Racing exhaust stacks for small motors?

[Squeakit]

Evidently, you used to be able to add racing exhaust to small hp motors for additional power - you removed the exhaust plate and bolted it on. Can anybody shed some light on this? Were these tuned exhaust setups like the type you can buy to put on small gas scooters to get more power/rpm's? Has anybody ever used these and do they work and is it possible to make something like this? Thanks for any info.

 

[Chinewalker]

Re: Racing exhaust stacks for small motors?

Hi Squeakit,<br /> Is it possible? Yes. Is it feasible? No. <br /> If you have the tuned exhaust headers on your motor you'd need a truly high performance drivetrain, etc. to harness it and put the power to the water. Adding pipes to Joe-Fishing-Motor would only make it louder. No one makes high performance props for your average 6-10hp outboard, and if you were to acquire a racing lower unit for something (Merc J/A motor, Johnrude 15A motor) you'd already be in the realm of high perf. Most motors out there already have a general purpose exhaust tuner built right inside the exhaust leg of the motor which helps scavenge the exhaust out of the cylinders. <br /> The guys who are running true tuned exhausts are spinning extreme RPMS on the lightest of hulls. I'm thinking APBA's modified division, where they can get over 100hp and 100+ mph out of a 49-cubic inch OMC 3-banger on pump gas.<br /> You cannot simply bolt on a pipe and start going highway speeds, as there is a whole science to matching pipes to specific motors. Tuned exhausts work on wave pulses in the exhaust to draw more exhaust out of the cylinders. The timing of those pulses and how strong they are are different for every motor and every pipe has to be "tuned" to fit that wave.<br /> Most of the bolt-on pipes or stacks of the 1950s were simply there to relieve back-pressure. On a typical 1950s motor the exhaust would back up in the motor leg, creating a positive pressure which would limit the amount of exhaust able to be pumped out the cylinders. Relieving that pressure allowed more exhaust to come out and more fuel to go in.<br />- Scott

 

[Squeakit]

Re: Racing exhaust stacks for small motors?

Hey Scott: Thanks for the great post; very informative. Got this idea from reading about Champion OB's which were evidently one heluva little motor in their day and came with optional bolt-on racing stacks. It was also mentioned that there were aftermarket stacks available for other '50's engines and was wondering if I could stick something on my old "Joe Fishing Motor" <G> 1957 'rude 5.5 to pep it up a little without burning it up. I've already stripped it down to basically a powerhead on a lower leg and it would look cool with some little pipes. It was just a thought - motor runs great the way it is. When you implied that the '50's motors had backpressure due to water backing up the lower leg, does that mean that opening my exhaust slightly either by making some crude "stacks" (even though they would not be tuned expansion chambers) or maybe drilling an opening in the lower leg would be of some help or would this just f---k up the running of the engine and/or cause damage? I know the Champs were designed for racing and my motor is not, but... ? Oh, and while I gotcha on here, I've heard that 2 blade props are faster than 3blade; the old'rude with a 2blade slightly outruns my old 7hp Eska (please don't laugh too much) and would sticking a 2blade on the 7horse make any difference? Thanks for sharing your knowledge.

 

[Chinewalker]

Re: Racing exhaust stacks for small motors?

Hi Squeakit,<br /> Personally, I would not bother putting too much effort into trying to modify the old 5.5. It's simply cannot withstand much added stress. It was designed and built to be a small fishing motor and the bronze bearings and aluminum rods will not stay together at high revs and high loads. Pulling aluminum shrapnel out of your back isn't fun... <br /> Before even trying to make some minor mods, I would borrow an inductive tachometer and find out what RPMs it's turning now. You don't want it to run much over 5000 RPM. However, if you're not reaching that max now, then you've got some work to do in motor tuning. Is timing set perfectly and 180-degrees apart? Is compression good? Is carburetor set-up properly and opening fully at wide open?<br /> If you want to stay with the platform you have you can try to find a 1960s 6hp powerhead which has needle bearings on the crank journals and will withstand a bit more abuse. It'll bolt onto your lower end, although the cowlings will no longer fit without some modifications.<br /> Better still, if you want to stay in the 5-6hp range, find a Mercury KF5, Mark 5 or Mark 6. These motors were raced in J-Class back in the day and there are numerous props out there for them. The bathtub racers out in Washington State also had them as the motor of choice. <br /> As for props - it's less a matter of blade count and more a matter of blade design and motor set-up. I use a 3-blade on my racing motors, as it offers better high speed stability. However, you're unlikely to have an issue with prop-walk even on your 5.5's best day. My props are surface piercing, where there's nominally only one blade in the water at any given moment. You cannot jack your motor up that high and expect to plane off and still cool the motor.<br />- Scott

 

[Squeakit]

Re: Racing exhaust stacks for small motors?

Thanks for the great info, Scott. Kinda figured the old 'rude wouldn't take much abuse, and sure don't want to blow it up; just get as much as it's capable of putting out safely. Will try to get ahold of a tach and check RPM - been wanting to do that for a long time anyway. Best wishes and thanks again!