Please teach me about engine compression.

Please forgive my ignorance on this, but I'd like to learn more about the effects of changing the compression ratio on an engine. It is my general understanding that higher compression gives you more performance. But I'd like to understand it in greater detail.

Does higher compression produce more HP at all RPMs, more top end, low end, middle?

Obviously there's a point where more is not better--damage to the engine, less "streetability", need racing fuel, etc.

In looking at the various aftermarket piston kits, there are different compression ratios available. My stock LTD came with an 8.7:1 compression ratio. When upgrading with a new piston kit, it seems that the most popular ratio for street bikes seems to be 10.25:1. I've also seen kits with 8.5:1, 7.8:1, etc., but it seems (from what I've read) that these are generally only used on turbocharged bikes. Why is that, and why do you want lower compression with a turbo?

If I ever wanted to install new pistons into my LTD, is there anything wrong with putting in a 1075/8.5:1 kit without a turbo? I'm not what you'd call a "performance rider". I like a little power sometimes, but I'm not generally an aggressive rider looking for a race at every traffic light. What would be the difference between the 8.5:1 kit and the 10.25:1 kit of the same size in terms of performance?

On my CSR, I installed a Wiseco 1135/10.25:1 kit. It's a very "spirited" bike, and really fun to ride. But frankly, I find that I ride my LTD more often because it has a "smoother" personality, and is more comfortable on longer rides. And, of course, I have to use premium fuel in my CSR, while my LTD still drinks regular.

With all the true engine gurus here, I'd love to read your opinions, and learn more about all the various factors to consider when choosing a compression ratio.

Thanks.

Skyman,
Check your library for a book ............
AUTO MATH HANDBOOK ,
by HPBOOKS

If they don't have it, buy a copy for $15.

Read it a few times and you will have a GREAT understanding of how and why engines work the way they do.

Thanks, MD. Checked with two local counties' libraries. No luck. :S

Places like summitracing or jegs should have it online.

Compression ratio is just what the name implies: it is a ratio of how much the piston is going to reduce(compress) the volume of air/fuel mix before it is ignited using "1" as the total volume of air/fuel when the piston is at the lowest point in its intake stroke.So an 8:1 compression ratio means that the fuel /air mix will be compressed 8 times from bottom of piston stroke to top before it is ignited.10:1 means it will be compressed 10 times before ignition.The higher the number,the more the volume of the air/fuel mix will be compressed or reduced..The more compression,the more power is generated. But no free lunch in physics.The more compression,the more heat is generated when the fuel is ignited,and the more force is generated on the combustion chamber and connecting rods,piston,crank,etc.Therefore,more wear on engine parts and more requirement for higher octane fuel to prevent "predetonation" when the fuel/air mix explodes prematurely due to excess heat and pressure.Always a trade off.So generally speaking,a higher compression ratio will give u more power in exchange for more heat,more wear, and the need for higher octane fuel.U pays the money,u makes the choice.Thats why these guys running Kawi dragsters are running 13:1 or 14:1 compression ratio engines,are using 101 octane av gas or alcohol mixes, and replacing heads and lower ends about every 10-20 passes down the strip.Meanwhile,us schmucks with 8.5:1 street engines are running bikes with 30,000 miles on them and NO engine work at all.Ur choice.U can raise compression ratio by raising the top of the piston (a dome) thus decreasing the space from piston top to bottom of the head, or mill the head a few thousands and decrease the distance from the bottom of the head to top of the piston.way simple but hope this helps.bj

hocbj23 wrote:

