998 head choice
Turbo Mini Forums

Good morning.

Now I've been speaking to the guy who gonna be rebuilding my cylinder head, now he's telling me I shouldn't use the 12g295 head I have and instead pocket the block and go with a 12g940 as they will flow better and make more power with the turbo.


Anyone have an ideas? Because I'd rather stick with a 295 but I want this engine to be drivable but also be able to use out on track aswell

Thanks Stephen

He's right.

What's your application ? Mild road motor, full race ... ?
For anything other than a balls out 998 the 295 will work absolutely fine.
The 940 will outflow the 295 but at the expense of port velocity. If you do use the 940 retain the stock port sizing.

Phil.

Edited by Turbo Phil on 10th Mar, 2017.

WWW.TURBO-MINI.COM

Gonna be fast Road and track day use

Engine spec so far

266 cam
+40 on a plus 998 block
Balanced wedged xdrilled back drilled and turfined crank shaft
GT1549 turbo
Variable boost controller
Ecu

So just head engine trying to work out

I would Probably stick with the 295 to save stripping the block down to have it pocketed (if its all built up right now)

Block is currently in bits

Having done it, the 940 provided much better performance.

I run a supercharger and I don't care the TB is on the wrong side.
VEMS + 12 PSI + Liquid Intercooler = Small Bore FUN!

Thanks

What was the driveablity like?

Does it rev better?

Was there torque gain? Over the 295

What was the power/torque difference between your engine with the 295 and the 940 Graham ? Do you have a graph for both ?
When I did the 940 for you I left the inlet port almost stock size, with just a good tidy up.

Phil.

WWW.TURBO-MINI.COM

I have a big valve 940 on a 998 metro with metro cam. Standard port twin carb. Its slower than the original till up to 1800 ish rpm then performs very well. Smooth reving etc. Must say compression was 12 to 1 with the 998 head and now its around 10.5 to 1. So can have something to do with that. Never tested it on a bench because... well here there is no one with a bench willing to test a mini

you can do anything if you set your mind to it...
i rather blow it up winning than keep it together losing.

finish date set for project 1... march 2018

My opinion - to get the 12G295 to work for an n/a engine it will need a fair amount of meat taking of the face. Making it useless for anything else. Better to sell that (££££) and put a 12G940 on. Probably work out cost neutral.

I have had a search and some of the graphs are missing for some reason. Anyway it looks like the torque figures are about the same, and its up 22bhp

I run a supercharger and I don't care the TB is on the wrong side.
VEMS + 12 PSI + Liquid Intercooler = Small Bore FUN!

Was it up 22bhp across thw whole range or overall??

Graham's is supercharged so losing a little bottom end from the 940 head wouldn't be noticeable I suspect.
Either head will work, but with the small turbo you intend to use it might run out of steam before the bigger port/valve become of real benefit. I know Graham revs his pretty hard and said the engine was willing to rev to. The 22hp he gained was peak.
If you do use the 940 keep the small 33.3mm inlet and keep the port sizing stock for best effect on the smallbore.

Phil.

WWW.TURBO-MINI.COM

Gonna use the 12g295 head with over sized valves and a little bit of porting then should be good

Thanks for the help guys appreciate it. Always helpful as always

What valves are you going to use in the 295 ?

Phil.

WWW.TURBO-MINI.COM

Gonna use single small collect 1275 valves turned down to fit. 3 angle valve seats. May run 33 29 on sizing though

If using the 29mm exhaust valves you'll need to pocket the block, or recess them and ensure there's adequate clearance from the block face.

Phil.

WWW.TURBO-MINI.COM

The 940 may be better on NA 998s but in this case you wont see any advatage over the 295.

Engine breathing is usually limited by a bottle neck somewhere in the system: intake, head, cam, exhaust, turbo.

Your engine will not be able to breath very well with a GT1549 creating a lot of back pressure. You need to change the turbo first before using the 940 head.

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."

For mine I used a reworked 998 head with 31.75 inlets and 29.5 exhaust valves

Matt W

What head?

Cam4180 12g202 12g206 12g295??

Thanks