Tech Tips

Chuck Sherman's thoughts on more power:
November 7, 1999

Re: modifications-

The three 'hot' mods are:

a) CVK's
b) Exhaust
c) Increased compression.

A reciprocating engine is a horsepower pump of sorts...it can be configured for many different combinations of power and performance, and simply 'bolting on' stuff or making a single modification rarely results in the desired gains.

Please note we are talking about a vehicle with a low power to weight ratio - there are approx 30 lbs to each HP with our URAL butts on board.  To get into the average family sedan class of power-to-weight, we need a number in the 21-22 lbs/hp range. Solving for this # yields a motor with a HP rating in the low forties.

There are a couple of ways to get there from here - make the motor spin faster, pr process more combustible mix at the same speed.  Since our engines are, well...crude, the latter is the better way to go.  By reducing inlet and exhaqust restrictions from the lip of the air cleaner to the tip of the exhaust, we process more mix - the carb, head, and exhaust mods help out.

The final trick is th 'squeeze' the mix harder - this is accomplished by slicing some meat off of the cylinder. I took 0.050" off of mine - It's a bit too much, as I get pinging if I run for extended periods of time above 70 MPH.  Yeah - I can run up there...been in the eighties many times now with the hack, spare tire and windshield all up. The best bet is a 0.040" mill - this bumps the copmpression to approx 8.8:1.

Ther result of these mods is a rig which pulls VERY well off of the light - as a matter of fact, it no longer feels like a URAL.  The increased compression allows me to shift at lower revs - I can run it down to thirty-five MPH in fourth fully loaded and still accelerate well.  The modifications do shine on the top end - the bike will accelerate well above 65 MPH, which was one of my goals when I started.

As for the Pettit cam, you can get 80-90% of the performance simply by lashing your intake valves at 0.004".  This reduces the valve overlap, thus increasing the 'squeeze' at low RPM. The downside is the motor clatters a bit.  Personally, I'd keep the stock cam in for this mod saves the need to pull the motor and change the cam.

Finally, I get all my jets from Recreational Leisure, a local company with boatloads of ORV stuff on the shelf.  The best bet is to buy a whole range of jets (3-4 sizes over the stock 117's) and keep trying them till you find one that works.  I think I'm on a 125 now, and will try the 130's in the spring.

The downside of the extra HP is it significantly shortens the life of the 3.89 final drive. Mine lasted 15K before it literally grenaded.  Someone once mentioned I said that figuratively...No it actually EXPLODED.  A tooth broke off at 45 MPH, caused the housing to shatter, and a tooth shot a hole thru the case with the energy of a .38 caliber shell.  If you decide to go the modification route, I'd de-rate the payload capacity to no more than 200 lbs, and make ring gear lashing part of the 2500km service.

The combination of mods have resulted in a rig which has allowed me to run door-to-door with a late model 3.8 liter T-Bird while three people were on board my rig.  Solving the power-to-weight equation yields approx 43 HP on tap - quite an increase over the stocker which barely wheezed up a local hill on its maiden voyage.

Chuck.