Pelican Parts Forums - Hours between rebuilds for 911 race engines

How often do you rebuild a race engine?
Good question.

As noted above and more:

Which parts and treatments went into it?
Weight of rod, piston, wrist pin, etc?
Stroke?
Stroke-to-rod-length ratio?
Displacement?
Compression ratio?
Cams?

How high do you rev it?
Have you over-revved it?
How high?

How is it warmed up?
How hot does it get (oil & heads)?
How is it cooled off?
Is it ever started other than to complete race laps?

What oil?
What oil pressure?
Does it ever starve for oil?
How often changed?

There is more.

I use cumulative at-speed race laps to determine track time.
Just add up all the green-flag, at-speed lap times.
That doesn’t include yellow flag laps or warm-up ‘out’ laps and cool-down ‘in’ laps.

I use 25 hours as my standard time-between-rebuilds.

I would never start a season without all fresh engines.

I have three complete race engines for my 914-6 GT-2.
One running in the car, one with dyno-time only in the truck
and the third undergoing a maintenance rebuild or finished
and on the dyno.

My engines are 66x91, modified 2.0 rods, Mahle race pistons, ‘sprint’ cams, 935 valve train and more.
It sees 8300 every shift, intentionally 8700 occasionally and 9000 unintentionally and rarely.
It is never run under about 6500 on track. It won’t run well at full throttle below about 4500.

It never gets above 200F oil temp and about 350F heads.
I have a propane ‘torpedo’ for warm-up on cool mornings.
I am very careful with the ‘cool-down’ lap.

I ice the intake manifolds between sessions to prevent any fuel boiling in the 46IDA3Cs.
This allows perfect starting for the next session.

I only start it for warm-up driving to the pre-grid, never other.
I have it where it starts on the first 1/3 turn and runs perfectly at 3500 during warm-up.

I start it briefly when cool at the end of the day in order to circulate cold oil that
has drained off (easier on the machinery the next morning).

With an only slightly modified 914-6 oil system, I struggle to not have oil cavitations
at very high revs while braking and turning. I have a type 771 oil light set at 45 psi.
I run the modified stock tank so full that it pukes about three quarts on the cool-off lap.
While the running oil pressure regulates above 60 psi, I can have the warning light blink
at me under certain driving conditions. Usually in the ‘heat of battle’ above 8000 rpm.

What is a ‘Maintenance Rebuild’?
Depends.

The first goal is absolutely to prevent a catastrophic failure.

Rod bolts and nuts are #1
Close behind is rod inspection
Next are rod bearings.
Then there are the (now expensive) main bearings.
After enough complete rebuilds, this can be done without splitting the case some of the time.

The top piston ring and a valve ‘touch-up’ preserves the power.

Everything else is detailed (and time consuming) inspection and measurement.
No part is insignificant enough to not carefully inspect and measure.
If in doubt, replace it with known (measured and tested) good.

New seals and gaskets everywhere prevent oil and other leaks at the wrong time.

In a race engine, every part has a lifespan.
While they are all different, eventually the entire engine is replaced.

The best philosophy is to do the Maintenance Rebuild too often.
At first, way, way too often.
You never want to find ‘almost’ failed or worn parts.

At some point, any engine should be taken out of ‘front line’ race service and another
use found – coffee table comes to mind.

The “BIG REASON” for all this effort is even a small initial failure can lead to a catastrophic engine failure.
Much of the time, nothing can be saved or re-used reliably.
I have 1/3 of a 956 F&S clutch that left the track in turn 1 at Laguna and
hit the adjacent rental car parked almost a mile away. Not much of the
rear of the car was left.

The goal is to win the race. If you don’t finish because of an engine ….

A secondary goal during maintenance rebuilding is you can incrementally increase the power by
edging up on things like compression ratio. Running piston-to-head and piston-to-valve clearances
can be worked down to ‘small’ but never touch.
There is no such thing as a ‘little pregnant’.

Learn from other’s mistakes and problems, don’t you make any mistakes.
Whenever someone has a problem, jump in and help.
Not only will you have more and better racing but also you can
usually learn what not to do.
Most track problems are self-induced, even in the ‘pros’.

Now, compare your racing effort to, say, the Porsche Cup Series.
The engines get a Porsche Motorsports maintenance rebuild every 40 hours or less.
New P&C, heads, crank, rods and even crankcase if too many hours total.
The cost? $35,000 to $45,000. Occasionally more.
Porsche won’t rebuild your engine unless they get to make all these decisions.
Your pocketbook is not a consideration.

Consider that these engines are almost 4-liter, turn 9000 rpm and put out LOTS of power -
almost as much as a 3.2, twin-turbo 935 at full boost and 8300 rpm.

So … let’s enjoy amateur racing at a level where your pocketbook is not stressed.
A damaged race engine is very high stress.
Maintenance rebuilds are not.

Best,
Grady