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theory behind oversized valves??


Guest touge

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Guest touge

So I'm building a new engine for the racecar (2zz of all things haha) and trying to weigh up valve options. Now i know that 1mm oversized valves work a treat on a lot of engines like the blacktop 4age etc. my understanding of why this works is down to poorish port design and to help it breath on the intake side mainly.

but what actually makes them work better?? Do they help spread the intake charge throughout the combustion chamber providing better mixing of fuel/air etc??

Let me know what theories are out there..

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Guest mikuni

I thought the same thing as flybrick, just less restriction in the intake tract.

It's worth researching your engine as much as possible, because as you mention, it affects different engines in different ways.

I've seen a dyno of the same type of engine I'm playing around with, an Opel C20XE which is a 2 litre twincam, and I can't recall what the standard valve sizes are, but with 1mm oversize in valves it gained nothing at all anywhere and lost a bit of lowend or midrange. The conclusion drawn was that valves only need doing once almost all other modifications are done and they are the final restriction in the intake that hasn't been opened up. Talking 250+bhp on standard valves. I think people often jump at them early because they are quite an easy upgrade while doing cams + lifters and open up potential for future mods without having to crack the engine again, but if it were my money I would hold off until after bumping up the CR with crazy slipper pistons :D

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Guest phatt20

with older two valve engines , bigger valves allow bigger seat inserts , which allow better shaped/flowing ports, on pretty much all four stroke engines the valve/seat area is the lowest flowing/most restrictive area in the intake. 1mm oversize usually allows a bit more leeway in cleaning up the throat/seat area and improving seat geometry etc. bigger valves can also increase shrouding too however which can lead to power losses if not addressed at the same time.

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Guest xsspeed

Just thinking aloud, haven't done the maths, but could a 1mm size increase have a greater/less effect on area on designs with smaller multiple valves per cylinder than others with less, larger valves...though I guess the engines in question are all 20/16 valve 4 pots

Otherwise, my thoughts would be as per Brad and FB

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Guest phatt20

generally oversize valves on their own do very little for most engines, unless the engine had poorly shaped or undersize valves for the seat size in the first place. the gains come about with the extra room the give you to mod the port/seat etc. also the back shape of the valve plays a big part.

are you planning on port/chamber work to complement the valves?

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Guest touge

Ok sounds a bit interesting.. my understanding of how they worked goes a bit deeper than that as in the 20v we did for dads ae82 we purposely didn't touch the ports at all, just used the bigger valve and a 292/304degree cam with 11mm lift. all done on a completely standard engine/compression ratio and it netted 128kw atw. This is much higher than any other blacktop I've seen with less mods and i put it down to an awesome cam and the oversized valves..

haven't found much info on 2zz tbh basing most of my decisions on what the trs engines run

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Guest mjrstar

There really is no generic rule, I did some research on the 1ggte boatanchor and next restriction on the yamaha heads is in fact the valve sizing, with porting a factory valved head giving minimal results wehn compared to an unported head with a set of oversized valves.

naturally these results would not be typical and would need to consider cylinder volume camshaft duration and compression ratio to really get the full story.

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Guest touge

Not sure on any port work at his stage need to have a good look at the head and see what can be done. my thinking is its not worth touching unless it is a good job. looking at standard cams for now but also weighing up getting a race cam made by kiwi cams who made dads cams

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Guest phatt20

Ok sounds a bit interesting.. my understanding of how they worked goes a bit deeper than that as in the 20v we did for dads ae82 we purposely didn't touch the ports at all, just used the bigger valve and a 292/304degree cam with 11mm lift. all done on a completely standard engine/compression ratio and it netted 128kw atw. This is much higher than any other blacktop I've seen with less mods and i put it down to an awesome cam and the oversized valves..

haven't found much info on 2zz tbh basing most of my decisions on what the trs engines run

Trouble with that is its hard to put any of the gains specifically down to the oversize valves when other mods have been done at the same time if you get what im saying.

most of the heads iv put on the flow bench dont see huge gains from valve changes alone, with the odd exception, but even then its minimal.

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Guest touge

Ok that makes sense no oversize valves unless i do some big cams and a head tidy up on it.. this decision is getting harder by the minute.. dammit

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Guest Rookie

TBH if you can't find information on the 2zz you should acquire it yourself. Get your head flowed and get a velocity profile, these will very quickly show up any short comings.

As mentioned earlier, the reason that oversize valves have been so common in the past is because they have quite often been the restriction, even up to the 90's where massive port were the norm, so they were a quick and cheap way of unlocking ponys.

I have no experience with the 2zz so I can't help you with any specifics.

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Guest touge

You are probably right i should do some bench tests if i go down this route. at this stage still weighing up standard engine with vvti vs race cams and a worked head. time will tell i guess..

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you'll be sinking mega cash to get a engine with big cams, valves.. etc. to work properly and make good power. will likely need to throw a good amount of cash at the bottom end as well. not just to handle extra rpm. but setting it all up to work with your big cams and valves, valve to piston clearances with correct cam timing etc..

+1 for figuring out its main downfall, and try remedy that without going to hard out. plus a few well thought out tweaks. wont drive you insane, and you'll have some cash to go racing

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Guest eke_zetec_RWD

Bigger valve = bigger seat. Seat circumference x valve lift = area for air to flow past. More area = more potential. Potential being the key word.

On 4 valve engines with small pistons valve shrouding is a issue with oversize valves. Need to look at stock combustion chamber....

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Guest Roman

Keep the entire engine standard, and spend the money on bits that bolt on.

That way, when it blows up you can just go get another one, and be good to go.

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Guest RUNAMUCK

Bigger valves wont always make more power.

It's critical to port the bowl throat to match. (and that doesn't mean to the same size either) But a larger valve/throat can reduce intake charge velocity. And sometimes create gay things like eddies, and port stall. Also, when valves start getting right up there, (Like a 2.02 inch valve on a 318) the bore wall can begin the shroud the valve, and impead the flow.

Pinto's love bigger valves though.

Have you considered Pinto?

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