25 replies to this topic

[Pauljb]

Hi guys and girls. I'm having my cylinder head polished and gas flowed for better performance. What's the ideal valve sizes I should be looking at getting, or keep them standard? It's for a 1098 by the way. Anything else I should look at doing to the head (uprating parts wise).
Thanks

[nicklouse]

you dont want the ports polishing.

 

what casting?

[Pauljb]

Can't remember off top of my head. Why not polished ports? Just properly gas flowed ports then?
Cheers

[MIGLIACARS]

The ruff grinding on the ports help the air and fuel to mix better.

 

The 295  or 206 heads are the ones to have modified, the 202 or the cam 4810 have smaller valves so not much gain.

You will prob need the head skimming to raise the c/r depending on the pistons you have.

 

In all honesty a 1275cc lump will be the best power for the £££  The 1100cc isnt the best lump to modify. But some did come with the 295 head. A better cam will see more power output.

[Cooperman]

With a 1098 engine you are better off seeking more mid-range torque rather than out-and-out top end power. The 1098 will not rev like a 998 or a 1275. You will need to change the cam to get the best results, but something like a 266 or Evo001 might suit well. There are other options you can consider, but avoid any cam which makes peak power at anything over about 5600 rpm as you don't want to rev a 1098 to anything much over 6000 rpm.

 

For the modifications to the head, you don't want or need to actually polish the inlet tracts. What should be done is to remove most of the valve guide bosses, clean up the short-side radius into the inlet valve, open up the exhaust tracts to reduce 'pumping losses' and, most important, grind out around the valves to de-shroud them and enable the gases to flow in and out more easily.

 

After doing that you MUST measure the sizes of the combustion chambers, calculate the size you need to get the compression ratio to what you want (probably about 9.75:1 for a 1098) then get the head skimmed to suit. 

 

As for valve sizes this will be determined by which head casting you have, what valve sizes are available 'off the shelf' and whether you are prepared to have larger 1275 valves machined down to suit if the optimum sizes are not available.

 

Have a read of David Vizard's book ''How to Modify the A-Series Engine'. It explains in detail all the things mentioned by me and others on this thread.

[timmy850]

I had my 202 head modified to suit a 1098 engine build. You might not need to get as much work done depending on the spec you are aiming for. 

(27.2mm / 1.07" exhaust valves, 31.75mm / 1.25" inlet valves, Unleaded exhaust seats, valve guides, Combustion chamber & port work)

 

Cavler ST have some small bore head combinations listed on their website. You can see the valve sizes available and what kind of work is done to them. 

http://www.calverst..../cylinder-head/

[Pauljb]

Thanks guys. I know 1098 are less powerful but more torquey than other engines. It needs reboring so thats why I wad gonna have the head done at same time (as it'll all be stripped down). Was possibly looking at a Kent cam and roller rockers as well but if there more for 998 then so be it.

[Pauljb]

It's a cam4810 head by the way. Just looked

[Pauljb]

And how does urs run timmy? More pull or more power?

[timmy850]

It's currently on the shelf as I've got the engine and gearbox reassembly to do first. It will be much better than the tired old 998 in the car now. I'm aiming for around 70hp 

[Pauljb]

Cool. I've got a 202 head at in my garage as well. Was just gonna sell that as standard as the 1098 has the 4810 head on it.

[timmy850]

Over here the 1100 engine had the 202 head standard. It can be a better starting point for modifications as the valves and ports are slightly bigger

[Turbo Phil]

The 202 can be made to work well and was the standard head for the 1100. It saves quite a bit of work when porting, rather than using the 4810, as the ports are already larger and better shaped. It also features a larger inlet valve over the stock 998 head, though most of the benefit of this is lost due to the incredibly shrouded chamber design. Removing this shrouding and shaping the chamber is the number one piority when working these heads.
Regards
Phil.

[Pauljb]

Through skimming?

[Turbo Phil]

Through skimming?

No. The chamber needs to be shaped/cut back by hand (or CNC if you're rich) to move the chamber walls away from the valve to reduce the shrouding effect.

Phil.