Different strokes for diferent folks - anybody know anything about custom cranks?

[kenikh]

I am curious if anyone has experience with custom stroke configurations on Porsche 911 motors. 66m and 70.4mm are the oldest standards, but has anyone seen anything in between or have any knowledge regarding factory testing of other strokes?

[911st]

Not an expert but some points I picked up along the way.

Rod length is an important consideration in motor architecture. Longer strokee means shorter rods which is not a good thing. Modified 3.6's often raise the wrist pin to help.

Shorter strokee can allow access to higher rpm. If motor size is open, bigger is better. If limited to a given motor size playing these two off against each other may make for small improvements.

The 66 out of the box has less issue with oil starvation of the center rod bearings and can spin up to 8000 with a good oiling system. The 70.4 should be drilled for over 7400rpm. Cross drilling and modifying the center crank bearing can fix this with longer strokee motors.

Higher rpm can be limited by the valve train as much as the crank.

Longer stroke motors of the same CC often make more TQ earlier.

The factory made a 2.5 on the 66 & 70.4. I am not sure but the SS might have made more hp but the long strokee proved more reliable. This may not apply to a motor built today.

A 3.5 turning making 270hp is much less stressed and will live a long time compared to a 2.5 making 270hp that spins to 8000rpm plus.

Do not know if any of this helps.

[kenikh]

Thanks for the thought - these points aren't ones I was looking to have answered, having already been through them. I do think they will help other readers, so thanks!

My questions are functionally irrespective of rod ratio, given that you can offset grind a 66mm crank to SC/GT3 or 2" Chevy journals and keep a 2 liter rod length by moving the journal center anywhere you want within the area of the original (larger 66mm crank) journal.

This of course assumes custom rods.

My question stems from the fact that I am going to be under-grinding a 66mm crank that is out of spec to SC journals. Thus, I have a lot of latitude on stroke: You can pretty much come up with any combo between 60mm and 70.4mm by offset grinding a 66mm crank to smaller diameter, due to all of the meat on the old rod journal.

What I am trying to figure out is: has anyone seen any relaibility issues on a custom stroke? For example, the 70.4mm crank w/ 6 bolt mains sheds its flywheel catastrophically due to resonance issues above 8K RPM. I am looking for these kinds of data points.

[911st]

[kenikh]

Perhaps I am asking too much?

[HelmetHead]

I like where this is going

[911st]

I would not say you are asking to much. There is a lot of knowledge here that is way past my ability.

Again, I do not know stuff at the high of level.

If you are regrinding a 66's main's this dose not effect stroke and might create problems with the angle the oil passages. If it is the rod journals then you can effect the stroke.

However, if you are not going to play with the piston diameter or are not targeting a given size like 2.3 for a class, going the longest stroke you can get will make for a bigger motor that dose not have to be as stressed to make the same Hp or can make more HP.

Also, longer stroke can make more compression. 66's are so strong as there is a big overlap between the main and rod journals. It is reliable as the oil dose not have to fight as much centerfical force to get through the crank.

If staying under 7500rpm, I would think it is better to go for a larger motor.

Good luck. I hope to learn something to.

[kenikh]

My motor will see 8K RPM and is 2.3L in size (based on 66x85). I don't like 70.4mm cranks at these speeds due to the flywheel bolt shearing issue, so I am merely wondering how far I can go on the stroke to keep 8K as OK. Mains are STD.

FWIW, I like the feels of the shorter cranks - even 70.4mm can feel a little lazy to me on small bore motors, so as you can see, I am really trying to play things close to the line here.

[911st]

Oh that sounds like fun.

I have alwas wondered what a 66 crank in a 3.0 turbo case with 98 or 100 mm pistons would be like?

Best of luck.

[Flieger]

I think the 1.5 liter flat8 typ 754 engine from the 804 F1 car in 1962 had a stroke of 54mm. I know it was less than 66mm- maybe 58mm? The typ 771 2.0 liter flat 8 from the 907 and 910 was a development of this engine. I can look it up in Fere's The Racing Porsches later.

Interesting thoughts, Kenik.

[kenikh]

BTW, the 935 baby was 60mm.

[John Schaefer]

Subscribed for the inevitable tutorial lessons that will pop up here. Good one Kenik.

[jluetjen]

Originally Posted by kenikh

Thanks for the thought - these points aren't ones I was looking to have answered, having already been through them. I do think they will help other readers, so thanks!

My questions are functionally irrespective of rod ratio, given that you can offset grind a 66mm crank to SC/GT3 or 2" Chevy journals and keep a 2 liter rod length by moving the journal center anywhere you want within the area of the original (larger 66mm crank) journal.

This of course assumes custom rods.

My question stems from the fact that I am going to be under-grinding a 66mm crank that is out of spec to SC journals. Thus, I have a lot of latitude on stroke: You can pretty much come up with any combo between 60mm and 70.4mm by offset grinding a 66mm crank to smaller diameter, due to all of the meat on the old rod journal.

What I am trying to figure out is: has anyone seen any relaibility issues on a custom stroke? For example, the 70.4mm crank w/ 6 bolt mains sheds its flywheel catastrophically due to resonance issues above 8K RPM. I am looking for these kinds of data points.

kenikh; I didn't answer earlier since we've discussed much of this on earlier threads. Now that I see where you're going, let me share a few $0.01's. What you're really talking about is crank design. Here's a few points to consider...

1) You really can't discuss the crank in isolation, without considering the rods and even the Pistons. If you change one you often need to end up changing the other(s).
2) Your 70.4 mm crank is already underground once. By doing this, you are also reducing the bearing area supporting the rod. Now engine designers include significant margin in this area, but increasing revs reduces this margin. Undersize grinding the crank will do it some more. At some point you'll start to run into issues like increased wear of the bearing. Longer stroke cranks also increase the side loads on the rods, which make more demands on the bearings.
3) Keep in mind that picking a stroke between 66 mm and 70.4 mm may not even net a result which you will notice. For example the long-stroke and short-stroke versions of the 911ST (2.3) both had about the same HP and torque at comparable engine speeds. The big difference is the crank vibration which you alluded to. As far as performance, there wasn't much between them. So if you decide to pick a custom crank with a stroke somewhere between the two, the end result is going to be...???
4) The crank vibration thing is something that is hard to predict at our level of calculation. Nowadays there is sophisticated software that allows engine designers to model the vibration characteristics of an engine. This is one of the key technologies that allowed F1 engineers to design a high-rev'ing V10 since the vibration pattern of a V10 is significantly more complex then a V8 or V12.
5) Now you could for example make a 60 mm stroke crank, which will allow the bottom end of the engine to withstand more rev's then the 66 mm crank. The problem is that I don't think that the bottom end is what limits the revs of a 911 engine, it's the valve spring technology. So even though you'll be able to spin the crank safely to 10K or maybe 12K RPM, the valves springs still won't let you get much past 8500 RPM (taking into account the cam that you'll need to spin to those levels). You'll also find that you'll lose CR as you reduce the stroke. So that means you'll need a bigger piston dome to fill the big hemi-head combustion chamber of a 911. But 911 race pistons already have big domes and it creates combustion quality issues. Basically it's hard to cleanly burn all of the fuel in the charge since so much of it collects in the deep recesses of the stretched out combustion chamber shape. This costs HP, efficiency and smooth running.

To make a long story short, if you end up trying to develop a custom crank with a stroke between any of the normal 911 choices, you're going to spend a lot of money developing the crank, as well as the associated rods and pistons. At the end of the day, you'll most likely find that you have an engine which doesn't feel any different then a regular 911 engine, or in some cases loses the flexibility of a factory configuration.