Thoughts on some different engine configurations

[jluetjen]

but I'm curious where the balance point is in 911 engines between stroke and comfortable RPM.

Andy, it's not clear to me that there is any sort of a stroke versus rpm trade-off on 911 motors. As you mentioned, in general terms 911 motors have shorter strokes then most comparable engines. The reduced stresses on the bottom end I believe has contributed to Porsche's legendary toughness in endurance racing.

Near as I can tell it's usually a top end failure that results from overrev'ing. Specifically some sort of valve/piston interference issue. This becomes a bigger issue as the valves get bigger (and heavier) with the increased bore size and capacity, and when more radical cams are used, or a combination of both of these. These problems are not insurmountable, but then you're talking about stronger valve springs which have the knock-on affect of increasing rocker and cam wear.

There are other subtler combustion related issues that could occur with big bores and short strokes, but they may only show up as you try to extract the very last HP out of a given sized engine or pass emissions testing.

[Henry Schmidt]

When Porsche first designed the 911 engine they started with a clean sheet of paper. They designed the engine with basic engine principle known throughout the industry ( bore stroke ratios, rod length cylinder thickness and so on) What they did was add their own twist. Air/ oil cooled, horizontally opposed 6, 6 individual carbs. All very cool stuff. As they began to change the engine they had new considerations, primary among they was cost. They wanted bigger engines ( bigger is better, right? ) but they didn't want to change everything. So they compromised. First they bored and them they stroked. Each time they changed the engine they compromised something else. Every time they stroked the engine they made the rods shorter instead of longer. Some things even got weaker. Case in point engine cases. Alum to Mag. Lighter but? With the 70.4 crank, the rod was too wide which made the flyweights too thin and at high RPMs the cranks broke. Ask Frank Beck, he broke one of my 2.7s trying to run 8200 with it. That's why the RSR had a very special crank. That's why I would only built a 3.0 and larger engine with a 9 bolt crank. Next they changed the 3.2 & 3.3 rods. When they put the larger journal on the rods they discovered a clearance problem with the oil pump. Solution was to make the rod smaller, this required a smaller rod bolt. Now we have a whole new set of problem. My solution, change the journal back. Why do that you ask? Well I saw a 962 crank and it had 3.0 journals. Being a fan of Porsche engineering I thought it made cents. In summary, under 3.0 = 66 stroke. Over 3.0 9 bolt crank with 3.0 rod modification. Why not just build 3.0 SC engines? WE do and we love them.
PS; please don't email me telling me your 2.7 has lived at 8000 RPM for years. I believe you, I've heard that story but it's not my experience.

[dd74]

Henry: I can at least attest to your stock 3.0 engines. How many years have I had that 3.0? Two years, I think? It must still be breaking itself in, because it feels stronger each time I drive it.

Didja you ever see the dyno results on it?



I won't preclude a little horsepower massage in the future, though. Just 210-220 hp. Nothing radical, however; I wouldn't want the reliability/driveability too fly out the window.

What's the stats on the SC motor in the blue car?

[Shuie]

Wow! Thanks again Henry, this is good stuff.

When you use a 74mm crank to do a 3.4 or a 3.5, do you guys ever go to a better rod like the Pauters or do you always try to use a 962 crank? The 962 cranks cant be real easy to find

[Henry Schmidt]

Stats on the blue car engine are, 70.4 x 98. 9.5 to 1 JE. 39 mm intake 37 mm exhaust. Big port heads, SSI, Euro 2.7 fuel dist. Taylor wires, Recurved dist. aux air regulator removed ( 74 car), 20/21 cams, 022 warm-up regulator ( 3.0 turbo), 3.2 oil pump. This car is street driven by the same owner for 25 years.


Shuie ........I'm sorry for miss leading you. We don't use 962 crank shafts, we modify stock 3.2 & 3.3 cranks to use a 3.0 rod. I got the idea from measuring a 962 crank.
I think you're right when you say "The 962 cranks cant be real easy to find ".
As for after market rods, I have used them in the past when a customer insist but it's rarely my idea and I really like stock porsche rods when prepped appropriately ( ARP rod bolts on 2.0 rods, and ARP or 935 titanium bolts on 2.2, 2.7, 3.0 rods)

[Shuie]

Henry, I just want to make sure I understand this.

