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Old 10-21-2018, 11:55 PM
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Default Hemi heads, valve shrouding, and YOU

I thought someone might be interested in seeing the difference between Honda's inline valve head and the hemi 212 from a piston's perspective.

Part of the hemi design's appeal for performance is that it allows larger valves to fit in a smaller cylinder. Not such a big deal with the 68mm bore of a GX200 or the 70mm hole of a 212, but it is with the GX140's little 64mm jug.

This is the milled 140 head with a 28.6mm titanium valve. It would nearly touch the cylinder wall given enough lift which means bad shrouding. That caused me to abandon it for this project.


This is a junky no-name 196 clone head. Lots-o-room.


And this is the money: a hemi Predator head with stock 27/25mm valves. It looks like the valves are more centered too, which moves them closer to the widest part of the cylinder. They also move away from the cylinder wall as they open.


Based on a stock 212 hemi making peak power at about 4800rpm, this head on a 140 could make peak power up around 7000rpm with no massaging. The problem is compression with the huge chamber bringing it down to about 6.8:1. There are ways to work around that though...
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Old 10-22-2018, 04:19 AM
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I have learned a lot hear on DIY. Thanks to all the experts and everyone else. ? what part on the engine is called the JUG? I only heard this word ( jug) twice since I've been a member hear and working with mini bikes and go karts.
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Old 10-22-2018, 06:34 AM
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The jug... Is the cylinder section of the motor...
In this case, it is integral to the crankcase.
Alot of V-twin motorcycles have removable cylinders.. Harley to name one...

So as to bettter understand this... The shrouding you are referring to is the material around the valve seat edge??? And in this case, why is this a big matter?? (The top head pic)
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Old 10-22-2018, 08:15 AM
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Man, thats a really small exhaust valve in the first pic .86"?
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Old 10-22-2018, 02:14 PM
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Quote:
Originally Posted by pearl111 View Post
what part on the engine is called the JUG? I only heard this word ( jug) twice since I've been a member hear and working with mini bikes and go karts.
Jug, hole, barrel, lung (as in "one-lunger")....all slang terms for the cylinder.

Quote:
Originally Posted by mckutzy View Post
The shrouding you are referring to is the material around the valve seat edge??? And in this case, why is this a big matter?? (The top head pic)
Partly, yes. The valves sit in the chamber and the chamber edges can shroud the valve which obstructs air from flowing around that side. However, at higher lifts, the valve can move down far enough to then be shrouded by the cylinder wall. That's where hemispherical heads come in: they eliminate the chamber shrouding and send the valve out away from the cylinder as it opens.

The rub is that they typically have large chambers and not any quench. High compression requires domed pistons that aren't optimal for flame travel. The lack of quench and tight spaces means that hemis play well with nitromethane though....for those that dare.

Quote:
Originally Posted by KartFab View Post
Man, thats a really small exhaust valve in the first pic .86"?
Yep, chamber, valves, ports.....everything about the head is small.
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Old 10-22-2018, 04:25 PM
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I see we have touched on the hemi and traditional styles... Do the flat head motors follow similar philosophy in the valve shrouding effects... Or they different due to parallelism to bore?
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Old 10-22-2018, 06:48 PM
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Flatheads aren't something I do as much with, but a common sentiment is that it's generally easier to focus on getting air in and out rather than trying to squeeze it more. Milling tends to hurt flow through a flathead when using stock castings. Shrouding is still a consideration with them and aftermarket heads usually have bigger valve reliefs.
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Old 10-24-2018, 07:17 PM
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And just for kicks, here's one with twice as many valves.
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Old 10-30-2018, 08:17 PM
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Quote:
Originally Posted by 65ShelbyClone View Post
The rub is that they typically have large chambers and not any quench.
Someone asked me what quench is.

The machined flat area of the head is visible in the cylinder, especially in the first picture around the spark plug hole. That's the quench area. When the piston approaches TDC, the air/fuel mixture gets squeezed there and out into the chamber, causing further turbulent mixing of the air and fuel. It improves burn speed, efficiency, and reduces the octane rating requirement of the engine.

The last picture shows no visible quench area because the chamber is larger than the cylinder.
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