mini 998cc valve details
Turbo Mini Forums

Hi guys

just trying to figure out the valve seat contact width for the intake and exhaust valves for mini 998cc..>?

so valve seat angle 45 degrees with tapers to optimise flow

valve face 45 degrees with 30 degree back cut

what are the ideal widths?>

thanks

For a turbo, aim for at least 1mm on the exhaust. You could less on inlets but it'll hurt longevity.
You should have the seat cut with a PROPER three angle cutter, or even better, a radius cutter. Ask your head man.

Also, on a 998. You need to use the A+ valves, as A ones are soft as butter.

A back cut of any form will help flow measurably. That said I'd personally go with 0.050" on the inlet and 0.060-0.065" on the exhaust. If your budget allows why not fit the stainless valves, especially on the ex, then all the work is done for you.

Phil.

WWW.TURBO-MINI.COM

+ yup any back-cut seems to work

For the exhaust i don't think it matters all that much if it stays stock.For the intake the valve itself has more effect than the seat. A back cut on the valve is easy and cheap and gives a consistent gain in flow even if you leave the seat as is.

A 998 (202)needs quite a lot of surgery to make it flow really well. But you can do
A few mods that are easy and will
give good gains for a minimum amount of futzing about.

Edited by Sir Yun on 30th Dec, 2016.

That sir, is not rust, it is the progressive mass reduction system

http://aseriesmodifications.wordpress.com/

well russelengineering in australia reckon the 998 stock head can be made to flow better on the road than the 295 head, but I am going to be turbocharging and fuel injecting it so I am not worried about the 998 being a bit crud compared to the 1275, once the mods are done I should have over 100whp which is all I really want for 500kg

well I think I am going to do the seats myself with sioux valve seat grinding stones, i'll do new valve guides, ream and then make custom pilots for the sioux grinder kit. so I am thinking 45 on the contact part of the valve seat then 60 and 30 on the inlet and outlet areas respectively I think. if I can afford new valves I will.

3 angle carbide cutters are very expensive as are full radius cutters so I am going to go with the stones I think - they are more accurate and give better finish with less material loss than standard valve seat cuttters i'll just turn the OD's and angles with a diamond on the lathe

sounds like youve done your homework,

One thing to double check is compression ratio, i'm not particualrly up to speed with small bores, but can you get a large enough chamber in the 998 head rather than the 295?

The chamber is very deep so once you open up the shape to get it to flow You need to take off about 2mm off the head to get the CR up for NA use.

I take out quite a bit off the guide bosses it needs to be done on a 202 especially on the ex. If you do new guides ( they need to be reamed afterwards) i leave the old ones in and grind the port that waynyou have less risk of a burr wandering into the guide hole.

Edited by Sir Yun on 1st Jan, 2017.

That sir, is not rust, it is the progressive mass reduction system

http://aseriesmodifications.wordpress.com/

thanks for the info everyoen
sir yun do you have problems with the old guides not giving a straight cut on the seat? when you hand lap your valves in to check your fit does the band look uneven/wobbly?

yeah I need to do some research into what CR everyone is running with turbo 998s, thinking people are running 8.5:1 or more?

anyone else feel free to throw in on the CR's they are using.