Ideal A/R ratio?

[hosedup]

I've been doing some research and wanted to confirm a couple things. Fron what I gather, turbos with a scroll and turbine A/R of .7 and higher are better suited for our purposes and modern quick spool hybrid turbos don't work well.

I ask because I may have an opportunity coming up to pick a turbo from several salvage (big truck) units that are usable and likely won't have time to research each one. I'm trying to put together a kind of rule of thumb for picking an ideal unit or at least not picking a less than ideal unit. Any help is greatly appreciated.

[racket]

For big truck turbos try and have a turb scroll A/R ~50% bigger than the comp scroll A/R , but thats only half the story , the turb wheel exducer AREA needs to be larger than the comp wheel inducer by several millimeters , eg, a comp inducer of ~75 mm with a turb exducer of >80 mm , ............a bigger hole out than in .

[hosedup]

Nov 12, 2019 15:25:05 GMT -5 racket said:

For big truck turbos try and have a turb scroll A/R ~50% bigger than the comp scroll A/R , but thats only half the story , the turb wheel exducer AREA needs to be larger than the comp wheel inducer by several millimeters , eg, a comp inducer of ~75 mm with a turb exducer of >80 mm , ............a bigger hole out than in .

So, turbo AR 50% larger than compressor AR. If for example, I find one that has turbo AR of 75, I want to make sure the compressor side AR is roughly 50.

I'm assuming the AR is usually stamped on the side of the scrolls? As for the exducer being larger in area than the inducer, I am assuming that means measuring the diameter of the exposed part of the wheel, not the diameter of the funnel (if equipped) or the diameter of the part of the wheel inside the scrolls.

Is there anything else I need to watch out for? I'm assuming that variable vane units like in 6L Ford power stroke engines are a bad choice. Ball vs sleeve?

Sorry for the newbie questions, I want to make a more educated choice for a turbo unit so I have something better to work with than the T3T4. (The T3T4 worked great until it destroyed itself due to oil pressure loss)

[racket]

Yep , its the actual wheel dimensions that are important NOT the housing , so a bigger diameter/area of visible turbine wheel exiting gases out of the turbo than the comp whees area going into it .

Variable vane units are an option, though you'll probably end up fixing the vanes in one position eventually .

Ball or sleeve , I'm a "brass bush" guy , we don't have any need for the benefits that balls can provide when accelerating the rotor from low rpm , we're never at low rpm other than at spoolup , balls will make it easier to spoolup but I'm not a fan of balls .

To prevent oil pressure problems , have a Hobbs pressure switch in the oil line set so that fuel is cut if oil pressure drops below at set point , they're cheap insurance and a must have bit of equipment .

Cheers
John

[hosedup]

John, thank you again for sharing your wealth of knowledge. Last question for now

Let's say a you can pick any reasonable turbo out there to make an engine, what do you pick? By reasonable I mean something commonality available, not stupid expensive and can tolerate some abuse.

[CMDR_Boom]

I can definitely tell you what you'll most like encounter on ebay in the big diesel segment, outside a horde army of Chineseium.
HX35-HX60's are plentiful and cheap if good/used or rebuilt for our purposes, as well as any of the Borg Warner SX400 line.
Add: as of late a lot of Cummins units have been showing up as well. Not sure if that's indicative of suspect build quality or just because they're plentiful in the OEM arena, or show up as unused fodder from the upgrade process.

[racket]

LOL......."how long is a piece of string " might be easier to answer than your question ,............generally its simply a case of whats available at the time, and this can vary enormously on where you live on this planet .

I've never thought about the subject very much, having always been chasing the largest flow I can get my hands on, rather than more "sensible" options .

A big consideration would be what purpose you wanted to put the engine to , if only to make a bit of noise and flame then the cheapest turbo possible , but for any other reason there'll be more specific turbos that would be more suited .....................so think about the destination first before deciding on the turbo to get there .

As for cheap Chinese turbos , they have their place , and considering the fact that most of the "reputable" turbo manufacturers have had chinese manufacturing plants for years , sorta speaks for itself .

There are some very reputable chinese turbo manufacturers and I'd be considering one of their larger Holset/Cummins HX82 or HC5A models if I ever wanted to make a turbo based engine in the future , there are simply more parts available, and at more reasonable prices than for the large Garrett turbos...................Garrett has gone down the path of unsuitable "performance" turbos whereas Holset/Cummins has stuck with more traditional (read cheaper) designs which are better suited to our needs , there is also a lot of aftermarket comp wheels that fit the HX82/HC5A turb shaft quill .

Considering the duty cycles our engines will have , we don't need "million miles" warranty , a "cheap knockoff" if treated "respectfully" would serve our purposes , ....... every turbo , no matter how high its quality can be easily destroyed with careless use..............abuse will destroy both turbos equally as quickly .

Hope this helps answer your question .................if not , ask another :-)

Cheers
John