[the_texan]

Ok, let me first state that I know very little about the actual functional specifications of the internals of an engine. Though I am trying to learn, it is pretty much a foreign language to me (computer tech/network engineer). What I am trying to understand, is engine compression, but not a basic concept - that I think I have (pre-supercharging, change the pistons to reduce the compression to allow for the post-supercharged compressed air volume), but more of a "why"?

Let me explain where my confusion stems from (using an E-Force for reference).

The stock 3v Mustang GT compression ratio is 9.8:1 (according to various web sites). The E-Force (and many other superchargers) boast as a "bolt-on", not really eluding to the need for changing the internals (to forged), nor dropping the compression (a Livernois block I looked at was a 9.1:1 or 9.7:1). I assume in the Livernois block mentioned, the 9.1:1 is for supercharged applications, and the 9.7:1 is N/A.

So, understanding that boost=more compressed air=more air in the cylinder to fire/expand on ignition, why do you need to lower the compression; in other words, why lose horsepower/torque to gain it again. Why not just leave the compression the same?

Assuming (yes, I make a lot of assumptions) this is due to some natural limit of maximum compression in a cylinder before it essentially blows apart, is there a compression ratio-to-boost "chart" anywhere that could possibly give a guideline as to what compression ratio to aim for for a given range of horsepower?

Reason for asking: I recently called an engine builder here in Houston and got a quote on swapping out the internals in my stock 3v block to forged. They mentioned dropping the compression (don't remember to exactly what), but I'm wondering if they base their compression ratios on the 1200-1500 HP engines they've built in the past. Perhaps the compression doesn't need to be AS low for me (I feel like I'll probably be happy with 600-700 HP), and making it that low would actually "cost" me some power.

On the flip-side, is there any HP "level" where it is essentially understood that you DO need to change the compression ratio? If the E-Force and many others advertise themselves as applicable "bolt-on" power-adders, then they also (in my opinion) state they are "safe" for the stock compression ratio of 9.8:1. I'm assuming this is for the stock low-boost pulley only, but when would it then truly become necessary to drop the compression ratio inside the cylinder?

As I said, I know very little to nothing about the inner workings/specifications for the function of the engines. The only way to gain knowledge is to ask questions, and though it may seem like a stupid question to some, I wouldn't ask if I honestly didn't have the slightest idea. I only hope I didn't make this too confusing for those who would like to help me understand.

 

[boxcabmike]

Supercharging an engine is no big mystery. You take a given volume, (your cylinders) and ram it full of air under pressure(boost). More air means you can burn more fuel. More burning fuel and more release of BTU's, (energy in the form of heat) Do you need to build an engine for a supercharger? Yes and no. If you build up the internals, rods pistons crankshaft, you make that engine more able to cope with the pressure gains in the cylinders. If you don't, you have to keep the boost down to about 8 PSI max with all the stock parts. What's the real limiter? Your head gaskets actually. If you have evaer heard the term" o-ringing" the heads it means just that. you take a copper o-ring and copper gaskets and seal that head gasket down tight. Lots of machining and big bucks. These tire burning rear end flailing drag pictures everyone advertises with are usually the result of lots and lots of cubic dollars or a one tim grenade with the pin pulled. The reality on the street is that unless you o-ring those heads and limit your compression ratio with an otherwise stock engine yours will probably end up with a gigantic thud and a spill akin to the Exxon valdize at some point. You want teeth grinding HP? Buy a 5.0. The bottom end will easily take 10LBS of boost and the parts are available. 4.6 or 4.o? save your money and trade up.

 

[Guest]

I think detonation is the reason for lower compression and quench is the problem. Google quench.

 

[Guest]

I think detonation is the reason for lower compression and quench is the problem. Google quench.

Detontation does play apart in this since the majority of these builds are for guys that want to be able to run on pump gas. A lot of it comes down to physics 101. The increased volume of air and fuel inside a space which is decreasing in size creates heat. To try and alleviate some of this heat, drop the compression making the combustion chamber larger making more room for expansion. Proper cooling, increased fuel (acts as a coolant inside the combustion chamber) and higher octane fuels can over come these problems.

The idea behind dropping C/R is that the additional boost will make up for the power loost due to the drop and then some but I think this is starting to change from what I have seen lately...the new 5.0 is a good example. The type of induction system should also been taken into consideration when deciding on the C/R of the engine. Centrifugal and turbos need to keep the C/Rs up especially in street applications. This keeps the car from driving like a dog while not in boost.

Previously and still today, the majority of engine builders recommended lowering the C/R (8.xx's) for a positive displacement blower. These blowers compress a given volume of air per revolution every revolution and are not dependent on RPM. Recently, I have seen builders leaning towards C/Rs in the 9.xx's. Personally, I run a Whipple making +/- 16 psi on a 9.2:1 C/R making over 600rwhp...and no problems with blowing head gaskets. MLS gaskets and good head studs/bolts prevent the heads from lifting and blowing out gaskets. A quality rotating assembly (especially good pistons with deep ring lands for boost), prepped block and good components will get you where you want to be. There are plenty of guys running 20+ psi on similar setups making a lot more power!

 

[908ssp]

Another thing to remember is engine development is evolutionary. Years ago keeping a proper fuel to air ratio was more difficult so lowering the compression and pumping more air allowed the then primitive fuel injection systems to properly set fuel ratio. Now with modern electronic fuel injection this is easier and a higher CR can be maintained. The better your tuner the better your results.