rods. The stock rods won't clear the block with the longer stroke. Plus the Carrillos are the best rods you are likely to see.
944S2 or 968 Crankshaft.

The wrong way:

Stock pistons
4.5mm shorter rods to fix the piston problem.
944S2 or 968 crank.

This is really screwed up. The CR is way too high and the rod angle situation is much worse. This engine will never make any power and always want to detonate.

It is too bad Porsche didn't make all the 951 2.8 liters. It would not have cost them any more and it would have made an almost-supercar into a supercar.

Chris Cervelli
Technodyne Inc.

Konstantin

</font><blockquote><font size="1" face="Verdana,Tahoma,Arial,Helvetica,Geneva">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">Originally posted by Chris Cervelli:
<strong>
The wrong way:

Stock pistons
4.5mm shorter rods to fix the piston problem.
944S2 or 968 crank.

This is really screwed up. The CR is way too high and the rod angle situation is much worse. This engine will never make any power and always want to detonate.</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">&lt;the rod angle situation is much worse
what do you mean with that? can you explain it further?

&lt;This engine will never make any power and always want to detonate
why will not make any power? After all it is still a 2.8L engine with more dispalcement than a 2.5L engine
Detonate yes but at which boost? I can not imagine that this engine will detonate at 16 PSI??

Konstantin

John Anderson

ON the 2.8 liter bored motor I built fopr my 951, torque was not too bad..I only have two dyno sheets from the day I tuned it ( we started at 10 psi and moved up each run 3 psi at a time) so I cannot say what it put down at one bar. At 16psi it was making 390 hp at the tires and 460 lbs torque. The torque comming in at around 3700 rpm for its peak. If I remember right, at 13 psi it was easily over 300 at the tires with the upper 300 lbs torque.

I have never just stroked a block, but I know it does work. I like bore, or bore plus stroke as in our NA race car. I can say one of the big reasons I liked bore to start with was the fact that it let me feel comfortable reving the motor hard, but thats not really an issue, and I have learned that.

I'm not arguing with anyones input, just offering my own. Infact, I just got a call yesterday that went like this:

ring...ring...ring... Hello?...."MAN THIS MOTOR IS OFF THE HOOK!"...Who is this?? This is Aaron...so the conversation goes. I have not heard from Aaron since I shipped his block last year, but he went on to tell me the story of him beating his buddy in a stroke 2.8...Aaron's car is and NA, and so is his buddy's..

I really believe both setups work..I personaly, like bore before stroke.

Take Care

David Floyd

Thanks for the info Chris and John A, as they say there's more than one way to skin a cat.

I will post some Dyno numbers in the near future.

Chris, 60k miles on your engine speaks well for your design.

My engine is a John A design and I hope for as many miles.

Matt Sheppard

In theorey, Chris C is right. Usually one goes for longer stroke for bottom end torque - where our cars are, well, . . . wanting.

In my case what happeened is the P.O. got a call from his mech - "Hey, your cylinders are all scored to #$%#! from a leaking injector or something. You can get 1st over Porsche pistons and hone it or sleeve it and get more cc's for the same or less money - what do you want to do?"

From experience, I've had (2) 2.5's and a 2.7 sleeved motor and I almost feel like auto X in the 2.7 turbo motor ain't all that bad where as I'd be a fool to go out in my 2.5. It is definately friskier down low than any 2.5 I've driven. I understand that longevity is a real concern - well just have to see - only 14K on my sleeves so far.

Chris Cervelli

&gt;&gt;Chris, 60k miles on your engine speaks well for your design.

I wish I could claim the design. Andial pioneered these in the early 90's. They had enough pull with Mahle to get the pistons made, which is the key to the whole thing.

To answer Kon's questions:

The high CR is bad for power. With a high performance turbo engine you generally are going to turn the boost up to the knock limit. Low CR with high boost has the same knock limit as high CR with low boost. More boost makes far more hp than higher CR.

I know the new 996 TT has a highish CR of 9.4:1. I think they did this for these reasons:

A street car spends most of its time a part throttle. The high CR helps efficency at part throttle, which reduces fuel consumption and emissions.

