Ok, just for some fun here in chat, you have the same cams, carb, pipe, headwork, valves, CDI, 1 quad with 14 to 1 compression stock sized cylinder VS a quad with a 720/730 displacment at 11 to 1 compression? I hears some of both sides chat in here on this but not on the same page as of yet.

Love to hear everyones thoughts, Even DSnuts

 

OMG a serious topic... makes me think about the old days, when people on this forum had conversations about real performance topics.LOL Anyway, I would take the displacement simply because the next off season when I do my yearly teardown I could get the higher compression picton. I always look to the future.[img]i/expressions/face-icon-small-wink.gif[/img]

 

No replacement for displacement [img]i/expressions/face-icon-small-smile.gif[/img]

 

Amen.

 

Compression can give you more torque, but increases in displacement (or a facsimile thereof) will give you the big HP numbers.

 

Ok so Torue is what gets over the top of the sand dunes, what is the need for HP in the Dunes??

 

Really? So whats the difference between a 50cc motor with a 14:1 compression piston and a 650cc motor with an 9:1 compression piston????

Torque allows the horsepower to be utilized under load. Take me, im a big guy - I ride an EX - and the thing is a torque monster. But she didnt come that way from the factory.


But incase you want the true definition .... "The moment of a force; the measure of a force's tendency to produce torsion and rotation about an axis, equal to the vector product of the radius vector from the axis of rotation to the point of application of the force and the force vector. A turning or twisting force."

OK, now Im R E A L L Y ready for some tater salad.

 

Ive been thinking about the same thing lately with this stock bore higher comp high flow motor. I like the higher compression IMO. The bigger piston is bigger displacement yes, makes more torq im sure.. But If I can get a smaller bore piston with lighter components and a awsome flowing head I would think I could push air faster and MAYBE rev a little faster. Its a toss up. I sure would like a 14:1 730... You can tune your carb around rideability IMO. I see how well the race built 450s run, My goal has been to get a fast reving 650 do act like that, Then the math and the proof is on the hill... I run against them all the time.


I think it all comes back to VE as freez would be happy to clean house on..lol


I agree, This thread is great and has many of discussion points!!My curiosity comes down to internal weight and efficiancy of parts and how its assembled along with a MILLION other little things. My point is that if they can make these little motors rev real fast we can make ours do that to. Higher comp should rev faster, bigger displace make more torq, Now who can rev to peak torq faster is the question.

 

Displacement is what increases torque base. If you have a similar build that is 650cc and you increase the bore making it a 730 without changing anything else........You can almost bank on a 10% increase in torque since the displacement increase is roughly 10%.

Why this is important is......it happens throughout the entire rev range until the heavier 105.5 mm piston starts to impact the upper rpm acceleration..........that is compared to a lightweight, forged stock bore piston. The forged piston you get for a 720 or 730 kit will mostlikely be lighter than stock piston so this effect probably won't apply in that case.

The point is you won't just have 10% more torque at peak........you will average about 10% more throughout the rev range.

Compression also effects torque because the cumbustion kicks the piston over harder. Torque will increase by a bigger explosion (more displacement) and from compressing the same amount of fuel and air in to a tighter space at ignition. Whatever forces the piston over TDC and down the hardest will put the most turning force on the crank which will create more torque. Stroke also increases torque mostly by utilizing a longer lever at the crankpin (that is where the rod attaches to the crankshaft) Stroke adds distance from the axis of the crank to the crankpin. The longer lever requires less force to turn the crankshaft, thus when using the same force, it turns the crank shaft harder producing more torque.[img]i/expressions/face-icon-small-smile.gif[/img]

Compression does something that bore really can't do for you. Compression will do much more for making a motor rev faster than the increase in torque could really explain. It has to do with how much quicker it kicks the piston over TDC. Bore will rev some faster because if you ET a DS motor on the dyno then add torque, it will post a faster ET in most cases......but not as much faster as adding torque with compression.

Horsepower is nothing more than a mathmatical equation. It is a result of what the torque is doing. If you make big torque and crappy horsepower that means you motor peaks torque early and drops off quickly. You will have a very narrow powerband. As long as you stay in that powerband, you could pull a very heavy load up a sand dune, lol. You just wouldn't do it very fast. The longer you hold out your torque curve, the more horsepower you will make as the rpm's climb.

Ok, which makes more torque? A 14:1 stock bore or an 11.5:1 730? I would guess the 730 does. There is not enough compression difference in 2.5:1 to make up for the lack of displacement. If we were talking 9:1 730 vs 14:1 stock bore...........I would have to put them on a dyno to know which had more peak torque. I can say as a race motor I would take the high compression stock bore any day. As a dune motor I would take the pump gas motor with the displacement[img]i/expressions/face-icon-small-wink.gif[/img]

You asked for it, ERBE[img]i/expressions/face-icon-small-smile.gif[/img]

 

Thanks Ron!
So a Big Bore wins with lower compression VS a Stock Bore with High Compression. Kinda wanna see em on the dyno now dont ya to compair??