7 deceleration rate theory

[Tony C]

I've seen several mentions of the noticeable rate of deceleration when the throttle is lifted-off (much more noticeable than the average tin-top) being due to the 7's brick like aerodynamics - bin thinking about this.

 

Having driven the tin-top today, the boredom gave me plenty of time to consider that maybe the deceleration is due to the light flywheel of the 7 engine. I've noticed that if I don't change gear 'smartly', I can't get a smooth gearchange because the engine revs decrease so quickly. Whereas in the tin-top, reving the engine requires much more time to be taken when gearchanging for a smooth change - allowing the revs to drop to be 'right' for the next gear up.

 

Seem to have confirmed the light flywheel theory by slipping the 7 into neutral at high speed - giving 'normal' deceleration.

 

I quite like the increased braking when backing off the throttle in the 7 - I find it allows very easy and accurate speed control - on the road 😬

 

BRG Brooklands SV 😬 It is only with the heart that one can see rightly; what is essential is invisible to the eye. (Antoine de Saint-Exupery)

[Peter Carmichael]

Nah Tony,

 

The flywheel is minimal in effect on acceleration/deceleration - possibly equivalent to 11kg extra vehicle weight in 1st gear.

 

The reason for the noticeable deceleration of the Seven under engine braking is a simple matter of the low mass of the Seven. It takes a certan torque to turn over the engine . Because of our old favourite F=ma (and its rotational equivalent, T=I x angular acceleration), the lower the mass of the car the more it decelerates.

[Tony C]

See what you mean Peter - so why is there no significant difference in deceleration between my tin-top and 7 when I pop them into neutral and let them coast, yet the rate of engine rev drop when lifting off the gas is greater in the 7 than the tin-top

 

BRG Brooklands SV 😬 It is only with the heart that one can see rightly; what is essential is invisible to the eye. (Antoine de Saint-Exupery)

[sjmmarsh]

Isn't the extra drag due to engine braking? At, say, 70mph a 7 in top gear will be revving around 4000 rpm, whereas a tintop in top will be around 2500rpm. As you lift off, there is a lot more drag from the caterham engine.

 

If you repeat the tintop experiment in 3rd gear you should find it slows much more quickly.

 

Another factor may be due to electronic throttle control - when I got my 7 I realised that I had got sloppy in the way I backed off the accelerator - I am convinced that my Jag S-Type overrides a sudden release in the throttle to give a smoother (and presumably more fuel/emission efficient) response. I could be talking out of my 🙆🏻, but it fits my experience.

 

Steve

[Foxy Smith]

Remember you also have to allow for higher compression ratios in tuned seven engines this will enhance engine braking. You'll get the same effect in a n/a diesel engine.

[Tony C]

I tried to compare my tin-top (Polo GTI) in 4th with my 7 in 6th, both giving 80 mph at 4000rpm - pretty much. In gear, the rate of deceleration of the 7 is much more noticeable.

 

I'm really thinking about the engine rev drop when depressing the clutch while gear changing - the 7 drops engine revs VERY quickly, whereas the tin-top engine revs drop at a noticeable slower rate. I have to give the revs time to drop in the tin-top, whereas in the 7, if I changed gear that slowly the engine revs would have dropped too much, giving a very jerky change because of the marked mismatch between engine revs and drivetrain revs. I just had a thought that the rev drop between gears in the 7 is only about 500 rpm, but that still doesn't explain the noticeable difference in engine deceleration when the engine is not connected

 

I agree with Peter that for the same deceleration "force" from both engines, the lighter mass of the 7 will give greater car deceleration - but again, this doesn't account for the almost insignificant difference in rates of deceleration when coasting from the same car speed, nor the more rapid rev drop with declutched engines, in the 7.

 

A conundrum, no less

and the compression ratio of my tin-top engine is 11.5:1 and the 7 engine is only 10.5:1

 

BRG Brooklands SV 😬 It is only with the heart that one can see rightly; what is essential is invisible to the eye. (Antoine de Saint-Exupery)

 

Edited by - Tony C on 4 May 2004 21:35:26

[Peter Carmichael]

Three things here:

 

1. Engine braking: the same decelerative force from an engine gives greater deceleration in a lighter car

 

2. Coasting declutched: engine braking is no longer an issue. The drag forces are dominated by a. aerodynamic force at high speeds b. tyre rolling forces at low speeds. The loads on the tyres are proportionately lower with a light car, so not much difference between heavy and light. At high speeds the poor aerodynamics of the Seven will cause it to lose speed rapidly.

 

3. Drop in engine revs: Very much depends on the engine installation and the ECU strategies for closed throttle. A plenum can give soggy responses. Given that the engine may be delayed in responding, whatever the engine loads, a light flywheel will result in quicker change of engine revs.

[Sheds Moderator]

Bear in mind that most ECUs are set for a "soft land" of the engine when the throttle is abruptly shut. IIRC this helps emissions, probably reduces wear (?) and it certainly makes it easier to drive smoothly. A lot of tintops I've hired have a facility where they seem to hold revs at around 1500-2000 for a second or so when changing gear, and since most "normal" drivers (whoever she is) change up at 3 or 4000 and into a gear where they are likely to be doing 1500-2500 revs, the engine will be turning at the right speed to get a smooth gearchange without too much clutch juggling. I suspect this will contribute to lack of engine braking.

[Tony C]

Good thinking batman - the rev decay on my tintop was tested again this morning - bloody weather - and it does seem as though the slow rev decay could be artificial, it's VERY slow 😬

 

Seems as though Peter has hit the nail on the head that there are several factors involved.

 

When the weather improves I'll try some comparative deceleration runs - good excuse as any 😬

 

BRG Brooklands SV 😬 It is only with the heart that one can see rightly; what is essential is invisible to the eye. (Antoine de Saint-Exupery)