[VelociRaptor]

When it comes to building engines for all out performance for any application is for one is to look at the average rpm band the engine "see's" the most. Do you fire road ride, tight trail ride, round track, MX, drag, dune or simply a mix of all of the above. Make a decision on what fuel you want to run in the machine and base the compression of the build on that octane. For a build to be a success for the owner it needs to be better in most if not all areas than it was before, there is no reason for someone to run a large cam if the engine rarely revs over 6000rpm(limited by terrain not power curve).
Compression:
Compression is simple really. Take your hand and put it over your mouth. With all of the air out of your mouth, use your tongue like a piston and try to draw air in. You will notice a vacuum will build instantly trying to get air through your mouth which is against your hand.
Now with your hand still over your mouth try this. With your mouth being half full of air do the same thing with your tongue. You will notice more delay of vacuum building in your mouth- the more air currently in there, moreso on the delay.
This is compression working on the intake stroke. The more compression you run in an engine the less "air" will be left in the chamber/cylinder after the previous exhaust stroke. With this then the higher the compression the faster the chamber/cylinder will pull vacuum and bring fresh air/fuel in. In return on this the fact the fresh air/fuel will be less contaminated by residual gas left over from the previous exhaust stroke the higher compression you run. The exhaust works better by removing more burned gases from the chamber/cylinder with higher compression and the intake has a faster recovery time on vacuum with less area in the chamber/cylinder to be filled at TDC.
Another thing about compression is also trying to be as effecient as possible at attaining it. Say you have an engine thats got a .040" thick head gasket, 60cc chamber volume, and the piston being .040" from the top of the cylinder at TDC and you have another engine with a .040" thick head gasket, 60cc chamber volume, and a piston being dead on .000" at the top at TDC. Both engines having the same static compression ratio which will make more power? The second engine because mainly the flame travel involved and the quench the second engine has by removing air/fuel as much as possible between the cylinder head deck and the top of the piston to get it into the chamber were it can be as close as possible to the spark plug. There will also be an advantage of having a smaller dome on the piston for flame travel and airflow across the piston.

 

[VelociRaptor]

Now that you see the advantages that more compression plays in the exhaust and intake cycles of the engine now lets look at this. Say you have an engine thats perfectly matched in cam profile, rpm use, compression, porting, intake assembly and exhaust assembly. Take that same engine and increase the compression and you can reduce the overlap of the camshaft, make more power everywere and have better driveabilty all at the same time.....wtf you say? Heres why, remember I said that the more compression you run the less residual burned gases are left behind after the exhaust valve has closed. In return you have less deluting of the fresh intake charge thats coming in. Overlap is just that, its taking up the slack of this residual gases being in the way and also getting a head start on the intake port/valve delivering the fresh air/fuel.....all while the piston hasnt really done anything but sit at TDC while both the exhaust and intake valves are open(did you know that happens?). Yes, its cheating mother nature by creating a siphoning effect of the exhaust valve being open all this time and the burned gases rushing out in that direction. The intake valve cracks open just before the exhaust valve closes and with all of this motion toward the exhaust going on in the chamber it siphons(sucks) some fresh air/fuel in by the intake valve and by the time the fresh air/fuel reaches the exhaust valve its closed and the piston starts its way down the cylinder creating a low pressure point(vacuum) in the chamber and the intake valve continues to open and delivering the fresh charge into the chamber/cylinder. The problem with this is designing the right amount of overlap in the cam profile to get the right amount of timing events to make that happen without fresh air/fuel going straight out the exhaust before the exhaust valve closes or not getting enough overlap to get as much as possible of the residual gases pushed out the exhaust valve before it closes.
What you have to understand is it takes time to get air and fuel to move, the faster the engine revs the less time it has for all of this to take place. The faster the engine is spinning(rpm) the more amount of overlap thats needed to effeciently do this....to a point. The more compression you have, the less residual burned gases are left over to be moved out of the way so in return there is less need of overlap.

