Another 998 compression question
Turbo Mini Forums

I've used the search feature and have not found an instances where someone is running a low compression in a 998. We are about to build/bodge a 998 turbo for an endurance "race" and are looking for some wisdom.

STD A+ dished pistons, pocket block, and a cleaned up 12G940 head is giving us 7.8 or so compression (+/- .5). Initial plan (need to keep costs down to skirt spending limit rule) is to just run a MG metro turbo manifold, turbo, and carb/plenum/intake setup. It's got some other blowoff valve on it, and I think a team mate knows how to adjust boost.

I think we have plans to potentially fab up an intercooler and we've also go access to high octane race fuel during the event.

Our N/A engine we are replacing put out 52 hp at the wheels and we just want a little more than that with an er on the side of reliability over outright hp bragging rights.

For cam we're trying to find an MG metro grind, but I can get a 256/256 with .324/.324 lift or a 256/264 with .324/.318 lift cams (lift assumes 1.22 rockers) locally for cheap.

Is the compression simply too low for a 80-ish hp, predictable temperament road racing motor? Any other bits of advice with regards to the combination of parts we are using? Boost suggestions to start off with?

I'm in the US, so parts are not readily available. I can, however, substitute a 1.3 intake for the 1.4 intake 12G940 head and I can possibly get a, unskimmed 12G295 head to use, but I think that would lower our compression even more!

Sorry for the long post and thanks for any help that's offered. (And if there are specific posts that I've overlooked while using the search feature and that you think I should read, please point me to them using a link)

I've never seen A+ dished pistons on a 998 and I don't believe that they were ever fitted as standard on the A+.

If you only want 80bhp, then 9.5:1 with a good intercooler and about 7 psi boost will do the job.

Choice of cam is good, but i'm not sure that you need to use a 12G940 head. The unskimmed 12G295 will do the buisness.

I'm running an unskimmed 12G295 with flat topped pistons, 12 psi boost, good intercooler and a T2. It's putting out around 120 bhp now that I've stopped the rev limiter coming in before it reached the power peak.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Metro Van low comp 998 had dished pistons. I have a damaged example of a dished 998 piston.

Yep, A+ 998 came with dished pistons in some forms, all 6cc. I've see both slipper and fullskirt type.

I can't see that the CR you have calculated is right though. Typically pockets are 1cc, but the 12G940 head is 21.4cc (compared to 24cc in the 998 head). My calculations would suggest that you'd have more like 9:1 CR.

Which as Paul points out is ideal for what you want, especially as the T3 takes some spinning up.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

www.twin-turbo.co.uk

If you're using a lot of boost, 8 or 8.5:1 is better.

If you need reliability, fit uprated/forged pistons, standard A+ pistons will break-up/melt. (I ran a N/A MG Metro engine with std pistons for a few years & a chunk of a piston crown disappeared!)

Fit uprated Mains & Big-end shells.

Fit turbo head-gasket & manifold gasket. Torque head-nuts (oiled) to 55lbf ft

Fit turbo oil-pump.

Fit Grey clutch cover & AP race clutch plate (if you're using pre-Verto clutch.) If you're using Verto clutch, use Turbo clutch cover at least, or use RTS (Roberts Twin Spring) Verto cover.

Fit lightened flywheel.

Use Millers Classic Mini 20W/50 oil or similar which is formulated for engine/box/diff combination .

www.minispares.com for all the above bits + advice.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

my thoughts exactly paul

however, can i add that you can get modified turbo oil pumps to fit smallbores from PaulH on here

"Turbo's make torque, and torque makes fun"

"did you know you can toast potato waffles?"

Thanks for the feedback. You have me questioning my compression calcs now, too. I did it several months ago and because the compression factored out so low we ended up building another block using flat tops. Using _.030 flat tops and the same head I had initially cc'd for the calcs for the turbo 998 we've ended up with like 11:1, Hmmm. I'll have to re-measure.

RE: Big bore head-- it's what I've got already and 12G295 heads are tough to find. And not that I can speak for lawrence, but perhaps because I mentioned I was going to use a big bore head that he advised I used a turbo gasket (??). Would be some odd machining for those rings in a 998 block, no?

RE: Turbo pump-- I was just going to make up a spacer plate to fit the 1275 metro turbo pump in it. but the turbo pump is thicker, so may not fit inside of the flywheel housing. Hmmm. The other thought was push the pump pin in further, cut off the excess that fits on the inside, and file off the support snout on the inboard face of the pump. Wow, that types out pretty easy! Haven't looked to see if it's can be done like that, though.

If I can't find a metro cam, do either of those others sound ok? For the turbo app, do I want dual pattern cam or the identical inlet/exhaust?

Thanks again.

WWW.TURBO-MINI.COM

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

www.twin-turbo.co.uk

Rusty by nature

Sorry to hi jack this post, but I have some questions regarding this topic and rather than flood the forum I thought I would "recycle" an old thread!

