The V8 Owners Forum -

This is my first real post on this forum and hopefully you guys can help!
Is there any advice for lightening the Rover V8 flywheel. I have to get it faced (yours truly will be running the machine) so I was thinking of taking some weight out of it too.
I believe its a standard SD1 item (how can I confirm that) and its going into a Stag.
In summary....
The engine block is NOT a stronger block but its from an early SD1. Its been skimmed to piston tops and std bore size 10.5:1 pistons are fitted. I have a Holley carb, to go onto a Buick 215 manifold (hand carriage back from the States! ) It will get an uprated cam (TBA)and it will need a custom exhaust to go in the car but the headers / manifolds are already made as the engine was fitted albeit broken when I got the car. The heads are from an older RR I believe 40mm exhaust valves and 35mm inlets?
Double valve springs are planned (are they necessary) so I will have to modify the heads to accept the larger od of the outer valve spring.
I have to fit an uprated oil pump and an old style drive to the dizzy as the front cover i have is from a P5. (It has a new style seal fitted)
All in all this engine is a bit of a pick and mix but I thinks it all matches together so far. I have had it loosely screwed together with old bearings and rings and the standard cam shaft and it all turns ok. I was bothered about the block being skimmed as I dont know how much was taken off.. again done before I got it.
So the questions are to repeat meself
1) How much if any do I bother taking off the flywheel
2) Have I left myself any pit falls in the block heads arrangement...(crystal ball time!)
3) Do I need to do that mod to the heads to take the double valve springs?

In my humble a block skim and 10.5:1 pistons is pushing your CR up way too high. Even the pistons on their own are really pushing the CR to the limit.

The combustion chamber design is 40-50 years old, the squish does not work well even if you get the edge of the piston 30 thou from the heads due to the dish in the pistons. The squish band will only be around 8mm wide. The old 10.5:1 pistons were fitted when 5 star fuel was also available.

You might get away with a high CR if you have a wild cam but then you will need to change a load of things in order to make use of the revs. (Like replacing the cast pistons!)


My mates and I are building a 3.9 lump at the moment, the CR will be no more than 10:1 and we have had to cut pockets in all of the pistons in order to avoid contact with the valves.

All just my humble!

I also have a set of 10.5:1 Pistons with valve pockets in them. Were in a pile of bits I got in a job lot of engine parts off Flea Bay. I could use them I guess the valve pockets would reduce the CR. I could also machine the top of the pistons back by ten thou to maybe undo what was done to the block and then use thicker head gaskets.... so far apart from buying the block and pistons... £40.00 the engine has cost me nothing. I have another +20thou block with a set of knackered pistons... maybe I should just use that and replace the pistons with 9.35s or something.
the engine that was in the car when I bought it had broken bearing caps so I scrapped the whole thing... seemed a good idea at the time

Using composite head gaskets might knock 0.5:1 off your compression ratio, or you could fit 9.75:1 vitesse pistons.

Sorry cant help with flywheel question, all i know is that they are bloody heavy!

jefferybond wrote:Using composite head gaskets might knock 0.5:1 off your compression ratio, or you could fit 9.75:1 vitesse pistons.

Sorry cant help with flywheel question, all i know is that they are bloody heavy!

That's a good idea!


I've done some maths but had to make some assumptions:-

Swept vol = 441cc
Tin gasket vol = 3.1cc
Head vol = 36cc
Vol above piston at TDC = 7.3cc

The above figures give a CR of 10.5:1 (This actually assumes that Rover were telling the truth with the CR in the first place!)


Right if you change the head gasket for a Real steel 1.3mm thick gasket then knock off 1cc to allow for compression the gasket vol will be 7cc.

Using the above figures but with a 7cc comp gasket the CR will be 9.76:1
which is not too bad! (Could even go a touch higher!)

The assumptions that have been made are that the CR that Rover state is correct and that the head volume is indeed 36cc.

I'll do some real time measurements when I get the heads back from the machine shop. they had to be skimmed to remove some old damage too. I always figured composite gaskets and knew things were going to be on the hairy edge... but hey thats where we live anyway!

thanks for the help on this. I will be doing the work on the flywheel this weekend so will take some if not much off that to reduce the overall mass.

Ill post again later once Ive found out more and then see if our calculations agree!

Thanks again for now

73stagman wrote:I'll do some real time measurements when I get the heads back from the machine shop. they had to be skimmed to remove some old damage too. I always figured composite gaskets and knew things were going to be on the hairy edge... but hey thats where we live anyway!

thanks for the help on this. I will be doing the work on the flywheel this weekend so will take some if not much off that to reduce the overall mass.

