4-valve v's 2-valve heads

[oldcorollas]

If you make the 4V port smaller you kill flow, they need to be bigger to get the right shape for efficient flow into the bowls, 2V only has to feed 1 bowl so the shape can be optimised for that one bowl rather than being compromised to feed 2.

Pumping losses IMHO would be very similar, the 2V airflow takes longer to get moving, but stays moving longer, so you end up with a very similar net effect.

and if you have 2 seperate ports.. one for each intake valve? then you are effectively making it a twin 2V, and can make the same shape to feed the bowl.. and make size for velocity? how can that not work?

re pumping.. the increased pull on the piston will be closer to mid stroke for the 2V, and the inertia effect will be near BDC, so the piston has less moment arm on the crank.. so it would still lose out on pumping losses..

edit: in sayin git takes longer time for things to happen for the 2V, that is saying "there is an upper limit to what rpm they can work at.
but to say that something that happens faster (4V intake) can't work at lower rpm seems backwards...

ie, if you have different burn time for fuel, then you adjust the time at which it ignites... but a fast burning fuel can still work at lower rpm.. it just needs less advance to reach peak cylinder pressure at the right time... whereas a very slow burning fuel, won7t completely burn if you go to high rpm

[TK]

Actually, I think I've got it - it goes back to the lift rate thing.

Most smaller 2V engines have not had anywhere near the aftermarket development of, say, a chev.
The result is that the parts required to run very rapid lift rate roller cams are simply not available.

If it were not for the massive lift rate & high lift achievable with a solid roller cam & rocker arm lift multiplication the 2V engine would not be able to match the 4V.

I wonder if you could make a 4V engine with solid roller buckets & get the rapid lift rate - total lift would still be limited, but I think a 2V would struggle to match it with even that improvement.

[TK]

The 2 seperate port thing has been tried, it didn't work all that well, swirl was almost non-existant & the required combined port cross-sectional area is still greater than that of a 2V.

It's all in the lift rate & achievable total lift.

[Billzilla]

Methinks you could spend a long time thinking about it Bill, there is an answer there somewhere, but I don't know what it is.

I'll think about it, but I can't promise I'll be able to explain it.

I guess at the moment about the closest thing we're going to get is the Nissan VK56 - 5.6 litres.
In the fullness of time I'll get one and build it up as a race engine ... hopefully someone will buy it for a Sports Sedan or something.

and if you have 2 seperate ports.. one for each intake valve? then you are effectively making it a twin 2V, and can make the same shape to feed the bowl.. and make size for velocity? how can that not work?

Some 7AFE heads have two inlet ports for each cylinder.
Repco Brabham also tried it and got some good results.

[TK]

I think that it's extremely amusing that the thing that most ohc devotees detest most about dinosaur engines (ie. pushrods & rocker arms) are the very things that allow a dino to match an ohc multivalve bucket & shim engine.

[Billzilla]

I wonder if you could make a 4V engine with solid roller buckets & get the rapid lift rate - total lift would still be limited, but I think a 2V would struggle to match it with even that improvement.

A couple of years ago a fair few of the F1 engines used things called 'finger followers' and they were used to both vary the total lift (up to 17mm) and the valve timing & duration, I think.
It could only be done with special coatings and so on, to reduce the friction because using rollers would introduce too much mass.

[oldcorollas]

If it were not for the massive lift rate & high lift achievable with a solid roller cam & rocker arm lift multiplication the 2V engine would not be able to match the 4V.

I wonder if you could make a 4V engine with solid roller buckets & get the rapid lift rate - total lift would still be limited, but I think a 2V would struggle to match it with even that improvement.

there are more than a few OHC engines that run followers... allowing variable lift/duration, as well as physically smaller and lighter cams.

by saying that a 2V needs higher lift and duration to match a 4V... isn't that just saying that the 2V can't flow as well as the 4V... ie, properly developed 2V or 4V are still very different in capability. but one you can tune down to match the other... and one can't be tuned up to match the other

[oldcorollas]

what about 3 valves? (from GM)

[TK]

by saying that a 2V needs higher lift and duration to match a 4V... isn't that just saying that the 2V can't flow as well as the 4V... ie, properly developed 2V or 4V are still very different in capability. but one you can tune down to match the other... and one can't be tuned up to match the other

I guess that's the biggest difference between the 2, the 2V flow is inferior in all respects other than port velocity, the only thing that allows it to match the 4V is the ability of the pushrod engine design to allow much higher lift & faster lift rate - without that advantage the 2V would get shat on.

