Optimal valve size for 47mm head?

[katt]

I am ordered 47mm kit will my old 39mm head be compatible with it?

What's the optimal valve size for 47mm head to uncover full potential of 47mm BBK?

Performance 47mm head - intake 23mm, exhaust 20mm.

vs

Standard 47mm head - intake 18.5mm, exhaust 16mm, will using this give any advantages over old 39mm head?

[humanshield]

As long as the compression is not reduced too much it should result in improved performance.

In order to fit bigger valves, they usually have to increase the squish area and make the head more of a Hemi head to fit the valves in. That lowers compression. I've seen big valves heads that lowered compression too much.

So to answer the question, it "should" increase performance, but "could" reduce performance also.

[fugaziiv]

In our experience, on a smaller head better porting is far more important than valve size.


Matt

[gouvis]

The stock valves (18.5mm in and 16mm ex) are optimal for a 50mm 139qmb that will top at 7.500-8.000rpm. You will need a big valve head if you want to go beyond.
But it is not recommended. Check this application www.gregraven.org/hotwater/calculators/valve-sizing .
I tried different engine from bigger motorcycles to some small cubs and also my scooter and the results where everytime pretty close to the valve size that the manufucturer had chosen. Also a big valve head, as said above, will reduce your compression and this is approved by test that other forum members did in the past. So i would recommend a head that meets you cylinder's bore if you don't want your compression be higher that stock.

[ratdog]

In building real M/C engines, we often found the increasing valve size but keeping stock ports did NOT improve your preformance. I think I may have the same thing with a big valve head on my 50 mm overbore setup. Using a 50mm big bore kit with head with oversized valves but stock ports, my bike is actually slower then a friends bike that we put a big bore kit with a larger chamber but stock valve head. We are both using the A9 cam. Tomarrow I should get the stock valve big bore head in and we will see if it makes any difference. If that proves to be the case, I will break out the tools to port the head on the oversized bore head and let you guys know the results

[fugaziiv]

dexameth can weigh in on this, but on his build, the valve size wasn't as important as big ports and a good cam. His head flow restriction was all in the porting. The valves themselves are far less important.

Matt

[dexameth]

I noticed a power gain in a 52mm setup going FROM a BVH with 23/21 valves and okay ports to a head with 21/19 valves and ports the best I've ever seen from factory. The intake starts at 24mm and smoothly tapers down to the valve. This helps speed the air fuel mixture going in the cylinder. The exhaust port I honestly didnt measure but it was so smooth, no seams from casting anywhere. Huge improvement over the other head.

[katt]

I have not seen any heads come ported, so porting is a manual process you opt-in to do yourself?

[ratdog]

I have seen a couple of “big valve, ported heads” but the price was stupid. Like $300+

There is a lot to porting, and frankly you can hurt your self by getting the ports too big. It all has to do with RPM. Big open ports are great when your running like 9000 rpm and higher, but big ports can really hurt you at low RPMs. Two simple things that help a lot are simply matching the port on the head to the intake manifold. The other area that makes a big difference is around the valve guide

Just taking out any sharp ridges ( like casting lines) can make a great difference. While I do exaust ports real smooth and polished, stopping at about 80 grit is better in the intake ports. The slightly rough port with promote better fuel air mixing. IMHO porting without a multi angle valve job is a waste of time. I have not seen anyone offering a multi angle valve job on GY6 heads. No, it would not be worth my time or trouble to do multi angle valve jobs for other people. Cutting the seats only takes a few seconds, it’s the setup that takes the time. I also don’t have my CBC lathe any more that made cutting the angles on the valves a quick one setup job. Damn I miss having a machine shop!

[dexameth]

All that is true ^^

But by my explanation of "ported from the factory" means that the casting was superb, hardly any sharp edges, and a nice taper from 24mm to 21 at the valve. not just a straight-shot like some, same bore throughout.

I'll upload some pictures of the head here in a few to try to show what I'm talking about.

Edit: Tapatalk keeps failing the uploads... worked fine earlier, dunno what the issue is.

[katt]

Aug 9, 2018 12:51:34 GMT -5 dexameth said:

All that is true ^^

But by my explanation of "ported from the factory" means that the casting was superb, hardly any sharp edges, and a nice taper from 24mm to 21 at the valve. not just a straight-shot like some, same bore throughout.

I'll upload some pictures of the head here in a few to try to show what I'm talking about.

Edit: Tapatalk keeps failing the uploads... worked fine earlier, dunno what the issue is.

Use insert image button on top left of post editing window.

[dexameth]

Aug 9, 2018 18:56:13 GMT -5 katt said:

Aug 9, 2018 12:51:34 GMT -5 dexameth said:

All that is true ^^

But by my explanation of "ported from the factory" means that the casting was superb, hardly any sharp edges, and a nice taper from 24mm to 21 at the valve. not just a straight-shot like some, same bore throughout.

I'll upload some pictures of the head here in a few to try to show what I'm talking about.

Edit: Tapatalk keeps failing the uploads... worked fine earlier, dunno what the issue is.

Use insert image button on top left of post editing window.

On my phone... Where all the pics are, using Tapatalk. I'll do it later if my next attempt doesn't work but thanks!

[dexameth]

Must have been Tapatalk's servers yesterday...
The head in it's entirety:



Not the best pics of the ports, but enough to get the idea.
Intake:



Exhaust:

[magoconnor]

Hi Ratdog, I would like to see your cylinder head, after you had done your work to it.

[ratdog]

If I take it off the bike again, I’ll post pics.  Hopefully that won’t happen, I hate paying for head gaskets!