Planet Minis

jrotten

the shop i work at just opened a scooter dept. and i was looking through catalogs and polini had some xr/crf 50 stuff-including a 4 valve head.anybody know if this is their own head?

moto234

its a pretty sweet setup from what i read about it a while ago

wearenet

found it

02 POLINI XP 4T STROKE RACING

The day I see an Italian mx win something though...



They know there stuff for street though.

from that link

POLINI PERFORMANCE CYLINDER KIT: VOYAGE IN THE HYPERSPACE
The cylinder is made of selected light alloy and the barrel has a nickel - silicium coating that ensures a very high resistance to heat and abrasion. The 52 mm bore keeps the 107 cc displacement. The piston is in aluminium die-cast at high silicon content. The head is made of aluminium alloy and gravity chilled and it has been designed using the new PRO/ENGINEERING three-dimensional system. It features a single-shaft distribution devise with 4 valves. It distinguishes itself for the smaller angle between the valves and for the burning chamber which is extremely small with the spark plug placed in the centre and adequate to the squish, assuring great performance. It optimises the power output and reliability. The distribution diagram enables to reach the best torque value without reducing the power output at the runway speed rate. The carburettor has been developed in co-operation with the Dell'Orto technicians and distinguishes itself for a specific calibration that improves the racing set-up.

firepower354

"single-shaft distribution devise", indeed...

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mexicanyella

Not sure how you get the spark plug in the centre with a single camshaft, but at least with a single camshaft you won't have that dang cam-and-bucket lash adjustment arrangement.

firepower354

Honda's had the plug in the middle of SOHC 4V's for decades. Even with 8 rocker arms in the way.

__________________
If it has my name on it I did it;)

FirepowerMinis.com
Custom FPM parts and engines.
Parts and service for all makes.
Honda Trail Bikes parts and accessories.
Pitster/Piranha/Thumpstar bikes, parts, engines and accessories.

mexicanyella

Dang it, I done shot my mouth off before being fully awake. Embarrassing.

Are you referring to the RFVC, as found on XR's of the 80s? I've not examined one of those before.

Our old '93 and current '98 Accord have a SOHC 16-valve arrangement and the plugs go through the valvecover (you might have seen an arrangement like that before somewhere) but they're offset/angled. Could be they enter the combustion chamber in the center, but definitely not straight in.

I've not been around other Honda car engines enough to know if this arrangement is their SOP. Our somewhat newer CR-V is DOHC.

breska-inc

Branyers

jrotten

yeah,i know about the search feature,just being lazy and asking instead of looking.seems like it might be good stuff,just wasn't sure if it was another brand sold as their own.i guess i will look further into their parts for my z's future engine mods

wearenet

Sometimes I feel like Sylar from that show Heroes when I look at that stuff.
I guess the part I don't get is why adding more moving parts and weight to the engine gives more performance.

mexicanyella

By Golly, I can answer that one.

There are several advantages to a four-valve arrangement. Probably the top one is you get more port area with two of each valve rather than one, so you can stuff more fuel/air mix through the engine. And you can do it with milder cam timing than what you'd need to use to achieve similar flow with two valves, since with the extra valves and port area you don't have to hold the valves open as long. Or as far, really.

With the milder cam timing you're likely to have a broader, less peaky powerband. And since each valve is physically smaller and lighter, you don't need as much valvespring pressure per valve to control it and it's easier to keep it from floating at high rpm, which allows higher revs.

Because of these smaller, lighter valves which don't have to open as far or for as long per cycle, you can design a compact, shallow combustion chamber without the necessary wide angle between the valvestems (necessary to allow the larger valves to open a greater distance and still fit in there without hitting anything). The narrower valvestem angle means a shallower chamber. Higher compression becomes possible with a flatter piston crown, keeping the combustion space compact. Better combustion, more turbulence, a central or near-central sparkplug location for faster flame travel, and depending on how the valves are arranged, good opportunities to build in a flattish "squish band" at the combustion chamber edges, which sort of squirts the compressed charge toward the sparkplug as the piston nears TDC, adding turbulence and speeding combustion.

There are three-valve heads in lots of things, usually with the intake valve duplicated since it's harder to suck air in than blow exhaust out. Yamaha has a bunch of five-valve engines, with three intakes and two exhausts. Some car company--Maserati?--had a six-valver, and Honda has made a number of gran prix race engines with oval pistons and eight valves per cylinder.

nielspbr

In addition to mexicanyella: Honda also had the roadgoing NR750 (V4, 32valve, oval pistons)