It is a pity that no-one has made a decent effort to develop a 4V head able to harness the rapid lift rate of a roller cam & use the lift multiplication of rocker arms.

There has been a few engines with ohc roller cams & small rocker arms, but they were poor efforts (mitsi did one, 4g63 commercial engine - I think).

Just think, if you could get heaps of lift at a rapid rate & combine it with 4V heads with a port design to work with big lift you could have all the benefits of much higher flow & actually be able to use it - without the need to go to excesively large port volumes/valve sizes to achieve it.

[edo]

I guess at the moment about the closest thing we're going to get is the Nissan VK56 - 5.6 litres.
In the fullness of time I'll get one and build it up as a race engine ... hopefully someone will buy it for a Sports Sedan or something.


Some 7AFE heads have two inlet ports for each cylinder.
Repco Brabham also tried it and got some good results.

I see the regs have changed now & 4 valve engines are allowed to run up to 6 litres...

Phil Irving did some weird twin port heads for some bloody thing...which is really helpful

[Greg Rust]

Good stuff fellas,

AA+++ will read again.

[Billzilla]

I guess that's the biggest difference between the 2, the 2V flow is inferior in all respects other than port velocity, the only thing that allows it to match the 4V is the ability of the pushrod engine design to allow much higher lift & faster lift rate - without that advantage the 2V would get shat on.

Okay, let's try the sim-engine thing again but raise the rev limit to 12,000 rpm with a short-stroke engine.

[edo]

Can you simulate bent pushrods?

[GTSBoy]

Lots of variables there - You can pretty much go to 35% of the bore for maximum inlet valve diameter without too much trouble, but what about the rest?
If you want to have a go, please be my guest.

I think you mistunderstand. If you guys can feed me nothing more than the bore diameters, valve diameters and lifts that you are using for these 6L engines, and then also the same numbers for a couple of comparable small bangers, then I can attempt to produce a table of ratios that should be able to demystify some of this stuff.

I am sure that the secret lies in the way that the available real flow area at the valves changes with bore.

I also wish to input 2c on the basic argument, as addressed most recently by Tony, and asked about by oldcorollas. My take on the separate port for 4V engines thing is this. Think about the old argument about exhaust systems. Twin vs single. You can have the same area for flow in both if you choose twin pipes that add up to the same area as the single system. But the circumfrential length of the twin pipes is more than the single. The single is a more efficient provider of flow area per unit length of wall. So the twin system will have higher wall friction losses and ultimately, less flow.

Extrapolate that idea back into the inlet ports (and I suppose the exhaust ports) and you can see that you really do need more port area (and hence volume for the same port length) to get the same flow. The difference might only be small, but it is there. And, here's the killer. If the 4V valves offer more flow area, and at some engines sizes they certainly will), then you will also need even more port area to support that flow.


Here's a further rub. Tony was talking about the 2V engine not neing as unhappy to run a bigger cam. I look at a bigger cam as being broadly equivalent to running more ignition timing. The amount of timing you can put into a motor is an indication of how (in)efficient the motor is. The more you do to improve the breathing efficiency of a motor, generally the less timing you can put into it. So for cams, if you have to run more duration it is an indicator that you need to provide more breathing - which obviously is what you are doing with the extra cam duration. Now Tony is allowed to run much more lift. So the ratio of curtain area to valve area (and obviously port area) is going to be different to Bill's....but maybe only slightly because they have agreed to use the lifts that they have agreed to. I reckon that what they should do is chose lifts that keep the ratio of curtain area to valve area constant between their engines, then see what the effect of the (different) port areas really have, and also, how much cam duration difference is then present.

Ideas?

[oldcorollas]

i guess that is one part of your discussion i don't understand.. why can7t you make a 4V with the same port velocity as a 2V?


this is the mitsu one right?


BMW drag lever actuation


golf VR6


rolls royce merlin


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plain Vtec


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toyota/lotus elise 2zzge



for reducing valve train losses, and for allowing variable lift.. roller finger followers are being adopted by many manufacturers..