 

[VelociRaptor]

The compression of an engine is were it gets the power to move the piston down the cylinder on the power stroke. The more cylinder pressure you can create the more force there is against the piston to move it down and in return turn the crank(torque). Horsepower is something that cannot be measured, it has to be calculated off of torque remember. Compression sounds like one of things you cannot have too much of but its not. Remember, diesel engines have roughly 20:1 compression and they dont have spark plugs for a reason- the cylinder pressure along with adding the fuel at just the right time basicly is all it uses.
Preignition will kill an engine.
Even with top of the line parts all your doing is delaying the kill. Spark knock is were theres either too much compression, too much ignition timing, too much load against the engine or too lean of a air/fuel mix or a mixture of all. What happens is there is so much cylinder pressure building up in the chamber/cylinder that the air/fuel ignites itself usually in the far edges of the piston and chamber and the flame travel starts heading toward the spark plug. By this time the plug has fired and ignited the mixture around itself and the flame runs into the flame coming from the other direction and bang, its lit too soon and tries to push the piston down before it reaches TDC and thats bad...and noisy.
On the other hand the more compression you have the strong and more violent the burn is to create energy to move the piston down the cylinder to create torque, thats right, torque through the crank. If its too violent then you know what happens.
The catch in all of this is there are two types of compression in all types of internal combustion engines:
Static and dynamic.
Static compression is what most of you are used to dealing with. The actual measured amount of displacement of the cylinder and chamber. How much amount is in the cylinder and chamber at BDC divided by the amount thats in the cylinder and chamber at TDC.
Dynamic compression is static compression with the amount of cylinder pressure thats bled off by the cams overlap and duration of the lobe profiles. Yes, its a little more involved but its a more precise way of showing how much compression an engine really has.
Remember, power is generated by cylinder pressure in its simplest terms. The larger the camshaft profiles are the longer the valves are open and in return of all that good stuff is you also have more and more pressure bleed off. How did all of those '60s muclecars run around on the street from the factory on pump gas and 11:1 or more compression along with iron heads? The cam was bleeding off cylinder pressure and the use of leaded gas. How does all of these crotch rockets and450s run around doing the samething today? Camshaft profiling and technology in FI and combustion chamber shapes.

 

[VelociRaptor]

Camshaft basics.
Lift- The measurement of how far the valve travels.
Duration- the measurement of long the valve spends open.
Lobe seperation- the distance from the peak lift of the exhaust lobe and the peak lift of the intake lobe.
Duration is usually the measurement that applies directly to the rpm useage of said camshaft. The more duration a cam has the higher the engine needs to spin to bring it into its effeciency range(powerband). This also means that the higher the engine has to rev to bring in the powerband the more rpm down low that becomes less useful(more compression is used to bring this back in line....to a point). Remember when I said it takes time to get air/fuel to move and that the higher the engine revs the less amount of time it has to get the mixture to move. This is were cam lobe design comes into play. The longer a valve is held open the more amount of time the engine has to fill the cylinder and in return keep making power as the revs get higher. Drawback on this is the larger the lobe(in general) the higher the engine can rev under power the more amount of power thats traded off down low. VTEC Honda engines are something designers and engineers have created thats actually two cams in one. It has a small, low end torque cam that the engine runs off of up to mid RPM and once the engine passes a certain RPM its swaps to a larger more aggressive horsepower cam lobe....almost best of both worlds. The more duration a cam has(without changing anything else)the more amount of overlap it will have. The only way to bring the overlap down to its previous level(compared to its smaller version) is to widen the lobe seperation angles. Overlap is a tuning tool for the cam engineer/designer to taylor said amount of duration to a particular chamber/port/compression combination. Change any one of those things and you can drasticly change the effeciency of said cam in that engine. Another thing about duration is, in general, the larger the engine is the more amount of duration it can use and be equally effecient compared to a smaller engine. Generally a cam that makes peak power at 7000rpm in a 686cc engine will make peak power at 6500rpm in a 804cc engine if nothing else is changes except the bore and stroke. Reason why is because of the bigger demand of air the 804 requires compared to the amount of air required in the 686. The cam has the same amount of time to deliver air/fuel in both so it will run out of time quicker the larger the engine size gets. Sure, port design and other factors can deviate this but if nothing else being changed this will most of the time be the case.

 

[OzLinc]

all good stuff there..........

 

[vfm2010]

Great write up Bo! You are 100% right on the money about building for the application not just for max HP. The components of an engine build should compliment each other to achieve the desired result (ie. don't build a drag motor for a rock hound).

 

[Britalican]

This was an interesting read.

 

[Xstriker]

Awesome, thanks Bo, great write up for those of us who are not as mechanically inclined.

 

[Baxter]

I miss Bo because I enjoy this stuff and he makes it easy to understand.