I would love to say I'm comfortable and competent at reading flow maps for turbos, but I haven't really read enough to be totally confident quite yet. I'm building a B18B1 right now for boost, with Wiseco 9.4:1 pistons, Eagle H beams, and fresh ARP hardware throughout. Stock bore, stock valvetrain. My goal is 400 WHP. I have all of the supporting mods hammered out - miniram manifold, 3" custom exhaust (baffled, catted, and muffled), ID1000's fed with a Walbro 255, CTR transmission, solid poly mounts.

The part that's tripping me up is the flow charts. I have a friend who SWEARS a GT28RS will get me to my goals, but I'm pretty sure the flow chart disagrees with him, a lot. Garrett's compressor maps are the easiest to find, so I've been using them to get an idea of what size I'll need. I don't know if I'll need 20 PSI, or 25 PSI to get to my goal, so I've been doing the math for both. I'll be tuning my redline to 8000 RPM, but I've been doing my math at 7k.

These are the three turbos I'm eyeballing right now. I calculated my pressure ratio to be ~2.3, and CAF to be 36 for 20 PSI, and 41 for 25 PSI.

First, the GTX3071R. Both cross-points are pretty much right inside the middle island.


Second, the GT3071R. Both cross-points are to the right of the middle island. That means this turbo is too big and will spool slower, right?


Third, the GT3076R. Ignore the circled cross-points, I used the wrong PR for those. The 20 PSI cross-point is within the middle island, but the 25 PSI cross-point is to the right, two islands out. Now I'm just getting confused


So my questions are these:
Is a pressure ratio of ~2.3 roughly correct?
Are corrected air flows of 36 and 41 roughly correct for 20 and 25 PSI?
From a turbo similar to this, would it take 20-25 PSI to reach 400 WHP?
I did my calculations at 7000 RPM, but will be tuning the engine to rev to 8000. Did I mess my numbers up by using 7k instead of 8k?
I tried to reverse the math on CAF to figure out what RPM I would be at for 10 PSI, and came up with 7500...which seems backasswards. Did I mess up my math, or can you not do the math that way?

And most importantly - Is there a turbo I didn't even bother looking at, that I should consider for a car oriented towards daily driving, with weekend use on GT and road race courses?

 

if you can get a gt28rs to make 400whp I'll give you sexual favors because you're a miracle worker lol

it's going to take a good bit of pressure to make 400whp on a straight ls with a stock head, so your estimate of 20psi is close, ultimately it might be closer to 25.. they just don't flow well and the stock cams are the biggest limiting factor in your build, also the stock intake manifold isn't helping any... I would at least look into an aftermarket intake mani like a skunk2 pro. the bigger plenum and slightly larger, longer runners will help with power production and midrange torque

as far as the 3071 vs 3076 the difference looks much bigger on paper, in actuality you won't really notice much between the two on identical setups. they both share the same turbine wheel, the only difference is the 3076 has a 5mm larger exducer on the compressor side (i think one model/part number of the 3071 *might* have a slightly smaller inducer but don't quote me on that)

the 3071 would fit your motor to a t but if you want a little wiggle room for a slight power increase you cant go wrong with the 3076.

if the budget allows or if you can find a good used turbo, the gtx2867 through gtx3076 would make the power you want and depending on which turbo you picked would allow for much higher power levels down the road if you ever wanted more

 

ROTFL! Your opening line really cracked me up!!


***EDIT***

 

^threadjack much?

 

Yes, start your own thread...continue to thread jack this one and all of your comments will be deleted

 

Oh hey, that's where this thread went to! I saw your response when you posted it, and thank you for answering the most important question wantbost. 3076's are all over the place in various conditions, so that's what I'll probably go with when the time comes. I know I'm rather limiting myself with the P75 IM (but hey, at least I'm not using a PR4), and I won't bore you with my reasons. A pair of cams will go in eventually, but that's down the road, and I'm mainly aiming to open up the torque band with the cams when they come.

My "what turbo" question was directly answered, but what about the other minor questions? Are my PR and CAF numbers roughly correct? Why did my attempt at reverse-calculating spool characteristics fail so miserably?

Also, continuing to educate myself created another new question. When going through the math for airflow through intercooler piping, I found that I'd be on the top end of 2.25" piping before the air hit the point where it would be turbulent. Is that correct, or should I step up to 2.5"? The style of intercooler piping (specifically the section between the intercooler and throttle body) that I visually like seems to wrap around the gear housing, do a 180 at the tip of the case, then come back around and up near the transmission. In case I'm explaining it poorly, it looks like this -



The majority of "kits" out there end up looking like this, coming up through behind the headlight, though -



Is there a benefit outside of appearance between one kit or the other?

Question cliffs:
Are my PR and CAF numbers correct?
Can I reverse calculate spooling with PR to find new CAFs at specific boost levels?
2.25", or 2.5"?
Intercooler piping routing - form, function, or both?

 

you cant really gauge spool from a compressor map, you have to be able to look at the turbo as a whole, from the housing sizes and designs, to the design and function of the compressor and turbine wheels, as well as their weights,etc. to properly gauge response times.

problem is most people will never really know all of that because they just want a turbo that works... I think me, TheShodan, and a select few are the only ones who can look at a turbo and go "it'll do this like this but not that" I buy failed turbos and take them apart just to learn about them, from blade angles, tip heights, etc.

but I'm weird lol. I have at least 25 turbos right now, most work, some are in pieces haha.

the intercooler piping at your flow rates should really be 2.5", 2.25 is just too small and when you take the bends into account you'd have severe turbulence issues, the way you like works better as well because the piping is slightly shorter in length overall vs the typical setup

 

also you have to know the engine very well...

bore, stroke, functional rpm range, compression ratio, cylinder head flow, cam design, cam timing, intake manifold design, mechanical ignition timing, how aggressive the tune is, fuel used, turbo manifold design (runner length and diameter, collector merge angle, collector length, etc)

take all of that into consideration along with the turbo as a whole and you're on the way.. it's an awful lot to have to calculate but it's very useful info to have at times.

it's to the point where I have maps memorized, I have friends who walk up and ask what turbo to use, I ask them all of the info above and in a couple of minutes I can mentally plot a map :/ I think I should get paid for this lol