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Author Topic: Big 16v valves  (Read 416 times)

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Offline Robin_16v

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Big 16v valves
« on: July 10, 2012, 04:54:05 pm »
Hi folks,

Ok who has tried big valves in a 16v on a std bore of 82,5 mm, i was thinking of 34mm inlet valves and 29mm exhaust valves or so, if it's fits...

What should be better, kr or abf cilinderhead. Maybe the kr is better, because of the abf waterways?? no risk of breaking into?

Robin

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Offline Tim Moll

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Re: Big 16v valves
« Reply #1 on: July 10, 2012, 06:27:30 pm »
Too many valves for me Robin...lol

Offline Robin_16v

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Re: Big 16v valves
« Reply #2 on: July 10, 2012, 09:42:55 pm »
Hahaha, no comment  ;D

Offline JMR

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Re: Big 16v valves
« Reply #3 on: July 10, 2012, 09:47:31 pm »
I think Rob t on here has big valve's in his...in fact fairly sure...but dont know the size...he may spill the beans if we ask nicely...tho totally understand if he's rather not.

The problem with valve's that size Robin is gunna be the cost...yr talking around 600 minimum to start with for valve's & inserts, plus the porting work unless you were gunna have another go yrself.

You can get well past 200hp on std valves tho Robin, so generally big valve's are used on ultimate spec engines.

We will probably be doing a big valve 16v for Paul's hillclimb MK1 in 1800 form tho so will pop up the flow detail's as progress is made.

Offline Stooza

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Re: Big 16v valves
« Reply #4 on: July 10, 2012, 10:02:07 pm »
Is there much power to be had from this mod? I always thought the valvers had great breathing capabilities... but I am here to learn :)
Mk2 2.0 16v GTi (built for track day fun)
Mk3 16v GTi (standard)
1970 1915cc type 1 beetle (rebuild on hold)

Offline JMR

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Re: Big 16v valves
« Reply #5 on: July 11, 2012, 12:25:18 am »
They're fairly small by many 16v standards...Vauxhall XE 33mm, Peugeot 306 GTi-6 34mm, I think the modern Ford Duratec is 35mm as standard.

Bigger valves will always work better, it depends on the level of power or performance your looking for v's the cost involved...std valves work well in most normal applications tho.

Offline Stooza

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Re: Big 16v valves
« Reply #6 on: July 11, 2012, 02:59:15 pm »
Fair enough, is there such a thing as too much airflow tho? I guess if the flow outweighs the fuel in it could be a problem, but what about on the exhaust side?
Mk2 2.0 16v GTi (built for track day fun)
Mk3 16v GTi (standard)
1970 1915cc type 1 beetle (rebuild on hold)

Offline JMR

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Re: Big 16v valves
« Reply #7 on: July 11, 2012, 09:09:21 pm »
Nope...introducing the correct fuel ratio is easy to go with whatever amount of air is flowing through. .either in carb or injection form.

Exhaust flow is an odd one to explain. Within a certain window a given exhaust flow ratio works well. Under running conditions, the engine spec can have a big influence.

At some point, as things grow here, I'll try and put some basic explanations together for flow ratios & targets, how engine spec can effect valve ratios needed, carb & fuel injection sizing etc.

Offline Stooza

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Re: Big 16v valves
« Reply #8 on: July 11, 2012, 11:12:21 pm »
That would be awesome matey. I won't pull this thread any further off topic.. sorry about that hehe
Mk2 2.0 16v GTi (built for track day fun)
Mk3 16v GTi (standard)
1970 1915cc type 1 beetle (rebuild on hold)

Offline JMR

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Re: Big 16v valves
« Reply #9 on: July 12, 2012, 11:59:10 am »
No sweat Stu...I think it's all fairly related to Robin's post.

Big valve will in general give more flow potential up to the point that no more air will pass through the induction system or the port itself. Plus as the direction Robin is thinking above, bore shrouding has to be taken into consideration. The more air, and fuel mix, you can introduce into the cylinder the better. Big valve's are not just a top end thing either...the bigger the opening area (the curtain area) as soon as the valve starts to lift off the seat, the more potential cylinder filling can happen earlier on on the cycle...so that happens at all rev's.

Likewise more flow does'nt always make more power...if there's a combustion issue then introducing more fuel/air mix is'nt always the answer. The other effect can be in the engine spec...a very wild engine can have enough/too much, again for the combustion process available...I've seen full race engines make little to no extra power with mega high flowing heads over very good ones, yet when the cam spec is brought back (a tamer cam profile) the peak power stays but the mid range comes up!...where dropping the cam back before would have lost power everywhere!..Ford Pinto.

Big valve heads may also not improve peak power numbers, or even low/mid range power, but can allow the power band to be extended to higher usable rpm where before it was falling away early...I've seen that on race/hillclimb spec 8v Golf engine's.

It's like the whole engine power/torque curve/spread...it's very much horses for courses, and every engine type/make & spec can vary.

Offline RobT

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Re: Big 16v valves
« Reply #10 on: July 13, 2012, 12:24:27 pm »
Mine are 1mm bigger in and out, no point going bigger until i get a 90mm crank and can fit bigger pistons to stay within class cc limit - shrouding like has been said.
2008 Audi A6 2.0TDI S-Line
2004 Audi A4 Avant 1.8T Quattro S-Line
1996 Seat Ibiza 16V F1.5 Race car

Offline JMR

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Re: Big 16v valves
« Reply #11 on: July 13, 2012, 09:26:45 pm »
Ah thanks Rob. My machinist is fairly confident an ABF will take an 84mm bore ok...we'll try one at some point...not looked at any of the short block 2ltrs.

Offline RobT

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Re: Big 16v valves
« Reply #12 on: July 13, 2012, 09:56:44 pm »
84mm is fine on an atmo car, but it pushes you over the 2000cc limit on a 92.8 std crank. You wouldnt want to use 84mm on a boosted motor though, unless using block braces. 94mm cranks are available which strike an interesting compromise for a bit more torque, but if cc limited for racing, mean a big piston cannot be used, and subsequently smaller valves. Gearbox is what tips the choice. Decent box means more revs can be used, shorter stroke, more power. Less optimal box, better with a longer stroke.

Are currentlt weighing up a massively destroked 1420 turbo, using a 1.8 shortblock as base. Oh...and a 8 bar fuel system.....
2008 Audi A6 2.0TDI S-Line
2004 Audi A4 Avant 1.8T Quattro S-Line
1996 Seat Ibiza 16V F1.5 Race car

 

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