bigger valves or port nd polish

[Ä‘uro]

can somebdy tell me what would be better or what is better; make both - biger valves nd P&P o just port nd polish or just put bigger valves ?
I spoke with one tunuing expert who told me that bigger valves arn't better every time nd I should do just port nd polish

what's your thinking ?

[hayesey]

hard to say without info on the rest of the engine, the porting/polishing that will be done, what valves will be used etc...

In general from past experience of what people have done here in the UK:

- biggest gain from decent porting
- slight gain from larger exhaust valves
- tiny gain on top of that with larger inlet valves as well

but it's such a vague generalisation that your results may differ massively.

[Jezza-7]

Was just reading this and thought, is it only worth getting the head ported and polished?

[hardchargin40]

I remember chatting to a 'head' bloke in the past who said enlarging the inlet valve wasn't necessary and that exhaust valve max size should go was 2mm larger.

A BVH would make it more top endy (is that a word? ), If not going for out and out power I would choose just a 'normal' port and polish for driveabiliity and power increase throughout.

[breadman]

From what I've read, the Polo head responds really well to proper porting work on standard valve sizes.
As with all these things though, experience is of utmost importance if your gonna get your money's worth. Someone who knows exactly what "works" rather than just opening up the ports and polishing it up to make it look pretty is what matters.
Whilst fitting larger exhaust valves does seem to be the icing on the cake, it's also very expensive. Whether the extra cost is worth that much in real terms can only be proven with a back to back test on a flowbench and ultimately on a dyno.
I'm gonna keep the standard valve sizes in my ported head and spend elsewhere.     

[Jezza-7]

I think that sums it up for me then. Just a good port and polish.

[Ä‘uro]

what do you think about this porting ?




 

[Nick_S]

That looks pretty good, although probably a bit over kill taking that much out at the opening. Stick a gasket over them and you'll see what I mean

[polo classic]

Here's a pic of my big valve 38/33 head. Looks like I have smaller port runners

[dub-disaster]

From a previous post about jezgt's research I think the inlet ports/ runners are oversized already and shouldn't be opened up any more as you loose air velocity.
      Have you run that big valve head those valve seems pretty big didn't know that there was enough space to enlarge both inlet and exhuast without massivley weakong the seats be intresting to see if they crack between the seats.

[Nick_S]

Yep, bigger ports don't always give the best gains. Polo heads are best modified by shaping around the valve guide insert and on the opposite surface to that.

[Yoof]

Auro- without knowing the rest of the spec you can't say too large. They are very big though, and I'd expect the distance between the water jacket and port to be very thin.

Porting quality looks good though- lots of time been spent on that.

Antony- you'll be fortunate to find a polo head that's not cracked between the valve seats as standard.

[Raw]

Following on from Nick's point about port size, I guess alot of good work can be undone by sticking an unmatched gasket and/or manifold on there.

[dub-disaster]

 sticking oversize valves in both inlet and exhaust cant help and already cracked head then no ?? i will not prtened o know anything realy on head design at all just did a bit of research when getting my head done and a few people told me that there wasnt enough space to but 2mm oversize valves in both inlet and exhaust without comprimising longevity of the head due to weakening , i think standard valves are inlet 36 and exhuast 29 ?? so that means less clearance between the two meaning more hance of inlet going out the exhuaston overlap ? and  does this not cause shrouding with the cylinder walls for both valves as heres not a great deal of clearance as it is ?

[Yoof]

When you go over 2mm exhaust, and larger on the inlet too you'd need offset guides.

Frode's head (not his shiney bald one) has offset guides, more money, and more cost if it goes wrong- pay the cost to be the boss 

The above job is also specialist, and I doubt you'd find many companies wanting to setup tooling for a one off.