Thread: compression ratio

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Im looking for some information on compression ratios. ive been looking on the web but i cant really find what im looking for. if anyone cares to explain, id like to understand what the relationship between horsepower, torque and the compression ratio is. What are the pro's and con's of high and low compression ratios. and any other info you can offer would be great too!

Thanks
Nick

OK, first, compression ratio is just that, a ratio. It is not a definitive number given to a certain pressure. It is the relationship of a certain volume of gases that is squeezed into a smaller volume of gases. If you take 9 parts of volume and compress it into 1 part of volume, then you have realized a compression ratio of 9.0:1 or 9 to 1.

Generally speaking, there are several factors that limit the compression ratio that you can use in an internal compression motor.
1. The fuel used. All fuels have a limit of compression ratio with which they can be used. Ideally, you would want a fuel to explode slowly to prevent spiking the cylinder pressure and injuring parts of the motor. You want the mixture to go booooooooooooom instead of BOOM.
2. The intake closing point of the camshaft. Compression cannot begin in a cylinder until the intake valve has come to rest on its seat and has thus sealed the cylinder. This point will be expressed as a certain number of crankshaft degrees after bottom dead center (ABDC). Closing the intake valve later will allow the bleed-off of some of the mixture that has been drawn into the cylinder. The piston is on its way back up the bore and will push the mixture back up the intake tract, disrupting the signal at the carburetor venturi and contributing to stand-off, the fog of mixture you can see at night sitting atop the carburetor. Venturi signal disruption is what causes the rump-rump idle characteristics. The venturi can't effectively meter fuel with air going both ways and doesn't know whether to sh** or go blind.
3. Squish or quench or whatever you want to call it, but it is minimizing the distance from the piston crown to the underside of the cylinder head with the piston at top dead center. The tighter you can make this dimension without crashing the piston into the head, the lower the octane fuel you can use to make the same power that you can with a higher octane fuel. Generally speaking, a figure of between 0.035" and 0.045" is used as a minimum. The tighter the squish, the more violently the mixture is squeezed and shot out of the dead space across from the combustion chamber and the better the mixture is homogenated. This also blows the mixture past the spark plug as it is firing.

Power in a naturally aspirated motor will be increased by about 4% for each full point of compression between 8:1 and 12:1. If you're making 300 hp at 8:1 and you raise the static compression ratio to 12:1, you might make an additional 16% power, assuming all else is equal. That would put the power at 300 times 1.16 or 348 hp. Now, this is very simplistic and it wouldn't necessarily be this easy. For instance, you wouldn't be able to use the same camshaft in the motor that you did at 8:1 if you were going to use the same fuel, because the motor would rattle like a can full of rocks from detonation. You would have raised the static compression ratio and so you would have to extend the intake valve closing point to bleed off some of the compression

Up until the point of only having unleaded fuel as a choice at the pumps, pump fuel was available in a variety of octane blends and you could pretty well run race motors on the street. The addition of tetraethyl lead to the fuel did two things. It introduced little "blockers" into the the mixture and also coated the valves and valve seats so that no welding could take place as the valve returned to its seat. You can't weld a dirty junction and the lead made the juction dirty. The little particles of lead in the mixture got in the way of the flame propogation as it spread throughout the chamber and cylinder and slowed it down to the previously mentioned booooooooooom. Think of the flame kernel as the runner with the football and the lead in the mixture as members of the opposing team who are trying to tackle the runner. The runner must jig and jog, moving laterally sometimes to avoid being stopped by a tackler. You can see how this will slow down the runner. Same with the flame kernel. That's what tetraethyl lead did for fuel, so that we could run some astonishing compression ratios on the street. Much the same thing can be accomplished with water injection today. The molecules of water get in the way of the flame propogation and slow down the burn.

Although there are exceptions to this, it is generally accepted that with today's unleaded fuels, you should build for a limit of 9:1 with iron heads and 10:1 with aluminum heads in a street motor that will see idle to 5500. The exceptions would be the camshaft used, fuel used, squish used and whether or not you were using some sort of detonation reducing system like water injection.

To muddy up the water even more, we must also consider Dynamic compression ratio. That takes into account the rod length and intake closing point of the camshaft. If you were to build a motor with 8:1 static compression ratio and use a camshaft meant to make power from 4000 to 7500 for instance, then the motor wouldn't pull the hat off your head. That would be a gross mismatch of parts. On the other end of the spectrum, if you were to build a 12:1 motor and install an RV type camshaft, the motor would detonate so badly that you probably wouldn't get the cam broken in until the motor would be junk from broken pistons and rings resulting from excessive cylinder pressure.

I have alluded to fuels as one of the limiting factors and you can do your own research into the qualities of different fuels. I had fun recently playing around on the Dynosim and using different parts and pieces in a motor for Dave Severson. He is planning to use E85 and so can probably build an effective static compression ratio into the motor of around 15:1 (centrifugal blower) and drive on the street with it, filling up at the corner pump.

