Free-revving engines - Andrew Mullin
Hi everyone! As you may have noticed I’m fairly new to the forum and I’m sorry if you’ve covered this one before. Everytime you read a car review in the press these days, or watch one of the car programmes on TV, there is a lot of talk about free-revving engines. Could you please explain to me how you can tell if an engine is free-revving? I realise this may be a stupid question, but I am just curious. Are Vauxhall engines generally classed as free-revving for example, or what about the engines in the new MINIs? I drive a Cooper S (not allowed to rev it yet though, only done 1000 miles!)! Honda engines seem to be generally classed as free-revving…..but what does it mean?

Thanks everyone, Andrew.
Free-revving engines - Daedalus
Basically its how quickly the engine will go from idle to say full revs by bliping the throttle. Our old Polo will only go upto half max revs on a blip if your lucky, the V6 Mondewoe is not much better. My Honda Blackbird bike will go round to about 80%. My Harris Magnum with an almost race spec engine goes almost the whole way round. The quickest reving engine in a car I have ever driven was in a works TVR at Oulton Park. I suppose its the way of saying that there is a lack of inertia in the engine allowing it to pick up speed quickly.


Bill
Free-revving engines - Jonathan {p}
Doesn't this depend on the size/weight of the flywheel.

Some people I know have put a smaller flywheel in to get a better "off the line" acceleration. This allows the engine to pick up quicker.
Free-revving engines - Nortones2
I would say an example of a free revving engine was a Honda 250 cc 6, on warm up. Nothing else comes close.


Free-revving engines - GGH
I would describe a free revving engine as an engine which sounds ands feels happy to rev. I would expect the engine to be able to breath well at high rpm, have a shortish stroke, have well balanced and light moving internals and most importantly a light flywheel. All the exact opposite of my Golf GTI 8V!
Free-revving engines - racingkarts
free revving - that's an easy one - it is describes as the engines ability to change speed - revoloutions per second. this depends on a few factors - all of which effect each other.

firstly its stroke - long stroke - slower pickup
secondly its flywheel weight - more mass- slower pickup
thirdly its a combination of tourqe and power -

the most free revving engines i have seen have been

a 125cc rotax kart engine- 1000 rpm to 13500 in about 2 secs
a six cylinder 3 litre 900bhp skyline engine - very quick
and a 6 cc model aircraft engine

it all depends on several factores
Free-revving engines - MikeC
Agree with GGH and others. I also think 'free-revviness' depends on 1) its peak power rpm and 2) peak torque rpm in relation to the max engine speed? Ie the higher these are up the rev range the happier it will be revving.

Hired a 1.6 Zetec Focus this week, that has peak power at 6000rpm and peak torque at 4k. This engine seemed to be happy revving and to get the best out of it and found myself regularly revving to 5, 5.5, 6k. Yet it didn't seem unduely stressed.

I would never do (or need to do) this in my 2.0 8v Cavvy as its very un-rev happy but because the max torque is produced at only 2600rpm and max power at 4000rpm I very rarely need to rev above 3500 which is good as above there it feels distinctly unhappy and sounds as if somethings going to go pop! May be comparing a new engine to a 150k one is not a fair comparision though.

Free-revving engines - racingkarts
the load on the engine as stationary also applies -eg - fanbelts - water pumps, oil pumps, -all that lot - at the end on the day - a engine with a supercharger- or turbo, is going to rev faster that the same without - also, fuel injection is better than carbs, in most exapmles anyway
Free-revving engines - M.M
Possibly the most free revving engines were small to medium capacity 16V engines pre-cat and pre-ECU.

MM
Free-revving engines - Andrew Mullin
Just wanted to say thanks to everyone for your replies. I’ve been away for the weekend so I’m just catching up! Makes a lot more sense now.

Andrew
Free-revving engines - John S
MM

Not just the 16valve engines - remember the push rod FIAT 124 and the OHC FIAT 128?

Regards

John S
Free-revving engines - racingkarts
ever heard a fully balance 998 a series (mini) engine, with a decent cam and a race head, very free revving indeed
 

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