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Old 10-04-2007, 10:24 PM   #1
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Default Thinking of going from SC to Turbo, little tech help...

I'm tossing around the idea of moving from my SC to a turbo setup. Just tossing around the idea right now, but thought I'd investigate a little. I read through the Garrett website, should read it again. Looked at BEGI and FM websites to see what turbos they offer.

My thoughts are since I already have a TDR IC, and Gary's making tubing to run his IC with an FMII, I could just get an exhaust manifold, turbo, downpipe, an electronic boost control solenoid for my Hydra, some odds and ends, and I'm off and running.

First order is what turbo should I get? Engine is fully built 1.9l, my cardomain site has the details. 99 head is ported/+1mm valves, Webcams, etc etc. Hydra and 550s. The car sees weekend street, some autocross, but more open track days than autocross. I really enjoy the torque of the SC, so I don't think I want to experience much lag, but then I'd like the option of pushing more than 250 rwhp if I feel like it. The engine will easily handle it.

Garrett 2560, 2860, 3071? Those are the ones I've seen on either BEGI or FM's site. Someone mentioned the disco potato isn't such a great turbo model, but that's from limited research on my part at this time.

Jeremy at FM had another idea I wasn't aware of, that makes sense while on track, is the thermal efficiency of the turbo while on track. The larger turbo will create less heat in the engine, and make more power with lower boost than the smaller turbo. I enjoy comparatively low running temps while on track, and I'd like to keep it that way for longevity's sake.

Anyway, help is much appreciated.
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Old 10-05-2007, 12:32 AM   #2
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just a little to add, low lag and highly efficent turbo + high HP are are opposite ends of the spectrum.

in order to figure out which turbo you'll like best, you'll need to calculate your flow of air that the engine would normally make, then choose a pressure, then you can find which ones will be most efficent at those choices. of course, your boost is going to be determined by what your entire system can handle, so its a little of circular reasoning to figure out the best turbo.

usually the disco potato is a bit too big for a 1.8L engine, it doesnt hit efficenty for that flow rate until very high boost levels. a disco potato is great for a 2.0L engine however, as my friends MR2 can attest. you'll be somewhere in there for track duty, depending on what manifold and downpipe , and engine management you get.

this is a pretty cool site http://www.squirrelpf.com/turbocalc/index.php if you know about your current setup(or can guess) REALLY cool to see all the compressor graphs. i still dont know exactly what each characteristic means, but if things line up near the peak efficency ratings, youre looking at the proper turbo for the job.

oh yeah, and read maximum boost by corky bell. amazing, i even picked it up at the local library...(dont worry corky, you'll probably still end up taking my money)
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Old 10-05-2007, 01:01 AM   #3
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IMO the disco potato gt2860rs would be flippin perfect. spools at less than 4k and can make 300whp.
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Old 10-05-2007, 09:05 AM   #4
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Therein lies the rub. If you're looking for absolutely no lag with the option of pushing 300rwhp, an MP62 or 1.2L Whipple-based supercharger system is tough to beat.

I think you'll be sorely disappointed when you see how much lag is introduced on a similarly capable, 300+rwhp turbocharger setup on these small engines.
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Old 10-05-2007, 09:43 AM   #5
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Originally Posted by Brian View Post
Therein lies the rub. If you're looking for absolutely no lag with the option of pushing 300rwhp, an MP62 or 1.2L Whipple-based supercharger system is tough to beat.