Ill post again later once Ive found out more and then see if our calculations agree!

Thanks again for now

Sounds like you are going to do this properly by measuring everything with a burette which is great and in my humble the only way to do it.

A bit of useful info for you....every 8 thou off the heads is worth 1cc out of the chambers, this is not the same for the bores or the head gaskets though because they are round, the combustion chambers are not!

I dont have access to a burette but I do have easy access to ml marked syringes which we use at work for tiny measurement of glue ratios.... same idea though

Because I cant rest I have done some measurements with what I have here. I have a pretty accurate syringe nicked from work and a sheet of glass across the top of the bore. The top of the piston fits 7cc exactly. The Gasket as per your input thanks for that again is 7cc and the heads have been reduced by 15thou according to the machine shop (but he still has the other one to do!) so thats 2 cc from standard leaving 34cc

All in all I get 10.18:1 CR

By the time I have cleaned up the combustion chamber and given it a bit of a polish I reckon that will end up pretty much bang on 10:1...

Just OK I think

Re flywheel, you would only skim the front and take any weight off from the back. You would then have to get it balanced again. Personally I wouldn't do it to an old cast flywheel. If it breaks up its your feet that is in the way. Far better to go buy a steel flywheel the weight you want. That said some have done what your looking to do.

Regarding the lightened flywheel, I would consider whether you need it, i.e. what are you going to use the car for, drag strip, sprints or cruising the countryside.

Lightened flywheels permit the engine to spin up more quickly that the std flywheel, but reduce the amount of stored energy, so can spoil the cruising ability as the engine is always "working"

In my experience, I have lightened flywheels in 2 of my 1.3 Nova's (I know they aren't V8's (engines are both well tweaked) and go well, but cruising they return 27/28mpg on a steady run (yes 1.3 engines!!!) whilst my TP Vitesse returns the same mpg on a run, 3.5 V8 std flywheel

As I understand it the lightened flywheel would allow the engine to "spin up" more readily and so reach its maximum power output more quickly. This would help with overtaking, drag strip times, acceleration even at motorway speeds.
There are always disadvantages along with advantages and many of the mods we do to our cars are compromises. The cost of "improved performance" is nearly always fuel economy. If that were our goal then we would do other things... lighten the car, drive more slowly, change gear ratios to reduce RPM at top speed.....which would then compromise acceleration and reduce performance.. Ive gone the full circle

I have also read a few posts and the arguments for and against lightening flywheels are fairly well balanced (no pun intended) because the balance of compromise depends on the final requirement. The Stag will never be terribly fuel efficient so its up to achieving the best power to weight ratio per pound spent or removed LOL
I wouldn't be going daft and if I had to pay to have it done I wouldn't bother. As I will be doing the machining in my own time myself then I thought it would be worth taking 2 or 3 pounds out of it if possible as long as the strength of the things isnt compromised and that is where my question was really pointed.
Where to remove the material from the back face obviously and how much can be removed.
However after surfing for hours tonight and last night it would seem that the information is not readily available.

A while back I ran the numbers for switching from an iron to aluminum
flywheel for my Triumph TR8 with Rover V8. The effect was surprisingly
large. There are a couple of approaches to doing the math. The more
rigorous approach is to calculate the polar moment of inertia for the
two different flywheels, adjust for the square of the overall gearing
(transmission, final drive and tire diameter) and convert to an
equivalent linear inertia. The second method (the one I chose) is to
start with a known linear to rotational equivalent and ratio from there.
The known relationship I used is a solid disk rolling on its edge.
It has an effective inertia exactly 1.5 times what it would be if it
wasn't rotating. That means the rotational component is 50% of the
linear component. Adjust for the square in gearing and you have the
answer. I wrote a little Fortran program to do the calculations.
I assumed a 12" diameter flywheel which is the Buick/Rover diameter,
less the ring gear. The circumerence of a circle is the diameter
multiplied by pi. So if you roll the flywheel along the ground it
will move 37.7 linear inches per revolution (= pi * 12). A 205/50/15
tire has a diameter of approximately 23.1 inches. My TR8's final
drive ratio is currently 3.45:1 and first gear is 3.32:1, so one
revolution of the flywheel results in the TR8 moving approximately
6.3 inches. Ratio the squares and take half ((37.7/6.3)**2)/2 = 17.9.
So each pound removed from the flywheel (equally across the face) is
the same as about 18 pounds of weight removed from the car when in
first gear. So if you remove ten pounds from the flywheel (equally
across the face), the result is equivalent to removing 180 pounds of
vehicle weight in first gear. The effect goes down for each higher
gear, of course. Removing weight farther from the rotational axis
has a more pronounced effect. If the weight is removed from the
outside of the flywheel only, the effect is about 2.78 times as strong
since a solid disk has a radius of gyration of 0.6 times the radius
(1.0/0.6)**2 is 2.78). 2.78 * 180 is 500 lbs equivalent weight
reduction. A non-trivial effect, particularly in a lightweight car.
I ran the numbers a couple of ways to illustrate. For my TR8,
assuming a 3.45:1 final drive ratio, 205/50/15 tires and LT77 gear
ratios of:

