Oversquare engine design - 8000 redline

[Tonger]

Thinking about building a motor using a 3.0 case and 70.4 crank. I was considering a 3.2 short stroke but the cylinders are not amenable to machining to 98mm and I'll probably wind up with a set of Nickies. I contacted LN Engineering and they said that I could also go to 100mm or 102mm if I machine the case creating a more oversquare engine.

What do y'all think? 100mm yields a 3.3L and 102mm creates a 3.45L.

If you were buying P&C's (plus rods) anyway, what would you do and why?

Thanks

[Catorce]

There is a lot involved in machining the case to accept a 102mm piston and I don't recall how much meat is left around the head studs after this. Even on a stock 3.6 case there is extremely little meat when you go a 109mm spigot.

Maybe someone can chime in here.

On our cases we make we moved the case through bolts a hair to get more meat.

[K24madness]

I would stick with the 100mm pistons and try and find the lightest rod/piston combination that gives you good service life. That would really help with the engine spinning up.

If you’re open to non pump fuel crank up compression and run 100oct or E85. The later really LOVES compression and has the additional benefit of keeping things cool. I know it’s a hassle with either but the benefits are well worth the inconvenience IMHO. If EFI you can also do a pump tune for the long journeys. Most of us tend to stay pretty close to home with our weekend toys.

[Trackrash]

Don't the heads need to be machined as well when the case is opened up? Of do Nickies cylinders only require the case to be opened up?

If you are going with Nickies, I would think 100mm slip-ins would be the way to go.

BTW, what valve train would you use to get to 8000 rpm?

[Tonger]

Engine will be twin plug and Charles Navarro suggested the machine in 100mm cylinders over the 100mm slip ins. I think he was a little concerned about the cylinder wall thickness.

The 102mm pistons definitely don’t leave much room for cylinder seating as y’all said. I’m just thinking the additional 0.15L might yield a little more torque.

As for valve train, I’m definitely open to suggestions but I imagine a light setup would be best.

Do y’all think low end torque will be adequate with the 3.3? Can we achieve 8000 rpm safely with the ‘big’ pistons?

Thanks!

[Walt Fricke]

You might want to read up on the problems the 70.4 crank had in holding the flywheel on at 8,000 rpm back when Porsche was using this crank in its race engines.

It can be solved.

[Peter M]

Tonger,
What are you actually trying to achieve?

What crankcases, crank and cylinders do you have ready access to now?

And what's your actually budget.

Answer these 3 questions adequately and I think the rest will be easy.

[faapgar]

The 70.4 RSR crank was 6 bolts to clamp the flywheel.They had harmonic issues and could shed the flywheel.Missed shifts would contribute to that.Henry at Supertec has a beautiful 66mm forged 9 bolt crank he sells that is light and cures that problem.Neil Harvey has some very nice pistons with smaller wrist pins and light rods but I have never seen their cost.Use the 100mm cylinders and machine the case so you have a thicker cylinder for strength.Make the rod length 132mm then you have a 2 to 1 rod sroke ratio.That will make 3.1 and 8000 rpm is not a problem.That is an oversquare build.You can use 3.6 heads if you narrow the stud spacing slightly and it will breathe.To finish it off you will need my stainless step headers that Brian at M&k is working on.Ciao Fred

[dannobee]

The heads don't need machining on most of the big bore kits. The head side of the cylinders are 113mm. I'll qualify that and say "usually," I'm sure that some out there do require flycutting the heads, but not for the most part.

[Tonger]

The 3.0 core short block that I have is a 930/16 from 1982. It has a 9 bolt flywheel.

Since I was looking to increase the displacement, I was going to buy a set of L&N Nickies cylinders (100 or 102mm) and suitable pistons/rods.

My thought was that the 3.2 SS uses 98 mm pistons and since I was buying p&c anyway, my thought was to just build a larger displacement version making it more oversquare. Right or wrong, my hope is that adding some more displacement will help with low end torque.

My goal is something twin plug and 10.5:1 with EFI and up to an 8K redline. We can decrease the redline as needed but would ideally still be >7K. I also would like to do ITB if the budget allows. To be clear, I am not building the motor myself, it will be done by someone with very experienced hands.
Thanks!