I think you'll be sorely disappointed when you see how much lag is introduced on a similarly capable, 300+rwhp turbocharger setup on these small engines.
I agree, a roots supercharger like eatons is insta-power at every revs... even at 2000 or minus. I'm speaking from experience

But if you don't drive like a poor old man at those revs... just go turbo, otherwise if you drive at those low revs, just downshift and driving will be more enjoying
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Old 10-05-2007, 10:00 AM   #6
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Is "lag" what we're really speaking about? I think low rpm power is what the SC has over the turbo in most cases BUT for the track, I can't see it making much of a difference. What's the lowest rpm out on the track? 4500? I'd aim for something that's going to reach peak boost prior to that. My t3/t4 on a 2.0 will make 17+psi before 4500. That's with a DIY setup, journal bearing, OE internal wg turbo on a TIGHT budget (compromises). And the power is very linear- boost onset has occurred as low as 1300. That built 1.9 should have excellent flow with all that work. Use a ball bearing turbo and it'll work even better.
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Old 10-05-2007, 10:47 AM   #7
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+1, Rob.

In his initial post, it sounds as if his driving is split up 50:50 between weekend toy/autocross and track driving.

If that's the case, then perhaps he wouldn't mind the extra lag the 50% of the time he uses it during weekend toy/autocross driving. It certainly won't matter during track driving.
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Old 10-05-2007, 10:53 AM   #8
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i agree with m2cup about most of this stuff.

you can eliminate almost all disadvantages of a turbo if you carefully manage the goals of the system. if youre on track, then most likely you will be seeing the upper half of the rev range most of the time.

boost onset and lag are different things. with a good turbo and engine management(flat throttle shifting?) the time between pressing the throttle down and feeling the acceleration can be minimal. just make sure to keep your footwork nice and smooth. thats lag. boost onset is the time from when you push on the pedal at low RPM to when you feel the acceleration due to the turbo. a supercharger is gunna beat you every time at this, but what the heck are you doing playing around at 2000 rpm anyway? this is where turbo sizing matters most, too big and you'll be at redline before you get enough flow to push the turbo, and too small and you'll have the turbo spinning at max RPM at idle(thats really small! but i'm just trying to make a point, so...)

the disco potato is just so damn efficient. 77% if you size everything right? dang.
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Old 10-05-2007, 12:16 PM   #9
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...but what the heck are you doing playing around at 2000 rpm anyway?
Driving around town. That's where you'll find yourself at low revs, certainly not at the track.

I know the prevailing argument there is, "well, when I'm driving around town, I never let the RPM drop below 4500! low-revs are for pussies!"

However, not everyone likes to go screamin' around town, calling attention to themselves from stop-sign-to-stop-sign. For this kind of "tooling around town/normal driving," the supercharger or the teensy turbo is going to give the best results.
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Old 10-05-2007, 12:32 PM   #10
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At 3000RPM, if the driver has the patience to wait another 0.5 seconds for the turbo to spool to the same boost, he can enjoy another 10% more power and torque at the same boost and generally less heat. :gay:


FWIW, to clearly lay out the terms...

Lag: the delay between a change in throttle and the production of noticeable boost when the rpms are in a range in which boost can be achieved.

Boost Threshold
: The lowest RPM at which boost from the turbo will increase power over the engine's atmo. evq....Simply, the lowest rpm you notice boost.


Ex. I have a boost threshold of 1500RPM and spool to 12psi at 3800RPM in 3rd gear.


when a lot of people say lag, they really mean, shitty *** boost threshold and slow spool.
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Old 10-05-2007, 12:48 PM   #11
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Werd up, even my huge *** s256 will have a boost threshold of 2k rpm. Full 22psi by 4500 rpm. Sure its 2500 rpm, but how long do you think its gonna take to move that range? Me thinks not long.
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Old 10-05-2007, 03:15 PM   #12
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Thanks for all the responses. Main reason for the thought of changing over was the last dyno session was rather disappointing for the level of work in the engine itself. I admittedly do not beat on the car on the street, there's a lot of short shifting and fairly low rpms around town, meaning below 4k, maybe 3500. The engine just makes enough torque down low that reving it around town doesn't do it for me. The dyno graph showed the torque hitting 200 lb/ft a little over 2000 rpm and holding there until 6k. It was just flat. The hp was only hitting 250 rwhp, and we spent some time tuning the cams. Boost was 13 psi, and hit as much as 14 after adjusting the cams. I was hoping for something like 260-270 with the amount of work on the engine and the porting I had done on the SC.

