10 replies to this topic

[limby2000]

I know this is a very confusing and complex subject which there may be no exact answer as various solutions may be right/work. I have mini 998 a+ that I am converting to a 1098. I have the 1098 crank,I have a set of 10+ thou oversize nos tarabusi pistons,that have a small dish in the top, still in the leyland box. I also have a standard unskimmed 12g295 head off an mg1100 engine. Now I know I've got to get the cr down, I am aware of the gallery issues with skimming the head but as i,m having new valve seats and guides I could get skimming done. Should I have the block skimmed?. Also would I be better off getting flat top pistons as I know keith calver,s doing them on his site.Also I have specalist components injection system, does injection favour a particular c/r?.

[Dusky]

Get flat tops, easier cr.
You will need to measure everyrhing though. Zo get the exact cc of the head, and measure how deep the pistons sit down the bore on tdc.
You can skim upto 80 thou on a 12g295 I am told, but measure the head and galleries first. On a 1100 you wont need that much of a skim anyway.

[limby2000]

Yeah I know the mg1100 had dished pistons with this head, but they run 4 star with lower c/r. In theory with the 1098 crank and pistons it should run ok, as it was factory c/r.

[Cooperman]

You can over-bore a 998 or 1098 block to +0.080" which, with a 998 crank gives 1060 cc. That gives a better result than using a 1098 crank as it can rev so much higher without the risk of crank breakage. However, if using a 1098 crank with an effective rev limit of c.6000 rpm, you will be looking for a higher torque unit, so why not consider going to +0.080" and using the 1098 crank. This will produce a capacity of 1167 cc and thus a really high-torque engine. Flat-top +0.080" pistons are available as well, so it's win, win.

After doing the first 'dummy build' the block deck should be skimmed to ensure that the pistons come right to the top of the bore.

Once that is done together with any 'cleaning-up of the combustion chambers and ports in the head (slightly larger inlet valves would help a motor with this spec) the compression ratio calculations and measurements can be done and the amount to be skimmed from the head worked out. By measuring down the oil gallery hole and comparing the hole depth with the head thickness it can be confirmed that there is enough metal on the head to allow the skim required.

That would be an excellent engine with very high torque and good power at up to 6000 rpm, especially if an MG Metro cam were to be used.

 

Good luck with the build.

Edited by Cooperman, 01 January 2015 - 01:29 PM.

[coopdog]

You could always leave it with a low cr an go forced induction which will give very good results even with low boost ;)

[coopdog]

You could always leave it with a low cr an go forced induction which will give very good results even with low boost ;)

[carbon]

Limby2000 - depends on the dish capacity in your set of +0.010 1100 pistons. If this is the shallow MG1100 dish (about 3cc) then the compression ratio with an unskimmed 12G295 head will be close to 9.0:1 which is pretty good starting point for a fast road engine.

Edited by carbon, 01 January 2015 - 05:31 PM.

[Spider]

There's really no way of knowing what CR the combination will give until the bottom end is assembled, then you can CC the Piston in the block, CC the Head and look up what the typical CCs are for the Head gasket that you intend to fit, do your calculations and go from there.

 

You'll usually get the best performance getting the pistons closer to or flush with the deck.

 

As for an actual CR number, you would get the best - and safest - results doing a Dynamic CR Calculation rather than a Static one, this will also take in to account the Camshaft Profile, as it is very easy to Over CR an engine that runs high Static CRs with Mild Cam Grinds.

 

<Edit: You'll also find it very hard to get a machine shop to take the bore out only 0.010" (and it probably won't clean up at 0.010" either). Have a look at the pistons that Mini Spares have on offer, they are considerable better than the others you've looked at>

Edited by Moke Spider, 01 January 2015 - 07:30 PM.

[limby2000]

Yep moke spider, someone said to me that it might be difficult to get a 0.10 thou rebore, perhaps that why you only see piston sold in 0.20 thou steps. What I forgot to mention that this engine build is for an automatic transmission, as such its main design is for torque and power delivery not exceeding 6 k, the cam I've got is a sw5. Ultimately I guess at the end of the day there no quick fix for engine planning and ill just have to sort out the burett and oil. It would have been a lot simpler to go down the 1275 route, however I did fancy something different and as far as I know no one has done a 1098 injected.

Edited by limby2000, 01 January 2015 - 11:12 PM.

[Stiggytoo]

Correct me if I'm wrong but I didn't read anywhere that you have 1100 rods?

[limby2000]

Your right, I did,nt mention rods. 1098 and 998 rods are same length, only difference are the A+ are push fit as opposed to full floating pin. The difference is in the crank throw and piston crown height.