4-valve v's 2-valve heads

[CussCuss]

ok so why hasnt someone designed an offset cam that would sit at where the top end of a pushrod would sit and then act on a rocker so you can have high lift on a 4v motor and get both the benefits.

Of is the area so much greater that high lift isnt needed?

edit, ok that honda one was what i was thinking, that can do high lift?

[TK]

^ that's where I'm going with the ohc with roller cam & rocker arms - to allow the 4V engine to use similar lift to the 2V.

The result will be devestating to the 2V, it looses one of it's biggest advantages & the flow available from the single valve will not get close to allowing it to keep up.

The fact is, that using currently available parts/technology the 2V engine does have the lift rate/total lift advantage & there is SFA the 4V people can do about it.

I'll upload Bill's engine file & jam .750" lift into it - gonna have to fudge the flow figures tho, coz I doubt he has them for lift that high.

Even then it's not gonna be a proper comparison, the ports of the 4V head will not have been optimised with that much lift in mind, but it should give a bit of an indication.

[TK]

Nup, crap, can only figure out how to download the file as a text document.

Run the sim would you bill - use .750" lift & fudge the flows so that it'll work.

[CussCuss]

^ that's where I'm going with the ohc with roller cam & rocker arms - to allow the 4V engine to use similar lift to the 2V.

The result will be devestating to the 2V, it looses one of it's biggest advantages & the flow available from the single valve will not get close to allowing it to keep up.

The fact is, that using currently available parts/technology the 2V engine does have the lift rate/total lift advantage & there is SFA the 4V people can do about it.

I'll upload Bill's engine file & jam .750" lift into it - gonna have to fudge the flow figures tho, coz I doubt he has them for lift that high.

Even then it's not gonna be a proper comparison, the ports of the 4V head will not have been optimised with that much lift in mind, but it should give a bit of an indication.

why has this not been done yet, surely its fairly obvious if i can pick it up within 5 minutes of reading.

And isnt that honda setup above doing something close to it?

[TK]

Actually, I'll take Seans ported 1FZ figures & use them with big lift - but it'll have to wait, I've got the parts back from balancing for the chev I've gotta get done, so I have to knock over the short before I go home.

[Greg Rust]

Ford GT 5.4L 32V

[Billzilla]

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.

Some six litre stuff - 4" bore, 2 x 1.57" valves, 0.6" lift - That one mades lot of power with the big cam.
The seven litre LS7 - 4.125" bore , 2 x 1.57" valves, 0.55" lift.
Two litre 3SGE - 3.386" bore, 2 x 1.417" valves, 0.51" lift
1.626 litre 4AFE - 3.228" bore, 2 x 1.283" valves, 0.445" lift - This the screamer engine I'm building.
1UZ - 3.444" bore, 2 x 1.41" valve, 0.42" lift
Suzuki V6 - 3.464" bore, 2 x 1.36", 0.445" lift - rally car engine.

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.

I can see that logic and will have to think about it.
Previously, I mentally seperated the port into two basic sections - the port above the valve throat area, and the valve(s) throat area. For a given amount of power, the airflow through the port-above bit shouldn't have been any different, I would have thought. The dynamics of the airflow into the valve throat area bit varies greatly for a good 2V & 4V head though, and I would still favour the 4V as there's less turning for the airflow.
I'm also not convinced that swirl is such a huge advantage - We all know the water-down-the-bottle trick. But try it again with the bottom cut out of the bottle so the airflow doesn't have to gurgle up through the neck of the bottle.

Here's a further rub. Tony was talking about the 2V engine not neing as unhappy to run a bigger cam.

Certainly on the smaller engines you don't need or want as big a cam as on the 2V engines. The 4V ones make good power with relatively small cams.
That being said, I happend to see a two litre Supertourer engine apart (promised not to tell who is was sorry) and they were running 0.580" lift cams on it. The cam lobes looked like party sausages, and I asked the guy how long the valve springs lasted.
"Two hours."

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?

I'm not sure the modelling of EAN is accurate enough to show the differences if we keep the curtain area the same on both types, but I'd love to give it a go.

[oldcorollas]

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 would like to see that (hmm.. if i had a copy of EAP myself.... :O )

Re: the twin intake ports.. for same cross sectional area, of course the twins will haev larger will friction losses.. but if you are free to up the size, then even with that wall loss, you have the same total flow, and you can address the "aiming at the port bowls" thing..

porting an existing head is also a tradeoff between reducing boundary losses, vs velocity and flow etc... so someones bigger ports with more wall loss, will work as well as a smaller port with better walls.. etc...

