Paulyy

I don't think i've even seen anyone here with a high RPM capable motors making power over 7000 rpm

I know these motors are larger for a I4 compared to 1.8 - 2L motors.

A lot of people here go to extents to rebuilding their engines with lighter forge pistons and rods (much lighter then stock), lightened & cross-drilled cranks, they get bigger valve heads with better flowing intakes even huge turbos that supports a lot more power then target power achieved.
But i haven't seen anyone utilise the 'large bore small stroke' we've got.

Large bore small stroke = RPM>Torque
Small bore large stroke = Torque>RPM

With all the modifications above, by getting a solid lifter setup (to stop valve float) would enable the motor to rev more higher with also a custom camshaft matched with an intake to allow airflow at the higher rpm.
OR Is the problem with the 8V head not able to flow enough air at the cylinders?
I know going with a 16V head on a 2.5 would get quite costly with modifications and even just locating a working 16V head isn't easy.

I've also read that 8V heads would produce more torque then horsepower compared to a 16V head.

Or is it that much cheaper to build a 8V or 16V 3L engine to achieve the same amount of power from a higher RPM output motor?
OR
Is it it just a reliability issue? (heat/cooling, spinning bearings ect)

I know more power causes more heat in the engine.
More Torque then Horsepower breaks Drivelines
More Horsepower then Torque spins bearings?

When you Track rats (NOT Auto-X) build your engines, do you try make as much power as possible through out the rpm range? Do you really care about the low end torque? i doubt you spend any time lower then 4k rpm with a 6500 red line.

What are you more worried about?
breaking gear boxes/snapping drive shafts/breaking torque tube shafts?
OR
spinning bearings, cooling issues(other things im not sure about on a higher rpm engine)?


Just so you all know i'm not planning to build a race car, more to know why not many people go for horsepower at real high rpms.
IICR corleone built a high rpm 2.5 and 3L engine, that's it.

Please discuss.
Thanks

lee101315

I tend to follow the direction the engineers went when I modify an engine, and in this case I feel they put an emphasis on torque. Heres why I think so:

Cam profile
4-2-1 header design
60lb crankshaft ( long and heavy for a 4cyl)

Many people try to keep these engines under 6000rpm for longevity, especially on a 3 liter ( no one wants to spatter the internals on the pavement with a 15k build!)

You can build the head for whatever you like, but I dont think the 2.5s will enjoy being revved to 8k

Richgreenster

I think you are getting into More money that it is worth.
How fast do you want to spend/spin?

95ONE

Its just that RPM has such a large slope of diminishing return in the HP game. I was going to have a 7500 rpm 3 liter. I ddin't feel Like I was going to need to go any higher. That's what my boost is (was) for.

TurboTommy

Keeping the torque value up into higher RPMs defiinitely has a dramatic effect on increasing hp. It's just that there's so much more involved.
Solid lifters, very agressive cam (maybe, not quite so much with a 16V), heavy springs, head work, possibly a different intake mani. Then you need very good exhaust flow; headers, big turbine housing. (So, together with the aggressive cam, low end power and response will be less then desirable. Lower gearing would be a must to really feel the extra hp)
Then there's the oilling issues; need dry sump.
Then; even though we do have a large bore configuration, which helps; it's still four big cylinders with the corresponding high reciprocating masses. The parts are subjected to high stress, compared to the same displacement divided into 6 cylinders, for example.
There's lots to consider; and money$

I guess it's just more effective to increase power by increasing the torque across the exsisting power band.

Paulyy

i guess it is more effective to just increase the torque from the existing powerband.

maybe some people have more money the brains lol!

V2Rocket

Here's a thought:

It has been said here that the stock exhaust design on the 951 (pipe length/size/bends) is the reason why the 951 has such a mountain of torque in the mid-range - the restriction brought on for packaging (or marketing vs 930) reasons limits top-end power but makes the car a bit nicer to drive in the 3-5000rpm range on streets.

If you are spinning faster on a built engine it is obviously going to be pushing more air through all those pipes. More air generally means larger pipes. So size up the pipes to reflect the airflow that will be going through at higher RPM/displacement but limit it in a similar way to the factory. So maybe the engine will rev to 7500, but power will be falling off there. Peak power could be at 7000 and the torque could be riding high until 6000-6500, for example. Would likely cost lower-end driveability though, too big of pipes = low exhaust velocity/slow spooling turbo.

Maybe not ideal, and definitely not quite so simple, but it could be worth looking into further.

blown 944

Spencer, that is one of the first things to be done. Hell Patrick has a 5" exhaust. It's more about the oiling and rotating mass.

V2Rocket

I meant more the crossover and turbine housing than the rest of the system, to clarify. Definitely changing the rest of the system is one of the first things to do, if not the first.

blown 944

Even then, the engine has serious limitations (which I'm exploring).

I'm hoping these smaller bearings and lighter reciprocating assembly help out a little.

67King

I'm worried about oiling, but frankly, the intake manifold tunes sufficiently low that it won't allow the engine to breathe well at higher RPM's. A custom short runner intkae would move the power curve up, and different cam profiles would, as well. Also, the 2V's are going to be limited by valve area. A 16V with 37mm intake valves should be capable of higher RPM, but a 3.0L would require the 968's 39mm valves.

But at the end of the day, without a dry sump, a reliable high RPM engine is an oxymoron.

odurandina

as long as you have a $120~150 K engine budget (so you'll still have some money left over when you quit after the 3rd, 4th and 5th engines have blown up) and return back to earth.... keep in mind, the Germans, Americans, Japanese and everyone else already tried this with hundred million dollar budgets, and engineers with lab-coats, fancy degrees and billion dollar equipment.... you're already driving a considerably undersquare engine built to survive in an what is already a horrific environment (i.e. more than 2.2 litres and 6,000 rpm +).... 4 cylinders and high rpm are oxymorons... Porsche and Lotus presented very close-to-the-limit examples in their production cars... there have been a few exceptions over the years, but they were all dry-sump, big budget projects.... getting into a Cayman or 911 would be money far better well spent to stay in "Porsche," as there are engine and turbo options right off the shelf. there are many examples of guys who laid out big bucks to stay in the 4-cylinders when they had plenty of money to move up to 6-cylinders.... or if you were going V8, not only would you NOT be beating up the crank/bearings at 6,700~7,200 rpm and 500 hp, but you'd have dozens of options starting with the crate motors and be able to move up to forged parts and the required oiling to get you where you wanted to go without twisting the engine apart or going broke... but what you're describing, by scale is almost akin to entry-level champ car budgets and lots of places to fail in an extremely challenged (2 up~2 down) design.



.

67King

Our engines are horrendously OVERsquare, and the number of cylinders has NOTHING to do with it. If anything, an inline architecture lends itself to RPM. Yes, they are lower displacement, but most sport bikes are 4 cylinders. The Harley V-rod has the same displacement/cylinder as ours do, and it screams. Ducati's are pretty close, and they are 12,000 RPM V-twins. The Ferrari 458 is a 9,000 RPM engine that is more like an inline 4 than any other V-8 out there. Even the lowly Mustang Boss 302, which is wet sumped, it'll turn 7500 RPM all day long. With hydralic lash adjusters, and roller finger followers (they could make it turn about 9,000 wiht a valvetrain change).

You want to turn RPM on a 944 engine? Fix the oiling problem. The rest is easy.

V2Rocket

NASCAR-style drysump...bouncing around 9000RPM for days...