The four-valve, pent-roof combustion chamber is really good at resisting knock. This made it easier run high boost with the high CR. Plus the engine controls are really sophisticated, so they can run at or very near the knock limit with total reliability.

Porsche's power goal of 450hp was easy to meet with this engine. That allowed them to compromise the design for fuel economy and emissions without taking away power.

I think if the engine needed to make 800 hp the CR would be down at 7.5:1 and the boost would be 1.6 bar. The fuel economy and emissions at part throttle would be worse and the power potential would be greater.

The rod angle thing is something else entirely.

If you divide the rod length by the stroke you get a number called the rod ratio. It is better to have the rod ratio higher rather than lower.

Examples:

951: 150mm rod 78.9mm stroke, ratio is 1.90

968: 150mm rod 87.9mm stroke, ratio is 1.70

911 3.6 air cooled:127mm rod 76.4mm stroke, ratio is 1.66

911 3.6 watercooled:130mm rod 76.4mm stroke ratio is 1.70

The 3.6 911 engines have a rod ratio problem. The mechanical loads are very high due to the low ratio. That is why the high revving (8500+) water-cooled 3.6 engines need extremely light rods and pistons to survive. If the rod was longer, the mechanical loads would be lower, and conventional steel rods would be ok.

The 911 was originally designed for a 66mm stroke. The distance from the crank centerline to the cylinder top has not increased, (except maybe in the water cooled engines?) so the rod couldn't get longer unless the piston pin moved up in the piston. Eventually the pin gets too close to the rings, and can't go any higher.

short rod 2.8 951 engine: 145.5mm rod 87.9mm stroke gives 1.655 ratio.

The rod ratio also affects how air flows in and out of the engine because the piston speed is high near TDC in low rod ratio engines. These engines do not breathe as well as high ratio engines due to this.

Hopefully that clears up the issue instead of confusing it more.

Chris Cervelli
Technodyne Inc.

Konstantin

I agree, BUT if someone wants to run just 16 PSI it will be better to go to 8,5:1 CR for better off boost performance.
The question is: will the 3 L 8V Turbo engine engine detonate/knocking at 16 PSI with 8,5:1 CR and will destroy itself at a 6500 RPM maximum?
I agree for more boost and higher RPM is not good but how at this "low" boost and low RPM?

Konstantin

dmoffitt

thx for all of the info!

i'm going w/ the shorter-rod method as it's all i an afford (don't have the $ for pistons AND rods) right now - however i plan to run 10-12psi, 15 tops, so i'm not too concerned w/ detonation, i just wanted some more low-end grunt on the street before the turbo had fully spooled, and i doubt i'll take the car near 6500 often / ever, in fact it hardly sees 5000/5500 as it is...

with the 3.0 crank, shorter pauter rods, and fly-cut stock pistons, where do you see that power and longevity wise (on a fresly built short block and new head)?

Jeff Lamb

</font><blockquote><font size="1" face="Verdana,Tahoma,Arial,Helvetica,Geneva">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">Originally posted by dmoffitt:
<strong>with the 3.0 crank, shorter pauter rods, and fly-cut stock pistons, where do you see that power and longevity wise (on a fresly built short block and new head)?</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">What are "fly-cut" stock pistons?? I haven't heard this term before.

Thanks,
Jeff

Matt Sheppard

Jeff:
One cuts a recessed "bowl" out of the piston top to lower the compression ratio - since your adding swept volume to the chamber but essentially keeping the compressed combustion chamer volume the same with a 4.5mm shorter rod.

Dmoffit:
Why not just reshape the chamber in the head with a dremmel. You can cc the chamber before and after pretty easily with a 50 cc syringe and plexiglass. Many perf. co.'s sell kits for that, but also easy to make yourself. you only need to drop it a few cc's to come back to stock CR, not like yould do an entire reshape of the chamber.

If you do cut the pistons, I'd look into the Swaintech coatings for them. Might help keep temps down at the piston since you will be removing some of it's heat dissipation capabilities. I understand $ is an issue though.