Compression ratio is just what the name implies: it is a ratio of how much the piston is going to reduce(compress) the volume of air/fuel mix before it is ignited using "1" as the total volume of air/fuel when the piston is at the lowest point in its intake stroke.So an 8:1 compression ratio means that the fuel /air mix will be compressed 8 times from bottom of piston stroke to top before it is ignited.10:1 means it will be compressed 10 times before ignition.The higher the number,the more the volume of the air/fuel mix will be compressed or reduced..The more compression,the more power is generated. But no free lunch in physics.The more compression,the more heat is generated when the fuel is ignited,and the more force is generated on the combustion chamber and connecting rods,piston,crank,etc.Therefore,more wear on engine parts and more requirement for higher octane fuel to prevent "predetonation" when the fuel/air mix explodes prematurely due to excess heat and pressure.Always a trade off.So generally speaking,a higher compression ratio will give u more power in exchange for more heat,more wear, and the need for higher octane fuel.U pays the money,u makes the choice.Thats why these guys running Kawi dragsters are running 13:1 or 14:1 compression ratio engines,are using 101 octane av gas or alcohol mixes, and replacing heads and lower ends about every 10-20 passes down the strip.Meanwhile,us schmucks with 8.5:1 street engines are running bikes with 30,000 miles on them and NO engine work at all.Ur choice.U can raise compression ratio by raising the top of the piston (a dome) thus decreasing the space from piston top to bottom of the head, or mill the head a few thousands and decrease the distance from the bottom of the head to top of the piston.way simple but hope this helps.bj

WAY,WAY,WAY too simple, simple enough to get a weird running bike or broken parts.

Read the book,,,,
It will help you understand how to put together a good combination that works for you.

Okay, I understand that part.

Maybe I'm wrong, but it seems I've never seen anyone talk about increasing displacement (with bigger pistons), but leaving the compression ratio down in the ~8:1 range, like the stock pistons. Is it just because for most people, the purpose of increasing the bore is to get more power, and if you're going to replace the pistons, you might as well also boost the comp ratio?

The reason I ask this is that with my LTD, I've been thinking about replacing the pistons at some point. When I first got this bike a few years ago, I did a top-end rebuild. It was my first attempt at this type of project, and I had no idea what I was doing. The engine had 40K+ miles on it, and the cam chain guides were worn out/broken. Since I was already in there to replace the guides, I figured I would also replace the rings--thinking that was all I needed to do to "freshen up" the pistons. I also removed the valves and cleaned up the seats, replaced the seals, etc. Unfortunately, I never checked the ring gap when putting it back together.

The bike ran well afterwards, (and still does), but the cylinder compression is on the low side (around 100-110psi across all 4). I did a leak-down to confirm it wasn't a valve issue. I'm guessing that putting new (STD) rings on to the original pistons (with a light honing of the cyls) didn't fix the problem of cylinder wear, and that I really should have re-bored and up-sized.

So...as I said, the bike still runs well. But the low compression numbers still bug me sometimes, and I start thinking about maybe replacing the pistons. But I don't necessarily want to hop it up like I did with my CSR. I kind of like its more mellow temperament right now.

So, if I did decide to replace the pistons, is there any reason I should NOT get something like a 1075/8.5:1 kit?

Dude didnt ask for a Ph.D thesis on engine design,he asked for
"various factors to consider when choosing a compression ratio>' Those are factors to consider:heat,wear and fuel requirements versus longevity,dependability,and economy.Weird running bike?Broken parts? Only if compression ratio is your ONLY consideration and of course it isnt.BJ

Ask yourself a question: given that the street bike market is geared towards horsepower, don't you think Kawi or Honda would drop in higher comp pistons if getting more power was that simple?

What I remember (for example on my Jaguar) was they had an optional "C" type head (C stood for competition) that had higher compression. An, it also had bigger valves, different cams, and was fed by three carburetors instead of two. So raising compression alone ain't the story, it all has to play together.

Look through some web sites,
Find a kit you like the looks of,
And call and ASK QUESTIONS,
If the rest of the bike is stock,
Pipe, cams, filters, valve sizes, porting and all that.
The people who work for GOOD companies will know what will work for you and your bike for the intended purpose.

If you make the wrong choice then it ends up costing extra money,,,,,

It could be just having to buy octane booster,
Or it could be having to put in different valve springs because it makes its power boost at a higher RPM.

The point is ,,,,,,
DO YOUR HOMEWORK NOW.

If you make the right choice, you will end up with a sweet running bike that you will fall in love with all over again.

It's interesting that 8.7:1 was the standard comp.ratio on a '78 Kaw. My '77 KZ650 has a standard comp.ratio of 9.5:1; I wonder what the engineering considerations were on the larger enginein 1978. Ed

Very good question.
It does make you think though.
I wonder how different the cam timing is ?