40 replies to this topic

[70K1100]

I'm starting to rebuild my 1970 Morris Mini K with the 1098 engine. I have a 295 head which has had 80thou shaved off it. My question what else would I need to do to the engine to get a compression ratio of close to 10.1? Any assistance would be much appreciated.

[carbon]

All depends on what pistons you are planning to fit. Use the bowl size is in the pistons to work out the compression ratio.

 

And for a road car 10:1 is a bit on the high side unless you are using non-standard cam and planning to fill up with 99 octane.

[Spider]

What CR can you get now?   

 

And as Carbon has raised the point, what Cam are you intending to run?

 

What pistons do you have and are you 'locked in to' those?

 

Carbon, I'm not sure what method of Octane measurement you guys seem to have in the UK nor can I presently remember what we do here, however with stock cams, from my own experience, we can run up to around 10.5:1 on 98 (readily available in most cities), however when I was last up NOT850's way, they had 100 Octane very readily available on pump.

 

I've seen it pop up a few times when I mention the CRs around the 10.5 and above (not that I use that method anymore) concerns form others in the UK. One car I have in the fleet happily sings along with 11.8:1 CR on pump fuels we have here.

[carbon]

UK pump petrol is sold based on RON octane number. I think Australia and NZ are same?

 

US and Canada are different, using (MON+RON)/2 for pump ratings so the UK 95 octane would be about the same as US 91 octane.

 

As well as CR and cam duration, other key limiting factors for CR include intake air temperature, engine temperature, an altitude (charge density). The hotter the engine gets the less CR you can run before you get risk of detonation. And if you do a great porting job on the head you may not need as much CR as a more restrictive design.

[Cooperman]

I'm starting to rebuild my 1970 Morris Mini K with the 1098 engine. I have a 295 head which has had 80thou shaved off it. My question what else would I need to do to the engine to get a compression ratio of close to 10.1? Any assistance would be much appreciated.

When doing any engine work involving cylinder heads, pistons and bores it is VITAL that you are able to measure and calculate it for yourself. No two head/piston/block configurations will be identical.

I, and others, have posted how to do compression ratio measurements and calculations, so you need to do a search as we can't keep on repeating this for obvious reasons.

So measure, calculate, measure again then machine as necessary. It needs to be right as I'm sure you realise.

Let us know if we can help further

[Spider]

UK pump petrol is sold based on RON octane number. I think Australia and NZ are same?

 

US and Canada are different, using (MON+RON)/2 for pump ratings so the UK 95 octane would be about the same as US 91 octane.

 

As well as CR and cam duration, other key limiting factors for CR include intake air temperature, engine temperature, an altitude (charge density). The hotter the engine gets the less CR you can run before you get risk of detonation. And if you do a great porting job on the head you may not need as much CR as a more restrictive design.

 

Yeap, just looked it up again http://www.environment.gov.au/topics/environment-protection/fuel-quality/standards/petrol  and we do go work off RON method in these parts.

 

All valid points carbon. Just in regards to cam duration, and if I can add, Inlet Valve closing angle, that is what I work to when working out CRs these days, I use the Dynamic method and found considerable better and less erroneous results while optimising what you can safely get from an engine.

 

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.

[Anthony30]

All depends on what pistons you are planning to fit. Use the bowl size is in the pistons to work out the compression ratio. And for a road car 10:1 is a bit on the high side unless you are using non-standard cam and planning to fill up with 99 octane.

My 998, high comp, is running 10.3:1, as standard. No point in going much higher on a road engine IMO.

[70K1100]

After reading the project small bore articles by Graham Russell where he used 998 pistons which were turned down 140thou at the crown which left the pistons sitting flush with the top of the block, I'm not sure if this would make the compression too high. Cam wise I'm looking at Graham Russell's new 266 cam. Mostly because a lot of people such as Cooperman, who from reading a lot of topics on here, know what they are talking about recommend a cam of similar specs as being ideal for the 1098.

[Cooperman]

A 266 cam, or the very similar MG Metro cam, will be ideal in a 1098.

C.R. of between 9.8 & 10:1 would work well.

[carbon]

For a road tuned 1098 the Kent MD266 cam should work nicely. I've used this cam in 1275 motors and I like the decent torque at low-mid rpm but can also spin up when needed.

 

However I think the Graham Russell 266 cam may be a very different grind, might not behave like the Kent MD266?

[timmy850]

According to my CR calculator:

Bore 1098 (oversize +040 to 1131cc): 65.588mm

Stroke 1098: 83.73mm

Head capacity (estimate): 27cc

With flat top pistons you're looking at around 10.3:1 (allowing for the gasket volume etc). You'd have to measure the volume of your actual head. Standard they are 28.4cc

 

That's using flat top 1098 pistons in the smallest overbore (+040).

http://russellengine.../mini-products/

 

Have you read the new 1098 build by GR too? it's got some updated tech since the old build.

http://www.ausmini.c...php?f=2&t=87949

 

http://russellengine...ct-68mm-part-1/

http://russellengine...ct-68mm-part-2/

http://russellengine...ct-68mm-part-3/

[70K1100]

Cooperman I've read the new engine build using the 68mm pistons but I'm a bit reluctant going that far on my engine. I'm thinking going out 40thou would be enough.

[70K1100]

Sorry Timmy850, I'm a little hesitant about going out that far.

[Cooperman]

Cooperman I've read the new engine build using the 68mm pistons but I'm a bit reluctant going that far on my engine. I'm thinking going out 40thou would be enough.

That should be fine.

There is no point in fitting a cam hotter than a 266 as a 1098 engine won't really rev like a 998 or a 1275. For engine safety it is wise to limit the revs to about 6000 rpm and a 266 is great for that as peal power will be at around 5600 to 5700 rpm.

The 1098 has super low and mid-range torque, so best to build it and gear it for those revs.

I built an Austin-Healey Sprite engine for historic motor-sport and I used a 1098 block, bored +0.040", fitted a 266 cam with twin H4 carbs, lightened and balanced everything, fitted a 295 head which was gas-flowed with slightly larger inlet valves and it gave around 68 bhp at 5800 rpm. But is was very nice to drive at lower revs, especially between about 3500 & 5500 rpm.

[70K1100]

The spec I am looking at is:
1098 +40
266 cam
Twin HS2 carbs
Flowed 295 head with slightly bigger valves
Cooper S rockers
Balanced bottom end.

I would like around 70-75ftlb of torque.