How far can you stroke a 911?

[Wrecked944]

If I understand correctly, my 964 has a 100mm bore and a 76.4mm stroke. But I see at least one of the guys on the 914 BBS contemplating an 86mm stroker flat four. In my car, I think that would produce an honest 4.0 liter motor with gobs of torque. I see guys putting 102/103mm jugs onto 3.6 liter motors to get 3.8 liter motors. Any reason why we can't stroke them as well?

[masraum]

Not sure, but if you're not careful it may spew oil all over the floor if you stroke it too much.

Sorry, just couldn't resist.

Check out BA's book, that may have more info on stroking. Seems that these days more of the upgrades are from bigger jugs. (which may lead to more stroking, and then more oil on the floor, sheesh)

[jluetjen]

Janus, keep in mind that the Type 4 Flat 4 engine is a completely different animal from a 911 engine. The rev range is much lower and will rarely see 7000 RPM without a complete redesign.

If you really want to go with a longer stroke, there are some things that you'll need to consider which will most likely result in a set of problems that are just not worth solving.

* Pistons speeds (and the associated forces on the rods).
* Rod length (and the associated affects on engine wear from the increased sideways forces on the pistons and vibration issues from the higher sideways accelerations).
* Interference issues inside the crank-case with things like oil pumps and other stuff that's in there.
* Crank strength as the rod journals become more offset from the mains which means that there is less metal in between holding them together. (Also go back and visit the earlier issues with vibration and piston forces on this potentially weakened crank.)

And for what? I have yet to see where a "long stroke" engine makes significantly (any?) more torque then a "large bore" engine of the same capacity.

[tsuter]

I hate to generalize... but...... bore makes TQ, stroke makes HP.

[jluetjen]

Originally posted by tsuter
I hate to generalize... but...... bore makes TQ, stroke makes HP.

I'd hate to go over it all again , but I have yet to see any supporting evidence for that "urban myth" in regards to 911 engines. I've looked at the data in regards to stroke, piston speed, rod:stroke ratio, air speed and any other way that has been brought up and I have yet to find (or had anyone supply any) any empirical evidence that supports that conclusion. There's no replacement for displacement, but as long as your piston speeds stay reasonable, the rod:stroke ratio isn't too extreme and the crank is solid, the 911 engine doesn't seem to care, cc's are cc's -- wether you add them to the stroke or the bore.


(Note to self -- do not get drawn into this again!)

[beepbeep]

hmm...103mm jugs and 86mm stroke...that would be one helluva strong engine.

Actually, I don't see any problems with more volume as long as it's reasonably undersquare. I would rather drive a car with engine that produces 200HP @ 6000 RPM than one that produces 200HP @ 8000 RPM.

The less they rev the smaller friction and wear is the more effective engine becomes.

Hell yeah, go for it if you have the means! It would have nice torque curve! And it's the area under (power) curve that counts, not how many peak HP is printed on your dyno-sheet

[Bill Verburg]

Who you gonna get to stretch the throws on a 76.4mm crank?

[Dan J]

Armando has done it/will do it any way you like

[Quicksilver]

I remember somewhere an ad for 4 liter engine parts. CNC Engineering seems to pop into my head. Never heard of anyone endorsing it so I suspect it was a handgrenade.

Feel free to test it out and get back to us!

Wayne

[Kaama]

First off, torque and horsepower are numerically related, always, and a change in one causes a change in the other. By definition

A) Torque = HP * 5252/RPM

B) Torque = Force X Moment Arm

C) HP = Torque* RPM/5252

I like to think that an engine produces torque, and HP is sometimes a result of that torque- not the other way around.

If you put 10 Lbs of force on a 1 foot long breaker bar you produce 10 FT-LBs of torque, using equation B.

-If the object (bolt, etc) that you are trying to rotate does not move, HP = 0 because RPM = 0 using equation C. There is no horsepower, only torque.

-If the object does move, then horsepower is being made as a result of the torque being high enough to overcome the load.