I found that the disco potato has a variant with a lower turbine a/r, wouldn't that help spool more quickly?

My power goal is a solid 270 (I'm talking real power, not correction factor BS), fun to drive on the street, and be able to keep the temps down while on track. I was able to run 40 minute sessions the last time I was at Sebring, so I got the most for my money. My coolant and air temps were kept in check most of the time, below what a lot of the turbo guys were reporting.

Based on doing some pricing, I could probably buy the right sized turbo, a manifold, downpipe, and EBC for about $2000 give or take. I could sell the SC for that and still keep the IC.

I thought the disco potato would be a nice option, esp when I found the info on the variant that had the lower a/r number, but I could be misunderstanding things.

Thanks again.
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Old 10-05-2007, 03:21 PM   #13
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something in the 2860, 28rs, 2871 would probably be best.


yes the smaller the A/R the faster the spool (increased velocity), as a compromise it will effect the top-end as the turbine can choke out faster.

Last edited by Braineack; 10-05-2007 at 03:39 PM.
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Old 10-05-2007, 03:29 PM   #14
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You'll feel like you're in a rocket ship
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Old 10-05-2007, 03:41 PM   #15
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Solflarick, i forgot I have a previous version of one of your last dyno runs before you blew up. I could over lap that onto of a few various turbo setups so you can see the power difference.
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Old 10-05-2007, 05:02 PM   #16
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Okay, thanks. That was with a stock 96 engine, other than a header and the modded M45 setup.

On the 28 series, since I wouldn't be pushing it to 300rwhp, you think the lower a/r turbo would still allow for thermal efficiency at high rpm? I set the Hydra rev limiter to 7500. I need it to make it around a couple sweepers at Sebring without running out of rpm or having to shift into fourth gear.
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Old 10-05-2007, 05:05 PM   #17
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Originally Posted by neogenesis2004 View Post
You'll feel like you're in a rocket ship
That's one thing the SC doesn't quite do. I can tell from the rpm and speed that the car's accelerating pretty quickly, but the push you in the seat isn't there much after initial acceleration, guessing because the torque curve is so flat.
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Old 10-05-2007, 05:05 PM   #18
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i wouldn't go bigger than a .63 A/R


Garrett posts the turbine maps on thier site.

Engine air flow in CFM is [(RPM)*(Displacement in cubic inches)] / 3,456. So for say, 7500rpm we have [(7500)*(115.938)] / 3456 = 251.60 CFM /14.472 = 17.38 lbs/min.

So at 7500rpm with 12psi boost (1.88 pressure ratio) you simply draw horizontal and vertical lines respectively for the turbine flow and pressure ratio. Where they meet on the map tells you how efficient the turbine will be at that speed.



if you notice, that's puts you close to ideal efficiency. Any bigger and you aren't very close. but as you can see, you are pretty borderline as well.

Last edited by Braineack; 10-05-2007 at 05:20 PM.
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Old 10-05-2007, 05:17 PM   #19
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i wouldn't go bigger than a .63 A/R

I agree, if a 350whp SR20 pulls strong to 8K on a .64, a < 300whp miata will not choke a .64 housing. A .86 would be painful.
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Old 10-05-2007, 05:43 PM   #20
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First of all, don't listen to the word lag if you're looking for 300 hp or even 250hp. If your turbo is fully spooled by 4k, you'll be doing just fine at the track or street for that matter. I've run a gt2554r and I'm currently running a gt28rs on my setup. The gt25 spooled instantly and the potato is fully spooled by 3700rpms. Both are great turbos respectably. Sounds like you were thinking about the potato. I can tell you from experience this is a great choice for the 1.8 or your 1.9. You'll quickly forget about the .5 seconds braineack is talking about when your car pulls all the way to the redline.
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