Sound set up .
Combustion temperatures can become a huge issue so using the right piston/liner technology becomes a necessity if you expect reliablity and longevity under any level of sustained high HP use. Ceramic coating, beam fusion technology are all part of this expensive manufacturing process....the list is becoming endless. I'm sure porsche would be using high end parts in racing applications when extracting high levels of power from this mezger engine. Accordingly, the engine must also become a sum of its parts .

 

You mentioned "piston" technology ........ and reliability and longevity. I'll provide a little food for thought. This is open to anyone who cares to participate.

Question: The piston wrist pins on the Porsche 996/997TT stock piston as well as most aftermarket custom pistons are center lined in the middle of the piston (side to side) for all pistons on both banks. Is this the best position for ultimate balance, maximum rod length, maximum redline and maximum power?

 

To my knowledge the centre pin position has no relativity to the balance, power, or rod length, max rpms etc. Power is generally determined by other things such as compression ratio, stroke and so on... Centre is the position generally used in performance applications (and most others as well these days) as there is less load on the thrust side....therefore one could fairly say it is the "best" position. The main purpose of an offset pin is to make the engine run quieter. An offset pin in a performance application may see an offset of up to @ 30 thou, and again there can also be an offset in the crank too....of up to @80 thou in some cases. This however is not the case in P engines though....from what I'm aware. Btw, why do you ask? If there is something else you wish to clear up i can ask one of my close associates from design and manufacturing (Mahle) and come back to you tomorrow. It is no problem .

PS. In performance engines particularly, reliability is of first and foremost importance, strength is secondary, so its best to keep that it mind. If the engine is not reliable then it basically doesnt matter how strong it is made .

 

At what power level did that happen? Looking back, there wasn't much of a noticeable noise from the engine on this car, but like I said, it had a race exhaust, etc..so there were so many other noises going on..lol.

By the way, I like what you did to the motor when you rebuilt it. That's some pretty bad-a$$ stuff, and very similar to what we're doing on our 3.8L GT2 build. I should hopefully be able to update that thread within the next few days.

 

The true center of the piston is not the true center of alignment of the motor. The wrist pins are offset to different directions on each bank to obtain true center. This offset allows for a smoother running motor. We have proven on an engine dyno there is power gained with this being the only variable. By offsetting the wrist pins you lengthen your rods which raises the piston to rod ratio which slows down piston speed.

All the above adds power and reliability.

 

Much of the reason i like wiseco pistons.. They come offset standard.. Take off more of the side load even if you stay stock length.. What would be wicked to have is a base PLATE to move the jugs up and lengthen the rod.. Would need some custom stuff like lengthened timing chain but a long rod 996tt engine revving 9k would be pretty nasty..

 

Tom,

Thanks for the kind words.

We had a special 90 mm exhaust, however it was not an open pipe race exhaust.

With the exception of the modified Porsche Spanish case, there isn't an Porsche 996tt part in the motor. We utilized the Porsche case but the liner squirters were from the 962 and the old locations were welded up and new ones drilled and tapped. Case was machined and piped to accept a GT1 three index oil pump. The case was also shuffle pinned, aligned bored and bow tailed. The interior was polished and coated to shed oil and disperse heat. It was also machined with deeper and different threads for the 12 mm head studs and cut washers. The case was machined to accept 105.7 mm steel liners.

 

We looked into raising the deck height, but the issues with the timing chain among others issues were mind boggling. With a 82.4 mm crank, the longest rod we could physically utilize was a tad under 130 mm's. We utilized CP pistons and engineered the offset. We would have liked to utilized a 133/4 mm rod but it wasn't possible without raising the deck height and getting into a real can of worms and mega dollars. With our current build we had to settle for a P/R ratio of just under 1.58 to 1. The stock 996TT utilizes about 1.64 to 1.

 

Tom, the power level was about 660 rwhp and about the same torque (slightly less). This one is for you Tom!

 

Great Post Tom,

great info Chad and Speed21

 

[quote=cjv;3190927]If the crank is not central in the block then that is true. However, I am not aware of the position of the crank in a porsche block at this stage to comment.

However, in fundamental principle the thrust face of the piston onto the liner is the better performing position. Relieving the thrust face by using an offset pin does make the engine run quieter and smoother....but to a lesser degree, but the smoothness is also dictated by other things as well such as tuning of the advance ignition curve, skirt/piston design etc. This is possibly where you may be achieving a degree of the "smoothness" you refer to.

You are saying the crank is offset in the Porsche block then?

Also, you are saying that you have one bank of pistons with the opposing sides/skirts (non thrust side that is) being positioned onto the liner in favor of the correct thrust side of the piston? That is not a very desirable situation at all. Running the pistons back the front (eg offsetting the position of the pin to the upper side off centre) will cause the non thrust side/skirt face to drag and slap back onto the liner after combustion on downstroke. That can/will crack the skirts. I wouldnt be doing that at all .

As i said previously. The basic principle of running an offset (correct offset...not back the front that is) is to quiten the engine, and yes it will achieve a degree of smoothness in that process....true. But fundamentally a central pinned piston performs better as it places the correct thrust side of the piston against the liner in the right direction.

To be honest cjv and with all due respect i just cant see it but then again my core knowledge is not from being directly involved in piston design. Are you absolutely certain there were no other adjustments made in the tuning of the ecu, boost and so forth?

May i ask what your compression ratio and advance curve is? Also what offset the crank has in the block or if its dead central. Same with the pistons....what offset is being used in thou. Also exactly how much extra power are you claiming to have achieved by using an offset piston?

As a matter of interest i will run what you have said past one of my close industry associates who was in piston design and will come back to you on Monday with his thoughts on your claims of power from this excercise.

Wow! We are straying off the beaten path of Toms thread somewhat although it is interesting what you are saying and worthy of investigation.

Also, my understanding of piston manufacturers is that Mahle is the far superior pick out of what brands have been suggested here so i think Tom has certainly got the jump there with his pistons .

Tom, do your custom Mahle pistons run offsets? I doubt it.. Are you also using the Mahle liners?

 

Relieving the side load of the piston from liner contact is actually what reduces the noise however removes the piston from its optimum position. If you have noise issues to contend with and are prepared to compromise performance then offset pinned pistons will run quieter but will not perform better.... Reducing noise is the fundamental principle behind the offset pin.

 

So why the rods got bent???

 

[QUOTE=speed21;3191116]speed21,

I will be away this weekend, when I return, I will answer your questions as best I can without disclosing exact numbers.

 

Great thread and kudos to the moderator. A lot of great information. Will keep following it.

Cheers.