Learning...got a compression question

[9fourteen]

I'm trying to understand how the bore, stroke and compression ratio are related. I looked up the formula for compression ratio and the only variable that I don't understand is the volume of the combustion chamber. I think I understand what it is but what I don't understand is how it varies from lets say a 2.0T and 2.0S engine. Those engines have the same bore and stroke but the S has a higher compression ratio. I assume they use the same head. What is different in the S engine? High domed piston?

I also have another question, is there anyway to calculate the horsepower (or torque)? Like a theoretical based upon engine specifications. Or can it only be measured like on a dyno?

[Flieger]

Yes, the 911S has a higher dome than the 911T. The dome fills the combustion chamber and makes the volume of the gases in a compressed state smaller than with a flat piston. Smaller compressed volume means greater compression ratio. Minute changes in the compressed volume can have a huge effect on compression ratio compared to a larger expanded volume from something like a larger bore. Simple geometry and math.

You cannot predict torque becuase there are simply too many variables in mechanical components- let alone weather, atmospheric pressure, temperatures ambient and engine, oil type, fuel quality, engine wear.

A supercomputer may be able to use a complex formula but the best way to predict power is ask a knowledgeable engine builder about similar engines tested on the dyno in the past.

[Eagledriver]

You can make rough estimates of engine performance based on engine specs and math. The torque of an engine depends almost entirely on the displacement and volumetric efficiency. In other words if it takes in 3 liters of air it will produce x amount of torque. Volumetric efficiency of a well designed engine will be around 100percent over a small range of RPM and 80-90 percent over the power band. They can even get better than 100 percent with proper intake and exhaust tuning.

Power is just torque times RPM. An F-1 engine is 2.4 liters and probably makes under 200 ft lbs of torque. With a large portion of this torque available at 19000 RPM you get something like 700 HP.

This is how you can often tell if someone is lying about the power they can make. An SC motor makes around 200 HP with CIS at 5800 or so. With carbs it can make 240 or so but at a much higher RPM like 6600. If someone tells you their SC motor makes 240 at 5800 then you know that can't be true because it would have to have a volumetric efficiency that is not possible.

-Andy

[psalt]

I also have another question, is there anyway to calculate the horsepower (or torque)? Like a theoretical based upon engine specifications. Or can it only be measured like on a dyno?


The answer to your question is horsepower can "only" be calculated.

Torque is what is measured. Torque is instantaneous twisting force and what is measured by a dyno. Horsepower is a calculated abstract using the formula (rpmxTorque)/5250. Horsepower is a rate of doing work and involves a time element. Andy is correct that torque is a function of displacement and the NA limit is around 80 ft/lb per liter, regardless of engine design. Horsepower per liter varys much more because of the rpm factor. Wally Hassan, Jaguar XK and WC F1 engine designer, explains this clearly in his 1960 SAE paper on engine design. Torque is more of a factor than horsepower for a road car, the industry standard for operating conditions is 85% of time below 30% load.

[9fourteen]

Thanks for the responses. It's good information. I have no plans on becoming a designer of race engines or anything, just want to understand how things works and why I might want to make a certain modification. Hopefully, sooner than later I'll start rebuilding an engine for my 914. At least that is the goal. Anyways I found a article on the web that talks about the physics of an IC engine. I thought it was easy to understand for beginners like me. It has some simplified calculation on how to calculate torque and power that will give you some ball park numbers. Here's the link is anybody is interested.

http://mb-soft.com/public2/engine.html