Bottom line is that you can't just pick a camshaft out of thin air and expect it to work in your motor. You must know all the measurements and specifics of your motor before making a cam choice. Use the dynamic compression ratio calculator of Keith Black's site to determine the camshaft to use if you want to choose your own cam. Of course the bulletproof way to do it is to call the cam grinder, but if you want to do it yourself, then use the calculator.

Using the same fuel, as you go higher on the static compression ratio, you must change the intake closing point of the camshaft to bleed off some of the compression generated so that the motor will not detonate. I played with some different static compression ratios and some different intake closing points at 0.050" tappet lift just to show the changes necessary. If you were going to build a motor with a 8.25:1 static compression ratio and you wanted to stay around 8.0:1 on the dynamic compression ratio so that you could run cheapo pump gas, you would want to choose a camshaft with an intake closing point of around 10 degrees after bottom dead center. Follow on down for higher and higher static compressions ratios. This was calculated on a 350 Chevy......
static 8.25, intake close 10, dynamic 8.010
static 8.50, intake close 20, dynamic 8.012
static 8.75, intake close 27, dynamic 8.022
static 9.00, intake close 33, dynamic 8.018
static 9.25, intake close 37, dynamic 8.061
static 9.50, intake close 42, dynamic 8.029
static 9.75, intake close 46, dynamic 8.016
static 10.00, intake close 49, dynamic 8.038
static 10.25, intake close 52, dynamic 8.043
static 10.50, intake close 55, dynamic 8.029
static 10.75, intake close 57, dynamic 8.069
static 11.00, intake close 60, dynamic 8.022
static 11.25, intake close 62, dynamic 8.038
static 11.50, intake close 64, dynamic 8.042
static 11.75, intake close 66, dynamic 8.035
static 12.00, intake close 68, dynamic 8.017

If you don't understand what I'm showing you here, please ask.

Well Said Techinspector!

Listen to the man, Nick - you just had a $1,000 college course, complete with the Cliff Notes, compressed and given to you as a "gift"

That's what's so great about this site!

Holy cow, that is some awesome info there, thanks for taking the time to write that out!

Why do you recommend a compression ratio of no less then 9:1 and 10:1 with iron and aluminum heads (respectively)?

One thing that is a little fuzzy... A higher compression engine requires a higher octane fuel? And with out that higher octance expect detonation?

And... With high performance engines, the higher compression ratios result in the fuel and air being compressed real tight, and when they combust throwing the piston back down the cylinder harder resulting in more pep?

Thanks
Nick

Originally Posted by NLMoschitta

Holy cow, that is some awesome info there, thanks for taking the time to write that out!

Why do you recommend a compression ratio of no less then 9:1 and 10:1 with iron and aluminum heads (respectively)?

One thing that is a little fuzzy... A higher compression engine requires a higher octane fuel? And with out that higher octance expect detonation?

And... With high performance engines, the higher compression ratios result in the fuel and air being compressed real tight, and when they combust throwing the piston back down the cylinder harder resulting in more pep?

Thanks
Nick

Nick, it's not a matter of recommending no less than 9:1 or 10:1, it's a matter of recommending NO MORE than that with a daily driver pump gas motor. Like I said, there are always exceptions to that rule. You can jockey the whole mess around by extending the intake closing point, tightening up the squish to 0.035", polishing the combustion chamber and piston crown to eliminate hot spots that could cause pre-ignition and optimizing the spark timing to prevent detonation. There are many iron-headed motors out there working every day at 11.0:1 static compression ratio, but everything must be optimized to make it work on pump gas. Then, the first time you pump in a load of bad gas, you're in big trouble. Better to build the iron head motor at 9.0:1 or a little higher and leave yourself a cushion to allow for variances down the road.

Yes and yes on the fuzzy thing.

Yes on the pep thing. Sounds like you have a pretty good understanding of how it all works.

As an aside, you can run a little higher static compression ratio with aluminum heads because of the higher heat rejection of aluminum over iron. The whole idea of higher compression is to keep the heads cool and you can do that better with aluminum. Do some research into the LT1 motor that Chevrolet produced. They used a reverse flow cooling system so that the heads got the coolest water that was available in the system before any of the other engine components. Traditionally, the heads and manifold are the last to get water and by the time it gets to the heads, it's already hot.

Again, great info! thanks for sharing the knowledge, its hard to find things like this explained so well. I have a much stronger grasp on the concept now!

Nick

One point missed in the foregoing discussion is the fact that a gasoline engine operates on a 4:1 expansion ratio, that is, with a compression pressure of, say, 100 lb., the pressure will be 400 lb. pushing the piston down when it fires. So you can see that a higher comp ratio will benefit the power output.

Originally Posted by R Pope

One point missed in the foregoing discussion is the fact that a gasoline engine operates on a 4:1 expansion ratio, that is, with a compression pressure of, say, 100 lb., the pressure will be 400 lb. pushing the piston down when it fires. So you can see that a higher comp ratio will benefit the power output.

I was not aware of that. Thank you for posting it.

Hey! I knew something about engines that Tech didn't! Made my day!
"I never met a man so dumb that he couldn't teach me something, and I never met a man so smart that I couldn't teach him something." Mark Twain, I think.