t25 vs t3

I am getting ready to put my classic Saab back together and I am wondering what Turbo to put on it. Factory it came with the T3 and a tiny intercooler...

As I am rebuilding the car, I am adding a larger intercooler (from a Volvo 745) and running T5 injection from a later model 9000. While the T3 is factory, the T25 came off of a Saab GM900 that I carted off to the junkyard. Both cars put out similar power.. but I am after drivability and less stress on the transaxle over ultimate power. If I do increase the boost, it will only be to go from 185hp to 200hp.. This will be an occasional Daily driver, not an autocross or track car

Use whichever turbo is in the best condition or has the least amount of mileage. I wouldn't even turn up the boost to keep the reliability as high as you can make it.

I dont know saabs in particular but I do know that t3 and t25 have different size flanges so there maybe a problem there

Exactly. T25 and T3 is a very vague distinction - it's like discussing 4 vs 6 cylinder engines. But they do use different flange sizes. If the car came with a T3 flange turbo, I'd stick with one with a T3 flange.

assuming the T3 is actually larger than the T25 (this is not a given, but likely), the T3 will provide the same pressure and flow at lower shaft speeds, therefore increasing turbo life. the T3 turbo will lag more, though, giving you a smoother, yet spiked torque curve. the T25 will have a more use-able boost window, spool earlier, and unless the engine outflows the top end of the pressure chart, will still make the same horsepower at similar boost levels/flow rates. a larger inter-cooler on the smaller T25 will make the turbo FEEL bigger, due the the higher density of the charge air.

me? id go T3. they are both factory turbos designed for roughly the same power output, so the difference will be minimal at similar boost levels anyways.

-J0N

ok.. so here is a real question then (I am still learning about turbos here) if I stick with the T3 that came with my saab.. and just add the larger intercooler (Going from the stock sidemount to the front mount Volvo unit and necessary plumbing) will that "soften" the power hit when it finally begins to provide boost to the engine?

the NG900 I had was the first turbo car I had.. and I hated that car. The car would laglaglagomgcurb! as the boost hit and the torquesteer would send it steering for the shoulder. I am hoping the Classic 900 with it's equal length halfshafts and A arm suspension will not have that issue.

I doubt you'll notice much change in power delivery. It takes a fairly significant amount of volume change to effectively add lag to the turbo, and I don't think you'll have all that much more there. Part of the key to getting rid of the omgcurb feeling is moving that hit down in the RPM range. Think 2 stroke dirt bike power valve if you have any experience with that. If it spools and comes into boost near the engine's natural torque or power peaks, you have a much larger step in power than if it comes in at 2-3k rpm, below the engine's powerband. Speaking to the dirtbike analogy, if the PV opens at low RPM when the engine is really below where it makes power, you'll get linear delivery to max rpm, but if it comes in when the engine is already in its powerband, you're in for a nice surprise.

No offense intended, but you might be starting to over think this a little and are splitting hairs.

A t25 vs. a t3 (both of unknown specs, those are vague descriptors) will both spool quickly and support similar power with similar torque curves. I loved the t25 on my 9000 for its instant spool and smooth delivery / powerband. Use whatever bolts in easiest and live with it a while to see how it works for your driving style.

Same with intercooler size and plumbing. The volvo unit is similar to what was used in the 9000 and should be sized well for your needs as a streetable car. It will slightly increase lag versus a smaller unit with less plumbing but really not enough to make a noticeable difference. The change in the rpm where the boost hit comes on will not be very quantifiable.

t25s and t3s were used by SAAB between many models and trim levels and delivered only minor changes in power and driving experience based on tune and boost limits.

Equal length half shafts will help with torque steer to a good degree, but not eliminate it. Having the entire front end in good shape will make a noticeable difference too; high quality bushings and additional bracing are a good idea. I do not think that you can ever really dial it out of the car but it should be of a level you can live with if you are only looking for ~200hp.

A good downpipe and exhaust (2.5"ish) and a reflash of the ecu (depending on what you are using) with T5 suite (freeware ftw!) should easily push you to 200+ lb/ft of usable torque. I am a big fan of smaller turbos on street cars and staying in boost most of the time. A very quick spool keeps the hit minimal vs. the normal torque curve through the rpm range and boost becomes very transparent and unobtrusive, if that makes sense? These engines make good off boost torque and flow well enough to spool either turbo very quickly so you cannot go wrong here... just remember to use the right spark plugs and change them often!

Are you still in EHT? I wouldn't mind trying to stop by and look at your collection of toys sometime when I am in SP / Linwood...

In reply to mad_machine: I'm planning to T5 mine as well. If your T3 is healthy stick with it, at least for for getting the T5 conversation up and running.

Also, one thing to consider is that T5 is very tunable, and even in OE baseline tune will run the engine better than the LH system ever could. If the power delivery is still too peaky with T5, I would try tunning it out first before making hardware changes.

A c900 with a healthy front end has very little torque steer, good tires help too.

I knew the C900 had better torque control than the NG.. I had a 2.1 900s that was a blast to drive.

As for turbos.. learning more and more

I dont see any reason to go with a t25 over a t3 on a 2.0l+ engine unless maybe it is an automatic with a very low stall converter.