Two Stroke Vs Four Stroke Longevity

I’ve always wondered why the 2 stroke karting engines are so poor when it comes to longevity. When you look at outboards they are getting thousands and thousands of hours on a 2 stroke. We’ve seen some racing applications where the engine is on the 3rd season.

Why can’t the engine manufacturers build something that lasts?

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Maybe they aren’t revving to the same RPMs as a kart engine? I have no idea. I’m just guessing.

They are saying new piston at 20hrs. Unless I am reading wrong? Some of the racing off shore outboards will see 8-9k continuous while drag engines see north of 10k. Some over 12k

I meant someone could probably get a piston to last last 80, if we didn’t rev them to 15k and tuned the carb different. I think another piece is these are so small the components might be more delicate

Direct drive application in a kart has some unique challenges vs outboard motors. Outboard can basically chug at (more or less) the same RPM. It doesnt have to pull strong over a wide range.

A kart motor has to operate over a larger rev range. Outboards also (usually) have better oil injection systems, EFI and other trick stuff that’s cost prohibitive for karting.

Also keep in mind that loads on components increase with the square of rotational speed. So the stresses on components in a motor turning 10K are much lower than one turning 12, not to mention 16 or more.

The size/weight of a kart is a factor as well. You can only pack so much punch into a 30lb block of aluminum, you can be (proportionally) more aggressive with a racing boat’s motor.

I think that’s part of it. Onboards aren’t pushing the limits of their engineering capacity, whereas many karters look for those extra tenths of a second from the motor. We could probably very easily tune a kart engine to last for 80 hours, it’s just that the performance wouldn’t be there.

All good points! I know the Animals tend to not last either when built. Typical racing engine…

2 strokes on a karting application are smaller and built to tighter tolerances. However you could have a 2 stroke engine last a long time before top end rebuilds if you jet it really rich and don’t rev it high.

A lot of the vintage guys with bomb ICA motors (which back when engines were wear and tear items spinning >20k rpms) run lower revs (12k) and the powerplants last a really long time.

You have to do this when replacement parts are almost IMPOSSIBLE to find!

It’s a good question to pose though, it rarely hurts to try and make motors last longer for racers.

I thought the bombs were Formula K 135cc motors?
What you say is true on longevity though, if I kept my FA motors under 15K they would last a pretty decent time. Get into 19K+ territory and any more than 40mins on a piston is danger zone.

Those are the B-Bombs, yes :slight_smile:
I meant bombs as in crazy engines in general. Sorry for the confusion :brap:

FA motors, as in…?

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Its a combination of RPM and head volume (compression ratio). FA and ICA revved to 19+ and you’d be changing pistons at least every hour. KF1 at one point were changing pistons between heats and running them in on the stand because they had the head volume so tight.

Two different failure modes though, one left you open to a cylinder wall-piston seize, the other left you open to pitting and a hole in the piston.

But when comparing 2 strokes really the only major difference is induction and that has little effect of longevity. So for all intents and purposes a KT100 and a Formula K 135cc are under the same types of stresses, just at different RPM.

So you can get a good idea how long an engine will last just by cutting down the RPM. Moped engines last forever, chainsaw engines last forever, they are all two strokes, just without high rpm or tight head volumes.