bikecarbfred
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| posted on 24/9/15 at 09:19 PM |
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Valve Job - Size Increase
Guys, see picture. What max size increase you would get out of this. 1, 2 , 2.5mm?
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snapper
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| posted on 25/9/15 at 06:00 AM |
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You done have much room around the plug so about the current valve seat width, open the port throat by the same and blend in.
I would unshroud the valves where they are close to the chamber sides, measure chamber volume and skim to return compression to standard or above then
look at back cut valves with waisted stems.
3 angle valve seats.
Knife edge the inlet port walls between ports at the manifold face
On most engines you only need to make the inlet valves bigger unless going full race or turbo
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I breath to pi55 my ex wife off (and now my ex partner)
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chillis
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| posted on 26/9/15 at 11:14 AM |
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doubt you'll anything bigger than =1mm, which will not yeald any great improvement in air flow and would poor in terms of hp per pound spent - I
would look at other ways to increase power that represent better value for money
Never under estimate the ingenuity of an idiot!
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bikecarbfred
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| posted on 26/9/15 at 11:18 AM |
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1mm oversize valve with porting around the pocket with polished exhaust port not going to get me 7-10bhp?
Already have schrick cams, carb setup on megajolt, 4-2-1, n sport exhaust system
Doing a rebuild with performance rods, standard pistons, new valves etc can't think of anything else. pitty
[Edited on 26/9/15 by bikecarbfred]
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Griff51
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| posted on 26/9/15 at 07:00 PM |
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Depends what you want to do with it, (what motor is it)?
Just looking there is room for improvement in those ports and you could get a slightly bigger valve in there.
"Good" head work is expensive, if it's for a full race motor, every little helps. Otherwise does the cost warrant it?
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bikecarbfred
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| posted on 26/9/15 at 09:42 PM |
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been quoted £250 for for all 16 valve ports from a rep* machine shop.
how would you improve it.
The engine is AFH 1.4 16v: N/A with CBR600 front feed induction box
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tegwin
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| posted on 27/9/15 at 09:53 AM |
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Theres no substitute for cylinder volume.... ditch the 1.4 and get something bigger if you want more power.... By the time you have fettled with
the 1.4 you will have spent a fortune!
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Griff51
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| posted on 27/9/15 at 07:48 PM |
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£250 is very cheap, just as an example the race heads we use for an 8 valve four pot, start at 1500 quid!!!
You can see the cut marks in the back of the pocket/bowl where the cutter has passed through, that needs to go
and can't see the short turn, so am guessing that could do with a tweak, if it's a street motor will you notice?
I doubt it, but all those little tweaks do add up.
Sort of agree with Tegwin, but if that's what motor you are using fair enough, but do remember generally if bigger valves
are fitted unless you are very clever, flow goes up, velocity goes down. So you end up with a powerful but gutless rev monster.
either way good luck and have fun (lots of info out there on various Polo forums)
Griff.
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bikecarbfred
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| posted on 27/9/15 at 08:13 PM |
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If velocity is going to go down with bigger valves whats the blummin point. why people do it lol
just for turbo or supercharge setups ?
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mcerd1
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| posted on 27/9/15 at 09:12 PM |
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quote:
Originally posted by bikecarbfred
If velocity is going to go down with bigger valves whats the blummin point. why people do it lol
just for turbo or supercharge setups ?
Unless the stock valves are undersized compared to the potential of all the other main components it won't be worth it...
With forced induction setups your changing things enough that it might need bigger valves to get the last few hp out of it, but it would probably need
to be a fairly extreme setup for the small increase that you'd be able squeeze into that head...
As a rule any engine is only as good as its weakest part, but if the rest of the parts around it are over spec'd it can make it worse rather
than better 
Lots of people fall into the trap of doing the wrong work because 'bigger must be better' but that's not always the case....
I don't know your engine so I can't comment on any specific mods...
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Griff51
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| posted on 27/9/15 at 09:22 PM |
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The trick is matching what your engine needs, with careful porting you can increase flow whilst maintaining velocity, or
if you are very good increase both. Most modern heads flow ok but are heavily restricted with the manifolding, hence put ITB's
on an otherwise stock motor and see a large increase in power. As with modifying anything it's always "how fast can you afford to
go"! And there is generally a trade off somewhere.
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bikecarbfred
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| posted on 27/9/15 at 09:48 PM |
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"hence put ITB's
on an otherwise stock motor"
That's why i run carb setup because it gave the engine more air and fuel.
I am seriusly considering not doing the valve porting after this discussion.
I have a re-build i am doing with all new parts including, pistons, rings, valves, guides, stem & h-beam rods etc. Just thought while everything
was in bits I could get the best flow through the head with a bigger schrick valves etc but suppose i will just have to buy oem valves.
infact one of my friend members is sending me an AFH head for me to practise porting.
This means I will have two AFH heads. Is there a way to do flow tests without valves in and just block of the chamber.
then i can compare. how would i got about this please?
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ettore bugatti
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| posted on 27/9/15 at 11:35 PM |
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Here is a pdf about building your own flow bench, looks like fun.
http://www.dtec.net.au/Tech%20Articles/DIY%20Flowbench%20Design.pdf
I don't think it is a bad idea to clean up the head, but extra gained flow really starts to pay off with higher valve lifts.
Also with bigger ports you may end up going through water ways, if unlucky.
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Griff51
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| posted on 28/9/15 at 04:33 PM |
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If Schrick or any other recognised VW specialists do replacement or oversize valves, go for it. Will probably have a better profile and/or be lighter
than stock, with all the other work you are doing no point in not going the whole hog with the head.
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bikecarbfred
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| posted on 29/9/15 at 07:16 PM |
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Exactly that's my thinking.
But schrick oversize valves need their length cutting and a groove putting in.
Blank valves.
Need to build my self a diy benchflow (need to know how to do that aswell) and use the spare head to learn more about flow after porting.
edit: thanks ettore
[Edited on 29/9/15 by bikecarbfred]
[Edited on 29/9/15 by bikecarbfred]
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bikecarbfred
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| posted on 5/10/15 at 07:44 PM |
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@ ettore. I'm building one! thanks. 
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snapper
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| posted on 5/10/15 at 07:58 PM |
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First of the head worker is any good then get a before and after flow reading
It's all about the little gains so don't just obses about valve size or head work
I said in an earlier post about cable heads cut back to a penny on a stick with waisted stems, worth more in flow than many are aware
Matching manifolds to ports for a smooth flow
You can get good flow increase by those jobs that don't seem sexy and you can't brag about in the pub but for real engine builders these
things are the edge they get above the mediocre
I eat to survive
I drink to forget
I breath to pi55 my ex wife off (and now my ex partner)
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