20 replies to this topic

[Denz100]

Hi, I’m currently (slowly) rebuilding a mini, shell is stripped and been blasted and being welded, but the spray is going to be on hold for a long while. So I’ve started stripping a 1098, some tuning is planned, a 295 head, cam, +60 thou bore, pistons plus other bits,( if any of this seems wrong please say I am a novice and going on what I’ve read and heard)
The question is which carb would be best or neither, I was given an su 1 1-2” carb with the car but needs to be refurbished, but I was also given twin 1 1/4” Webber’s and a polished manifold with heat shields that were refurbished a number of years ago and never fitted. Would either of these be able to fit, would I need a different head? Or is there a better option?

Any advice will be much appreciated

[blacktulip]

I would be looking at sticking to the 1 1/2" su you have and just rebuilding it or going for a hif38 which is also around 1 1/2" (38mm to be precise). The pros with the hif38 is it has the fuel bowl integral so no fuel starvation on hard cornering. The cons are it needs to be fitted to a shallow depth inlet so it can clear the bulkhead. The mg metro inlet is perfect for this.

[Cooperman]

A pair of HS2's would work well on a nicely flowed and matched alloy manifold. Once set-up properly they will give excellent throttle response and good smooth power right through the rev range up to around 6000 rpm.

With a 1098 you don't want to be regularly going much over around 6200 rpm due to the crankshaft limitations of the 1098.

[Spitz]

I have a simialr spec.  1098, +.040, 12g295, SW5.

 

I ran twin 1.25" and it was a fantastic set-up.  I even autocrossed the car......keeping the revs down a bit like Cooperman mentioned due to the long stroke.
I've since switched it to an HIF38 ( I can't remember why...I may have needed the twins for another project ) and it works pretty good also..... a tad slower at the low end.
If you do use a 1.5"... either HIF or HS4.... make sure you get the right needle for your build. AAA, AAM or other etc.  A stock 1.5" from a 998/1098 won't be rich enough on your build

[Denz100]

Perfect thank you very much, do you think roller rockers and double valve springs would be a good idea too?
Have been told they do help the engine rev higher but as I’m aware and has been mentioned the 1098 shouldn’t be driven over 6000 odd rpm often.
I’d like to have a good pull of power lower down the Rev range because of this reason, the car is only a project for occasional use, for filling a child hood dream if owning and building my own mini

[Denz100]

I have a simialr spec. 1098, +.040, 12g295, SW5.

I ran twin 1.25" and it was a fantastic set-up. I even autocrossed the car......keeping the revs down a bit like Cooperman mentioned due to the long stroke.
I've since switched it to an HIF38 ( I can't remember why...I may have needed the twins for another project ) and it works pretty good also..... a tad slower at the low end.
If you do use a 1.5"... either HIF or HS4.... make sure you get the right needle for your build. AAA, AAM or other etc. A stock 1.5" from a 998/1098 won't be rich enough on your build

[Denz100]

Thanks for the info comparing the two set ups, sorry to have 101 questions, did you have much work done to the block itself?
Been told to fit flat too pistons with the 12g295 head, but think this is more for a 12g940?? Any ideas??
Many thanks

[timmy850]

The 12G295 head was actually designed for the 1098 engine. With flat top pistons you'll be in the right area to get a decent compression ratio (around 9.5:1) - although this will need to be measured properly on your specific parts and checked before you get too far with the build. You also need to match the cam profile and other parts to make the engine work really well as an overall unit

 

My 1098 is:

+080 flat top pistons

12G202 head with 31.8/27.2mm valves and lots of porting/chamber work

RE266SS cam with standard rockers and double valve springs

DHLA40 Dellorto sidedraft carb (similar to a Weber)

Lightened and balanced standard flywheel

 

Mine goes really well and has lots of low down grunt, I could probably get more top end power with some larger chokes in the carb but I think for a 1098 it's better having the power lower in the RPM range. I have a programable ignition box and a limiter at around 6000rpm and it's very easy to hit as it revs very smoothly! 

