First off I have to say its been a LONG time sense I've posted on the boards (last time I posted, it must have been the old forum operating system). Just been searching for most of my questions but I have one specific question for ya'll. I'm planing a 305/350 SBC (305 to start with, 350 when I blow the 305 up) in a 1987 Pontiac Firebird/Formula and i have a question about compressor/turbo sizing. I'm not super new to turbocharging vehicles (you can find my previous build thread here: https://www.theturboforums.com/thre...win-Turbo-Buildup-UPDATED-4-20-09-with-*Pics* but I have some specific questions on this one turbo. I am trying to figure out if this Garrett TP38 will work decently on the 305 and mostlikely future 350 engine. The compressor map by itself:

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The turbo as stated is a Garrett TP38 off of a 7.3L international diesel off of a school bus (scored it for $80.00 at the local junkyard). I was able to take the compressor side apart and verified its a 56 trim turbo, 60mm inducer and 80mm exducer, 1.10 A/R compressor housing as stated in the compressor map. Now I was not able to take the turbine apart (didnt want to destroy the rusted out 12 point bolts Garrett used to assemble the thing). From eyeballing the turbine exducer with a micrometer its around 68mm exducer, unknown inducer, 1.0 A/R turbine housing. I went ahead and plotted out my setup on the map (well both setups at both desired boost levels). I dont plan on running anymore than 5k rpm on these motors, and looking to be able to support 15-20psi. Here is the map for the 305 at 15psi, and assuming I can get it to full boost by 3k.

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Now the same 305 @ 20psi, same assumption fully spooled by 3k rpm.

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and for the 350 at 20psi again assuming I can fully spool the thing by 3k (and yes, I know the spool time "should" be lower because the larger cubes, just using as an example).

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Heres my questions:

A: Going up to full boost, I am awfully close to the surge line.. would it be safe to be this close to the surge line IF I make sure I run plenty of bypass/blowoff to vent off the pressure once I let off the throttle.

B: With the 1.0 A/R exhaust turbine housing, is it safe to say it would be spooled by 3k? sooner? later? Just looking for people with past experience running this "kind" of turbo size and seeing where in the RPM band they were fully spooled (yes I know all setups are different, and whatnot, just looking for a close guess if my thinking is correct or not).

I figure if this does not work, I'll run the K26 and Garrett TB0348 (buick GN turbo) I have laying around collecting dust, but would rather run a single due to packaging if at all possible with this Garrett TP38 turbo. All thoughts/responses/tips welcome. Thanks in Advance!

Bowtie Guy

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ok the last time i tried to help a guy with turbo choices it was said i trashed the guys thread.....

but ill swing it for you..... with the maps you are showing its hard to read because of the low quality of the the pic. with out even really seing it i think both turbos are going to be to small and from the looks of it both engines would choke with those small turbos. have you given any thought to the $245 ebay gt45. on the 305 it may be a lil slow to spool but on the 350 should be right at home and make more power than both turbos u listed on lower boost. can you find and post a better turbo map

 

plenty of mustang guys run the gt45 give the old chevy a gun rather than a knife

 

They are actually all the same map/same turbo, just different plot points, and its the best one I can find thats actually for the TP38 version I have. The main reason I want to use it is because I already have it.. just looking mainly for people who have sized up turbo maps before if A: im running to close to the surge line and B: if this darn 1.0 A/R turbine is going to spool at all before the 5k rpm redline.. Thanks for your reply though. I'll have to look into the GT45 for another project my buddy is going to be doing soon.. on a 350 SBC

 

You don't need to be worried about the surge line. Nor the 1.0 a/r; it'll spool.

 

that's what I wanted to know. Thanks!

 

The turbo will spool for sure. But it will create more heat than a properly sized turbo will. Plan on a good intercooler

 

Should work decent for the 305 undersized for the 350 probably unless it is stone stock or darn near so. Would look into a da66 eventually if you do it. An hx 40 off an 8.3 Cummins is dimensionally similar and standard rotation making it way easier to upgrade later. Peope seem to get about 400whp plus a bit from an hx40 on about 5l so you would probably be in that neighborhood. Just keep in mind your upgrade paths are limited.

 

I misread these compressor maps. This turbo is oversized for these engines. The more cubic inches you can do the better spooling the turbo will be. Looks like it could use another 50 Ci or so to be closer to where it needs to be. It won't create excessive heat but it will lag more than you will want it to

 

Lol this is 145% wrong.

 

Tell me why I am wrong. I would like to know?

 

The plotted points for the 350 are already running off the end of the map at redline. The plots for the 305 are pretty much perfect though. If the points are correct for the 350 it would be out of it's efficiency range well before redline rpm, and would just be pumping hot air.

 

what about the plotted points to the left of the Map. those are very close to the surge point in will create a very slow spooling turbo it seems bet this turbo was not design 2 work at higher boost levels and flows more CFM at lower boost Then is needed

 

Meh. The left points all depend on when it will actually spool, and depends mostly on the turbine. In my experiences, small singles on larger (300ci+) engines dont take 1000rpm to go from 11psi to 20psi, and it also most likely won't be building 6psi at 1500rpm. To me, when you plot on a map you worry less about the surge line and more about where you are in the efficiency range at your given airflow required.

 

Turbine housings aren't an issue.

http://www.dieselpowerproducts.com/...g-options-for-94-97-73l-ford-powerstroke.aspx

There's a .85 and a 1.0. The biggest issue as said is hitting the right side of the map. If worse comes to worse and the turbo is to the point of surging some head porting, a bit more cam or anything of the sorts should be able to push him off the left side. The problem is him skidding over to the right.

 

It'll should spool fine. I'm using the .8? A/R turbined version on a stock 318 magnum converted to a blowthru holley. Lights off ~2500rpm. The internal wastegate isn't capable of keeping boost under control for low boost tho. Mine is just a spare parts build. It works great boosting a motor that makes prolly 200whp n/a, but you'll run out of compressor quickly on a real motor. Given the expense to upgrade, and the fab it takes to build for an oddball turbo, you might reconsider. If you had a pile of 305's it could be fun for awhile though