Engine Design

OKC_Kent wrote:

Shoe48 wrote:

, Over square is the Bore Larger than Stroke it will make HP at lower RPMS as where Under square engine Where the Bore is smaller than stroke makes HP at higher RPM ,,

I thought it was the other way around. My 650 is oversquare, revs 9000 rpm high to make power. My Harleys had long strokes, made all their power at low rpm...:whistle:

Yeah, I'd always understood that a long stroke meant low end power...

guitargeek wrote:

OKC_Kent wrote:

Shoe48 wrote:

, Over square is the Bore Larger than Stroke it will make HP at lower RPMS as where Under square engine Where the Bore is smaller than stroke makes HP at higher RPM ,,

I thought it was the other way around. My 650 is oversquare, revs 9000 rpm high to make power. My Harleys had long strokes, made all their power at low rpm...:whistle:

Yeah, I'd always understood that a long stroke meant low end power...

Yeah i agree with you guys as well. At least thats how it was explained to me.

I remember years ago, when Honda had those little four stroke 4 and 6 cylinder racers. They were definitely oversquare, and would spin 15,000 to 20,000plus rpms'

oversquare is big bore short stroke higher rpm motor,undersquare small bore long stroke stump pulling motor;)

OKC_Kent wrote:

I remember years ago, when Honda had those little four stroke 4 and 6 cylinder racers. They were definitely oversquare, and would spin 15,000 to 20,000plus rpms'

The RC166 of 1967 is the same bike as the one of the previous season, the only difference being an increase in power : now 62 bhp at 17,000 rpm.

Six cylinder 250cc :woohoo:
vf750fd.com/Joep_Kortekaas/1967.html

As far as trying longer rods on bikes; they generally have pretty long rods from the factory. Average rod/stroke ratio on American V8s: about 1.6-1.7. Bikes tend to hang around 2 or more to 1.

BTW, shorter rods have some benefits as well. They weigh less (always important) and provide better leverage against the crank throws for the same reason they wear-out cylinders and make pistons rock in the bore. The increased angle from vertical, like most things in engines, is a double-edged sword. However, the power differences (at higher and lower rpm) from different rod lengths are pretty minor compared to the wear differences. I have read a couple articles (on new-model bikes) mentioning rod-length being shortened by the factory to change power delivery, however.

Hey gang....O.K. so let me get this straight ...the best combination is not an 810 but a 302 Chevy with a 900 crank 'cause it'll rev quicker when it is between 6-12 degrees outside. But if I want a square stroker to hang a little longer in the combustion chamber I've got to lengthen my rod. O.K. I got it but just don't tell my wife about the square stroker part...she might not understand like I do

Every engine design has its positves and negatives and their own individual applications or uses. Some powerplants are designed for low end grunt, some for mid-range and others for high rpm power. It is difficult to design a motor that gives you the absolute best of all three.

Let's not forget that the Chevy 302 (290 horse factory rating-under-rated but actual hp. was in the high 300's) was light on torque compared to the other bowtie mouse motors, but it was one of the highest revving small blocks of the muscle car era and really made the Z/28 scream. It was introduced by Chevy in 1967 (327 block, 283 crank) and was designed to fit under the SCCA 305 cu/in. class maximum. The little small block gave the Ford 302's a good run down the straights, but coming outta the corners at lower rpm's it was underpowered according to Roger Penske who campaigned the car with Mark Donohue behind the wheel. Under 4000 rpm the motor was lethargic but once it started to fill its lungs...watch out!

Cheers-Colin Firth-Ontario Canada

yep!