If you look into equation A or C, Torque and horsepower are always numerically equal at 5252 RPM. If the peak horsepower occurs below 5252, the the torque will generally be numerically higher. If the peak HP is above 5252 RPM, the torque will generally be numerically lower.

Generally, when you increase the stroke, there are several options- I cannot say if any are done or not done on any particular crank, etc.

1- Stroke can be increased by offset grinding the crank, and using rods with a smaller big-end diameter.

2- Steel cranks can be stroked by overlay welding the rod journals and remachining to the original diameter, but offset from the crank centerline.

3- Rod length is a concern- shorter con rods put more side thrust on the piston skirt, and also lower the rod-to-stroke ratio. The piston reaches maximum velocity when the rod and moment arm of the crank are at 90 degrees. The piston accelerates to this point, then starts slowing down. Theoretically, this affects cylinder filling (breathing), and some argue it is not hardlydetectable on a dyno comparison. Sometimes there are aftermaket pistons available wherein the wrist pin is moved further up the piston so that the stock rod can be retained.

A longer stroke results in higher piston speed at a given RPM- The result is that the engine reaches peak HP and torque at a lower RPM.

A larger bore results in a larger piston top area. A given cylinder pressure over an increased area = higher force. Once again, the increased torque results in increased HP.

[JeremyD]

I believe that LN makes a big bore "kit"

[724doorE]

You can only stretch it as much as the inside of the case will allow.

Another way to do it is to offset grind the rod journal to a smaller diameter thus effectively lengthening the stroke but not enlarging the package. If the rod journal is 57-8mm like on a 3.2/.3 crank, offset grind back to 50mm dia. That would get you nearly to 80mm stroke. A further trick the V-8er's use is to offset grind to a smaller common size and use that journal diameter bearing and rod if the rod legnth is correct or close.

This is one of those subjects where there are truely many possibilities and some serious knowledge of past made parts and sizes for all manufacturers would be useful.

I'd be happy to be the test pig if anyone has a rod journal damaged crankshaft to donate

Kaama, isn't there an unwritten prefered minimum rod length/stroke ratio of 1.5 with up around 1.7 prefered??

[Oldporsche]

I do know that there has been some serious development done on the Chev small block to develop a "long rod" motor. I believe that the "long rod" motor has the ability to develop the torque at a lower and more useable rpm for street use.

David Duffield

[Lukesportsman]

Long rods leave the piston at TDC a little longer and "can" help cylinder fill. It also helps reduce friction by reducing side loads and results in a lighter piston combo.

CMW is the company that your maybe thinking of in the 4.0 adds. They actually make/made a 4.2 kit at one time. I'm still tempted by their custom billet heads, but don't know anyone who has ran them yet. It would be nice, as with a SBC, that we could buy aftermaket heads and cam combos to increase the efficiency.

[Kaama]

The longer the rod. the better, within limits of piston configuration.

The 540 CI offshore race motors in my Scarab produce about 675 HP and 700 FT-Lbs of torque- with 9.5:1 CR. This is due to aftermarket heads, etc, and tall deck race blocks that are .400 taller from crank centerline to head decks. You can stuff in a 4.25 inch stroke crank, 250 over rod, and still have room to deck the block!

The bores are 'small' at 4.5 inches, and can be safely taken out to 4.625. These motors can be bumped to 632 CI. There are also aftermarket tall deck small blocks that can be used to build stroker motors, too.

[jluetjen]

Rather then hypothosize or use V8 examples, let me add some real 911 data regarding stroke and rod length.

A couple of facts to clear up...
1) Peak piston acceleration occurs just off of TDC and BDC.
2) Peak piston speed occurs around 78 degrees after TDC (plus or minus a couple of degrees depending on rod length (and the resulting rod:stroke ratio).
3) Regarding rod ratio and "linger" time at TDC; I just don't see this happening per the data. The difference is where in the cycle the piston reaches peak speed, which can make a difference in cam selection or may help an engine with certain flow charactoristics.

Them's the facts -- but they're certainly open for revision if you can spot an error that hasn't been fixed yet or have more data to add.

[Doug Steinel]

John,

myths about Porsches are not "urban myths"... they are "suburban myths".