comp ratios

Hi, I noticed by going through the specs of several bikes, and by looking at big bore kits, that the older bikes tend to max out around 10.25:1 comp for street use, and the newer bikes have higher comp ratios (in the 12-13:1 range). Why is this, and how do the newer bikes handle the higher ratios and not have to run higher octane "race gas"? Hope someone knows, and will take the time to explain it. thanks

First off the new engines are water cooled so they can disapate more heat, and secondly static compression determines what octane to use. The cams will have a direct effect on octane too. Just because you have 12.1 does not mean you have over 250 static comppresion. This I do believe is correct. Anyone else have ideers.

I think you mean dynamic compression instead of static.
Dynamic compression is what the engine actually "sees".

Back in the 60's Chevrolet HiPo motors had a static (advertised) compression ratio of 12.5:1, but due to valve timing overlap (allowing compression to bleed out the exhaust) the actual (dynamic) was closer to 9:1.

One of the Nascar classes runs a 17:1 ratio, but due to carb restriction the actual (dynamic) is ~10:1

Static ratio assume that the cylinder is completely full (atmoshereic pressure) at BDC, and during actual operation that is seldomly the case.

Liquid cooling plays a big part in carrying away heat along with new materials which can resist the heat. Combustion chamber shape with muti-valve heads is another. Also as much as 30 years of R&D from factories and gas companies. Back in the 70's still running leaded gas and had only 2 valve heads. A 4 valve engine with centrally located plug, liquid cooled and precise fuel injection is heads and shoulders above a 2 valve, air cooled motor with a plug set off to one side because that is the only place it will fit. It's akin to comparing a 1940's car engine to an early 70's era muscle car. Back in the 40's side valve engines where in use. You would be lucky to get 100 HP out of an 8 cylinder engine. Go ahead 30 years and you get 250 HP overhead valve small blocks.

The only thing new is the use of computer controlled fuel metering, overhead valves, four valve heads, etc were developed a "long" time ago (prior to WW II).
The limiting factor is peak cylinder pressure, and the limiting factor there is the octane rating of the fuel used (92 octane normally today).

For more info, please read the following:

www.sdsefi.com/meltdown.htm

Engine Performance

I should have remembered Speedomotive

"Compression Ratio" as a term sounds very descriptive. However, compression ratio by itself is like torque without RPM or tire diameter without a tread with. Compression ratio is only useful when other factors accompany it. Compression pressure is what the engine actually sees. High compression pressure increases the tendency toward detonation, while low compression pressure reduces performance and economy. Compression pressure varies in an engine every time the throttle is moved. Valve size, engine RPM, cylinder head, manifold and cam design, carburetor size, altitude, fuel, engine and air temperature and compression ratio all combine to determine compression pressure. Supercharging and turbo-charging can drastically alter compression pressures.

In 1936 the Enfield 'JF' was considered a superb, smooth machine with a very modern and efficient four-valve engine.

And a 100 mph 350cc motorcycle from a British company named Chater-Lea

The company's last motorcycle sales were in 1936. Their best machine was a 350cc ohc which at 100mph was the world's fastest 350cc machine

And Excelsior (another British company)

The four-valve 250cc Manxman was released in 1935, later produced in 350 and 500cc sizes, as well as a 250cc model with fully-enclosed, water-cooled engine.

Notice the "Four valves and water cooled"

www.ianchadwick.com/motorcycles/enfield

The AJS 500cc V-four was launched in 1935, initially as a roadster. It had chain-driven single overhead cams, front-mounted supercharger. Initially air-cooled, it became liquid-cooled in 1939

www.ianchadwick.com/motorcycles/britbikes/brit_a.html

And that's just the British.

I better quit before this post becomes two pages long :laugh:

Post edited by: steell, at: 2006/01/31 22:26

those guys were well ahead of their time if they had the suspension and tire technology they would have really had something.ive allways said the Z1s and kzs have more than enough power its the suspension and rubber of the time that held them back! goodluck,happy wrenching!

Wow, thats a lot of great info! I love learning about this stuff!! thanks a lot
Scott

I agree with compression ratio being a useless number. I usually use it with V.E. (volumetric efficiency) to get an idea on whats happening.

Here are my thoughts (which may not be right - so feel free to correct).

So a bike might be designed for say a 12:1 compression ratio, but in real life only 85% of the cylinder is being filled on the intake, which means the real dynamic compression ratio is much lower - say roughly 10:1

Secondly, when a gas is compressed it heats up - compress it enough and it will self ignite - this is the principle that diesel engines used. It also means that there is a maximum compression ratio limit for petrol based engines. Its roughly about 14:1 or 15:1 (the lower limits of diesel engines).

Plugs are designed to wick heat out of the engine thru the cylinder head, and to self clean. Fuel is also used to cool and lubricate the engine, so you need to match the fuel and plugs together. The range you want is between 400C (the minimum temp that a plug will self clean) and 800C (the maximum plug temp before the tip melts or it cannot transfer heat) [I assume ngk plugs].

Octane is a retardent put in fuel - it doesnt really burn (its the hexane and O2 that does), so the more octane in the fuel (can never be more than 100%) the more resistant to burning the fuel will be. So if straight cut (unrefined) fuel is 87 octane - it will burn at about 400C, 98 octane will burn roughly at 800C (these are not the correct numbers, just to highlight the issue).

The spark plug only initiates the fuel burn (since its not diesel and not compressed to its flash point - it needs an assistor). If the fuel wasnt pre-heated by compression it would just put out the spark (like a firehose) - this is what happens when the engine floods with too much fuel. Once its compressed and ready to go the spark plug, initiates the burn.

The more mix you have compressed the bigger the bang [actually burn since bang is not terribly good] when its ignited, so the more powerful the power stroke, and more torque is produced on the crank. So more compression = more power but at a higher temperature.

You can also look at it another way. If you want to keep the power the same, then rasing the compression requires less fuel to be used - so it becomes more economic - both on fuel usage and fuel production.

As to why it wasn't done ages ago. First of all fuel couldnt be refined as well, then the materials (cast iron) couldnt cope with the heat produced, nor could the cylinder head design cope with the pressures involved.

wow!:blink:

TwoSeven is right ..as far as he goes..
So is Steell...........
This CAN easily turn into a multi page post...
Check out a book "hot rod math"
It will either help you out OR drive you nuts.

I guess I'll restart this post instead of adding another new one...

I've searched and haven't been able to find a straight answer to the question below -

What's the minimum octane fuel needed when running high compression pistons (12:1 or 13:1, drag race type pistons) in our KZ's? I'm considering going to the higher compression pistons but I'm not sure if I can run on pump gas. Anyone have experience with this? Thanks