Are you saying that you can modify a 74mm crank from a 3.2 Carrera to use the rods from a SC? Ive never heard this before. This might be really good news for me

Thanks again!

[Porschekid962]

Has anyone here heard of 9M, nine meister racing in London?? I followed an engine build they documented online and the results were pretty impressive. The case was a 964 as were the heads, crank from a 993, custom pistons and cylinders, i forget what rods they used, twin plugged with a motec ecu. The engine was a 3.8 and their goal was 100hp per liter. I checked up and the last number they quoted was 396 horses. They did not say if it was at the crank or wheels. They said most of their gains came from headwork and exact balancing of the crank after it was knife edged.

It goes to show you that you can have huge displacement but more remarkable gains can be had in the details.

[A Quiet Boom]

While we are on the subject, what is the bore/stroke of a 3.6? I believe the stroke is 76.4 which means it needs a 100 bore to achieve drivability, how does this figure in with the statements made earlier about 100mm barrels being too thin? I'm assuming Porsche went with thick wall barrels and just opened up the case. Is it possible to put a 964 crank and rods as well as barrels and pistons onto a 3.0 or 3.2 case? The reason I ask is that I have a potential line on a 3.2 with a spun rod and a 964 crank that will fit. I'm just wondering about taking it all the way to a 3.6. The other possiblity is to just buy the crank and rods and some big bore P&C's and install them on my 3.0 block if that is even possible. I'd guess that since the 3.2 crank can be swapped into a 3.0 and a 964 3.6 crank can be swapped into a 3.2 then the 3.6 crank would fit a 3.0? or am I crazy? Just thinking of taking rdane's motor one step further.

[Henry Schmidt]

I believe you'll find the 3.6 Carrera II crank has a stroke of 76.0mm.
That small fact aside it will fit in a 3.0.case.
The reason we talk about 100 mm 3.5 cylinders being very thin is because they are thinner than 3.6 100mm cylinders.
In order the fit the 100 mm cylinder in the 3.0 and 3.2 case / spigot it had to be thin and we have to open the spigot . This is very similar to the 92 and 93 mm cylinders in the 2.7 case. When you stretch these engines to their limit, things get thin.

[Henry Schmidt]

We'll I'm wrong.
In the previous post I said that the 3.6 crank has a 76.0 mm stroke.Well I'm wrong. As stated by Christian the stroke is 76.4. I could have edited the post so I wouldn't look wrong but I'm only human and everyone make mistakes. Sometimes it's a good idea to just see it and except it.
Sorry for the bad info.

[Bill Verburg]

102x76.4 is going to be hard to beat tjough 104x76.4 is available.

[cstreit]

Short stroke 2.5 – 90mm bore x 66mm stroke. Early aluminum case 2.0 with spigots bored out to accept 90mm P&Cs. Including all of the case upgrades it seems like a lot of machine work, but still a very neat engine.

Basically a 2.7 engine with the smaller crank. You could make this engine by using the 66mm crank and rods with the rest of the engine being a 2.7. This would give you a higher-revving engine.

I would like to hear more about this motor. A few questions:

Which rods? Is there machining required to any components for this?
Preferred intake?

[jluetjen]

Which rods?

Either the 2.0 or the 2.2 Rods. Do a search here and you'll hear about the difference in regards to the rod weight, design and bolt availability. It's been covered within the last 6 months.

Is there machining required to any components for this?

If it were me I'd go for some high CR pistons, especially if you were going to make a race engine. If you use the 2.7 RS pistons with their fairly mild 8.5:1 CR, the CR will drop even further, I'm guess to around 7.5 or 8.0:1. If you were to start with some 10.5:1 2.7 pistons, the CR should wind up at a "streetable" ~9.5:1.

