A 22.5 to 1 compression ratio engine with 10 PSI boost has a final compression ratio of 37.8 to 1.

Kick the boost up to 15 PSI and you are looking at 45.45 to 1.
Now lets have more fun and go to 20 PSI, you are now looking at 53.11 to 1.
25 PSI and you are looking at 60.76 to 1.

 

So what exactly would be overkill on these old dogs?

 

That depends on how much you do to the engine before you install the turbo.

Milled pistons and head studs, I am running over 20 PSI to mine with my ATS turbo.

Hey Cheaper....."I think Stans Headers can make just as much power as a turbo for a lot less money". Who said that?

 

So if the turbo increases the compression...wouldn't that make the truck harder to start in the cold?

 

Not me. I think I said something more along the lines that STAN said that his headers, with dual 3" pipes, and a turned up IP can make as much power as the FACTORY turbo setup.

Ain't NONE of that applies to what you've done to your mill.... that would be like comparing apples to cantalopes.....

'Course I can't say that Stan is wrong either - I haven't tried his headers.

 

Nope. Turbo is spun by exhaust gas - so it doesn't kick in and start compressing air until after the engine is running. Most don't create any significant compression of the incoming air (a.k.a. BOOST) until the RPMs get above 1800 or 2000 RPMs....

 

Dave, how are you figuring out that compression ratio? It sounds a little high for my thinking. If 10:1 builds close to 200 psi and 22.5:1 builds around 410 psi.... see where I'm going here? Are you saying that adding 15 lbs of boost is running the pressure up to somewhere near 800 pounds???

 

Ya that does seem at little bit effed up haha.

 

I'm a little confused on where you're getting your numbers, Malcom. The pressure increases linearly with the compression ratio. If the compression ratio is 10:1 then your final pressure is 10 times the starting pressure - assuming the temp stays the same. In this case we know it doesn't, but since the temperature doesn't change that much on the Kelvin scale between intake and max compression (NOT including combustion - we're only looking at this problem UP TO the point of max compression) for the sake of argument we'll ignore temperature. Atmospheric pressure is ~14.7 psi, so if the compression ratio is 10:1 the pressure at max compression is 147 psi.

With a 22.5:1 compression ratio then the pressure at max compression is 330.75 psi (22.5 CR x 14.7 psi). BUT, if you start with air that is already compressed to 15 psi above atmospheric pressure, your pressure at max compression is a little more than double that - 668.25 psi [22.5 CR x (14.7 psi + 15 psi)]. That's significantly less than 800 psi. On the other hand, at 20 psi boost you get 780.75 psi [22.5 CR x (14.7 psi + 20 psi)]. Pretty close to that 800 psi number.

Now that's the theoretical. But we know that an NA actually produces 400-500 psi, because we've measured it. Mine has about 450 psi average across all 8 cylinders. If you double the pressure of the incoming air with 14.7 psi boost then the end result is going to be twice the final pressure - 800-1000 psi.

I don't really find this all that incredible. Take a look at the pressures a few milliseconds AFTER combustion begins but before the piston has really begun to travel downward and relieve the pressure. The psi spikes well into the thousands - though it lasts only for a few milliseconds. Even a gasser with their mild 8 or 10 to 1 compression will spike up around 1000 psi at the very start of the power stroke (due to the higher explosive power of gasoline).

Why do you find 800 psi pre-combustion pressure so hard to believe?

 

Not being a math wiz like some of you, I'm using real world numbers that I know from experience. The numbers you are coming up with mathmatically don't seam to jive with real life. You know how much compression your truck had, how do you explain the difference?

 

Note that after thinking about it a minute I added some "real world" info to the post above. The difference is undoubtedly due to the temp increase. After the initial posting I got to thinking about it and realized that since diesel has a flash point of 143 degrees F, and our engines are igniting the diesel solely through the temp increase created by compression, I came to the realization that the temp increase IS significant enough (even on the Kelvin scale) to be an important contributing factor in raising the compression.

Sometimes its hard to set aside the old gasser notions when thinking about these diesels

 

http://www.supermotors.net/vehicles/...tors/index.php

This is the only link I have handy for effective compression ratios after adding boost.
The numbers it comes up with jive with the calculations we made with the machine shops comperssion ratio calculator.

 

That all looks good on paper (or screen) I guess. Just wondering what it would take to prove it. I don't think my compression tester is up to the task.

 

Cheaper, you are more like 475 degrees for diesel to auto ignite.
The flash temp is where the fire would flash over from another ignition source if it were heated to that temp.

Malcolm, bring your compression tester over to WV.
I wanna watch the look on your face as you sit on my engine while I run the boost up to 22 PSI.

I guess we could take the hood off so you could sit up straight.

 

i also went with banks seeing as that what i have installed on my truck!