Longevity - Race vs. "Stock"

[Cory M]

We've been racing 911's for a while now. Always in classes that required stock engines (3.2, upgraded internals and chip ok, but stock CR, cams, heads, etc) or were competitive with mild engines (3.4, bigger cams, 10.5:1 CR, carbs, etc. These engines have been pretty bulletproof over the years running 100 octane on single plugs, with a 7k redline.

Recently we picked up another car that has a more of a true race built engine. It's a twin plug, custom pistons, 12.5:1 or 13.5:1 compression, ti rods, extra large valves, larger bearings, 3.6 oil pump, etc. Custom cams and carbs, running 110 leaded fuel, with an 8k redline. I have heard similar race engines need overhauls every 40 hours and have heard of some vintage guys with high strung motors who rebuild even more frequently. We are used to going a lot longer between overhauls with our "stock" engines.

What exactly wears out on the engines that requires the short rebuild cycle? If you don't do it what breaks first? Seems like this one has lots of bearing and oiling upgrades that should make it last longer, not shorter. I understand the higher revs and CR will increase the wear substantially. If this engine was driven with a lower rev limit it seems like that would that extend the life? Is the frequent rebuild really needed or just considered cheap insurance to keep from grenading an expensive motor? I know the factory cup cars recommend a 40 hour overhaul cycle too, but many club guys go 100+ hours one them and still show strong dyno, leakdown, and compression numbers.

I'm not looking to justify neglecting proper maintenance of the engine with this post. I'm really just looking to learn more about what makes the race built engine life so much shorter. Also wondering if there are things I should do to maximize the life, like short shifting in non-competitive settings.

[SoyRacer]

I could go into a long explanation but I'll keep it simple.
Key wear points -
High compression, aggressive clutches, engine braking are Murder on thrust bearings.
Low friction rings have a tendency to have oil rings that have a short lifespan.
Stretch bolts, studs are only trustworthy for as long as the compressed material maintains its integrity.
and so on and so forth . . .

[Evan Fullerton]

If the water-cooled GT3 motors are any indicator, they seem to have blow by issues (oil tank pressurization) long before any other major problems barring lots of over revs. Valve guides and rings seem to be the first to go which isn't such a bad failure mode IMO. Any air-cooled motor with over 12-1 compression I think is going to be pretty short lived though. If they are high quality pistons and cylinders, it might last a little longer but JE pistons.......... I wouldn't be overly optimistic from what I have seen. Conservative tuning particularly to timing will help but of the air-cooled race motors I know of that have gone there with compression ratios......... none have made it a season.

Piston speed is hard on rings. The 3.8L Cup motors seem to go maybe 75 hours where the 4.0L go maybe 40 hours. While the 3.6L ones with a little less cam, power, RPM and slower shifts seem to go 100+hours. 3.6L and 3.8L are the same stroke, 4.0L being longer stroke.

[KTL]

Good question. This is something i'm always curious about, so thanks for asking the forum.

Something worth considering is how the hours are actually measured. What I mean is, people will install an hourmeter and when it reaches their set number of hours it's time to rebuild. Within that number of hours may be a lot of time that's not stressing the engine- idling, warming up, sitting on the grid waiting to be released, cool down laps, etc.. I'm of the opinion that the hourmeter should be triggered by an rpm threshold. That'll give you a more realistic indication of how long the engine has been run hard. Nice thing about that is nowadays with modern technology an rpm-activated switch is pretty cheap.

[Cory M]

I use a simple digital hourmeter that gets a signal off the sparkplug wire. It's very conservative though, for example last event I went out for a 25 minute session; but with the warm up, time on grid, and cooldown, it registered 0.7 hours. In reality only 20% or so of that time was probably spent over 4-5k rpm. I don't have a very sophisticated data system with extra analog inputs, so I can't log it there. I'm sure the life limits are related to number of cycles versus just running time.I'll look into the rpm activated switches, should be better than just applying a percentage to my hourmeter.

I'd still like to hear more about what wears out in race engines vs street. I work on large turbine engines so I don't mind geeking out on it. On turbines and it's well understood what components are likely to time out first, how and why. The industry is moving away from simple overhaul cycles based on the number of hours and more into analytical life prediction based on how the engines are operated.

[Steve@Rennsport]

Good question and there are a lot of variables at work here.

Hourmeters, aka Hobbs meters, are used to measure engine operating hours and normally connected to oil pressure switches. Since race engines are not idled long, its an accurate representation of total hours.

Insofar as TBO's, camshaft profile (peak revs), compression ratio, as well as the quality of parts used, all play critical roles in deciding how many hours between engine rebuilds.

As you can see, its NOT a one-size-fits-all proposition and each engine will be different.

[Cory M]

Evan- do you know what went wrong on the ones that didn't last a season?

[rbogh901]

yes, what usually happens? do they lose power or fail catastrophically?