Yamato Pipes

[Systemoverload]

I wanted a simple megaphone but it ended up looking like this:

I'm shooting for 7,500rpm or maybe a little bit higher. Do I need to add more oil at this rpm level? Now at 30:1 using Seadoo XPS.

------------------------------------------------

[Systemoverload]

Hmmm, so even oil ratios are secret too? C'mon!!!!

------------------------------------------

[proprider01us]

I use 16:1 on all my engines, stock and modified. I use a synthetic/petroleum blend. Never have had any lubrication issues and spool the mod engine over 8,000rpm

No secrets in my trailer.

[David Mason]

It is not a secret. Personally, I wouldn't change your oil mix but instead would fart around with your jetting for the addition of the pipe.

Everyone has an opinion on what ratio to mix for their engines, it sometimes depends on what type of oil and gas you are using, as there are a lot of different theroies I have seen posted on here and also Hydroracer on the very subject. Best bet is to go testing and read your plugs.

[Systemoverload]

Thanks. Not so many people into this over here so I have no one to ask.

Since it's a home-made pipe I anticipate it will take several iterations to arrive at the optimum.length & stinger diameter, ignition timing advance, jetting, etc.

Can someone please tell me what to look out for? I have read Jenning's book ~30 times to try and get some pointers in tuning this and I know the potential damage I can cause to the engine. Besides that, how do you know whether the prop is loading the engine too much or is the pipe is too short? I hear it singing but is it the right "song"?

I use a tach and GPS all the time, 13-ft vee-pad hull..I was getting 6,800rpm on one prop, 7,200rpm on another prop, no other changes made. 53mph tops, that's it and so I made another pipe but have yet to try it out.

-------------------------------------------------------

[Mark75H]

You meant how can you tell if the pipe is too long, not too short. If the pipe was too short it would be for even higher rpm. If the pipe is too long and the final cone is steep enough, it can stop the motor from revving. Alky racers use changeable pipe length to control motor speed instead of throttle ... long for slow and short for fast.

The only way to know if the length is the major player is to change the length of the pipe.

Yamatos seem to show their pipe sensitivity on the lower cylinder.

Here's the part that is the hardest for most people to grasp: The tuned pipe expansion chamber is only one part of a system. This system includes high exhaust porting. Without high porting, the effect of the pipe is dramatically reduced.

They way my brother puts it ... the expansion chamber is a crutch that allows the motor to produce good power when the exhaust porting would otherwise be too radical

for power, use as much oil as you can ... one guy I know (Ron knows him well) with more dyno time than just about anyone in racing uses 18:1 with his Merc ... he also has a lot of wins under his belt in almost every category in outboard racing

[Bill Van Steenwyk]

is if the motor slows down or loses RPM when you pull the pipe up (shorten it). If this should be the response when pulling the pipe up get it back quickly as you can burn or stick a piston quickly with too short a pipe.

Reading plugs will give you a lot of information, and the Jennings book you have, although somewhat out of date, is a gold mine of information.

There are also web sites that have information on correct lengths, angles, stinger and header pipe size for various applications, but the most specific of those formulas require disassembly of the engine and careful measurement of various internal dimensions of the engine, plus a target RPM range you want the motor to run in.

Gordon Blair is one person who has computer programs on expansion chamber design.

Good Ludk

[Systemoverload]

is if the motor slows down or loses RPM when you pull the pipe up (shorten it). If this should be the response when pulling the pipe up get it back quickly as you can burn or stick a piston quickly with too short a pipe.

Reading plugs will give you a lot of information, and the Jennings book you have, although somewhat out of date, is a gold mine of information.

There are also web sites that have information on correct lengths, angles, stinger and header pipe size for various applications, but the most specific of those formulas require disassembly of the engine and careful measurement of various internal dimensions of the engine, plus a target RPM range you want the motor to run in.

Gordon Blair is one person who has computer programs on expansion chamber design.

Good Ludk

Thanks guys, I do have Blair's book and have read at great lengths also. Will let you know how it turns out this weekend.


-----------------------------------------------------.

[Seagull 170]

You can make the pipe appear longer by squirting cooling water over it.

[David Mason]

Don't make to many changes at once. You won't know what worked and what did not. Make one change at a time and evaluate.

Looking at your hull and what not, I think the speeds you are seeing are in the ballpark. On a lightweight hydro with very little water friction speeds with your engine design achieve around 75 MPH. So with your V style hull and sitting down in it I think 60MPH is probably a real far goal, but doable if you find out what works for your applications. most of the advice on here for that engine will be from a hydro standpoint, and not a V hull. Mod engines like to be tweaked to a specific application. So what works for the 400CCMH group in APBA might not work for you. You are loading the engine more than we are with the hull. You might be able to rig up a simple water break dyno and simplify your testing somewhat. It would at least save you some time to find the ballpark and then tweak with actual test time on the water.