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MX5 1.8 Flywheel weight
DavidhMX5 - 1/12/18 at 04:10 PM

First job on the engine rebuild is to lighten the flywheel. Anyone have any experience on best weight to go down to.
Around 5kg seems to be the norm if bought off the shelf but wonder if this might be a little light as still want a usable road car.
For info it's a 1.8 1999 which will have forged rods and a 2554R turbo running variable boost maps.

APR - 1/12/18 at 05:12 PM

i'm running a 5kg flywheel with a stage 2 clutch in my Mnr on a gt2860r turbo and it is spot on. apart from the turbo which is in the process of getting changed to a Gt2554r
What pressures are you running on your turbo and what Bhp are you achieving
Adrian

pewe - 1/12/18 at 06:43 PM

I think mine is currently 5.9kg on my s/charged NA 1.6 but previously I ran 4.7kg.
The lighter one gave a lot of transmission shunt and the engine struggled when under the extra load of headlights etc.
Fitting the heavier one smoothed it out without any appreciable reduction in reaction time when flooring the throttle.
HTH.
Cheers, Pewe10

DavidhMX5 - 2/12/18 at 05:59 AM

quote:
Originally posted by APR
What pressures are you running on your turbo and what Bhp are you achieving
Adrian

Only just started building the car.
With a bit of work on the head I'm hoping for around the 270bhp mark.
Interesting that your changing to the 2554 from the 2860. I considered using that turbo but discounted in the end as felt it would suffer too much lag and have a fairly narrow operating band revs wise. How did you find it?

DavidhMX5 - 2/12/18 at 06:03 AM

quote:
Originally posted by pewe
I think mine is currently 5.9kg on my s/charged NA 1.6 but previously I ran 4.7kg.
The lighter one gave a lot of transmission shunt and the engine struggled when under the extra load of headlights etc.
Fitting the heavier one smoothed it out without any appreciable reduction in reaction time when flooring the throttle.


Thanks, just what I was looking for.
Was the issue around tickover where the SC would be taking power without giving anything in return?
I think I'll start at 5.5kg. Wont be an issue to make it lighter later on if need be.

pewe - 2/12/18 at 10:15 AM

Not really as the supercharger is blowing as soon as the engine is started.
There is however more mass in a supercharger (compared to a turbo) which saps power.
That power loss is only really appreciable at tickover where overall mass of the engine isn't compensated for by flywheel inertia.
When I was researching flywheels transmission shunt was apparently less of a problem with turbos for that reason.
Cheers, Pewe10

luke2152 - 2/12/18 at 09:24 PM

The stock one machines down quite nicely. It has a very obvious rim cast into it just for the extra inertia that you can machine off without affecting its strength. Not sure about relative weights but made a lot revvier and still drive-able in traffic once you got used to it.