Has anyone done an ULTRA Short stroke 3.11?

[joeblow]

What I mean is 66mm Stroke with a 100mm Overbore on a 3.2 block? It nets 3.11ltrs and has got to be a screamer...Just over 1.5 ratio too, has to be a great engine for a lightweight car.

Obviously we are talking very long custom rods which might be the issue (block contact).

Thoughts?

[HawgRyder]

We used to build 1557cc engines for VW racing.
It used a 64mm crank with 92mm bores.
They screamed at over 10K RPM and produced tons of power.
The main problem was keeping the flywheel attached to the crank.
The shearing action of the flywheel against the crank was too much for the best fastening system we could keep up with.
So...if you go ahead with this idea....plan carefully...and use the VERY best bolts, studs, and other fasteners available.
Bob

[joeblow]

Originally Posted by HawgRyder

We used to build 1557cc engines for VW racing.
It used a 64mm crank with 92mm bores.
They screamed at over 10K RPM and produced tons of power.
The main problem was keeping the flywheel attached to the crank.
The shearing action of the flywheel against the crank was too much for the best fastening system we could keep up with.
So...if you go ahead with this idea....plan carefully...and use the VERY best bolts, studs, and other fasteners available.
Bob

Sounds like a good argument for a multi disc Tilton clutch and mini flywheel.

Has anybody seen the clearance issues that might develop with the rod length?

[hesaputz]

How are you going to get a 66 crank in a 3.2 block (different main sizes)- sounds like a very expensive proposal with very limited paypack.

I'm no kinda engineer - but there seems to be a 'golden' range of bore stroke ratios for high revving track engines. Back in the day, that's why 917 and 962 motors are in the 66 by 90 to 66 by 95 range; particularly for air cooled motors, this allows good room in the heads for large valves and heat dissipation.

[jpnovak]

This can be done. Look at Henry Shmidt at SuperTec and his 2.8SS.

Use an early 930 case (or Carrera 3.0). The early 66mm crank bolts in and the case has the later stud spacing to accept a 100mm PC set.

The other option is the SuperTec "super crank". It was designed as a 66mm stroke for the 3.0/3.2/3.6 case with the larger bearing journals. Built to run with Pankl Ti rods from a modern GT3.

Yes, this would be engine porn! I can only imagine the sound of a large bore screamer.

[hesaputz]

Hey, I've got Henry's 2.8 SS - 95 x 66 on a 3.0 Turbo case; and I agree that would be the way to do a 3.11 SS, rather than a modified 3.2. Question is, what's the benefit? Is shorter stroke always better? Is angular velocity on the rods a problem? Who's built this, or a 98 x66 version, and how did it do?
Questions for an experienced engine builder/ engineer

[356RS]

I have been gathering parts and trying to calculate how much piston dome volume I can obtain for a 66 X 100 early 930 case engine with MFI I'm building. I have most of the parts except the cam choice. I'm talking with John Dougherty on that subject. I plan on starting a thread on the build soon.

[jcge]

66x100 MFI ... now you know i'm interested Mark...springs, flyweights and contours....
John

[356RS]

How about a #44 space cam in a 1/50R alum regulator body on a alum R3Y pump section. BTW removing 20 grams per flyweight gives a new challenge for mid range adjusting. Upper range works well.

[YTNUKLR]

Originally Posted by 356RS

How about a #44 space cam in a 1/50R alum regulator body on a alum R3Y pump section. BTW removing 20 grams per flyweight gives a new challenge for mid range adjusting. Upper range works well.

Mark, what are the differences in the regulator body and pump sections besides the material (mag/alum)? What is a #44 - 911ST? VERY interested to see your build-up

[jcge]

Do you really need all 6.3L ? Do keep us posted.

[YTNUKLR]

Oh, I get it ! I think that's the one to use.

[Quicksilver]

Originally Posted by hesaputz

Hey, I've got Henry's 2.8 SS - 95 x 66 on a 3.0 Turbo case; and I agree that would be the way to do a 3.11 SS, rather than a modified 3.2. Question is, what's the benefit? Is shorter stroke always better? Is angular velocity on the rods a problem? Who's built this, or a 98 x66 version, and how did it do?
Questions for an experienced engine builder/ engineer

Obviously shorter stroked rotating assemblies can be spun faster before they reach the same stress limits as a longer stroke version of a similar motor. If you are working with a fixed displacement limit then this can be used to adjust the working RPM range of a motor.

One of the most interesting high RPM advantages of a short stroke is related to what is commonly known as "Rod Ratio". (rod length:stroke) This isn't something that is generally played with in a motor because it can be very difficult ($$$) to change just the rod ratio because it requires custom rods and pistons, and possibly cylinders, etc.

