PRD Fireball & TMKC Engines?


(Gary Mangiapia) #1

I am looking at 2 different karts. One has a PRB fireball and the other has a TMK9c
I am wondering are they reliable say as the Rotex.
What do you recommend?


(Matt Martin) #2

What class are you trying to run? The TM K9C is a kz2 shifter engine and has pretty frequent rebuild intervals. and the PRD fireball is a TAG engine. The Rotax is yet another class.


(TJ Koyen) #3

The PRD isn’t going to be as reliable as a Rotax either. You’ll be hard pressed to find an engine that has longer rebuild intervals than a Rotax.

As Matt said, what is your goal with the kart, where are you racing, and what class are you going to run? The 3 engines you’ve listed are totally different beasts, and not in the same class.


(Jack Mazury) #4

As TJ said you’ll be hard pressed to find any two stroke engine with the reliability of the rotax as far as rebuild intervals are concerned. That K9C you’ll probably be looking at around 9 hours on a top end. I can’t speak for the PRD however the rotax its unlikely you’ll have it stick even at as many as 50 hours. Keep in mind the rotax is dealer sealed and the rebuilds once the time comes is by no means the cheapest one around.


(Gary Mangiapia) #5

So rotax lasts longer but a top end is more money. Can you give a idea how much they charge?
For top end rebuild on each engine/ Thanks


(Matt Martin) #6

https://www.accelerationkarting.com/rotaxrebuildrates.html


(Charles Kaneb) #7

A TM K9B won’t have terribly long rebuild intervals - eight hours or so - but there are so, so many reasons to get one.

  1. The power is addictive. Very few experiences compare to straightening the wheel as you pour on the power in a 40-horsepower shifter kart
  2. You will learn a LOT about the friction circle, about heat management around a lap, about tire management throughout the race. I haven’t driven a great shifter heat yet - there’s so much you can do to control the behavior of the tires in a shifter, from how you brake, to the line you take in, to the speed you carry at the apex, to how and when you accelerate off the corners, to where you want to “float” the kart to keep the tires from overheating in the critical areas of the track where you’ll need the grip to pass or defend
  3. It is the most abuse-tolerant, slowest-turning, most reliable 2-stroke you can put on a kart, and a great illustration of the difference between durability and reliability. Starting at the axle, you have a chain that’s the right size for 40 horsepower, and it’s going around sprockets with at least 16 teeth on it. Both sprockets are small enough to provide good ground clearance and you can even put a dirt guard below the rear end to keep grit out of the chain and bearings. Next you come to an enclosed, internally lubricated gearbox with internals strong enough to tolerate curb-hopping and standing starts, then a clutch that’s not slipping trying to get the kart going in a 75 mph gear and is rotating at 1/4 engine speed. Finally you get to the internals - as you go around the track you’ll be between 9000 and 13300 RPM the whole time, so no sustained 15,000 RPM or Formula One worthy piston speeds. The cylinder pressures are high, so you have to change the pistons and crank bearings regularly, but most errors in jetting, driving, fuel system design will reduce your cylinder pressures rather than raise them, so the driver and lap timer will find the problem before it stops the kart (does not apply to roadracing where I’ve seen someone keep an obviously sick K9C wide open halfway around MIS)

Anecdote 1) When I rented karts for a season before going to graduate school, ICC rental rates were $100 less per day than TaG rental rates, just because of the reduced labor needed to keep one running.

Anecdote 2) My K9B started to slow down at the end of a session, so I pulled in. One of the head bolts had stripped and I had lost all of my water. I took the exhaust off for what I thought was a postmortem. No damage - just time for TimeSerts.

Anecdote 3) When I bought this K9B, it just had a fuel pump running right to the carburetor, with undersized jets and a bulging (!) emulsion tube used to try to stave off the flooding. It ran hard - it had a “quadrabog” like an old Firebird - but it had been raced for half a season like that before I got it and kept going, lean spots and rich spots and all.

Anecdote 4) After ten hours my of driving that K9B of unknown orign, it finally stopped running. Upon teardown, the piston-to-wall clearance was 0.010", the crankshaft was broken and cracked in two other places, and the clutch basket had been JB-welded together with some brass screws and washers holding the clutch center together from the back end where it had stripped.

Anecdote 5) Performance was underwhelming on a recently-acquired K9ES. I kept coming in to find a shiny piston and white sparkplug, so I got to the end of my ICC mainjets and started asking Rotax drivers for theirs. By the time some color got to the piston I had gone from a 120 to a 155. Miss by a quarter turn too lean on an ICA and you’re exiting the track backwards, miss by 35 mainjet sizes on a TM and it’ll be a little slow.

You will spend more on tires than powertrain parts if you buy a TM and sprint race it locally.


(Liam Sergeant) #8

Why does it need rebuilds so frequently given it’s as you say reliable in so many areas?

(I like the idea of a reliable low maintenance engine, although my Rotax is plenty of power currently)


(Matt Martin) #9

In my experience with a K9A; they aren’t that reliable outside of the [frequent] rebuild intervals - you have to be on top of the maintenance! At which point, if you are religious about maintenance, everything is reliable.


(Charles Kaneb) #10

It has a short rebuild interval because the cylinder pressure and temperature are high, giving a lot of stress on the piston, and the bearings are also heavily loaded. My TMs were the only engines where I didn’t have to rejet, check the clutch for cracking and dry bearings, reposition the exhaust, and realign and retension the chain and sprockets after every heat. I don’t have any experience with anything before the K9B.