Off Roading Forums banner
1 - 3 of 3 Posts

Discussion Starter · #1 ·
I know we had a similar discussion on the old board, but I thought I'd try again. There are a lot of "new" people here now who can learn from or add to the topic. I've seen some of the old "what's better" questions popping up again in the last few months, ie the 383 question.

I think a lot of these questions come from not understanding the basic relations between engine components. I know I don't fully understand it all, and I feel I'm pretty good at this.

I have some experience in engine building. Professionally I worked as a mechanic for a few years before and after my time in the Corps. I have worked in machine shops, starting as "that kid who cleans up the shop" to the point where I was helping to build high performance engines. I have also been involved in drag racing as a hobby, building my own engines. I do not claim to be an expert, but I think I have a pretty good grasp on the basics. Here is how I understand some of the more basic principles of a reciprocating piston engine as it relates to performance. If you see something that is wrong I want to know, I'm trying to start a discussion in hopes of making things more clear for everyone, including me. I don't know it all, but I'd like to.

Assuming all else is equal (chamber pressure, stroke, rod length, etc) a larger bore results in more power. This seems pretty simple, same amount of pressure exerted over a larger surface area equals more force pushing down on the rod. Small increases in bore diameter have a big effect on surface area. For example: boring a 350 Chevy SB (4.000" bore) .030" over (4.030") yields 1.514 square inches more surface area in the engine (.1892 sq " per piston).

Stroke is determined by the throw of the crank (stroke = 2x throw). The throw of the crank is like a lever acting upon the load placed on the engine, the longer the throw the more potential torque made by the engine. The longer the throw the more time the piston spends "crossing over" at the top and bottom of it's motion, with higher piston speeds in the middle of it's travel. This can allow a higher duration cam to be used. The engine can also benefit from better flowing heads or larger valves. Longer throws also tend to be heavier. Adding mass to the rotating (not the same as the reciprocating mass) assembly can improve performance at the very low RPM end of the spectrum, but hurts acceleration.

Connecting Rods:
Rod length is one of the areas where I feel I have a lot to learn. Here is how I understand the rod length issue. In addition to providing a flexible means of connecting pistons to the crank the connecting rods also act like a lever. The the greater the ratio between rod length and throw length, the more advantage over the crankshaft throw and hence the load. This provides more torque. The downside is that it also increases reciprocating mass. I've been told a lot of conflicting information regarding the acceleration characteristics of a long vs. short rod engine but as far as low end torque is concerned it seems the higher the rod to throw ratio the better.

Compression ratio:
Compression ratio improves horsepower. No doubt about it. However, it seems to move peak torque higher in the RPM range. The increased compression seems to rob engine torque at lower RPM to squeeze the mixture. The engine with higher compression will usually produce a higher amount of torque, but it will be at a higher RPM. The amount of shift in the RPM of the engines peak torque seems to be related to the amount of compression increase. The higher the CR the higher the torque peak and less torque at lower RPM than an engine with lower compression. This is not something I have been taught or told, it is something I have seen in dyno tests. Once I realized what was happening it seemed like common sense, but it is not something I hear mentioned when increased compression ratios are discussed.

Well, what do you guys have to add? Maybe another thread can discuss things like cams, heads, intakes, etc. but for now I'd like to stick with the basic components.

I am pretty good with cams, heads, and intakes and I would like to get into that later. I've spent a lot of time at a flowbench. Mostly ruining junk heads, after all you don't know how far you can go until you've gone too far. (in careful, controlled steps)

My other car is a BULLDOZER

3,478 Posts
A higher compression ratio also makes the engine more thermally efficient, meaning more of the energy released by combustion is converted into work. This is why, generally speaking, that diesels are more efficient that gasoline engines. Hopefully, my high school physics teacher will be proud that I remembered that./wwwthreads_images/icons/smile.gif/wwwthreads_images/icons/crazy.gif

Discussion Starter · #3 ·
One note that I would like to add is that a power increase is usually an increase in efficiency. A higher cr increases efficiency by increasing cumbustion chamber temp and thus pressure. The higher the pressure the more force pushing down on the piston.
Also, to my knowledge, increasing the rod length effects the torque output but allowing the crank to turn more degrees past TDC before power is applied. This may sound confusing but isn't. Maximum torque is being applied when the crank is 90 degrees ATDC. For every degree before and after 90 degrees ATDC the amount of torque applied is a function of the resulting angle. Combustion Chamber pressure continues to increse as the piston begins to move. Using a longer rod allows the crank to move further past TDC before combustion pressure peaks. This means that you will have higher cylinder pressures when the crank is near 90 degrees ATDC which produces more torque. I hope every one can understand what I said. Maybe some one with more experience in this area would be able to clarify it a little better.

1 - 3 of 3 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.