so whats the deal in power- ohv vs flathead?

[freezerburn]

it seems that the displacements (cc sizes) and power outputs are fairly similar in Stock forms in regards to the two engine styles.
please input your ideas as i am in the process of choosing a large engine to build a fat tire deep mud/ sand slinger for out in the tundra.
but not withstanding that, i want to here about your thoughts as to the two engine designs and / or others that may apply to make your own points. -freezerburn

 

[RobK]

I'm sure this will drive all kinds of opinions and comments. The bottom line is flat heads are essentially old technolgy. Old, but proven, reliable and easy to repair. Flatheads have fewer moving parts, but because the valves are adjacent to the cylinder they have larger combustion chambers and lower compression ratios (6.5:1 in a 5hp B&S Raptor engine), and as a result are less efficient at burning fuel. Lower effiency means dirtier exhaust. Flatheads dissapeared from the automotive world in the '40s. OHV engines tend to be smoother and quieter running. Because the valves are on top and open towards the piston, the combustion chamber is smaller and the compression ratio is much higher (8:1 in a Honda GX200), resulting in more power from a smaller package. They're also much more fuel efficient with lower emissions. That said, I'm sure you'll now get many comments on the pros and cons of each.

 

[freezerburn]

thanks robk,
i understand that they (ohv) are more efficient but whats with the cc's being in most cases almost identical? wtf? mileage? who gives a :censure:? its a mini...
im going to do all i can to instigate both sides to get all the info i can... so please forgive me in advance:bowdown: speaketh please

 

[loampey]

ohv=ugly flat head= cool looking
a four stroke engine is basically an air pump. so the more air you can pump into the cylinder for a given displacement the more power you can make. OHV's are better at moving air just by there design. so if you can get past the ugly ohv's put out more power.

 

[freezerburn]

haw, haw, haw, haw, haw! now thats the spirit! haw, haw, hee, hee, hoo hoo hoo hoo, heh, heh, hee, hee, hee, ha

 

[mmisterbungl]

haw, haw, haw, haw, haw! now thats the spirit! haw, haw, hee, hee, hoo hoo hoo hoo, heh, heh, hee, hee, hee, ha

pass that over here dude.

 

[RobK]

Not sure I agree with the "ugly" vs. "cool" descriptions, but displacement is more a function of design and engineering. Although the old addage "there's no substitute for cubic inches" is correct, and as a general rule HP goes up with displacement, it's not a perfect linear correlation. The 5hp B&S is 206cc's, the Honda GX200 (6.5hp) is 196. The new HF Predator 6.5hp is 212cc's, and the Northern "Powerhorse" 6.5 is 208cc's.

My opinion on engine choice is go for the most hp and torque from the smallest package you can. I though about trying to squeeze a GX340 or 390 in my bike, but then thought the extra 40 pounds would be a reall drag. I think I'd rather build a GX200 to 15hp.

 

[Neck]

I dunno, this looks kind of cool!

 

[RobK]

Uuuhhhh...., okay. Is that a Honda GC engine with B&S flatty sheet metal?

 

[Neck]

Uh huh.

 

[freezerburn]

but guys, guys, guys-
196- 212cc -a difference of 14cc's for the same output? ooooohh woooowwww... and basically the same weight too? this is negligible imho. i must re-emphasize Stock Form, as that is how they were engineered- what they are, as they are and what they are made to do all day long.
lets hear about it...
and btw- nice motor there neck -freezerburn

 

[RobK]

So what exactly is your question? Hp and torque is a function of many factors in addition to displacement; carburation, cam profile, compression, etc.

 

[freezerburn]

robk, observations both ways in regards to stock ohv and L heads. but really, the first sentence of the thread was just an observation... but as you stated- there are many factors involved in outputs. i guess i started with a leading question to anyones thoughts on the subject of what folks like about Any engine design :thumbsup:

 

[Oldsalt]

OHV motors have a greater and greater advantage over flatheads as the induction gas velocity increases with RPM. But for most practical applications for "lawnmower" sized engines [especially for minibikes which don't have a gear box] the requirement is low and mid-range torque. An OHV and a 5 HP Briggs perform the same because there is little to gain in cylinder filling at the gas speeds involved. Also the flathead Briggs will cool better and they are certainly more reliable.