Old racing outboard

[acdc96]

I remember a couple of years ago I saw a old mercury outboard ( I think it was a 1960's) but it was converted and modified for racing. It looked awesome chromed out powerhead, Custom machined exhaust pipe, but the coolest part on it to me wash the carburetor pipes. It looked like a trumpet (I think that's what their called).
I want to make a carb intake pipe for my Nissan 18hp. How would I make one?
I know I have to weld, bend some pipe, and make a flange to bolt it to the carb.
I have a tig and mig welder that I have access to relativity easy. (I own a mig, my cousin own a tig). I don't have any machining equipment (lathes, mill, etc.) I have a floor model drill press.

If Anyone built one can you give me some tips?

Thanks in advanced

 

[Chinewalker]

Re: Old racing outboard

Ummm, why? It will do nothing for your performance and it will necessitate you no longer being able to run the cowling in place.

 

[acdc96]

Re: Old racing outboard

I'm starting to get into boat racing.

 

[Scott Danforth]

Re: Old racing outboard

a 3/4" radius rolled edge flange would suffice. the long trumpet intakes were a guestimate back then and didnt add much if anything.

Porting and exhaust work is a major key to getting more performance. I also remember being told that while porting and you can go from a really nice performing screamer to a pile of junk in a slip of the porting tool

 

[Chinewalker]

Re: Old racing outboard

I'm starting to get into boat racing.

What kind of boat racing? APBA sanctioned or wildcat? Stock Outboard? If you're just setting something up to go fast at the local lake, you'd be better served spending your time and money on propeller selection, engine set-up and so forth.

 

[Bob_VT]

Re: Old racing outboard

I think you are actually referring to the exhaust tube. There were tuned exhausts on some of the racing outboards. I ran a Mark 30H which was a small and super powerful 4 cylinder in C class and on occasion people would use exhaust tuners more then velocity stacks.

Here is a good search that will get you hooked on performance ..... google this ThunderCat Racing

 

[Chinewalker]

Re: Old racing outboard

Bob - you were a Crazy-Stocker? I ran C-Stock Runabout for about 5 years, but with Yamatos, not Mark 30Hs. I have two 30Hs, though, and do enjoy getting them out to let them bark every now and again!

 

[WIMUSKY]

Re: Old racing outboard

Here is a good search that will get you hooked on performance ..... google this ThunderCat Racing

That is just plain nuts.......

 

[nwcove]

Re: Old racing outboard

That is just plain nuts.......

thank gawd they have cages around the props!!! yikes!

 

[acdc96]

Re: Old racing outboard

I'm gonna try doing something like this merc head I saw on eBay. I don't like to port the exhuasts. Had to Many failures on Briggs and Stratton engines only the exhausts for some reason I can make perfect intakes. Im gonna make an exhuast that I can remove so I can put the normal one on when I'm not racing. If possible.

 

[Bob_VT]

Re: Old racing outboard

Yep, exhaust is the way to go and the intakes are tricky since you are running reed valves. Now if you can get a plat slide carb to work from a motorcycle

Yes, the mark30h sounded like an angry bee from a distance and it could be heard! I only ran the C class when it was around here in VT. It was a fast boat....... since my recent double knee replacement I could never kneel like that again.

 

[Frank Acampora]

Re: Old racing outboard

acdc: before you go trying to make a tuned pipe, do your homework. There are many sites to look up which have information. HOWEVER, a simplified explanation is this: It is all math. The purpose of a tuned exhaust pipe it to REFLECT a pressure wave into the exhaust port before it closes. Since the bypass port is closed before the exhaust, it ---sort-of---supercharges the engine. However, tuned pipes are only good for a narrow range of RPM.

Thus, you must decide upon the ultimate RPM at which you want to boost the engine while racing and mathematically figure the correct pipe length for that range. Then, using the RPM as frequency, you must decide upon a good pipe length to use--A sound or pressure wave has a specific length at a specific frequency and you use a 1/2 wave, 1/4 wave, or overtone wave, depending upon what room you have and how long the pipe will be. It is very much like designing a pipe for a specific note on a pipe organ.

With a single pipe, the wave is reflected at the end of the pipe. With dual pipes joining, the wave is reflected (I believe) from the point where they join into one.

SO, you see: While it is do-able, many have, it is also not simple and you need to put a lot of effort into designing it and getting it correct. Just slapping something together will not help and may indeed hinder performance.

It will be easier to go to a porting site and learn how to correctly port both bypass and exhaust ports on your engine. There is excellent literature out there on "Mapping" ports and modifying them.

REMEMBER: Porting a 2 stroke is WAY different than "porting" a Briggs.

 

[robert graham]

Re: Old racing outboard

They're called expansion chamber exhausts and the length and diameter of each straight section, the cones, angle of cones, diameter and length of the stinger all are design features to give maximum power in a particular RPM range....you'll need to know exactly what you want and need....and a good set of earplugs!....good luck and find a lake or river with no noise ordinances....

 

[Chinewalker]

Re: Old racing outboard

Adding all these air horns and exhaust pipes is a moot point unless other modifications are done. Lower unit? Standard fishing lower unit and standard propeller simply aren't going to allow the motor to perform. Original carburetor? Not going to feed the added fuel that better scavenging exhaust will need. You can't just mod one part of the motor and expect more power. Everything is tied together and needs other systems to work in synch. You can't put more fuel in (air horn, bigger carb, porting) without getting more exhaust out (tuner, porting). You can't expect to put more power through a stock fishing unit and expect it to perform without addressing propellers, motor height, gearcase shape, drag, boat design, etc.

Bottom line, there is no simple way to hop up your motor... Better to go "all in" or maybe find something that's already "hopped up", .ie a bigger motor, or something already designed for racing. The Yamato outboards might fit that bill, aren't terribly hard to find, and really run well.

You haven't said what kind of racing you're looking to get into. Makes a big difference as far as motor choice goes.

 

[acdc96]

Re: Old racing outboard

Ok with all this info in mind I thick I'll find my self a old Mercury Kiekhaefer mark ### and modify that. I don't want to ruin my outboard.
Thanks for all the help.

 

[Chinewalker]

Re: Old racing outboard

There's a Mark 30H available over at AOMCI.org in their Webvertisements section, AND it's in CT, AND he doesn't want to ship! Wants $2000 for it, which is about the going rate these days for one needing basic repairs and tune-up...

 

[Scott Danforth]

Re: Old racing outboard

there is a racing exhaust for the 30H on fleabay for $600. has the inlet horns as well.

this thread almost want to make me get the old Mark 20 and Mark 25 running.