 

I am going to install a set of twin HS2 carbs soon, as that's what I've always wanted and I love the look of them. I do still love the Dellorto as it makes a really good noise, is very responsive and is also very easy to tune - the downside being that on very long trips it can be a bit loud. 

[Turbo Phil]

Thanks for the info comparing the two set ups, sorry to have 101 questions, did you have much work done to the block itself?
Been told to fit flat too pistons with the 12g295 head, but think this is more for a 12g940?? Any ideas??
Many thanks

The 12G295 head was fitted to the 998 Cooper which featured raised crown pistons, to compensate the head has 28.3cc chambers in standard form. Unless the head has been skimmed (which is often the case) you’ll have a significant drop I compression with flat top pistons and worse still with dished.
The 940 head castings on the other hand have chambers far smaller (21.4cc) than any of the small bore heads which would raise the compression.
It’s vital to know the compression ratio of your engine build, not only if you want it to perform its best but also to ensure it’s not excessively high which could cause issues with detonation.
You need to accurately measure the chamber volume in the head, piston dish, head gasket and how far down the bore at TDC the pistons sit. With the exception of the head gasket volume, which can be found online, never assume any of the other figures. Accurate measurement is the only way. Coupled with the known engine CC, or bore and stroke figures you can calculate the static compression ratio.
There are plenty of online calculators, though it’s easy enough to work it out yourself. Or put the figures up on here and either myself or someone else will do it for you.

Phil.

[timmy850]

The 12G295 was first used on the MG1100 and it works perfectly with a 1098 engine, like what the first post is asking about.

They used the 295 later on the 998 engines and had to use the raised dish pistons because the compression ratio was too low with the lower CC capacity.

[Turbo Phil]

I missed it was a 1098, I was wrongly assuming a 998. This still doesn’t detract from the fact that everything needs to be measured and the compression ratio calculated. It’s quite possible the head has been skimmed, the valve seats recut, or the new pistons sit lower in the bore, all of which will have a big effect on the ratio.
For example on a 998 engine with standard size flat top pistons sitting 0.010” down the bore, 2.8cc gasket, 0.5cc for the ring land and 28.3cc in the head the compression ratio would be 8.68-1.
On a 1098 engine with the same spec the compression ratio would be 9.46-1. Quite a difference.

Phil.

[robert.hedges]

AC dodd did a really good post on here about carb sizing against BHP and final drive. Worth looking for I think....

[Cooperman]

Don't bother with higher ratio rockers. They move the power band up and you get less bottom end but slightly more top end.

The best rockers are the early pressed steel type.

Don't use valve springs which have too high a rate. Fairly standard springs won't allow valve crash at much below 6500 rpm and with a 1098 you won't be revving that high - well, if you do you might be buying a new block & crank some time soon  .

I once had a Mini with a 1098 MG 1100 engine. The head was lightly gas flowed and CR was around 10:1 (5-star petrol in those days). The only problem was it had drum brakes!

[gazza82]

The only problem was it had drum brakes!

 

That did seems to be a BMC trait ..

 

 20191204_front_suspension.jpg   37.87K   0 downloads

 

I never did get to test these out .. these are 8" drums off of an A40 MkII that replaced the 7" drums on my A35.

 

 

Actually it's a good job I didn't try when we got the car out of the garage .. I found the shoes and springs in a cupboard!!    After 40 years I don't think the master cylinder was going to move much anyway!!

 

But at least it was easy to push down the drive and pull onto the flatbed to bring home!

 

It's getting discs as the engine going in has a bit more 'umph' than the 34hp 948cc that's in there now .. actually it might have been 37hp as the SU I put on was supposed to give it a whopping 3hp extra!!

Edited by gazza82, 25 February 2020 - 05:07 PM.

[robert.hedges]

In fact Russell Graham Engineering in the small bore part 3 technical info on their website have done testing with different types and inlet/exhaust manifolds and cards on a 1098. From memory I think the twin 1.5 su did well but to be fair there was very little benefit over twin hs2's at the top end.