Tuning engine for the low and mid end bhp.

So you have your engine which is top notch, but you feel that the power curve is very narrow and the feel of the bike is very abrupt. If you are in the ATV motocross (all those Apex PRO MXR and DRR 50cc, 70cc and 90cc engines) then you definitely have room for improvement for those small size engines that have to carry substantial weight on them. Or, if you are just looking more to enjoy cross country and trail riding then you do not need high revving engine.

If you are willing to try out some changes yourself, then below are some tips how to turn the high revving engine into torque monster.

Lets start with the basics. Below are some easier modifications that you can easily do and others require more skill, special software and tools.

Crankshaft

Increasing the stroke is probably the most effective way to increase torques as it increases the displacement of the cylinder and in effect decreases the peak rpm. An option is to go for an aftermarket crankshaft with the longer stroke reweld the conrod’s big end hole.

High primary compression crankshaft (HPC) is not suggested as the volumetric efficiency of the crankcase for the primary compression increases. It is the same with filling the webs or using the other ways of increasing the primary compression. By increasing the primary compression you effectively work on increasing the mixture movement velocity, which supports the high rpm engine.

Inlet and carburettor

Try reducing the size of the carburettor. If you have a 50cc engine then try out 17mm or 19mm carb. For 100cc engine 24-26mm is sufficient, 125cc engine 30-34mm. What this does is that smaller diameter increases air-mass velocity and improves flow characteristic.

Experience has shown that changing the reed valves does not have effect on the torque. Although some suggest that simple glass fibre reeds and reduced reed size is a good option for low revving engine then carbon fibre option woks as good (e.g. Moto Tassinari options can be found on ebay: here)

Ignition

I would suggest to keep the stock timing as the engine is revving lower then there is no need to advance or retard it.

Cylinder and crankcase

The easiest option to say is to go for a larger, big bore cylinder. If this not the case and you have maybe restrictions to do so then there are things that can be done on a stock cylinder.

If you have necessary tools then you can try to modify the stock cylinder. The cylinder ports are designed to produce a certain power characteristic over a fairly narrow rpm band.

First measure and calculate the stock engine’s specifications – the most critical measurement is the port’s time-area. A port’s time-area is a calculation of a port opening’s area and timing in relation to the displacement of the engine and the rpm. In general, if you want to adjust the engine’s powerband for more low to midrange then the following to things can be done:

1. Reduce the base gasket (keeping in mind the squish!) or turn down the cylinder base on a lathe to increase the effective stroke (distance from TDC to exhaust-port opening). This also retards the exhaust-port timing, shortens the exhaust-port duration, and increases the compression ratio.
2. Narrow the transfer ports to reduce the port’s time-area for a rpm peak of 7,000. The rear transfer ports should be angled so they oppose each other rather than pointing forward to the exhaust port. This changes the flow pattern of the transfer ports to improve scavenging efficiency at 2,000 to 5,000 rpm.

The timing is quite important for better low and mid range and as a rule of thumb the exhaust port should open at 90 degrees ATDC, transfer ports at around 120 degrees ATDC (for high rpm and narrow power curve engines the exhaust ports should open at around 80degrees ATDC and transfers at about 115 degrees ATDS)

Cylinder-head shape also affects the powerband. Generally speaking, head with a deep combustion chamber and a wide squish area is for a low and midrange while the head with a wide and shallow chamber and narrow squish area and reduced compression is targeting for the higher power curve. There is a software that you can use to find best option for you (see the cylinder head calculator link here)

Exhaust

Exhaust system consists of the pipe, and silencer. Pipe in essence has different sections that are critical to produce the desired powerband (head pipe, diffuser cone, dwell, baffle cone, stinger and the silencer).

Pipes are usually made of steel, aluminum and/or titanium. My experience says that there is no performance difference when it comes to the material of the pipe. So no need to go for a titanium option, which costs a fortune to repair if it cracks.

Exhaust silencer should be shorter with the smaller diameter. The idea of the silencer is to maximise the pressure and velocity in the pipe. If the pressure increases too much at high rpm it will increase the temperature back to the cylinder. First to take the hit is the piston and it could seize in the cylinder.

There is no good way to tell what to do and how to do as the pipe and the silencer should eventually be custom designed and built for your needs. Otherwise follow the manufacturers instructions as they have pipes designed for the purpose and have labeled them accordingly (e.g. motocross pipe, midrange powerband, trail pipe, etc).

Flywheel weight

In some forums you can read that adding flywheel weight increases torque. It does not! What is simply does is saving inertia and does not affect your torque level in any way.

Hope this helps you a bit if you are looking to improve the low and midrange of your bike, or ATV or gocart.