40 replies to this topic

[timmy850]

Cooper S rockers are normally a worse rocker ratio than the small bore ones, make sure you check them first

[Spider]

 

Here's an exersise you can do to see what I'm banging on about. We know that the stock Cooper S engine had 9.75:1 Static CR and the 1275 Engine (like fitted to the 1275 GT) had 8.8:1 CR, yeah?  Work out what the Dynamic CR is of these engines, you might be surprised.

 

They are almost the same.

 

While I haven't looked at small bore engines, I'd be confident in saying that they are also around the same numbers.

 

For 'safe' builds I aim for 8.4:1 Dynamic CRs and for higher performance, 8.8:1 Dynamic CRs. Both these will run on our pump fuels, though at 8.8:1 DCR, you do need to be very careful, it is in hand-grenade territory.

 

 

Now I'm in the office, here's a the numbers on the above examples;-

 

Examples;-

 

Taking the Stock MKI Cooper S;-

 

            Static CR                                             9.75:1

            Total Combustion Chamber                  36.36 cc

            Inlet Cam Timing                                  230 deg (as Advertised at 0.019” at the Rocker)

            Lobe Separation Angle                        107.5 deg

            Inlet full open                                        110 deg

 

Dynamic CR = 8.10:1

 

Taking Stock 1275 Engine;-

 

            Static CR                                             8.8:1

            Total Combustion Chamber                  40.8 cc

            Inlet Cam Timing                                  230 deg (as Advertised at 0.019” at the Rocker)

            Lobe Separation Angle                        107.5 deg

            Inlet full open                                        110 deg

 

Dynamic CR = 8.03:1

 

IMO, working from static numbers is only part of the story and doesn't tell you really where your at or how far you can really go.

[Spitz]

I am using a 12g295 head on a 1098.  The head was shaved .080"  ( I think this brings it down close to 25cc )

I used dished (6cc I think ) +.040 pistons

SW5 cam and a 3.1 FD  ( I originally had it on a 3.44 FD, which pulled away a little quicker )

 

The engine builder quoted me a 9.5 . 1 comp ratio.

 

Pulls along nicely

[70K1100]

I am using a 12g295 head on a 1098.  The head was shaved .080"  ( I think this brings it down close to 25cc )
I used dished (6cc I think ) +.040 pistons
SW5 cam and a 3.1 FD  ( I originally had it on a 3.44 FD, which pulled away a little quicker )
 
The engine builder quoted me a 9.5 . 1 comp ratio.
 
Pulls along nicely

You don't know what figures the engine produced?

[Spitz]

No idea…..no "rolling road" within about 1000km.  I'll guess at 60hp

[Vipernoir]

.080" off a 295 head will give around 24-25cc from memory.

Assuming flat topped pistons, a nominal deck height of .005", and 24.5cc in the head, your CR will be around 11.2 - so you may find that you need more capacity in the head than you already have... 

The 12G295 is ideal on a 1098 (and compromised on a 998) as standard. 

 

The +.100" engine I have on the bench at the moment had to have the standard 28.3cc head enlarged to 29cc to get the CR down to 10.3, so measure the chamber size and the deck height before doing anything else.

 

Numbers wise, the +.060" 1098 I threw together with an MG Metro cam chucked out 63 brake and 66 torques, so you will be in the same ballpark with your build.

[70K1100]

Vipermoir that sounds like a nice build.   MG Metro Cams are almost non existant in Oz unfortunately.   What carb set up is on that engine?

[Spitz]

On mine……. I've run twin HS2 and presently HIF38….no need for more

[70K1100]

Spitz,   how do you find the driveablility of the 1098 with the 3.1 gears?   I'm looking at 10" wheels and not sure how well it would drive around town with the 3.1 gears.

[carbon]

Town use should be just fine using 1098 with a 3.1 diff - as long as you're running a standard ratio gearbox and don't go wild on the cam timing.

 

For comparison the factory produced 998's using 2.95 diff.

[Cooperman]

Town use should be just fine using 1098 with a 3.1 diff - as long as you're running a standard ratio gearbox and don't go wild on the cam timing.

 

For comparison the factory produced 998's using 2.95 diff.

And they were not nice to drive. All the classic 'nippyness' of the Mini went and they felt sluggish and even slower than the early cars. 

[carbon]

Good point Cooperman, taller diff will make the car slower off the mark but quieter at speed.

 

I'm running 1275 with 3.1 diff and the slightly closer 'S' ratio gears, this is good for town & country. The 1098 has good reputation for torque, with the standard ratio gears it should be able to pull away smoothly using a 3.1 diff. But I have not tried this combination, so others may be able to comment further.

[Cooperman]

I think a 3.44:1 would be ideal with a 1098. The 1098 does have good torque, but not as much as the 1275 and not quite so low down, especially with a hotter cam.

I once had a 1098, in fact an MG110 unit with a 731 cam and twin HS2's. It had a 3.76:1 FDR and really went well in terms of acceleration although with the rev limit of a 1098 the max cruising speed was a bit low, especially for modern traffic. But then, a classic car is never going to be good in modern traffic, unless it's something like a Porsche 911 Carrera

 

By the way, running 10" wheels will make virtually no difference to the overall gearing as the rolling radius of the tyres is almost unchanged. A 145/70 x 12 or 165/60 x 12 is within a few per cent of a 145/80 x 10 or a 165/70 x 10 (do the maths yourselves!).

Edited by Cooperman, 29 August 2015 - 07:45 PM.

[Vipernoir]

A 3.1 is better with a 1098 than a 3.44 - certainly in mine anyway.  The extra torque more than compensates for the longer first gear and the longer legs help the engine last longer on the motorway.

 

The 998s that had a 2.95 fd also had the wide ratio HLE gearset. 

I have one of those fitted in my daily with a 3.2 fd, and first is nigh on useless (except in heavy traffic) as you're in third by 20-odd mph.
Having written a new visual gear ratio calculator, I can see that 2nd in that gearset is the same as first on my sccr box with a 3.1 in it.  Chuck a 2.95 in the HLE box and it effectively turns it into a 3-speed with overdrive.  Next time the box is apart I will be finding a 2.95 to give it a whirl.

[70K1100]

Cooperman I am contemplating running 12" wheels but I've heard a few people say that it ruins the ride quality of the mini. The only Mini I've driven was the one I owned when I lived in the UK which was a Mini City E on 12" wheels with a stage 1 kit from minisport. I don't know any different. However with the poor quality roads around here the higher profile tyres would be a better option.