Compression VS Boost....

[jski55]

Dont know where to put this question.. The IDI has higher copression ratio 21 to 1 or something like that vs the psd 17 to 1 or so. psd spool up to 25 lbs boost or more. Recommended boost for the IDI is not supposed to exceed 13lbs. How does boost add to teh compression number? Meaning by equation or direct add.
ie: 21 plus 13 would that be 34. and would psd be 17 plus 25 or 42?
Just curious... I know some of you gurus know this stuff

thanks

[RunRed]

Not sure if I know either but I will throw this out there. My ford being NA gets atmospheric pressure drawn into the cylinder (since there is nothing to boost it in, it may be less than atmospheric by a little actually) so since atmospheric pressure is I think 14psi, you take 14 and then you figure the compression ratio.. so it would be 21 times that, 14x21=294psi. Now when you add a turbo to my IDI and build say 10psi, this is 10psi in addition to the 14, boost gauges read 0 because they are cailbrated to start at 14psi, so in theory, there is more pressure in death valley than mount everest because there is more space between the atmosphere and the earth's outer wall. Just like in the ocean there is a ton of PSI at the bottom and barely any at the top.. So back to the turbo, we take 14psi+10boostpsi X 21compression=504psi. So a PSD with 25psi boost is 663psi. 15psi boost on an IDI computes to 603psi. So you are really getting to the same pressures inside the combustion chamber but I am sure boost pressures are lower on the IDI because of the older head gaskets, haed bolts, etc., that limit the pressure that the IDI can handle. Now the higher the compression ratio is, the easier it will start because the air gets compressed more heating it up more, that is why when you read about the people that lowered the compression to allow more boost, that they don't start worth a dam. So also in theory, a perfect IDI should start better than any PSD, mine seems to outperform all of them I see at work but the DI is hard to beat. Not to sure on all this, I'll be more confident in this when someone else agrees with this.

[arthurgoboom]

The important number here is the combustion pressure, i.e. the pressure in the chamber when the fuel is ignited. If all other things were equal (airflow efficiency, fuel volume per combustion cycle, combustion chamber design), if you increase either manifold pressure (boost) or the compression ratio, then the combustion pressure would go up. The math RunRed posted above is basically what answers your question. If you start at 14psia (N/A) and compress it by a factor of 21 you get that 294psi compression, or if you start at 39psia (PSD turbo) and compress by 17 you get 663psi. Then you shoot fuel into it and it ignites, and the pressure goes up much higher. There are formulas out there somewhere that let you calculate the mean cylinder pressure. Of course, the combustion chambers are completely different between an IDI and a PSD, so there's really no comparison.

The ~13psi limit on IDI comes purely from people pushing them and finding the limit before connecting rods bend or head gaskets blow. You could probably put more on it if you had a head stud kit, good head gaskets, and torqued the heads down tighter. Heavier connecting rods would probably be required. For all that work though...I think you'd rather have one of those engines with a big blue "C" on them...

[asauer]

Actually, increasing RPMs in an engine is alot harder than adding power as far as bottom end stress goes. When the cylinder fires and is on the power stroke, the momentum of the piston is already heading in the correct direction, so doubling the power is not stressing the bottom end as much as you'd think. On the other hand, the transient loads induced into the rotating assembly upon reaching the top and bottom of the stroke are much, much higher; and they only increasingly get worse as RPM goes up. It's pure physics- at the top and bottom of the stroke velocity is zero, but acceleration is at its highest. Also, the middle of the stroke, velocity is its highest, but acceleration is zero. Basically, when acceleration is the highest the bottom end sees the most stress applied to it; i.e. trying to get the rotating mass slowed down and direction reversed.

[jski55]

thanks guys that actually helps.... just lets me know that there is more to it than boost and compression ratio. +

[asauer]

Also, keep in mind that boost doesn't really mean anything. Think about it- boost is simply a build up in pressure. What is really important is air density and airflow. The more dense the air charge, the more oxygen molecules that are making their way into the combustion chamber. The same can be said with airflow. Think about an air hose with a blow gun nozzle on the end- it has high "boost" (pressure), but a small volume. Now, cut that air hose in half, and you have a massive amount of volume, but not the same 'boost'. Which one do you think flows more total air? Keep in mind that the scavenging effects can be higher with higher boost ('blowing' the spent gases out of the combustion chamber and more quickly filling up with fresh air) but usually this isn't the case. The turbo's exhaust housing will most likely be very restrictive to build high boost numbers, therefore presenting very high backpressure, and preventing exhaust gases from readily flowing out of the cylinders. This is referred to as 'drive pressure'. It's all a complex balancing act. Like everything in life, there will always be a trade-off.
Volumetric Efficiency is ultimately the name of the game. Nobody really mentions it often, but that is THE single most variable that affects power. When gas motor guys install bigger aftermarket heads and/or port their heads, they are simply increasing volumetric efficiency. By definition, volumetric efficiency is simply how efficient an engine is at filling its volume (cylinders) up with fresh air, and how efficiently the engine also scavenges the combustion out of its cylinders. A higher compression ratio, by definition, raises volumetric efficiency. This is why gas motor guys up their compression ratios up so high compared to stock.

[RunRed]

Hmm. I remember reading about the 6.2 GM diesel a while back, they said reports of 30mpg were not that uncommon because of some recardo or something head design.. Must have had something to do with scavenging. Pretty amazing that an old POS could get that yet today with computers mapping out every atom of the fuel molecules can barely scratch that with less displacement. Gotta love the EPA lol.

[macmaniac1]

Something to think about! I bumped into a neighbor of a gal. that hauled a four horse trailer around to showes all over the northeast. She had a mid-90's f-350 powerstroke. She ended up with an average milage of 30 mpg while towing!!!!!! This truck had no loss of power either. She never said anything to the dealership because she was afraid they would try and take it away. She had the feeling that it was an experimental model that got out into the regular fleet of trucks. Simpily a more effient head design, engine, and or computer mapping?

[asauer]

Or poor mileage tracking methods...

[Dave Haven]

Originally Posted by asauer

Or poor mileage tracking methods...

That sounds like a distinct possibility.