1st 3.32:1
2nd 2.09:1
3rd 1.40:1
4th 1.00:1
5th 0.83:1

along with flywheel weights of:

stock flywheel - 32 lbs
lightened steel - 22 lbs
aluminum - 11 lbs

The engine in the TR8 is essentially a Buick 215 aluminum V8 from the
early 1960's. The stock flywheels in those had a big ring around the
perimeter. Lightening the flywheel by milling off the ring is similar
to removing the mass from the perimeter (from 32 to 22 lbs). In the
numbers below, I didn't do it that way but a more accurate approach for
the aluminum flywheel would be to assume a reduction of 22 to 11 lbs
equally across the face and add that to the difference of the 32 to
22 lbs across the perimeter. In any event, a lighter flywheel looks
like a good thing to do for performance. Here are the numbers:

32 to 22 lbs (across face assumption):
1st 177.5 lbs
2nd 70.3 lbs
3rd 31.6 lbs
4th 16.1 lbs
5th 11.1 lbs

32 to 22 lbs (perimeter reduction assumption):
1st 493.4 lbs
2nd 195.5 lbs
3rd 87.7 lbs
4th 44.8 lbs
5th 30.8 lbs

32 to 11 lbs (across face assumption):
1st 372.7 lbs
2nd 147.7 lbs
3rd 66.3 lbs
4th 33.8 lbs
5th 23.3 lbs

32 to 11 lbs (perimeter reduction assumption):
1st 1036.1 lbs
2nd 410.6 lbs
3rd 184.2 lbs
4th 94.0 lbs
5th 64.8 lbs

Rotational inertia is mass multiplied by the distance from the
rotational axis (integrated over the surface). The effect is
stronger farther away from the hub. The best is from the
perimeter. Equally across the face is less effective and near
the hub is the least effective. In my example, dropping 21 lbs
from the perimeter is equivalent to over 1000 lbs reduction in
weight in first gear. Dropping the same mass from the face is
equivalent to 372.7 lbs.

Reducing the flywheel inertia does reduce the stored energy for
start from a stop. Torque follows displacement. Little engine
in big car with tall gearing needs more stored inertia at start.
Big engine in little car with short gearing can get away with
much less stored inertia. On the street with a ligter flywheel,
you may need to use more RPM and clutch slip. On the strip, you
may bog if you don't have enough excess torque at the rear tires
(more traction than engine/gearing). Remember that HP is the
measure of how much potential torque you can have at the rear
tires via gearing. If you have enough power to overcome your
traction, then a heavy flywheel is a loser.

A friend has an older BMW M5 that came with a 12 lb flywheel from the
factory. The car weighs 3800 lb, has the same size engine as the TR8
(215 cubes), has a 264 degree cam on 110 deg center and has a 3.53 1st
and 3.91 axle. It gets off the line very easily and, thanks to the
IR EFI, can spin the rear tires (275/35R18) at will in first gear on
dry pavement. Hooks just fine in second, though. Gary also had a Buick
215 powered Vega. It weighed about the same as the TR8, had a 3.36 axle
and a 3.41 first. He lightened the 32 lb flywheel to 22 lb (mostly from
the perimeter) and said the result was wonderful. He said he would have
gone lighter if he could. Of course, there is a point of diminishing
returns and a combination of a very light flywheel and a digital on/off
type race clutch can make street driving less than pleasant.

Dan Jones
St. Louis, MO USA

Wow. Respect.

WOW. that is a number crunching exercise and thanks for the info!
It is worth the effort and I reckon that even with a road car the reduced mass should be more efficient in the early stages of performance enhancement.
As you say it is a matter of diminishing returns but to take some off is better than to take none off!
As long as its done right!