4-valve v's 2-valve heads

[GTSBoy]

If you guys want to work out why this is happening, just punch your various numbers in a spreadsheet and see what the ratios of bore diameter, area, valve head diameter, area and curtain area come out like.

I'm sure that the relationship between some of those varying as the square of bore (and valve) diamater, and some varying only linearly with bore diameter will paint an enlightening picture.

You will need to do it for the engines you have just done, and probably almost any pair of comparable 1.3 - 2 litre engines as well. Obviously the 2V small engine would want to be one that has a decent head - no dungers like the Kent.

cheers

[TK]

Do you wanna do it?

Sounds time consuming.

[GTSBoy]

It's not that time consuming - just send me the sizes and I'll do it.

[CussCuss]

well this got constructive...

[HoonBoy]

Makes me so happy to go home to my 4v big bore engine and 2v small bore engine..

[Billzilla]

If you guys want to work out why this is happening, just punch your various numbers in a spreadsheet and see what the ratios of bore diameter, area, valve head diameter, area and curtain area come out like.

I'm sure that the relationship between some of those varying as the square of bore (and valve) diamater, and some varying only linearly with bore diameter will paint an enlightening picture.

You will need to do it for the engines you have just done, and probably almost any pair of comparable 1.3 - 2 litre engines as well. Obviously the 2V small engine would want to be one that has a decent head - no dungers like the Kent.

cheers

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.

[TK]

A bit of dribble here to let a few people understand a few things.

Bill's engine was using a significantly smaller cam (duration wise) than mine - which goes a long way to showing the effect of inertia ramming from intake velocity.

He would have found that using a similar duration cam to the one in 'my' motor pushed peak power above the rpm of the brief, by the same token I could not make the same power with a smaller cam.

Here's where inertia ram effect comes in - the smaller 2V ports have better velocity, allowing better inertia ramming, hence the engine can tolerate a bigger cam without loss of torque.
The larger 4V ports are able to flow more air, so they do not need as much cam, but on the flipside inertia ram effect is not as good, so they will not tolerate a larger cam without loss of torque.

2 very different approaches to a very similar end result.

Cool huh?

[oldcorollas]

why do 2V and 4V ports automatically need to be different sizes?
a 4V could have seperate ports for each valve (tho for packaging they probably wouldn't)

hmm... sayign that a 2V needs longer duration to get the same effect means that in the first X% of the intake stroke, they are worse, and give poorer velocity... and it is only when there is a larger pressure differential between the unfilled cylinder and the port, that it actually starts moving air... and once moving, needs more time to get air in..

which is what bill was saying in the first place?

2 very different approaches, but one will have more issues with overlap when not at WOT.. ie, emissions, loss of dynamic compression etc...

[arcane]

Could someone kindly explain what is meant by n-angle modifying a head's intake or exhaust ports? E.g. "The intake ports were 3-angled."

[TK]

Nah, not really, they will both behave very similar in terms of emissions, part throttle drivability etc.
The difference in duration is only around 4-6 degrees, it is far from huge.

hmm... sayign that a 2V needs longer duration to get the same effect means that in the first X% of the intake stroke, they are worse, and give poorer velocity... and it is only when there is a larger pressure differential between the unfilled cylinder and the port, that it actually starts moving air... and once moving, needs more time to get air in..

Yes, but by the same token the in the last X% of the intake stroke the 4V is worse - due to lack of inertia ram. So it still ends up much of a muchness in the end result.

[TK]

Could someone kindly explain what is meant by n-angle modifying a head's intake or exhaust ports? E.g. "The intake ports were 3-angled."

The angles of the lead up & exit of the seat, 3 angle is old tech, modern design seats use up to 7 angles & various radiuses to give a good blend into the seat & chamber.
The width of each angle cut plays a big part in how the port flows, different angle can bias the flow for low or high lift etc.
The angles can also act to help atomise the fuel better as it passes through the valve, 'tripping' droplets back into the airstream.

[oldcorollas]

Nah, not really, they will both behave very similar in terms of emissions, part throttle drivability etc.
The difference in duration is only around 4-6 degrees, it is far from huge.

Yes, but by the same token the in the last X% of the intake stroke the 4V is worse - due to lack of inertia ram. So it still ends up much of a muchness in the end result.

interesting

so would there be much difference in terms of pumping losses, due to the higher vacuum (if there was such a thing) in the 2V cylinder?

hmm, why can't you make a 4V port smaller, so it has the same velocity as a 2V, and thus has the same inertia effect as a 2V? or make the port smaller to restrict it, and get the same effect as the 2V port...

[TK]

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.

[Billzilla]

hmm, why can't you make a 4V port smaller, so it has the same velocity as a 2V, and thus has the same inertia effect as a 2V? or make the port smaller to restrict it, and get the same effect as the 2V port...

That's got me buggered as well - I know the small 2V engines need bigger ports than the 4V ones to make the same power.
What the hell is going on with the bigger engines? It should be even more the case, as to the smaller engines the air appears more 'viscous' and so a single opening (valve) should work better than two smaller ones.

I must remember to not talk myself into believing this just yet though ...

[TK]

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.