I had a few questions in particular.

why do forced induction set-ups use lower ratios?

also, sometimes when I read about tuning/tuning software.. there are usually options for different compression ratios.. why would you want anything other than the highest? Internal component strength im guessing?

im trying to get a better understanding on the entire concept so any other info you might want to throw in would be much appreciated!

 

Lower compression ratios help create a slower more controlled burn and less heat. Higher compression ratios create more heat typically and are therefor more prone to detonation, especially under forced induction. This is why you have to use high octane fuels on super high compression motors.

 

Interesting. thanks man

but heat is heat either way so why does it matter if you introduce more air into the cylinder?

thanks again - up for more

 

Because too much heat and air inside your combustion during the time of your "forced induction" will be too much to handle for the motor. By using certain pistons you lower/raise the compression depending if its either a dome shape or flat. This helps control the amount of detonation inside the chamber, as well as regulating the amount of air and fuel that is being dispersed. By lowering compression, it eases the load on the engine more less, allowing it to work with the turbo more efficiently.

Here is a calculator that would kinda help you grasp the conception concept. You can mix and match a number of different blocks, heads, pistons (flat domes for low compression setups) and head gasket combinations for different compression readings: http://www.ff-squad.com/technet/cr-calculator.htm

 

cool thanks alot man

 

There are basically 3 compression ratios you need to be concerned with.

The first is the one everyone knows about which is the static compression ratio. The static compression ratio is the ratio between the displacement of the cylinder vs the displacement of the combustion chamber. The formula for it is cylinder volume plus combustion chamber volume divided by the combustion chamber volume. This is the compression ratio people normally speak of such as "10:1" etc. Basically the higher the point you want to make peak torque the higher your static compression ratio needs to be, so more is not necessarily better.

The second is the effective compression ratio. This is the compression ratio measured at the point which the intake valve closes. The cylinder is not sealed until the intake valve is completely closed and the intake valve does not close at exactly bottom dead center. Using normal pump gas this ratio for an 4 valve engine with an aluminum head this ratio should usually be around 8.5:1. On a 2 valve head and cast iron heads this number needs to be lower because iron does not transfer heat as well as aluminum. You need to choose a cam that closes the intake valves so the effective ratio is around this number. Any less and the engine will not be very efficient. Any higher and the heat generated by compressing the mixture may prematurely light off the mixture and the engine will detonate. Calculating the effective compression ratio involves a little trigonometry. The 8.5:1 ratio is for a naturally aspirated engine.

The third is the dynamic compression ratio and is basically the effective compression ratio times volumetric efficiency. Its not something that is directly measured but is inferred by you brake mean effective pressure.

AS the engine compresses the mixture it's temperature rises. If it gets too hot in the combustion chamber the mixture will light off before the ignition fires the spark plug. This is generically known as detonation. When people ask what the highest compression ratio you can run on pump gas there is no real corect answer. You could have a static compression ratio of 15:1 and still run pump gas without detonation - if your effective compression ratio is around 8.5:1. This would likely result in an inefficient engine, especially at low rpm, because the engine would have to rev so high to make use of intake valves that close so late. The reason the intake valves close after bottom dead center is because the mixture that is flowing into the cylinder has mass and therefore it has inertia. When the piston stops at bottom dead center is tends to keep flowing because because of this inertia and if the valve remains open for a while longer you can get more mixture into the cylinder. This is known as inertial supercharging and it is the means by which you can get volumetric efficiencies higher than 100%.

A turbo uses a lower compression ratio because the mixture is already partially compressed by the turbo before it enters the cylinder. Also, by using a lower compression ratio by default the combustion chamber is physically larger. Therefore, there is a larger volume to stuff the mixture into and therefore you can get more mixture into the cylinder while still maintaining a good dynamic compression ratio.

 

wow that was alot of good info Scott Tucker. Thank you.

So to clarify, the effective compression ratio is due to the inertia of the air alone? or do the cams/valve springs (tension) effect it aswell? is effective compression something you can tune? (harder springs)?

It is interesting how you put it in terms of the air having inertia I never would have thought of it that way.. So even if the piston is on its way back up (compressing) air can still be comming in because it has nothing pushing back against it (yet) =inertia

very cool. I have to go revisit my highschool physics notes! haha

thanks again,very interesting

 

This is the only way a naturally aspirated engine can achieve greater than 100% VE.

Air is a fluid. It has weight, and thus inertia.

 

Well, not exactly. The effective compression ratio is the one measured at the point the intake valves close. It is a mechanical ratio and does not vary while the engine is running.

The only way to change it is by changing the static compression ratio, degreeing the intake cam (known as changing its 'phasing'), or by using a different cam with more or less duration. An intake cam with less duration will raise the effective compression ratio (because the intake valves will close earlier) and an intake cam with more duration will decrease the effective compression ratio (because the intake valves will close later).

Yes, air definitely has inertia and will keep filling the cylinder even when the piston is moving upward to a certain extent. There is a point when it will spring back and travel back out the intake valve though. The idea is to get as much air (and fuel) trapped in the cylinder as possible by the time the intake valves close. When that occurs, that will be the rpm point torque peaks and volumetric efficiency is highest. Below peak torque the air/fuel does not have enough velocity and some of it reverts back up the intake.

Where in the 408 area code are you? I am in Ben Lomond near Santa Cruz but I am originally from San Jose. I was supposed to be teaching a high performance engine preparation class at DeAnza College in spring but California budget killed that for this year. Hopefully the budget will allow it next year.

 

good info man thanks.

yea Im in San Jose. I never thought DeAnza would offer a class like that, Bummer on the budget cuts.. those are being felt everywhere.

again, Thanks for the info!

 

Yeah, that class has been offered since 1974. This is the first year we won't have it and would have been the first year I taught it, although I've lectured in it in the past. It's an awesome class, there is really nothing else offered anywhere else like it so I am really bummed. There are a lot of first class engine builders that were inspired by that class including Steve Dinan of Dinan Engineering, the BMW tuner.

 

Scott Tucker laid it all out. I learned something new myself