[Walt Fricke]

Got it - 9 bolt crank. One doesn't hear of the harmonic issues which caused the 6 bolt 70.4 to want to back the flywheel bolts out (unless you knew the secret, which Porsche apparently never did)affecting this one.

The nice thing about 10.5/1 and twin plug is you can run pump premium.

Redline can be a sort of arbitrary concept. If you use the parts usually used for a build like this, everything will handle 8,000 just fine. Ought not to have valve float or mechanical interference.

What you want, though, is the torque curve after it is built, which will allow you to use your transmission gearing to calculate the RPM at which each upshift (after 1-2, nobody has a 1st tall enough to make that efficient)should be made. I used to shift my 2.7 at 8,000 because the engine felt like it was still pulling strong, etc. Put it on the chassis dyno, and lo and behold 7,600 was the sweat spot for that engine with the gears I had. Most Porsche 915s have a pretty decent spread of ratios, with the optimum (for the engine) upshift points at pretty close to the same RPM.

Where do you plan to race?

[Tonger]

I'm not a racer, just wanting a nice engine for a sleeper / rat rod. When I do DE, it is at Road Atlanta.

I was thinking about using the SCRS ratios for the transmission since I have a bunch of short gears I could reuse from an early 915/02 that match those ratios almost exactly.

[Mark Henry]

I personally would stick to a thicker cylinder wall, smaller piston.
8K redline, are you racing? Street engine I'd stick to hair on the conservative side.

My '67 bug T4 conversion I could have used 103mm pistons (w/ nickies) X 78mm stroke for 2.7L, but I used 102mm instead which gave me 2.6L and a bit better safety margin. Thin cylinders walls are more prone to going oval IMHO.

[Carbster09]

TRE is building me a 3.3SS w 100mm P&Cs!

[smokintr6]

Something to consider with street engines... My old 3.6 air cooled engine had 12:1 compression, 3.8 RSR heads + cams, and ITBs. It revved to 7800 RPM and put 300 hp to the wheels in a 2600 lbs G body car. It totally sucked on the road. The torque below 4,000 wasn't that impressive, and it was so rare on the street to be singing up to 8,000 RPM that the car was almost never fun. I sold the engine, and went back to a stock 964 engine, that probably makes 75 less horsepower, but it it so much more fun on the road. The low end torque rips up to 4k, and it really makes the car fun to drive around town.

My 997.2 GT3 is also a blast to run on the street, but it has variable intake plenums, and vario cam on the intake and exhaust valves to maintain some semblance of a torque curve.

I guess what I'm saying is that if you're wanting to build something for torque, I'd be looking for something with a plenum instead of ITBs, and probably some cams that will cap the horsepower closer to 7,000. The aircooled engines aren't sophisticated enough (especially with ITBs) to make big low end torque numbers and carry it all the way to 8k RPM. If you want torque, sell what you have and start with a 993 vario ram engine. You will almost undoubtedly spend less money than building an ITB monster from scratch.

[Mark Henry]

I agree there's no perfect combo, a race engine sucks on the street, a street engine you will be leaving HP on the table for torque. For high performance street and occasional AX/track you want to build a torque monster not a high rev engine.

I've had to turn down jobs where the client has unrealistic combos ideas for the true intended use of the engine. It's been a long time (the 90's) since I've heard of someone wanting a "square engine", it looks good on paper, but I've never seen it work good in practice with air cooled engines.
Unless you have money to burn testing stick with proven combo's.

[Tonger]

What y'all are saying makes a lot of sense. I'm going to circle back and rethink the whole oversquare/short stroke on the street idea.

Thank you very much for sharing what you know!

[Trackrash]

You are now on the right track. Nothing wrong with a SS3.2. With the right cams, induction, and exhaust there is no reason why you can't have a nice torquey motor.

In my case, I wanted a motor that would pull out of slow turns, since I auto cross often. My 3.0 SC motor has JEs at 9.5 CR and a GT2 102 torque monster cam with Webers and a backdated exhaust. Starts pulling below 3000 rpm until past 6500. It will hit 7500, if I want to save a shift. I am often complimented on how my motor sounds and performs.

The key is matching all your components to how you want the motor to perform.