20 replies to this topic

[babsbrown]

Good evening guys,

Is this compression ratio going to be too low?

I'm building a 1098 bored out to 40+ so essentially 1132cc now.

12g295 head with 26cc Chambers
Stock 1098 crank
8cc dished pistons
Standard head gasket and 1tho deck clearance.

This gives me 8.6:1 compression ratio.
I'm wanting a torquey fast street motor and will be fitting an evo001 or Kent 266 cam.

I know I can skim the 12g295 head a bit more. But even if I skimmed it 40 thou, that will bring me up to 9.06:1

Would the extra work to have the head skimmed be worth the money and effort?

I also appreciate 1098 motors had low compression ratios from the factory.

Cheers, Babs

[Earwax]

For me , i think that low 9CR is going to struggle with your description of fast /street/torquey.

 

Probably too late or $$$ now

 

But if you want to keep the head - do the CR with flat top pistons ( 8cc better off)

 

Or change to a 202 head and do the CR ( probably 3 cc better off)

[Lplus]

For me , i think that low 9CR is going to struggle with your description of fast /street/torquey.

 

Probably too late or $$$ now

 

But if you want to keep the head - do the CR with flat top pistons ( 8cc better off)

 

Or change to a 202 head and do the CR ( probably 3 cc better off)

Keith Calvert does flat top pistons at +40 and very nice they are too.  I'm using a set with a stock 12G295 giving 9.1:1  Probably a tad low but it's a nos head I picked up so I was loathe to skim it

[Shooter63]

I think a change of pistons would be ideal,but you can skim the head to get what you need, the minimum head deck thickness recommended is 80 thou, but watch the oil gallery depth. You will need to re-set the rocker geometry and cam timing to get the best out of the engine.

Shooter

[whistler]

I used KC +0.100" flat tops and a head with 28.5cc and that gave me 9.75:1 which was the target CR. The block was also skimmed a little.

[babsbrown]

Thanks guys I really don't want to buy a new set of pistons as this was meant to be a budget build, I've already thrown good money away by buying cylinder heads I didn't need.
And the pistons I have are new.

Hopefully I'll get 60 thou off the 12g295 and get the compression up a bit. I'll aim for 23cc per chamber.

Equally I have a 12g940 head. I could get the exhaust valves sunk around 40thou to allow th valves to clear the head. They have a cc of 23cc.

I guess I'll have to do a few calculations and work out the best case scenario.

[babsbrown]

For me , i think that low 9CR is going to struggle with your description of fast /street/torquey.

Probably too late or $$$ now

But if you want to keep the head - do the CR with flat top pistons ( 8cc better off)

Or change to a 202 head and do the CR ( probably 3 cc better off)

Keith Calvert does flat top pistons at +40 and very nice they are too. I'm using a set with a stock 12G295 giving 9.1:1 Probably a tad low but it's a nos head I picked up so I was loathe to skim it

How does it drive mate?

[Shooter63]

I can tell you a way to pocket the block to run a 12g940 head, but would take absolutely no responsibility if injury happened, but I've done it successfully before.

Shooter

[babsbrown]

I can tell you a way to pocket the block to run a 12g940 head, but would take absolutely no responsibility if injury happened, but I've done it successfully before.

Shooter

How did you do it mate? Tbf my other 998 engine is pocketed. I did it with a cordless drill and a grinding stone. Can't be any worse than that idea.

Still undecided as to what to do really. Too many options Really. I shouldn't have bought so many heads haha

[Lplus]

 

 

For me , i think that low 9CR is going to struggle with your description of fast /street/torquey.

Probably too late or $$$ now

But if you want to keep the head - do the CR with flat top pistons ( 8cc better off)

Or change to a 202 head and do the CR ( probably 3 cc better off)

Keith Calvert does flat top pistons at +40 and very nice they are too. I'm using a set with a stock 12G295 giving 9.1:1 Probably a tad low but it's a nos head I picked up so I was loathe to skim it

How does it drive mate?

 

It drives fine, I do use 99 octane so no rough running now I've got the mixture right. It also has a BP55 cam and a 3.2 diff.  I suppose it could have a bit more torque below 2500 but that might be the cam or diff - still it's got plenty of go in a clubman estate.

[Ethel]

8.6 shouldn't be too bad, especially if you want to run on regular unleaded. A bit higher would be preferable, but taking the head off isn't that much of a drama if you decided to go the extra mile & skim it at a later date.

[babsbrown]

8.6 shouldn't be too bad, especially if you want to run on regular unleaded. A bit higher would be preferable, but taking the head off isn't that much of a drama if you decided to go the extra mile & skim it at a later date.

Ah that's reassuring thanks,

I'd started to worry I'd messed up. I can get a head skimmed for £50 here, so I reckon I'll do that. I was dreading anything around 8.6 being absolutely powerless.

Cheers again

[babsbrown]

For me , i think that low 9CR is going to struggle with your description of fast /street/torquey.

Probably too late or $$$ now

But if you want to keep the head - do the CR with flat top pistons ( 8cc better off)

Or change to a 202 head and do the CR ( probably 3 cc better off)

Keith Calvert does flat top pistons at +40 and very nice they are too. I'm using a set with a stock 12G295 giving 9.1:1 Probably a tad low but it's a nos head I picked up so I was loathe to skim it

How does it drive mate?

It drives fine, I do use 99 octane so no rough running now I've got the mixture right. It also has a BP55 cam and a 3.2 diff. I suppose it could have a bit more torque below 2500 but that might be the cam or diff - still it's got plenty of go in a clubman estate.

Excellent sounds like she pulls well then. I was after a decent bit of torque low down for getting away at lights and roundabouts etc... if I can get a bit more of that I'd be happy haha.

[Shooter63]

I can tell you a way to pocket the block to run a 12g940 head, but would take absolutely no responsibility if injury happened, but I've done it successfully before.

Shooter

How did you do it mate? Tbf my other 998 engine is pocketed. I did it with a cordless drill and a grinding stone. Can't be any worse than that idea.

Still undecided as to what to do really. Too many options Really. I shouldn't have bought so many heads haha

I used a W225 grinding point in a plunge router, which I converted to use a threaded rod with lock nuts for depth of cut. It's a bit animal but works well, it's main use is to equalise chamber depths if I'm modifying cylinder heads. It took a bit of time but done the job OK. Obviously if you are doing this in situ, tape off all the holes in the block, grease and pack the cylinders. The shoe or what ever its called on the router keeps everything nice and square just leaving you to steer it.

Shooter

[Cooperman]

Our old A-Series engines do thrive on higher compression ratios. I would suggest that you skim the head to achieve around 9.8:1 and if using an Evo001 cam you go for an Aldon Yellow distributor. (That cam is great for mid-range torque and I am just fitting one to a 1293 cc MG Midget).

Before skimming the 295 head, check the depth of the oil way hole and ensure that you have a minimum of 0.050" (fifty thou) remaining depth after skimming. With the larger capacity it's unlikely you will need to skim anywhere near to the minimum in order to get the CR you need. Slightly bigger inlet valves would be good, but not essential.

Yes, check the rocker angles after skimming, but I think that you will not need to shim the rocker pillars, but make sure the rocker pads onto the valves are OK. 

Measure and calculate accurately and all will be fine. Come back on here if you need any further advice.

Some say that you can't rev a 1098 engine, but they are fine up to around 6,000 rpm. I once had an MG 1100 engine  with the old 731 cam in a rally mini and I regularly used over 6000 rpm. It never blew up  .