I have a HC 998 engine (10.5:1?) and want to lower the CR down enough (do I even have to reduce it???!) run boost. Now If you retard the ignition doesn't this have the same effect as lowering the CR? I know I'll loose some power, but will it be that much? As long as I run an intercooler to keep to inlet air nice and cool not that be enough? I'm not after more than 100bhp. Thanks muchly

Retarding the ignition will not reduce compression. All this does is change the time at which the spark happens.
To reduce compression either fit dished pistons or a more suitable cylinder head.
If you dont do this you run the risk of automatically reducing compression by blowing up a piston.

Get yourself a 12G202 or a 12G295, you will lower your CR and get the added bonus of the bigger valves.

12G202 should be reworked thought ...

I found this post useful :
http://www.turbominis.co.uk/forums/index.p...6504&lastpost=1

Rusty by nature

Revisiting this thread and I guess would like someone to check my math: We're still looking to do the budget build using existing parts which is basically a +.020 998 with a metro turbo setup (carb and manifold). Standard boost likely.

Spec I've measured out:
Bore measured 2.563"
Stroke (assumed) 3"
ring land .25"
piston diameter @ top 2.54"
piston to deck height .020"
dish 5.5cc (measured)
head chamber (measured) 21.4-21.2
pockets (measured) .6cc
stock 1275 copper head gasket

Yes, we're using a 1275 head --12G1316-- and thoughts are to just a stock 1275 camshaft (non metro and non economy). I was reading in vizard that duration / overlap was an enemy of turbos (but what do I know?)

Anyway, I'm getting at about 9:1, even with the shallow chambered 1275 head.

I must have missed something, I still don't understand why are you trying to use the g240 head ? If you are looking for reliability then I don't think pockets are better than a std g295 head (I bought two of them in the US 2 years ago, $75 each, from Healey engines). Pockets are a potential source of ping as they present sharp edges everywhere.
You don't need neither big valves nor weird cams to get 80hp at crank, just increase boost a little bit and get yourself a precise ignition system. MD266 will do well.
CR is meaningful only when the cam is also known. These engines have a bad VE so you'll usually need a higher CR to get decent power.
If you don't look for huge power then spend more time on the turbo selection. With a small turbo you can get lots of torque at low RPM.

std 998 A+, g295, MD266, RHF4, 109hp @0.8bar/5400rpm

Main reason is I need to keep it below $500 total cost (including car purchase) and am trying to use existing parts-- stuff I already have stashed. I hear you on the edges/hotspots/detonation worries, for sure. Head rebuild and skimming (assuming no new parts and not including purchase price of the head) will run $185 US minimum. I have a good 12G1316 head now and compression will be around 9:1 as I've figured it (with the above supplied info). And I've already got the metro setup with the T3 turbo so can't go about sourcing another at this first bodge... er... I mean "try".

A 12G295 concerns me for a couple of reasons-- crack-prone is one of them. This car will be run for up to 8 hours non-stop in an endurance race for budget built cars and then be asked to perform for another 8 hours a second day in a row.

All things considered, the 12G1316 is the EXACT SAME head as the 12G295--ports are identical in size. Difference is the intake valves are .010" larger and exhaust are .015 larger. I figure (with no proof to back it up) that the exhaust valve shrouding as a result of the pockets is offset by the larger exhaust valve. I've done the 1275 head on 998 several times, sometime with pockets and sometimes without, depending on cam lift and how fresh/sunken the valves seats are. By far the best return for investment (time and $) I've experienced. I've got side-by-side comparison pics:




Again, I hear your concerns about the hot spots and will take note of this. Other concern is the tiny bit of head gasket that's between 2 &3 using a 1275 gasket.

In short, It's all just an experiment, and it may very well likely fail-- but if it fails, might be nice for it to fail bloody spectacularly.

Our revs will be spent mostly at 3500-5000 rpms for the duration of the event.

BTW: We've been running NA 998 motor with a Metro 12G940 head (larger intakes), 274 cam and about 11:1 compression using an HIF6 and metro inlet, lcb exhaust with reasonable success. It lasted 3 events plus some testing for a total of about 70 hours before it broke a spring retainer and took out 1 piston. (that motor is back together with 1 piston at +.010 larger than the other 3 and runs great again).

this would be my first attempt at running anything turbo, aside from the stock metro turbo motor that came with the setup we are to use-- and it kept blowing head gaskets.

I see.
Head gasket is another concern. It's still not clear if new style composite gaskets are good for turbo. Copper seem to stand if boost is not too high (under 10psi). People will correct if I'm wrong.
I think for endurance it's better to have lower cam lift, this would also avoid pockets.
I don't know what's the reason of head failures but if it's the temperature then you may work on a better (and better balanced between cylinders) cooling.

std 998 A+, g295, MD266, RHF4, 109hp @0.8bar/5400rpm