15 replies to this topic

[unburntfuelinthemorning]

According to Haynes, 998 CR's are as follows:

 

pre A+                                8.3:1

'83-'87                              10.3:1

'88 on (category C)            9.6:1

'89 on (low compression)   8.3:1

'89 on (high compression)  9.6:1

 

How are these differences achieved?

 

As far as I understand the 8.3:1 ratio engines had dished pistons.  If flat tops were used on these engines would that raise the CR to 10.3:1 or 9.6:1?

 

How is the difference between 10.3:1 and 9.6:1 achieved, by combustion chamber volume?

[Spider]

They also varied the gudgeon pin heights.

[nicklouse]

dont forget the raised D crown pistons.

 

list of pistons and CR they give.

 

http://www.somerford...age=page&id=112

[phillrulz]

Jesus, so my 86 City E is the highest compression! 

[unburntfuelinthemorning]

They also varied the gudgeon pin heights.

As in gudgeon pin centre to piston crown height? 

 

So if I were to fit flat top pistons to my pre A+ 998 with it's 12A1456 head what would the CR be?  It currently has dished pistons but I don't know the cc.

[Spider]

 

They also varied the gudgeon pin heights.

As in gudgeon pin centre to piston crown height? 

 

So if I were to fit flat top pistons to my pre A+ 998 with it's 12A1456 head what would the CR be?  It currently has dished pistons but I don't know the cc.

 

 

At a rough guess it would be around the 10:1 mark.

 

You'd really want to / should 'CC' it to be sure.

 

Yes, the Pin Centre to Crown Heights changed.

[carbon]

The Mini City E used very similar specification 10.3 CR engine to the Metro HLE. In both of these the engines the reason for the very high compression ratio was to achieve best fuel economy, not maximum power.

 

I think the City E and HLE used a specific carb (manifold ported vac advance) and distributor (advance can) combination, which effectively retarded the ignition timing at full throttle to prevent knock.

[unburntfuelinthemorning]

The reason I ask is that I've got oil in the cylinders.  I suspected valve guide wear as the oil seems to build up with engine off and plugs are clean after a run.  After removing head I have valve guide wear, valve stems diameter smaller than specifications, oil coating the port side of the valves below the guides, valve spring free length too short and seat recession on one and four exhaust.  By the time I've replaced the parts and had hardened valve seats fitted I'm approaching the cost of the Minispares standard replacement unleaded head which they're not stocking at the moment.  As I want to upgrade I might as well get a stage 3 type head now rather than throwing money at a head I don't want.  Ultimately I'd like a well sorted bottom end with flat top pistons block decked to bring them flush and a 270 degree cam In the meantime I'd like to use it on my current engine with standard cam but don't want the CR to drop below 8.3:1 or it'll negate any performance increase from the improved breathing.  So I'm trying to get an idea of combustion chamber size that will work with both engines.

[absx2]

The Mini City E used very similar specification 10.3 CR engine to the Metro HLE. In both of these the engines the reason for the very high compression ratio was to achieve best fuel economy, not maximum power.

 

I think the City E and HLE used a specific carb (manifold ported vac advance) and distributor (advance can) combination, which effectively retarded the ignition timing at full throttle to prevent knock.

 

They also need 97 RON to run properly but you can get away with good 95 RON super unleaded but expect it to run like a pig on anything else.

10 years of putting up with a metro HLE motor has taught me not to use high compression ratios on a daily driver.

[1984mini25]

The Mini City E used very similar specification 10.3 CR engine to the Metro HLE. In both of these the engines the reason for the very high compression ratio was to achieve best fuel economy, not maximum power.

 

I think the City E and HLE used a specific carb (manifold ported vac advance) and distributor (advance can) combination, which effectively retarded the ignition timing at full throttle to prevent knock.

 

With the 25 and sprite both getting the City E's engine and gearbox, mine (and an original sprite I've worked) both had a standard cast manifold and hs4 carb just like any other mini/City E. With a standard Lucas, or in my case a ducellier distributor. But what the City E engine did have that was different, was the 2.9 diff, rubbish for acceleration, but great at dropping the revs by about 500rpm for better fuel economy.

Edited by 1984mini25, 25 March 2018 - 08:23 PM.

[phillrulz]

 

The Mini City E used very similar specification 10.3 CR engine to the Metro HLE. In both of these the engines the reason for the very high compression ratio was to achieve best fuel economy, not maximum power.

 

I think the City E and HLE used a specific carb (manifold ported vac advance) and distributor (advance can) combination, which effectively retarded the ignition timing at full throttle to prevent knock.

 

They also need 97 RON to run properly but you can get away with good 95 RON super unleaded but expect it to run like a pig on anything else.

10 years of putting up with a metro HLE motor has taught me not to use high compression ratios on a daily driver.

 

 I use Shell V power, heard to stay away from tesco 99 momentum due to the ethanol. Mine is only a weekend car. 

 

The Mini City E used very similar specification 10.3 CR engine to the Metro HLE. In both of these the engines the reason for the very high compression ratio was to achieve best fuel economy, not maximum power.

 

I think the City E and HLE used a specific carb (manifold ported vac advance) and distributor (advance can) combination, which effectively retarded the ignition timing at full throttle to prevent knock.

Hmm i need to run a compression test for my engine as im having problem see my thread about engine not holding a tune. Any ideas what values im looking for? I know all the same value or close but what sort of figure. 

[unburntfuelinthemorning]

The Mini City E used very similar specification 10.3 CR engine to the Metro HLE. In both of these the engines the reason for the very high compression ratio was to achieve best fuel economy, not maximum power.

 

I think the City E and HLE used a specific carb (manifold ported vac advance) and distributor (advance can) combination, which effectively retarded the ignition timing at full throttle to prevent knock.

Of course in the mid eighties we had 4 star petrol as well and I presume the 10.3:1 CR took advantage of this.  998's in the seventies with 8.3:1 only needed 2 star. 

 

Don't all models with vacuum advance retard ignition at full throttle/advance ignition at part throttle?

[absx2]

 I use Shell V power, heard to stay away from tesco 99 momentum due to the ethanol. Mine is only a weekend car. 

 

 

 

 

 

Petrol octane is shockingly bad these days especially the supermarket stuff, I had to retard my ignition 3 times on the way home as the knocking was that bad after chucking in 10 ltrs of 95 Ron from Morrisons and it got to the stage that it wouldn`t pull up hills.

[phillrulz]

 

 I use Shell V power, heard to stay away from tesco 99 momentum due to the ethanol. Mine is only a weekend car. 

 

 

 

 

 

Petrol octane is shockingly bad these days especially the supermarket stuff, I had to retard my ignition 3 times on the way home as the knocking was that bad after chucking in 10 ltrs of 95 Ron from Morrisons and it got to the stage that it wouldn`t pull up hills.

 

Worth using these one shot octane lifting additives? 

[Spider]

 

Worth using these one shot octane lifting additives? 

 

 

I'll say a cautious yes.

 

Need to do your homework on what's available to you locally. Some only raise the Octane by 1 point and some up to 7 points. They most definitely are not all made equal.