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.

i disagree slightly.. you only need more port area and mor eflow if you want top end power.
if you are making taxis and shopping trolleys, then the port size/velocity/flow will be determined by the rpm range of the engine... of course if you make ports bigger, you lose some lower rpm velocity.. but same thing for runner lengths etc, and the measures that (for example) BMV have gone to with the snazzy inlet length devices.

the argument for 2V usually is "they are good enough for what they are".. but you can build a better flowing head down to that (detune/reduce port size) also..

agreed with the analogy to timing... "yeah bro i can run 60deg timing with avgas in my race engine, it must have more fasts"

re valve lift... i see it as a ratio between port area and curtain height.. (for port area vs circumference PiR^2 / 2PiR = R/2).. larger valve has mor eport area and larger port area/circumference ratio.

when you compare port area vs curtain area.. you get this.. (attached graph)

a 30mm valve needs 7.5mm lift to have a curtain/valve area of 1.
a 40mm valve needs 10mm lift to have a curtain/valve area of 1.
a 50mm valve needs 12.5mm lift to have a curtain/valve area of 1.
a 60mm valve needs 15mm lift to have a curtain/valve area of 1.

so it seems sensible that a larger valve needs more lift, so the air that is comign thru the port, has the same area to escape.... which is why 4V does not need as much lift as a 2V

ie, for same valve area, a 60mm valve needs 15mm lift for area ratio of 1
but for a 4V, same valve area needs 42.4mm valves and only need 10.6mm to get the same curtain area

two 42.4mm valves with 15mm lift will have 41% more curtain area than a single 60mm valve... even tho the head area is the same.....

[Billzilla]

The fact is, that using currently available parts/technology the 2V engine does have the lift rate/total lift advantage & there is SFA the 4V people can do about it.

How do you go with piston-to-valve clearance with such big cams? We often have to be very careful even with the smaller cams.

I'll upload Bill's engine file & jam .750" lift into it - gonna have to fudge the flow figures tho, coz I doubt he has them for lift that high.

I just used a generic flow co-efficient that I know a good 4V head can make.

Nup, crap, can only figure out how to download the file as a text document.

Save it as a text file, then use the command line window to remove the .txt at the end of it.

Run the sim would you bill - use .750" lift & fudge the flows so that it'll work.

Will do. I think the program will complain about the ramp rates though ...

[oldcorollas]

ok so why hasnt someone designed an offset cam that would sit at where the top end of a pushrod would sit and then act on a rocker so you can have high lift on a 4v motor and get both the benefits.

Of is the area so much greater that high lift isnt needed?

edit, ok that honda one was what i was thinking, that can do high lift?

BMW already did that with their throttleless engine.. where the valve lift is used to act as throttle.. i forget which one

[Billzilla]

Run the sim would you bill - use .750" lift & fudge the flows so that it'll work.

My convenient 'cheat' of just using a generic flow co-efficient seems to be biting me on the arse here - There's no difference in power between a 0.75" lift cam and a 0.600" one.
I'd need to make an accurate guess as to real flow figures, and I don't want to make any mistakes on that.

[GTSBoy]

I can't multiquote for some reason, so I'm going to have to paraphrase a bit.

>Bill's comment ref swirl.

Agreed. Obviously the swirling bottle flow is a trick to fool people who don't understand what they see, and having swirl in a pipe or port itself is not going to improve total flow. It can only cost total flow. But the swirl that they are talking about with ref flow into combustion chambers is a different beast. They are trying to induce swirl in the chamber, not so much in the port. The swirl creates a finer scale of mixing which does all sorts of wonders for combustion efficiency. Actually I should say that it CAN do all sorts of wonders. The RIGHT amount of swirl is still the right amount. You need to get the scale of mixing (ie how finely the droplets of fuel and molecules of evaporated fuel are divided up in the air) right to achieve your compromise (detonation limits, flame front speed, emissions, etc etc). This type of work is what the OEM engineers have been getting very good at in the last 20 years or so, but I strongly suspect that most of their advances are brought on by pure old fashioned trial and error type work. Trying to evaluate this stuff from first principles or by maths modelling would be a very tricky task.

Oldcorollas - When talking about needing more port area in a 4V to go with the better valve flow, I was talking about our aim to maximise power rather than to just make the same power. In this case I had gone back to the beginning of the argument which is that one head is better than the other. Which a 4V is no doubt, for some as yet undetermined range of engine sizes.

I will take Bill's numbers to task probably tomorrow. Would appreciate some comparable numbers from Tony's EAP model too.

[TK]

a 30mm valve needs 7.5mm lift to have a curtain/valve area of 1.
a 40mm valve needs 10mm lift to have a curtain/valve area of 1.
a 50mm valve needs 12.5mm lift to have a curtain/valve area of 1.
a 60mm valve needs 15mm lift to have a curtain/valve area of 1.

Thanks for working that out.

So, say you're using 2x 40mm valves, you'll need 10mm lift (or in reality a bit more) to take advantage of the valve size.
But, reality also says that if you aim for peak lift of 10mm the valve actually will spend SFA time at that lift - it'll be busy opening & shutting.
So, to make that valve work you're gonna need at least 12mm lift.

But that is not heaps (only .470" lift) - there goes the more lift idea.

So, it's gotta be in the ramp rate.

A quick fudge would be to jam some roller cam figures into the 4V sim.

Bill - try 246/252 on 112 centres, use 600 lift & put it down as a pushrod style agressive mechanical roller cam, with 1.6:1 rocker ratio - that should produce some interesting results.

[Billzilla]

Bill - try 246/252 on 112 centres, use 600 lift & put it down as a pushrod style agressive mechanical roller cam, with 1.6:1 rocker ratio - that should produce some interesting results.

Killed the power at lower revs, cross-over at about 6100rpm then a rapidly increasing improvement. You'd need more revs to see a big chunk more.
But then you're piston-speed limited, with the big stroke.

[TK]

Yeah, my bad, I thought about that after I posted it.
Go smaller on the cam 238/244