Crankcase Pressure

[Propjockey]

Ok...I've seen it done before, now I'm curious if anybody has experience with it.

Tapping the crankcase to pressurize the fuel tank by drilling horizontally through the engine mounting lug.

Does it work? Pros.....cons?

 

[Chris H]

A two stroke engine is basicly a pump, the better the engine is sealed the better the pump action will be.Why would some one want to drill a hole in it ?Regards Chris.

 

[eric ekern]

ive seen 1/8th (.21)scale gas cars do it. the supposed theory is to get more consistent tank pressure from full to empty tank and therefore a more consistent needle. i dont have any first hand experience, ive just seen it done in the r/c car mags. my 2 cents

 

[Hammerhead]

O.S has some aircraft engines that come with the case tapped from the factory. I have seen some of the guys at the races doing it with the .21s (nothing bigger) They didn't seem to be having problems, but I didn't notice any outstanding benifits either. Maybe they use two tanks the same size in their Hydros because of a lack of clearance in the hull. They thought the case would be able to pressure them more equally.

My 1.5 cents

Hammer

 

[Propjockey]

Well let me tell you what I'm working on, and maybe someone has another solution.

I want to pipe the cooling water from the head into the exhaust on a .15 engine (stop laughing). I'm hoping to accomplish two things here. First, cool the exhaust. Second, by dumping the water into the header just past the turn (side exhaust), I'm hoping the venturi effect will help draw more water through the head at lower speeds when the ram effect of the pickup tube is low.

Problem is, if I pressurize the fuel tank from a point 'downstream', won't water vapor be pumped into the tank as well?

 

[Hammerhead]

If your pipe is at the perfect length, then the un burnt fuel will be sucked back into the engine on the down stroke. This gives you a turbo effect. The heat inside the pipe is important for vaporizing the unburnt fuel to make this work. If you put water into your header, even if it vaporizes, it will get sucked back in. Water unlike the fuel will not compress in the combustion chamber, therefore your engine will stall.

I know that in the real drag boats, that they do inject water into the headers, but I think applications are different than they are on the 2 stroke nitros.

Anyone else have some input?

Hammer

 

[Propjockey]

Hmmmm....good point. Back to the drawing board.

Thanks Hammer.

 

[TimD]

Hammer is correct - you will end up pulling water back into the chamber if you inject water that close to the exhaust port. Some gas guys inject a small amount of water into the back 1/2 of the pipe for speed runs - (cooler pipe acts shorter).

The other thing is crankcase pressure can change dramatically as the motor is running - it can actually go to vaccum - and is not that reliable for boat use.

From your earlier post it seems to me that you are trying to increase your cooling of the motor and pipe. Is the motor running that hot? Generally most boat engines run too cold to burn the fuel properly.

What fuel / head clearance are you running?

 

[Propjockey]

I'm using Byron's Competition Blend (25% Nitro, 16% oil--80/20 synthetic to castor and adding 6 oz of Sig brand castor to a full gallon to get just over 20% oil).

Stock head clearance.

I broke the engine in with an airplane prop and an electric fuel pump pushing water through the head. Plenty cool that way, but the boat's not wet yet.

The reason for overkill on the cooling is, the engine is a Traxxas .15 Pro, designed for their Nitro Vee.

The Traxxas forum as well as other people I have talked to say the engine overheats when you start moddin' it (metal props, higher nitro etc.) One guy said the surface of his engine actually smokes after extended runs. That's too hot in my book.

The engine will be ported with a tuned pipe and header, spinning an Octura X440 on a 23" 'rigger.

 

[Hammerhead]

If they have that bad of a reputation, then I would recommend getting a different engine.

If you are dead and determined to use that one, then you should look at using cooling fins instead of water. You can also wind the soft aluminum fuel tank pipe around your header and cool it that way, if you want. Wrap it about 5 times.

Talk to MikeP! Send him some mail. He uses air cooling fins on his .15 outrigger.

Don't know what kind of engine he is using, though

Good luck!

Hammer

 

[TimD]

Propjockey,

I would just use pipe pressure for the tank, and if a single water pickup is not enough (which I would be very surprised) then mount a second water pickup on the turnfin. Also, being in a rigger it will have some airflow over it as long as it is not hidden under a cowl.

I would go for a octura 1938 or similar - the X440 will be too small in pitch and too large in diameter. Different story to a mono!!!! If the case gets so hot that it smokes it is either over-propped or way too lean (or both).

Oh yeah, set the head clearance down to under 10 thou or there abouts. should make the motor run better.

 

[Propjockey]

As always, thanks a ton for the info y'all!

Like I said, the boat's not wet yet (and I'm gettin' real impatient). I may not have any problems at all-I'm just trying to cover all the bases.

The engine seems well made and doesn't really have a bad rep...but it is a 'sport' engine and wasn't designed with race mods in mind. Pretty much all the problems have come from trying to soup it up one way or another.

 

[Hap Mulvany]

I would like to correct an earlyer statement. When you put water in a pipe the effect is that of a longer pipe. Note a whh wet pipe it is much shorter than the dry pipe