Bolens Diablo Rouge and Hus Ski Discussion Forums

Hi all,

I've discovered a difference in the motor cylinders that might answer some questions.

Have you ever wondered why on some literature, the 500 & 502 Hirth 54R motors refer to the engine as being a 16 HP and lists the 503 Hirth 190R as a larger HP? Also, have you ever wondered why the 502 mufflers are smaller than the 503 mufflers. All this even though both motors are 300 cc, have the same cylinder stroke, take the same pistons and the same cylinder head?

I have wondered for a long time and may have found a difference that can explain. While getting some cylinders bored out last week, the engine guy pointed out that the chamber ports were different sizes and wondered if I knew why. After looking closer at this today, it possible that the 190R motors are equipped with a cylinder with the larger chambers and therefore allow more fuel mixture into the cylinder thus boosting the HP.

This would explain why many 503's seem faster than the 502's and why the 503's are equipped with a larger muffler to reduce back pressure in the exhaust. Although the cylinders are interchangeable, the obviously larger chambers must improve the power of the engine.

Anyone have any other thoughts they care to share?

Bruce

Hi Bruce

Much larger transfer ports. That would definately add HP. I have a couple of 56R motors and bunch of 193R motors, basically the 292cc versions of the 54R and the 190R - 300 cc motors. I will look tomorrow at a couple of the cylinders. Also 500 rpm differences between these motors.

...Mark.

Bringing back an old post with an update that may further explain HP differences.

For all these years, I've thought the Tillotson HR3A and HR14A that came with Diablos were the same carb just made in a different year. Reading through some Tillotson literature, I was surprised to find out that the HR3A has a 1-3/32" venturi and the HR14A has a 1-3/16" venturi. This venturi is within the carb (not on the flanges) in the same area as the brass fuel check.

So, the HR14A has a larger throat and thus with the larger ports in the cylinder, it makes sense that this motor set up could produce more horsepower!

HR3A on the left, HR14A on the right.
Below is the HR3A
Below is the HR14A

Well by golly, I just found another possibility! Cylinder heads are not all the same!

When looking at literature, I see 54R, 190R and 191R motors all have 75mm standard pistons and 68mm strokes. The 54R, 190R and 191R are listed at 15HP, 16HP and 19HP respectively. Now I can see how the larger ports in the cylinder and the larger venturi in the carb give the 190R an extra HP, but 4 extra HP is hard to believe.

Today, I found cylinder heads with different displacements in them. A lot different!
. .
Measuring from the gasket surface to the spark plug hole on a typical head I am used to seeing is about 0.546". This other cylinder head only measures about 0.388" or 0.158" less. That is more than 1/8" shallower and thus a fairly more significant compression ratio.

The 54R is listed at 7.6:1 compression ratio. The 190R and 191R are listed at 10.2:1 compression ratio. I am still a bit confused why the 191R is listed as such a higher HP though. One side note is there are early 191R's with the ignition coil under the flywheel which is typical and later 191R's with the ignition coil outside the flywheel. Not sure if the later design has more go power than the earlier design though.

If you were to put a slightly oversized piston in a cylinder with large ports, use a shallow cylinder head to get higher compression and use a Tillotson with the larger venturi, what do you suppose would happen?