Couple of questions, Intake Air Temps and Compression

[unlimitedpower]

What sort of Intake air temps are ideal for the Nissan diesel I guess if it matters a RD28T
Ive just fitted a gauge to get some pre intercooler install figures, on a 23°c day i spiked to 90°c on a long up hill pull. That was as high as it got (i don't make a habit of that) it cruses at round 55-60°c
I know ambient air temps make a difference, I just want to know as the first intercooler i'm going to connect up is a small factory side mount, (engine is in a Nissan Stagea) After that i'll latter look at a larger front mount, If i need it,


Second,
How much of an impact does a longish extension tube (150mm X 6mm ID) plus the hose on compression tester have on figures in a compression test, My understanding is as the tube and hose etc are all added to the combustion chamber volume, As these are pre the non return valve,

Only reason i asked is i comp tested my engine a while a go just for curiosity sake really and it was a very even 200psi across all 6 cylinders #5 was maybe a little higher,
Now my understanding is this must be incorrect as diesel needs at least 350psi to ignite, Car starts and drives fine so im not really concerned just interested,

Brett

[mudski]

What sort of Intake air temps are ideal for the Nissan diesel I guess if it matters a RD28T
Ive just fitted a gauge to get some pre intercooler install figures, on a 23°c day i spiked to 90°c on a long up hill pull. That was as high as it got (i don't make a habit of that) it cruses at round 55-60°c
I know ambient air temps make a difference, I just want to know as the first intercooler i'm going to connect up is a small factory side mount, (engine is in a Nissan Stagea) After that i'll latter look at a larger front mount, If i need it,
The colder the air the better. The colder the air, the more you can compress the air....

Second,
How much of an impact does a longish extension tube (150mm X 6mm ID) plus the hose on compression tester have on figures in a compression test, My understanding is as the tube and hose etc are all added to the combustion chamber volume, As these are pre the non return valve,

Only reason i asked is i comp tested my engine a while a go just for curiosity sake really and it was a very even 200psi across all 6 cylinders #5 was maybe a little higher,
Now my understanding is this must be incorrect as diesel needs at least 350psi to ignite, Car starts and drives fine so im not really concerned just interested,

Brett

No impact. Your still compressing the entire area, cylinder and compression tester tube, with the same amount of pressure. You are measuring compression, not volume.

[unlimitedpower]

But the pressure reading comes from the compression ration, Eg size of cylinder intake volume vs the combustion chamber volume, if you had a very large combustion chamber, say half the size of the actual volume of the intake drawn in that would end up with a very small pressure reading as it would only be squashed in half?
I don't know if that makes sense it makes sense in my head.

Adding extra volume would effectively reduce the static compression ratio would it not? Extra volume was added due to the long length between the tester and the cylinder head.

im only 90% certain of this so if you could please explain your reasoning that that would be great, I know my actual reading can not be 200psi as it wouldn't run, So that could only mean a faulty gauge, I don't have another one to try unfortunately,

[the evil twin]

Any change in the total volume (assuming the swept volume IE Displacement, is constant which in this case it is) will result in a change in compression ratio.
The pressure is directly proportional to the ratio so, Yes, your tester will lower the pressure.
To roughly work out by how much... you need to work out the volume of the change as a percentage of the original (note that is only a roughie calc) the result could well be neglible in your specific case

Indeed, thats why shaving a head increases compression pressure, the same displacement volume is more highly compressed as the combustion volume where the Piston is trying jam all the air fuel molecules is now reduced.

Assume you have a 100cc cylinder and a swept volume of 50cc that is a compression ratio of 2:1
Reduce the capacity to 75cc with the same swept volume of 50cc and the compression ratio is now 3:1
Increase the capacity to 150cc with the same swept volume of 50cc and the compression ratio is now 1.5 to 1
The same holds true if you increase/decrease swept area by changing stroke.

Given the TB42 and TD42 have the same bore and stroke IE swept capacity but huge difference in compression this is achieved by changing the head so the combustion chamber is smaller and the compression ratio is higher even tho the displacement is the same IE 4.2 litres

[the evil twin]

So why is charge air temp important if you still have the same compression ratio and displacement and pull in the same volume?
Cause colder air is denser for a given volume and denser air for an engine means it has more oxygen and more oxygen means more power can be potentially harnessed