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Old 03-01-2013, 03:26 AM   #81 (permalink)
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Originally Posted by NeilBlanchard View Post
It would seem to be difficult to rework the way a combustion chamber is designed to work. They engineered it for a particular swirl and a certain flame spread, etc, and to change this after the fact would seem problematic. If you disable one intake valve or the other, and one exhaust valve or the other, you have a profound effect on the mixing and flow. In other words, the valve sizes and their placement and the shape of the chamber and the placement of the spark plug, that would have been better if it was designed for a 2 valve or a 3 valve to begin with, are very hard to get from a 4 valve design.

Also, the fuel metering and the intake and exhaust tracts are all designed for the flow through the 4 valves. It's hard to change any design, unless you can change all the things that are involved.

I see you've come to basically this same conclusion - it makes sense to me.
The 8-valve CRX HF combustion chamber is the same as the 16-valve chamber. Or at least very similar. My valvetrain is virtually identical to the HF. But you're right, the ports, manifolds and throttle body are no longer optimum.


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Originally Posted by renault_megane_dci View Post
Usually, cam lobes are made to lift the valve a quarter of its diameter.
I assume the 8V variant from Honda has bigger valves so the lobes are taller.
The valves are the same size and have the same part number. Only the springs are different. I can fully compress the 8-valve springs with my thumb.

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Old 03-01-2013, 12:09 PM   #82 (permalink)
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4 valves create tumble - no swirl! The combustion is related to port velocity. You want proportionately high flow with respect to the port cross sectional area. If you close a valve in A port designed for two valves, you HALVE the flow, and consequently halve the port velocity. Not only does this kill VE, kills fuel shearing and mixing, and reduces mixture motion - all of which slows flame speed, so you have to set ignition earlier, which hurts mechanical efficiency.

You can gain back your low rpm performance (and then some) by filling in the port with epoxy or aluminum solder. The port should be 86% of your valve diameter, and taper up to your valve. I can give you a figure if you tell me your valve diameter and stem dia. You obviously need a smaller runner intake manifold - and short runners (10" from valve to plenum should work well). plenum volume should stay the same. Throttle body can be smaller for better response and transient.

Fwiw an FSAE team just used a small intake manifold and ported the heads. They improved their FE by double digits iirc. Still not ideal, and perhaps easier.
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Old 03-01-2013, 02:23 PM   #83 (permalink)
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Wouldn't smaller and longer intake dia create more torque at lower rpms?
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Old 03-01-2013, 03:21 PM   #84 (permalink)
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Originally Posted by ERTW View Post
4 valves create tumble - no swirl! The combustion is related to port velocity. You want proportionately high flow with respect to the port cross sectional area. If you close a valve in A port designed for two valves, you HALVE the flow, and consequently halve the port velocity. Not only does this kill VE, kills fuel shearing and mixing, and reduces mixture motion - all of which slows flame speed, so you have to set ignition earlier, which hurts mechanical efficiency.

You can gain back your low rpm performance (and then some) by filling in the port with epoxy or aluminum solder. The port should be 86% of your valve diameter, and taper up to your valve. I can give you a figure if you tell me your valve diameter and stem dia. You obviously need a smaller runner intake manifold - and short runners (10" from valve to plenum should work well). plenum volume should stay the same. Throttle body can be smaller for better response and transient.

Fwiw an FSAE team just used a small intake manifold and ported the heads. They improved their FE by double digits iirc. Still not ideal, and perhaps easier.
Hth
Not familiar with Honda but wouldn't each intake valve have it's own port? My 24 valve Merc does.
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Old 03-01-2013, 06:25 PM   #85 (permalink)
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Originally Posted by Frank Lee View Post
Not familiar with Honda but wouldn't each intake valve have it's own port? My 24 valve Merc does.
I wish! The ports just split to feed both valves. The 8-valve engine has round ports, the 16-valve has oval ports.

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