27 replies to this topic

[Mini Cheddars]

'79 998 with stage one kit (Maniflow Cooper Freeflow manifold, Maniflow 1.75" single box exhaust and K&N cone filter), electronic ignition, standard engine block and bottom end.

 

On Monday, I should be receiving a fully reconditioned a 12G295 head, with a reconditioned rocker assembly.

 

In the interests of money saving, would I be able to able to install the head and give it a Colortune tune-up until the end of the month, when I get paid, so I can get it dyno tuned? Or should I wait and get the block deck checked over before proceeding?

 

I gave no instruction to the engineering firm reconditioning my head as to compression ratios, as I wouldn't know where to start. Would swapping the head and colortuning it give bad running results? The car is running fine as it is now (after hopefully sorting a rough running issue earlier - running lean).

 

Like I say, it would only be until the end of February so I can get it tuned at Slark Race Engineering.

 

I'm just an eager beaver!

[Dusky]

You will need to get the compression ratio right, cooperman wil explain how to do it;)
Carb needle will need a change but you can try an AAA needle orso to keep or on the road before the dyno tune.

[Bubblebobble]

Well it sound like you may as well not bother , as you are going to have to out it on and then off again . Plus the fact i would not expect a performance increase untill it is running at the correct C/R .

[Mini Cheddars]

You will need to get the compression ratio right, cooperman wil explain how to do it;)
Carb needle will need a change but you can try an AAA needle orso to keep or on the road before the dyno tune.

Has a brand new AAU needle in there at the moment, with SU's very own fancy dashpot oil.

 

 

Well it sound like you may as well not bother , as you are going to have to out it on and then off again . Plus the fact i would not expect a performance increase untill it is running at the correct C/R .

 

So early consensus is to just wait.

 

I know it would be ideal to get the CR spot on but I was just wondering if the drivability would be worse than good if I stuck it on 'as is'.

[timmy850]

A 295 has much bigger chambers standard compared to a 998 head. If you put it on without checking the C/R it's a good chance you'll have worse driveability.

[Cooperman]

The C.R. with a 295 head with standard 28 cc chamber volumes would be extremely low, maybe 7.5:1 (I've not done the sums). Even worst if the engine has dished pistons.

No point in fitting a potentially better head if the CR is lower than it should be. Ideally, a CR of around 9.5:1 to 10:1 is needed.

Measure (accurately) the chamber volumes of the 295, the distance the piston tops are at TDC and if it has dished pistons (if so what volume is the dish) and I'll tell you what CR that would give. All those figures are needed to work it out.

[carbon]

Pretty sure the 998 was running dished pistons and 8.3:1 compression in 1979. You may be able to just see the dish if you take a sparking plug out and use a small torch.

 

As Cooperman says, no point in fitting a standard 295 head to a 998 running dished pistons, the head would need to be skimmed a lot to get up to a decent compression ratio.

 

As a quick check on the 295 head you have measure the distance from the lower face to the top face, as standard this was 2.75 inches. If it's less than this then it has been skimmed in the past.

[Cooperman]

It really is necessary to actually and accurately measure the combustion chamber volumes. Those heads are all around 50 years old now and who knows what has been done to them.

As with everything in modifying engines, it is always necessary to measure and calculate. Such is the nature of engineering.

[Mini Cheddars]

Taking into consideration the replies, I will just wait until the end of the month and have SRE do the necessary calculations and fitment.

As the Mini is my daily, the turnarond time for the work needs to be pretty quick and SRE would have the necessary tools and knowledge.

Not only that, but the car has decided to continue playing up which means I don't fancy fitting a better flowing head if the car still runs like crap, but that's for another topic if I make one.

I may post a photo of the head later.

Quick edit: I have taken the head off before and it does infeed have dished pistons.

Edited by Mini Cheddars, 02 February 2015 - 07:09 AM.

[Cooperman]

When going to a 295 head it is best to fit flat-top pistons. I've never tried fitting a 295 to a 998 with dished pistons, but there would need to be a heck of a lot skimmed off the face of the head.

Might be worth thinking about fitting the 295 as part of a complete engine re-build at a later date when the right parts can be used throughout and the result will be much more satisfying.

[Mini Cheddars]

I do indeed have a spare 998 complete engine with a suspected buggered gearbox, which I could rebuild and put the block and head on.

Haha decisions!

Which pistons would you recommend?

[Cooperman]

Mini Spares do some nice flat-tops of excellent quality. If you wish, you can safely go to +0.080" overbore which gives 1060 cc. I built one like that with flat-tops and a gas-flowed 295 with slightly larger inlet valves. It had some other nice bits like a Howley alloy inlet manifold, an HS4, a 3-into-1 freeflow ex. manifold and a single box 1.75" diameter RC40 system. The flywheel was lightened slightly and FDR was 3.44:1.

It made 65 bhp at 6000 with an Aldon 'Yellow' dizzy and was nice to drive as it felt like the original 998 Cooper.

PM me if you want the full detailed specification.

[blacktulip]

i once fitted a 12G295 head to a 998cc with dished pistons. it gave a slightly smoother drive but it wasnt worth the aggro of swapping heads to be honest. i say its best to wait untill you know exactly what you want to do.

[Mini Cheddars]

Cheers for the offer, Cooperman. Still up in the air what I'm going to do yet, but I do remember envying this build.

http://www.theminifo...275-eating-998/

[carbon]

My first mini was 998, this must have been a unit from mid-70's with dished pistons.

 

I fitted a well skimmed 206 head (very similar to 295) to this, and using a 1300 spider cam, Oselli torquemaster and flowed HS4 it went very well. I never measured the head chamber volumes at the time but I still have the head in the garage somewhere and I will check the chamber volumes out of interest.