Race motor longevity

[skipdup]

As part of my continued education... Thought I'd ask here and try to better understand...
I currently race a stock 2.2 s-spec with webers in my '71 911. I have 12 race weekends and probably 12 de weekends on the motor.
I understand that (in general) the more power you make, the less longevity you get out of it.
As I contemplate going to a 2.4 race motor, I do find myself worried about longevity. I see some of my competitors (with real race motors) not practicing so as to not "waste the engine". That would be tortureous to me - I really, really like to drive.
So, I guess my question is... Is there a generally accepted sweet-spot between power and longevity for a 2.4? What things do I need to be thinking about as I discuss pistons and cams with my builder? What is a reasonable goal I should be looking for.
Basically, I'm realizing I'm ignorant, and looking for any way to get my thinking headed in the right and at least semi-informed direction.

Many thanks,
Skip

[PFM]

Skip,

I thought one of the pros would reply but I will give it a go. Three things shorten the live of an engine, any engine, heat, high horsepower per cubic inch or cc and RPM.

These are normally all interrelated. To increase horsepower we increase our volumetric efficiency, this adds heat and we normally increase the RPM as well. Heat we can attempt to control with better oil coolers (front mount), better VE is our goal so the last thing we can do is control RPM.

By example on endurance race motors we limit the RPM even though we know the engine can make more power with more RPM, you have to finish to win.

What I do is set a practice RPM limit about 500 RPM below my race limit, reducing the limit by 500 or better still 700 RPM can really help save your motor while giving you more track time.

The other optinon is use ultra light parts, best of breed valves and springs and such as if building a 9000 RPM motor but then cam it for peak power at say 7500 and just run it at 7500.

Remember speed cost mony how fast can you afford to go?

[KTL]

Do a little bit of searching with the search button here on the engine forum and you'll find some very good info. from the experts that build engines regularly. You'll find that RPM is a huge factor in longevity once you start getting past high 7000's. The loads seen by the pistons, rods, crank increase greatly as you approach 8000 and beyond. It's not a linear relation. It's an exponential increase and therefore the loads on these moving parts go up immensely when you are spinning things that fast.

[skipdup]

Thanks guys. I understand conceptually that rpm, heat, etc kill engines.
I've tried searching, but not sure I know how to look or what to look for - if that makes sense.
I think I'm probably asking what I should be educating myself on.
Essentially, p&c, cams and exhaust are free. So, other than the other go fast work that generically is done to engines (ie heads), what should I be concerned about.
Dawns on me I may be over thinking (wouldn't be the first time)... Maybe the question I should be asking is, for a 2.4, what's a good compression ratio and max rpm for the good mix between power and longevity. Is there a sweet spot? Or just a sliding scale of comfort.

Thanks again. I love flexing my ignorance for the world to see.

- Skip

[BURN-BROS]

Porsche tradition for endurance engines had been 10.5/1 with twin plugs.

[Plavan]

I have put 109 hours (includes warm up time) on my spare 2.5L race motor that saw 7700-7800 RPM. (10.5:1 compression)

I was orginally like you and worried about putting hours on and rebuild costs.
My motor guy just wants to inspect the filters after every 2 race weekends. He was even comfortable with me putting more hours on the 109 hour motor. The only reason I pulled it was to install 2.5L race motor #2 with more internal goodies.

Just watch the revs and temps. If you have a desent motor builder, you should be fine.