Effect of displacement on torque

[doctorgonzo]

I use the term "torque" loosely, as it maybe a totaly different force at work here.

Ok, so old story, bored out my 1000G engine and replaced with stock 1100G pisons so now 1074, not 997. What is that an 8% increase in displacement I think.

My question (and this my engine ignorance showing) is would the larger displacement tend to increase the torque with that same stroke distance? The reason I ask is man does that thing roll like a beast now when you twist the throttle. Sitting on the side stand with no rider twist the throttle from idle and it will roll all the way up to almost vertical and scare the crap out of you that it might come on over.

I mean I know it's a big inline 4, it always tended to to this, but I sure don't remember it being so dramatic. Maybe I'm just not remembering well.

Anybody care to give me a dummies guide to this stuff.

 

[rapidray]

YEP!!! You are experiencing a GAIN in torque!!! On a street bike, TORQUE is the FUNNEST increase you can strive for because it increases DRIVABILITY, throttle response, & accelleration! FUN stuff! Try a 1400cc GS 1100 or 1150 if you ever get a chance! Ray.

 

[Dan Ruddock]

Your engine is a 1054cc the 1100g had a longer stroke than the 1000g to give 1074cc. Torque is a function of displacement, HP is a function of what rpm the hp peaks at. Dan

 

[doctorgonzo]

Your engine is a 1054cc the 1100g had a longer stroke than the 1000g to give 1074cc. Torque is a function of displacement, HP is a function of what rpm the hp peaks at. Dan

Had wondered about that, 1054 because of shorter stroke, that makes sense.

 

[doctorgonzo]

YEP!!! You are experiencing a GAIN in torque!!! On a street bike, TORQUE is the FUNNEST increase you can strive for because it increases DRIVABILITY, throttle response, & accelleration! FUN stuff! Try a 1400cc GS 1100 or 1150 if you ever get a chance! Ray.

I'd LOVE to.
This build was a "halfway learning what the hell I'm doing" project.
I do want to go ridiculous big at some point in the future.

 

[posplayr]

I found this article; It does a pretty good job describing force, torque, horsepower and work. Even as an engineer it seems some of the explanation is more than you need to understand the formula.

Also the relationship is more simplified than it should be as it doesn't explain the factors ( stroke, combustion chamber size and CR) which directly effect MEP.

If I directly apply the formula, you would think that increasing stroke would increase both MEP and CID and that is the way to get more hp. So the formula is also ignoring other factors related to acceleration (i.e. rotational inertia).

This article is good at making you think you understand, but upon application realize how little you know once you try to use the formulas.

http://www.sporttruck.com/techarticles/0312st_engine_performance_horsepower_torque/index.html

The following equation explains and shows the relationship of how these variables influence horsepower output of a four-stroke internal combustion engine:
HP= MEP x CID x RPM/33,000 x 12 x 2
Here's a quick explanation of the relationship of the values of this formula. MEP is the theoretical mean effective pressure acting on the piston top through its stroke. Notice that cylinder pressure is divided by the work of 1 hp (33,000 ft-lb). These are the force units. Cubic inch displacement (CID) reflects piston top area and the crankshaft's stroke length, which is divided by 12 to convert the value to feet. And finally, the number of power strokes per minute for a four-stroke engine is the term RPM/2 because the cylinder fires every other revolution.
This equation predicts theoretical horsepower, not brake horsepower. It does not account for the frictional power losses of the engine. When you measure on an engine dyno, you measure the net power output of the engine after all losses. The reason it's good to know the math and physical reasoning behind power generation is that it shows you exactly where to make changes to improve the performance of your engine. If you work with the equation, you'll see that to increase power output you have to increase one of these variables. In other words, you have to increase the mean effective pressure in the cylinders, the stroke, the bore size, or the engine speed.

Here is a guy who does a reasonable job and also deals with the issue of inertia, but doesn't deal with how gear ratio effects acceleration.

http://www.mustangsandmore.com/ubb/DanJonesTorqueVsHP.html

 

[doctorgonzo]

Good info Pos, try and digest it all later. "Power" being a combination of compression, displacement, stroke, and RPM certainly makes sense.

I have increased compression (most likely) and displacement, but not stroke.

 

[tkent02]

How much of a power increase on the butt dyno?

 

[doctorgonzo]

How much of a power increase on the butt dyno?

Seems pretty good. Carbs aren't dialed in yet. It breaks up pretty bad when it hits the needles and before it gets to the main. This saturday is pull the carbs and change the needle height, then a few more miles before re-checking valves adjustment ad re-torquing the head. I haven't synced the carbs yet either. I don't want it to sit idling that long until I have the rings seated.

 

[Guest]

When you increase the displacement of an engine without increasing it's ability to breath more, in most cases it will not make anymore peak power, it just moves the power band down the RPM scale. So the butt dyno notices a big increase, which is a lot of fun.

 

[rapidray]

EXACTLY!!! Ray.

 

[Nessism]

Doc installed one of those D-port 1100 heads to go along with his overbore pistons, so ultimately it should be making more torque and HP - the question is how much more?

 

[doctorgonzo]

When you increase the displacement of an engine without increasing it's ability to breath more, in most cases it will not make anymore peak power, it just moves the power band down the RPM scale. So the butt dyno notices a big increase, which is a lot of fun.

That does seem like the effect. It used to not really start top pull until the 6000 to 7000 RPM range, and that seems more like the 5000 to 6000 range now. I still haven't had it over 8000. Been doing my break in by running it hard up to about 7500/8000 backing it down, running it up, lather rinse repeat.

The carbs aren't dialed in yet, so have a better handle on it once they are sorted.

VERY happy with APE's work Jay, and highly recommend you guys to everyone. The shipping made it a bit more expensive than a local shop, but the warm fuzzy of knowing you guys really know your stuff on these motors made it worth it in my opinion.

 

[doctorgonzo]

Doc installed one of those D-port 1100 heads to go along with his overbore pistons, so ultimately it should be making more torque and HP - the question is how much more?

I may dyno it at some future point when I have more money than sense.

I am also looking somewhat longingly at carb options. I want more streetworthy than smoothbore (and not that expensive). I've been trolling ebay looking at 36mm and 38mm GSXR carbs as a potential future upgrade.