12 replies to this topic

[Mr Dave]

Hello all,

 

Just started my 998 engine build and I'm a bit unsure with options for cylinder head, so hoping you clever people can give me some advice. 

 

As a bit of background, my spec so far is A+ 998, rebored +20 and pocketed and Kent 246 cam.

 

Compression Ratio

 

Having done a bit of reading on the forum and in the "bible", I am thinking a CR of 10:1 should be about right, but please let me know if there are any thoughts on this.

 

For the sake of the subsequent questions, I have used CR 10:1 and calculated the head chambers I need below:

 

Bore = 65.1mm

Stroke = 76.2mm

Swept vol = 253.6 cc

Target CR = 10:1

---->     Target clearance vol = 28.2 cc

 

Above piston vol (including pocket) = 1.9 cc

Gasket vol = 2.8 cc

---->     Target head chamber vol = 23.5 cc

 

Cylinder Head

 

So with the above in mind, I am trying to work out which combination of casting and modification I should go for.

 

The original head is an unmodified CAM4180 - nominal chamber vol on these (according to Calver) is 25.5cc, so would need a skim to hit my CR target even without any chamber shaping. As aforementioned, block is pocketed so 12G940 would seem like the best alternative (21.4cc nominal vol) and there seem to be a lot available, although this would effectively add cost over reworking the head I have.

 

Questions

 

- Is there a significant difference between fully reworked/ported/gas flowed CAM4180 annd 12G940 heads

- Could a CAM4180 head be skimmed enough to hit this CR (i.e without making it fragile)?

- Are there other castings worth considering?

- Are there any concerns with using large valves on 998s (i.e tumble/swirl/mixture prep)?

- Which companies / packages would you recommend?

 

Looking forward to your replies!

 

 

 

 

[timmy850]

It all depends on how much money you want to spend...

 

The standard 998 heads have the most amount of metal in them, and can be shaped to make the optimum flow and port velocity. The downside to grinding heads is the cost goes up..

 

The 1275 heads have good performance without having to do any work. They have larger ports than small bore heads, which flow better but don't necessarily have the same velocity. 

 

There are a few articles on this site worth a read.

http://russellengine...cylinder-heads/

 

[fwdracer]

If the block is already pocketed, this is a no brainer. Use a 12G940 1275CC Head. If you use a standard head with 33mm inlets it'll be fine. A light d.i.y clean up of the casting (blend in the short side radius of inlet and exhaust ports), use some good quality new valves and properly cut 3-angle seats will compare very favourably versus a fully worked 998cc CAM4180. In labour terms, It'll work out far cheaper!

Edited by fwdracer, 12 November 2015 - 11:11 AM.

[Mr Dave]

Hi Timmy

 

Thanks for the reply. They are indeed very interesting articles. Backs up my suspicion that flow velocity is a concern on the bigger heads/valves. It's a shame I'm 10,000 miles away otherwise I'd be knocking on his door!

 

As far as stuff in the UK goes, have had a quick trawl of the "off-the-shelf" packages and come up with these

 

https://www.minispar...px|Back to shop

 

http://www.minisport...inder-head.html

 

http://www.minispeed...ead-8509981100/

 

The Minispares one seems like a good package (and strangely has exactly the desired chamber vol) and is at the top end of my price range. Not a lot of info about the others, although the minispeed one is considerably cheaper for some reason. Anyone with experience of these?

 

Alternative would be go direct to machinist. Rob Walker would probably be my default as he did my block but open to suggestions/recommendations

 

[Mr Dave]

Hi fwdracer,

Am I right in thinking then that a standard-ish 940 head will be as good as a worked 4810, but additional work in the 940 won't improve it much further on a 998? Or is it just a question of the valve sizes? Just trying to understand how the balance between flow volume and flow velocity works.

If I go with 940 I will definitely need chamber work for a workable CR (although I guess there are big gains to be had here on 4810s so maybe that is the same either way). Is this a reasonable diy job or best left to pros?

[fwdracer]

I'm another advocate of not overdoing the amount of work on a head for road engine usage and would recommend a focus on torque over outright HP. The Graham Russell small bore tuning article is brilliant, he is a gifted and methodical engineer, but the really exquistitly worked small bore head he references in his article has so many man hours of work to achieve the flow rate and port velocity that I doubt many would choose to afford it.

 

Back to your application - Too much work on a 940 head simply won't benefit a 998cc motor unless it is winding big rpm eg. (8K+). For torque keep the port velocities up by not touching the standard 940 inlet and exhaust ports (the holes). Work the valve throat area only, a good generous short side radius and general cleaning up/lightly blending in the valve throat. Grind back the valve guide boss and install some bullet nose valve guides in a better material. All porting can all be done DIY-if you are comfortable/confident with a grinder. Dremels are surprisingly effective. Practice on an old scrap head if necessary (940 are ten a penny). Do not install the 35.7mm valves (Bigger 'S/MG Metro size valves), keep the standard 33mm. The results will be very acceptable versus the amount of time (money) required to produce a fully worked CAM4810 head.

 

For achieving the CR, don't touch the chamber shape, just head skim to achieve the correct Chamber vol, fit new valve/guides... get this done at a reputable engineering firm. I'd recommend Rob Walker, nice drive into the Cotswalds from Northampton...

 

What camshaft are you going to run? The above would work on anything that will peak around 6K. Beyond that you may/will need a bit more porting work.

Edited by fwdracer, 13 November 2015 - 10:25 AM.

[Mr Dave]

I will need to change the chamber shape rather than skim because the 940 volume is smaller than what I need - Calver quotes the 940 volume at 21.4 which would be around 11:1 CR on my engine - this would be too high?

 

I have gone for the Kent 246 cam, mainly for the reasons you have said, as I'm chasing torque over power at peaking no higher than 6K.

Edited by Mr Dave, 13 November 2015 - 11:34 AM.

[Mr Dave]

If I change valve guides, do the seats need to be recut to make them concentric?

[Dusky]

If I change valve guides, do the seats need to be recut to make them concentric?

yes

[fwdracer]

11:1 CR is too high for a road engine running on 95 Octane pump juice. With a short timed cam avoid much more than 10:1.

 

You can increase the unswept volume with a thicker head gasket - Have a check as compressed volumes differ markedly (you quote 2.9cc which is  on the smaller side for h/gasket cc value). The older Copper AF460 type is 3.4cc compressed (from memory) - New BK450 is 4.0cc...

 

Failing that a small amount should be removed from the chamber. Remember to balance and burette all 4.

 

In your CR calc's - Include a nominal 0.2cc for the volume above the top of the ring land.

 

As others have stated - if you are installing new guides, valve seats will need to be recut. Nice 3 angle seats. Do this after you've done the port throat work

 

[Mr Dave]

Sounds like a plan. Thanks.

 

If I have a 1275 head and a 998 block, do I use a 998 or 1275 head gasket?

[PhilipGCaldwell]

Bk450 gasket

[fwdracer]

As above - 1275 BK450 Gasket