Basically rod ratio can have an effect similar to a cam change. A lower rod ratio will tend to build low end torque and increasing the rod ratio will help move the power band upwards. You have to look at the way changing the length of the rod changes the way the piston moves. The shorter the rod is the more time the piston stays at the bottom of the bore. This means that the piston in a low rod ratio engine will have higher piston velocities at the upper part of the stroke. This will help induce higher port velocities at lower RPMs which will help with cylinder filling. A higher rod ratio will reduce the inequality between the piston velocities at the top and bottom of the stroke. At high RPMs this makes for a longer working portion of the intake stroke which also helps cylinder filling. (A side effect of a higher rod ratio is that it also reduces tension stresses on the rod and piston. Tension is the weakest direction in a piston/rod assembly.)
All of this is extremely dependent on all parts of the engine combination: cam timing, port length, port cross section, total port volume, head design, header design, etc...

Good books that I have seen explaining this are mostly related to drag, stock, and Indy racing development specific to American stock block engines. The best specific explanation of rod ratio that I have ever seen is in "The Chevrolet Racing Engine" by Bill Jenkins. Smokey Yunick has written lots about rod ration in various books and in a number of magazine articles. A good example of Smokey's explanation can be found in "Smokey Yunick's Power Secrets". (He likes long rods a lot.)

--
This is all kind of academic because this is only one factor in how an engine's specs are decided. Usually an engine's specs are decided by listing the limiting factors and then playing within those boundaries. A very short list of common limiting factors are:
- Money $$$
- Displacement
- RPM limits of components
- Heat rejection limits (cooling)
- Reliability needs
- Regulations
- Available components (see Money and Regulations!)
- ...

For power displacement is king. Everything else works out to a modifier of the displacement. If all other considerations are equal then the larger displacement will always produce more power. For most 911 engines that are built solely for power you would look at the RPM limits of the valve train and the rotating assemblies. Spinning the valves faster with reliability gets to be pretty iffy pretty quick as compared to how you can build a longer stroke rotating assembly to deal with the same RPMs.

When you balance all of this out it would be really hard to justify building a 3.11 with a whole host of custom parts when it wouldn't ever equal the power output of a 3.5 using the same bore. If you were building an engine that had to be under a 3.2 liter limit using an unlimited budget then it might be the way to go.

[stownsen914]

A gentleman named Fred Apgar used to build 66x100 engines when he raced a GTU 911 back in the 1980s. The displacement limit was actually 3.0L, so maybe he used slightly 98mm bore, not sure. I now have the car that Fred raced, but I know he kept the engine when he sold the car in the 1990s. I believe he revved this short stroke motor to 9000+.

Scott

[lindy 911]

Another benefit to a long rod motor is that the piston stays at TDC longer which builds cylinder pressure on the ignition stroke. Cylinder pressure = power. A naturally aspirated motor likes this, forced induction does not. We put together 468 CI BB Chevy's that ran in a boat. The first build was stock length rods with small blowers: 550 hp. The second build was with .250 long H-beam rods, 10.5:1 pistons and a cam change and no blower: 580 hp. The same 980 cfm carbs were used on both. The biggest difference was the long rod motors felt like they made twice the torque.

A limiting factor in a Porsche motor is the piston pin placement; not much room under the ring stack to move the pin around which is necessary when utilizing a longer rod.

Lindy

[356RS]

Thanks Wayne. Great information on "rod ratio" affects. I want this engine (66 X 100) to perform well in the 8000 range, but I'm very concerned about the piston weights of 500 grams at these high RPM's.

[joeblow]

Originally Posted by 356RS

Thanks Wayne. Great information on "rod ratio" affects. I want this engine (66 X 100) to perform well in the 8000 range, but I'm very concerned about the piston weights of 500 grams at these high RPM's.

I used to run my old 3.6 with ChMo Pauter rods and JE Pistons to 9000 RPMS with out any issues. Stock crank too, just X-drilled, knife edged and lightened.

[AlfonsoR]

I've never bought custom rods, which is what you're saying would be required, right? If so, do Pauter, Carillo, etc. give you specs for the rods for the application, i.e. max rpm per assembled rod with piston?

[joeblow]

Originally Posted by AlfonsoR

I've never bought custom rods, which is what you're saying would be required, right? If so, do Pauter, Carillo, etc. give you specs for the rods for the application, i.e. max rpm per assembled rod with piston?

Correct, you will need custom rods to make any of this work. I always had very good luck with Pauter Rods, ask for Brian if you call them. They have ChMo or Ti rods but be ready to break into the college fund for the Ti units.

I think this is the way I am going to go. Nothing quite like a flat 6 bouncing of 9000 RPMs. By the way has anybody heard the reversion wave after 8200-8400 RPMs? It sounds like a completely new engine after that! Hard to describe but there is a bit of a brick wall at 8200 that after punching through the engine wants to to head to 12K!! At least that was my experience on my 3.6 with TWMs. Simple erotic.