On a turbo map, the x axis is mass flow in lb/min. The y axis is pressure ratio, which is basically multiples of atmospheric pressure.

Many compressors will flow 28 lb/min. You ideally want one that will flow 28lb/min efficiently, rather than heating the air up a lot (which takes energy and is not efficient). So you would want to see where the point PR 3.1, mass flow 28lb/min falls on the compressor map. It could well fall within in a high efficiency "island". Also keep in mind we are talking about 4000 rpm, which your motor presumably rarely sees. So most of the time the required mass flow will be lower than 28 lb/min.

You should also get the biggest intercooler you can fit. Or two, like the gwagens and touaregs and sls cars do, one in front of each wheel.

Or just bolt your turbo on and turn the max fuel screw down until egt no longer scares you :-)

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One thing I don't understand about your calculations is you start with "iff you want 200hp..." What if I don't expect that much power - do the calculations change?


Also, how are you determining a PR requirement without intercooling as 3-4:1? The compressor map I'm looking at looks like it tops out at 2.6 at peak efficiency...

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This is eerily similar to exactly what I'm looking at. 34lbs/min at 69% efficiency/ I think max boost is ~21psi.

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How much power you get per cc of diesel depends upon brake specific fuel consumption, or bsfc. That number depends upon the motor itself and is a measure of efficiency.

Since you started with a pump set for 90cc, the power target is sort of irrelevant. You will have whatever power the motor can make from 90cc per injector per 1000 injections.

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The target DR is determined by multiplying the grams of fuel supplied by the air to fuel ratio, to get grams of air required. Then if you have no intercooler, you have to account for the fact that your air is heating up as you compress it, lowering the density as you are trying to increase the density. The higher the temp, the higher the pressure needs to be to achieve a given density of air at the intake. So more compression needs to occur.

How much pressure can be supplied at what volume is a characteristic of the compressor wheel dimensions and design. Some compressors can make 5:1 efficiently. Some cannot. Just depends what it was designed to do.

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I (sort of) understand what you're saying, but I mean how did you specifically arrive at the value of 3 or 4:1 non-intercooled? It sounds like you calculated a minimum PR of 2.08 - so was the extra 50-100% just a rough guess, or was there an additional calculation involved?

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the difference there is density ratio vs pressure ratio. you have to calculate DA vs PR based on compressor efficiency intercooler effectiveness and any other things that either add or subtract temperature from the intake charge. for example i measured an IAT of over 300* F with no intercooler running 20 psi from my HE221w. after getting my intercooler plumbed IAT dropped to below 100* which means the DA went from around 1.7 to 2.5 while at the same PR. (these are guesstimated numbers at best i dont know my VE exactly but based on exhaust gas composition vs boost and fueling its around 71% at 4000 rpm and i also dont have a good map for the HE221W compressor that i completely trust)

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1985 300D om617: 8mm M pump 175cc 5200rpm, holset he221w @ 30psi, large A2W ic, compounds on the way.
KD9AFT
A&P

Yes exactly what r3350 said. I corrected 2.08 for VE and then used the spreadsheet I provided to correct the air density for temp increase after compression of the charge air.

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One thing is that none of your calculations account for element size. Is that an irrelevant variable? Surely 90cc of fuel through 5.5mm, 6mm, or 7.5mm elements will have different characteristics, no?

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Injection time goes down, so egt is easier to manage. But the amount of air required to burn any amount of fuel is a strict stoichiometric ratio involving numbers of molecules of each.

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