I'm trying to pick a compressor for my turbo project, which means doing some math and then plotting some points. However, I've hit a snag.

Using MazdaManiac's "Rotary Math (http://www.rx8club.com/showthread.php?t=106294)" thread for guidance on the engine's characteristics, and the formulae at the end of a thread I found on turbo sizing (http://www.globalgoldtalk.com/everything-about-cars/48178-turbo-sizing-tech-finding-right-size-turbo-your-forced-induction-project.html), I'm getting some numbers that are well outside the range I expected. In fact, it's calling for PR's that are approximately triple what I would have expected. So then I decided to run the same calcs, replacing my HP goal with the 333hp the stock fuel system can support, and again, the PR from the turbo sizing article formula is about 3 times that listed in the Rotary Math thread.

The issue seems to be in the MAP calculation, where the turbo sizing thread says
MAP=[MFR * 639.6 (460 + Tin)]/VE(RPM/2)*CID.

What I found is that if I drop the 639.6 from the equation and multiply the result by 200, then the numbers start to look similar. So: MAP = [200*MFR(460+Tin)]/VE(RPM/2)CID seems to correlate approximately, but not precisely to what MazdaManiac's write up suggests.

Now, since MM's numbers are in line with what I was expecting, and he's successfully driving around with a turbo, I'm more inclined to trust them than this other thread. At this point, I'm really just wondering if I'm missing something, or if the turbo sizing thread I found is missing something.

I don't have the exact numbers, as I was doing the calcs on a desk calculator and not writing anything down. I'm just trying to ensure I understand what I'm doing in this phase before I try to use this info while looking at compressor maps.

Seems, wrong...

Can you rewrite the equation with the variable NAMES instead of just the numbers?

I always calc for MFR on the engine to get HP goal, then go from there to get the PR required for MFR required.

Here are some of the equations I use.

PR Compressor = (boost - Pressure Drop + ambient)/ambient
Ideal Temp Out =[(Tin + 460) * PR^.283]-460
Actual Temp Out = [(Tout Ideal – Tin) / Efficiency] – Tin
Density Ratio = PR * [(Tin + 460)/(Tout+460)]
Engine Displacement +[(#Cyl)*(Stroke)*(Bore)^2]/1.27
VFR100% = [(Disp)(RPM)]/ 3456 or 1728 rotary
VFRactual = (VFR100%)(VE)
MFR – NA = [(2.703)(PAmbient)(VFR)]/[TAmbient + 460]
MFR – FI = (MFR-NA)(DR)
Horsepower = (MFR)(60)/(A/F Ratio)(BSFC)
Or MFR * 10 = Horsepower (Rule of Thumb)

That affects the VFR as noted, the other equations are fine.

Werd, you da man CRH.

I am a legend in my own mind...... LOL

Unfortunately, all the variable names I have are in the post. Toward the end of the turbo sizing write-up, the author says "Don't worry about the constants like 3456, 60 or 639.6, they're simply there to tidy things up and convert units."

Yeah, I know rotaries flow double the air, that's why I tried multiplying the results by 200.

The raw number in question, 639.6, is some sort of conversion operator, but the source doc doesn't say what for.

I guess, if the numbers come out right after dropping the 639.6 and multiplying the results by 200 (2 didn't work, so I tried 100 thinking in percentages, that didn't work, so I tried 200, thinking double percentage, and it came out approximately where I expected), I can just use the formula that way.

MM can answer your question but it might take him a bit to get back to you.

I have spent some time learning the math and sizing lately.... took me quite a while sifting through different turbos to learn that this is as dead on right as it gets....

As I've noted elsewhere a dozen times or so, the Garrett GT2871 will bolt right up to the hot side.
You will need to make an adapter for the outlet of the cold side, which is not a major project.
That turbo is good for over 400 HP.

i dont remember the specific number off hand, but i do recall this turbo should spool right about 3,000 rpms; and hold way more boost to redline then we have ignition for ATM

The issue seems to be in the MAP calculation, where the turbo sizing thread says
MAP=[MFR * 639.6 (460 + Tin)]/VE(RPM/2)*CID.

What is the "639.6" and why are you trying to calculate of MAP?

Just calculate MFR and find the points on the compressor map that correspond. MAP will dictate itself and is, in the big picture, irrelevant.

I was trying to find MAP 'cause the turbo sizing write-up I found said to use that to find the Pressure Ratio. I have no idea what that "639.6" is. I'll have to try getting in touch with turbo sizing thread author again.

I like to understand why the math works before I trust it.

Thanks for the help.

Start with the air the engine needs to get your target HP.

Then you can size the turbo based on those air volume amounts. That is what my series of equations will do.

The pressure ratio at that point you can play with and see what HP it will bring.