Race Engine Forum UK

Hi Guy!


Happy new Year to You - Nice to see the site up and running again.

I am currently building a 128 1290cc race engine for an "amateur" race series - featuring old italian cars.

Engine specs:
128 1290cc engine
Compression 10:1
Cam steel billet 284`/ 10.5mm true lift / 3.5 mm TDC / 106` Lobe separation
2 * 40 DHLA - 32 chokes
4-2-1 exhaust manifold
32 mm exhaust valves
36.2 mm intake valves
Intake port taken from 25.5 mm to 32 at port face tapering down to 28 at port venturi. Material removed from port sides and roof only at venturi. SSR rounded off and Valve "bowl" enlarged.
Exhaust port now 27.5mm from valve throat and out.

Intake valve seat internal diameter 30.0 mm
Exhaust valve seat internal diameter 27.5 mm

A valve size of 36.2 mm, plus an increased port size, and bigger cam makes me consider to grind out the inlet valve seat ID to 32.5 mm. This to aid flow at higher valve lifts. For the exhaust port (32mm valve) I consider to bore out the seats to 29 mm and follow that diameter all the way out.

Is this a good idea?

Any other considerations regarding this engine greatly appreciated.


Best regards

Remi Lovhoiden.


Some pics of car and porting work progress:



Car now stripped ready for a little paint, fitment of Lexan windows, and a roll cage




Marking out initial port face size with felt pen using old 128 exhaust valve.




Intake ports roughed out to approximate sizes - now for finishong off with sanding rolls




Just moved to new garage some bits still missing. 5-56 and a carbide burr goes a long way. 128 exhaust valve (31mm), A112 Abarth intake valve (29mm) and coin (27mm) makes excellent measuring tools...... DHLA awaiting rebuild in background




Enlarging the valve "bowl" area and short side radius




Intake ports more or less ready.. no work to exhaust port when picture taken




DHLA manifolds ported out to 31.5mm to match to head face

Hi Remi,

Nice post, that looks good to me, I have drawn it in cad to assess the layout, see dwg below.

Now, ordinarily it's not necessary to do that, I'd be aiming for 1.5mm contact face width on inlet and ex, I'd put a wide top grind of 30 deg on the seat insert first, going right into the ally and recessing the top of the seat about 1mm down (this works very well won the SOHC head with a 30 deg back grind on the valve) then I'd grind out to true inlet valve diameter at 45 deg, dia 36.2, and narrow the contact face down with a 70 deg throat grind, leaving about half the insert parallel leading into the throat. As you can see your throat dim of 32.5mm is spot-on. Of course you'll never achieve the Olympian standards of accuracy that cad can, but done the way you propose the key features will be fine. It would be a good idea to tidy up the combustion chamber at 120 grit before doing the seat angles because all of them incl the valve angles and the 30deg top grind where it runs into the alloy of the chamber, must be sharp edged. If you radius them it will reduce the flow.

The 30 deg top grind with valve back grind reduces the turbulence thru the curtain area and nearly always (on sidedraft type heads like this) gives much improved flow at lower valve lifts.

The idea for the ex seat, well, you can check your dims on this, you want a plain 45 deg seat 1.5mm wide and into the throat you can use 70, 75 deg or parallel. The latter of course needs boring to get it right and of course gives the biggest throat area and you don't need to worry about breakaway and turbulence on the ex side. Does the throat need to be, say, 29mm? Well it depends where the controlling section is. If the throat outflows the port the port is too small. No point having a huge throat and a small port if you follow my drift.. Have a think about that.

I should point out, however, as I have done in my new DVD (see GC V/W) that a parallel throat is bad practice form one perspective - you don't have anything in hand when you come to replace guides and redress the seat. From that point of view a tapered throat is practically speaking - a much better bet. If the seat has gone out of shape and the new guide centre is 'way off' you'll find yourself replacing the seat inserts - but even then, there won't be much alloy section left to do it in. I've had to use big valves in some cases where that's been done, just to generate a sensible contact face width. Too small and the ex seat/valve combo will degrade fast.

Hi Guy.


Thanks for an excellent answer. Now back to working on the engine!

Further progress with the car and engine will be posted on this site - with pictures.


Best regards

Remi Lovhoiden