Topic

Well done on doing this Adam Might have a go at it myself! And kudos on the How-To

PQD is quite correct to state that on manual and auto transmissions the speed is determined by the gear ratios and the benefit you would see is better fuel economy (or worse) at the same speed before and after inlet modifications (or any other form of modification to the engine map, timing, camshaft profiles, exhaust, etc.).

Firstly, remember that a gear box is a torque multiplier. The torque produced by an engine is insufficient to start the car from standstill and accelerate, but once up to speed the torque required to maintain a constant speed is sufficiently delivered by the engine and hence the fact that top gear is a ratio of 1:1 and anything less than 1:1 is usually considered an overdrive gear. Hence for simplicity of manufacture, manual and stepped auto transmissions still prevail, but because each ratio is fixed, the engine revs must rise to increase the speed of the vehicle.

However the CVT is a totally different situation. Essentially, when you press the throttle pedal, what you are effectively doing is controlling the gearbox which in turn controls the engine speed by having an infinite number of ratios from which to choose so that the the engine rpm can be kept either at peak torque (or just below for fuel efficiency) for steady speed cruising or at peak power if wanting to accelerate quickly.

However, a CVT or IVT (infinitely VT) will basically select a gear ratio by adjustment of cogs or belts (think DAF 33 that could go as fast in reverse as forwards). In an ideal world the gear ratio selected by the transmission brains will be at an engine speed where the torque is sufficient to pull the car along at a constant speed with the torque required being determined by the resistance to motion of the tyres and the wind on a flat road. If on an incline, to overcome the mass of the vehicle to maintain momentum up the incline, the engine may need to rev faster as the gearbox will change the ratio selection, but going downhill the vehicle's mass will aid the engine so the engine will rev less than on a flat road.

Your modification has effectively altered the torque curve for your engine. Whether the Stepspeed is smart enough to realise this I have no idea not being familiar with the system but I suspect the engine management detects how much fuel is being used and the throttle position etc. to determine whether the ratio selected is capable of maintaining the speed.

Remember that an engine has a torque curve that is determined by the design (bore, stroke, etc.) and the efficiency of the combustion. The latter is what you have changed by allowing more air and fuel into the combustion chamber. Therefore you have probably increased the torque lower down the rev range, and now the CVT is able to hold the same speed at 70mph with lower engine revs compared to before. Before, your engine needed to be at 3200rpm to produce sufficient torque whereas it can do it at 2500rpm in order to maintain 70mph against wind resistance and tyre friction. A roling road before and after would confirm this showing the torque curve at a higher value lower down the rev range.

However, on a hot, dry day, the engine would not be as efficient so your revs may be slightly higher as the air is thinner and the mixture less dense, but I wouldn't worry, because from what I hear, it hasn't stopped raining in the UK.

Apologies for any confusion caused!

MartinW wrote: wrote:

The latter is what you have changed by allowing more air and fuel into the combustion chamber. Therefore you have probably increased the torque lower down the rev range, and now the CVT is able to hold the same speed at 70mph with lower engine revs compared to before. Before, your engine needed to be at 3200rpm to produce sufficient torque whereas it can do it at 2500rpm in order to maintain 70mph against wind resistance and tyre friction.

Apologies for any confusion caused!

What a clear and concise explanation that was Martin. I found it to be as near as damn it in laymans terms.

Just for the record, Whiz was 3200rpm and I was 3000rpm @70mph. That mysteriously went to around 25-2600rpm after doing the mod (checked it again on my drive to work last night and it was the same). Now, I'm quite sceptical myself about such claims. But I didn't believe that I could be 'that' mistaken or overwhelmed by such a simple mod. Either way, it helps me to enjoy the car more and brings me deeper into the Darkside. Appreciated.

The likely reason for the difference between the two cars could be any number of factors even down to the type of weather on the days you were comparing notes on engine speed and whether that was on a slight incline versus a slight decline. Also other factors like tyre pressures affecting the rolling radius (and thus the torque required), as well as what fuel and the state of the air filters etc., thus proving the Stepspeed is a clever little chap, or should that be woman? No, it's not multi-tasking, just playing with its gears so it must be a chap.

That is interesting regarding the operation of the CVT.

But there is a claim being made here that the manual gearbox driven car is also seeing as drop in revs at a set speed? This simply cannot be.