As far as intakes go, your standard 2.7 or 2.4S set-up (either carbs or MFI) should work just fine. I guess you could also do it to a CIS engine, but I'm not sure why you would bother since the 70.4 mm crank is perfectly adequate for the rev range of a 2.7 engine, and the 2.5 wouldn't rev any higher without going to more radical cams which would take you down the radical cam + CIS discussion again.

I would see it as being an interesting development path for someone with an early 2.0 bottom end and some 2.7 parts laying around, or else someone who's trying to stay within a 2.5 liter engine limit. The big benefit would be for someone who is trying to build an 8000 RPM + engine, in which case the 66 mm crank makes for a happier bottem end then a 70.4 mm crank. The longer stroke cranks can be made to work at those engine speeds, but it takes some special machining work. The top end issues would be the same for both engines. That's the only reason that Porsche made some of the ST's with short stroke rather then "long stroke" 2.5.

But aside from that I don't see any clear benefit or performance benefits that would justify the trouble. Basically the engine will have a little more torque and HP then a comparably spec'd 2.4, but less then a comparably spec'd 2.7.

[Shuie]

Chris, Im pretty sure that Tyson built that same engine w/ mag case, JEs, and MFI for Scruffy. There was a thread about the rods when Tyson was building the motor and the consensus was to use the 2.2 rods. I would love to put that motor in a SWB R clone or a stripped 914.

[cstreit]

Hmmm...

I've got most of a 2.7 with MFI pistons, cylinders, etc.. but no crank. I've also got a great line on a 66mm crank with Carillo Rods and lots of other associated nicey nice parts. I really don't have much to do with it, but I've always wanted to build one. I could run in some of the groups if I could mate it to a 915 gearbox. Is this as easy as buying the right clutch, or were the 66mm clutches and 70.4mm ones the same?

[jluetjen]

Is this as easy as buying the right clutch, or were the 66mm clutches and 70.4mm ones the same?

I believe that the crank had a different number of teeth, so you'd need a different clutch disk. I believe that the pressure plates are the same. I would get that confirmed though. I'll go count the teeth on my 66 mm crank if someone else has a 70.4 mm crank that they'd like to count.

[blue72s]

Which rods?

Did you know that with the advent of the E series 2.4 cars, the soft nitriding of the S rods were superceded by the Tenifer process?

So, instead of buying 2.0S rods, I think you could buy untreated 2.2 rods and have them Tenifer treated.

[A Quiet Boom]

Originally posted by jluetjen
I believe that the crank had a different number of teeth, so you'd need a different clutch disk. I believe that the pressure plates are the same. I would get that confirmed though. I'll go count the teeth on my 66 mm crank if someone else has a 70.4 mm crank that they'd like to count.

Teeth?

FWIW, I bolted a 2.0E 911 Clutch disk and pressure plate to a 3.0SC without any problems. I believe that the 2.0-2.7 used the same bolt pattern for the flywheel. Still not sure what you mean by teeth though unless you're talking about the starter ring gear or the trans input shaft.

[blue72s]

With the 70.4 crank, the rod was too wide which made the flyweights too thin and at high RPMs the cranks broke. That's why I would only built a 3.0 and larger engine with a 9 bolt crank. Next they changed the 3.2 & 3.3 rods. When they put the larger journal on the rods they discovered a clearance problem with the oil pump. Solution was to make the rod smaller, this required a smaller rod bolt. Now we have a whole new set of problem. My solution, change the journal back. Why do that you ask? Well I saw a 962 crank and it had 3.0 journals.

Were the flyweights re-thickened on 9 bolt cranks?
What you mean by journals? Are all 9 bolt 70.4mm cranks the same?

In summary, under 3.0 = 66 stroke. Over 3.0 9 bolt crank with 3.0 rod modification.

I quite agree.

[jluetjen]

Still not sure what you mean by teeth though unless you're talking about the starter ring gear or the trans input shaft.

Oops!

My bad. I was thinking about...

Oh never mind. I screwed up. That's what I get for trying to respond at 0'Dark :30.