I know the stock lt1 is 10.something:1 compression. What is the number derived from and how do you figure it out? My car has over 100k and I'm wondering what it is now. I know the car when stock didn't run close to what a new one did. I think the hp is 275 stock so i figure i was around 225 to 250 before my upgrades. I am trying to figure what may have worn out and if i can replace it or fix it to get more power. I have a tax return coming so i will have 1000+ to spend. What usually goes out or gets worn that causes performance to drop?

 

compression is based on the cylender bore, crank stroke, head CCs and head gasket thickness, that never changes in an engine, your still 10:1 compression, I see you have a blower on there so one of the first things that go are the rings, the extra pressure/heat, inside the cylenders forcing down on the rings wears them out, is it loosing that much HP? maybe just do a good tune up on it.

 

Compression is determined from certain dimentions like bore, stroke, chamber volume...etc. These will not change with usage.
Maybe it needs a tune up.

 

All fluids, Opti, plugs, pvc, wires, coil & air filter just done. Is there anything else to do besides the oil and filter for maintenance?

 

From what you have posted, it sounds like you are thinking of cranking pressure, not compression ratio (CR)? Two entirely different things. CR is simply the theoretical mathematical relationship between the volume above the top of the piston at bottom dead center (BDC) and the volume at top dead center (TDC).

volume BDC/volume TDC = CR

This is simply a mathematical relationship and can't be directly measured. However, you can measure the volumes and thus calculate the ratio. Usually though, it's estimated from knowing the volumes involved. Fairly easily done longhand, but there are a multitude of on-line calculators you can use for convenience. This one http://www.rosspistons.com/calculator.php includes a large number of variables, chiefly of interest just to illustrate the variables involved. You can be pretty accurate with a simple calculator like this http://www.wallaceracing.com/cr_test2.php which has just five variables.

OTOH, cranking pressure is a measured value. It is pressure in the cylinder measured while turning the motor over. Typically, a gauge is screwed into a spark plug hole and the engine cranked with the starter. The peak pressure is what you are looking for. If you try this, it should be done with the other plugs removed in order to get a decent cranking speed. Also, you don't want it to start, so disconnect the coil and the fuel pump if "somehow" you have left the other plugs in for some reason. If you somehow "forget" these two steps, the results can be "interesting" as I can attest to Also, the throttle should be wide open when testing.

Cranking pressure depends on both dynamic and static aspects of the engine being tested. Just as with static leak down testing, the better the ring and valve sealing the higer the cranking pressure. Unlike with a leakdown, the cranking pressure will also vary with the valve timing. The later the intake valve closes (the IVC point), the lower the cranking pressure. So, it is not an absolute value like leak down and cannot be used to directly compare two different motors. Consequently, the use of cranking pressure is primarily under two scenarios. It may be recorded for each cylinder when the engine is first assembles and monitored to detect wear (signified by falling cranking pressure). Alternatively, if it is a known spec (as it may be for a stock motor) it can be used as a single measurement to see if the motor is up to spec. Some people try to use cranking pressure as a way to determine if the CR is too high to avoid detonation. As a rule of thumb, many people use 170-180psi as a maximum. I do not use cranking pressure in this way, but many people do.

I bet someone here knows the stock spec for cranking pressure in an LT1, maybe they will chime in. But if you have access to a leakdown tester, you can use that instead to get an idea how "tired" your motor is. With conventional (non-gapless) rings a new OEM production motor will typically be in the 3-5% range. Above 10-12% and it's pretty tired. A carefully assembled race motor will be in the 1-3% range when fresh. Gapless rings will show near 0% leakdown on a fresh motor.

Rich

 

Damn, isn't that a great post if I do say so myself

Rich - who has had too much coffee and is psyched up as he is about to go out to the shop and finishing assembling his NEW race motor, ahhh, life is good! I live for this crap!!!!!

 

I like to do a leak down test over just a cranking pressure test.

 

That is a great post, going to have to save this one.

 

 

BTW: that 170-180psi rule of thumb refers to pump gas. Race motors are often way higher - up to 250psi.

Rich

 

How about you just use the Camaroz28 calculator lol. It tells you. I have a 388 ci stroker N/A with 12.1:1 compression and the calculator got it right on the point, so its correct.

 

not to mention aluminum heads you can run higher compression on pump gas, im doing it, but runs better when I run say 107 octane