Originally posted by RyanCivic2000
Bill, in regards to sparkplug placement, you are wrong. The SOHC placement is superior and allows for a more efficient combustion. There is a significant power loss with a centralized plug (especially with domed pistons) and the charge burn can be hindered upon combustion.
With the right valve springs, valvefloat will hardly be an issue. I would be more concerned with the stroke and piston speeds.
BTW, I didn't bring up the F1 car example, matraca112 did.
Umm...again I disagree.
While valvefloat can be addressed by stiffer springs, this in turn increases frictional losses as well as increasing valve-train wear. DOHC offers the better solution.
As far as the plug placement is concerned, there's a reason that the plug is angled and not centered, it's due to a cam sitting directly overtop. While I'm sure placement is ideal for the situation, it is not overall ideal. Regarding higher piston domes, yes, they do present more impedence for the flame front than that of lower domed pistons. However, this can be countered by advancing the timing. Couple this with the benefit of centrally located plugs (enable less advance) and you have a decent workaround.
But don't take my word for it. Below is the result of a quick google search. Please find me some information supporting your side, preferably more than one or two sources from BBS posts.
Article on Cosworth F1 Engine Development in the 60's (A revolutionary design...central plug)
These four valves would probably have given sufficient gas flow if they had been set vertically. However, mainly to leave room for the spark plug boss to be located centrally between the valves and provide smoother curves in the gas passages to and from the valves, the pairs of inlet and exhaust valves were inclined at 20 deg towards the opposite sides of the engine.
http://www.avant.co.nz/lotus/dfv.htm
Article touting the gains of a DOHC engine over the "Hemi".
The chamber of choice of the Japanese “super” bikes (the Kawasaki Ninja, the Honda Hurricane, the Yamaha FZR, etc.) and Indy and Formula 1 engines, the pent-roof permits a flat (or even concave) piston, a cylinder head that is only slightly domed, and a centrally placed spark plug. The effect is that the fuel and air mix are ignited rapidly because the mix is concentrated tightly around the plug. That allows higher compression ratios to be used with less fear of dreaded detonation—the charge quickly burns before the detonation
http://www.schubeckracing.com/Anewge...aceengine.html
Brief article on GM concept engine (2001)
"Using three valves-two inlets and one exhaust-gives you more room in the combustion chamber for optimal positioning of the injector and the spark plug, vertical and nearly central in the chamber," Indra said.
http://www.autospeed.com/cms/A_1193/article.html
Brief article engine design
DOHC designs have another advantage when coupled with four-valve cylinder heads. It allows the spark plug to be placed smack in the center of the combustion chamber, which promotes efficient combustion and reduces the engine's taste for high octane fuel. With an SOHC engine, the camshaft is usually in the middle of the head, precluding this optimal plug location.
http://www.jag-lovers.org/e-type/perf.html
Article on an interesting engine design
The design of the CSRV, which at first glance resembles an OHC cylinder head, allows for the central placement of the spark plug in the bore. By varying the spark plug location when referenced to the bore centerline, the most desirable position is in the center. This will allow the cylinder pressure build in the minimum amount of crank rotational degrees past TDC.
http://www.coatesengine.com/look_mom_no_camshaft.html
Article on good motorcylce engine basic design concepts
A centrally-located spark plug causes combustion's flame to reach deeper into the remote edges of the cylinder, turning more BTU into useful work, and eliminating the end-gases that often initiate detonation.
http://www.motorcycleproject.com/mot...t/secrets.html