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Doug Siegel
 
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turbo sizes

I've got a 62-1 turbonetics turbo that spools at 4k rpm. When it kicks in, it's pretty frightening.

I was told that the big turbo sacrifices airspeed at the low rpms, thus killing low end power and drivability.

Ported heads from what I am told has actually hurt my low end power but helps up top. Well, on a 4 speed street car that is perhaps a big sacrifice.

I've got a twin plug, efi, sc cam, elec ignition with 18* advance at idle (I think), as well as a taller 1st gear. If I use a smaller turbo, how small should I go and what kind?

I'd love some input on drivability.

Do you guys agree that smaller turbos create greater airspeed at idle and low rpm...better drivability around town?
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Old 06-01-2009, 11:15 AM
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GJF GJF is offline
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What is your static compression?
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Old 06-01-2009, 01:38 PM
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Corrupted by power
 
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After CR gets answered, decide how much HP you want and work back to create a list of the smallest turbos that will support it. Twin scroll boost faster, but require a specialized exhaust. Then you can look at the list of turbos and refine on characteristics to fit your priorities/budget.

There's also twin turbo, but that is a whole different beast in terms of turbo selection and exhaust.
Old 06-01-2009, 01:51 PM
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Doug Siegel
 
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I am using the stock 3.3 p and c. The crank is stroked....3.4.
I don't know what static compression is. (can someone define?)
bb exhaust 1 5/8, I think heads are 41mm, manifold too.

My engine builder tells me that at 1.2 or 1.3 bar, the car is pushing well over 600hp.
I've been running a 1 bar spring plus an adjustable boost up to 1.2 or 1.3.
I never turn up the knob.

I just bought a .6 bar spring for the wastegate.
I want a drivable car. I don't need 600hp. 400-450 would be good. Less is fine if I can get low end SMOOTH power.

I assume.....If the low end works as I hope it eventually will, then there should be no need to pine for a g/50/50.

Anyone have turbo suggestions?
Any other suugestions regarding low end smooth power?
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88' blk/blk 930: Haltech EFI, Twin Plug, Pauter Rods, Nascar Bearings, custom crank work, dowel pinned case, ported manifold and heads, Kokeln I/C, SC Cams, Turbonetics ball bearing 62-1, BB headers, RARLYL8 Zork, additional 993 oil filter, plx/inyourface gage, RS style coilover, Fikse FM10-17 wheels, TIAL 46mm 1 bar. (where the heck did all my money go?)
Old 06-01-2009, 04:20 PM
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Corrupted by power
 
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Quote:
Originally Posted by dsiegel360 View Post

I assume.....If the low end works as I hope it eventually will, then there should be no need to pine for a g/50/50.

Anyone have turbo suggestions?
Any other suugestions regarding low end smooth power?
Don't laugh, but I have a buddy using a 915 in his VERY HOT 3 liter turbo. It is strong and holds the power, but more importantly (he says), transformed the car from one that bored him to death (4-speed), to the love of his life (5-speed). Don't underestimate the power of gears. I think his car is more fun and feels faster w/ a 5-speed and 350ish HP than a 450HP+ monster w/ a 4 speed. The five speed work great at that power, but would probably grenade under 450HP - it's all trade offs.

That said, a Garrett GT30 with the right trim and A/R should spool like crazy and allow over 400HP. Just watch your surge lines on the compressor maps, as your 3.4 is going to pump a LOT of volume for a housing that small. A GT35r does make full boost at 2800 RPM on a 3.3, so I'd triangulate between the 30 and 35 for a very "N/A" feel to your car.
Old 06-01-2009, 04:28 PM
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Doug Siegel
 
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Great info. Thanks.

I spoke with someone today who said that the high HP doesn't destroy the transmission, it's using the wrong clutch. Does this make sense???

Regarding the 915, I would be willing to do without the top end HP to get a drivable low end. I suppose this also protects the life of the engine.

Patrick motorsports wants over 11k for a g50 conversion. Does a 915 tranny also require endless numbers of parts for the conversion?
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Old 06-01-2009, 05:06 PM
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What about re-gearing your present trans? This way you get lower gears that make it better for the city without sacrificing durability.
Old 06-01-2009, 05:19 PM
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Doug Siegel
 
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Quote:
Originally Posted by A930Rocket View Post
What about re-gearing your present trans? This way you get lower gears that make it better for the city without sacrificing durability.
Regeared 1st, 2nd, and 4th. First is taller. The idea was to keep it in the powerband at 4k at every gear. Doesn't work unless shifting at over 7k rpm.

Ist gear takes a while to get going. From what I gather, the big turbo prevents low rpm airflow.
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Old 06-01-2009, 05:39 PM
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A 0.82 A/R, Garrett GT30R ball-bearing turbo should take you to around 425HP and give you VERY quick spool-up and response, or step up to a GT35R for around 500HP capability.

Both are awesome, newer tech turbo's
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Old 06-01-2009, 06:24 PM
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I would not recommend the 915 if you produce over 350 ft/lb of torque.

The disatisfaction for your setup lies with the turbo. Once you replace that lag monster with something that has a threshold of <2500rpm you may like your 4spd tranny a lot better.
It's a system, the power curve needs to complement the gearing or vice-verca.
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Old 06-01-2009, 06:48 PM
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Hi Doug!

Have you considered a RUF 5-speed?

Scott
Old 06-02-2009, 02:10 AM
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Doug,

You could take the turbo off and look at the Hot side A/R number. You could reduce it by one size and that should get the turbo to spool up at a lower RPM. I'm not sure what A/R ratio you have on that turbo.

Bill Rudtner
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Old 06-02-2009, 03:45 AM
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What exactly is meant by the A/R size?
Feel free to get very technical because I am an engineer and would like to know the design aspects of this, but I am not a turbo designer by trade.
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Old 06-02-2009, 05:39 AM
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The A/R number is the intenal sizing of the hot housing. e.g. A hot housing (exhaust side) with a number of say 1.00 would flow more exhaust and spool at a higher RPM than let's say a hot housing with a size of .96 and then one step smaller of .84 But, there is a point of no return where the hot housing is now too small and it will choke and possibly over speed the turbo. Turbo size, both cold side A/F and Hot side A/R should be fit to a particular HP wish.
Air flow to H.P requirement. Boost levels are another consideration. For the Turbo Doug has his spool up should be sooner than 4,000. I have built his engine, sized his turbo and driven his car. It is an absolute animal. Sometimes there's a lot to look at when a 600 hp car comes on boost.
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Old 06-02-2009, 06:11 AM
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Quote:
Originally Posted by gtu935 View Post
Doug,

You could take the turbo off and look at the Hot side A/R number. You could reduce it by one size and that should get the turbo to spool up at a lower RPM. I'm not sure what A/R ratio you have on that turbo.

Bill Rudtner
The 60-1 and 62-1 is a very nice turbos from what I understand. I do not think the compressor wheel is any larger than the HF's that so many are fond of so I do not think that is your problem.

Do as recommend above and work on tuning the hot side of your current turbo. With Turbonetics it is easy to resize the hot side housing. The smaller hot side will spin up the compressor side quicker and bring the boost in sooner. This dose increase exhaust back pressure a bit so there is potentially a little trade off in total HP but not much unless you carried away trying to get full boost by 2200rpm of something. Watch for to small of hot side. This will show up in the form of boost creep so watch your gage to see that you are not over shooting your target boost in higher gears.

Sounds like you have a close ratio gear set. Might look at your differential ratio. this could help you off the line response a bunch. Just watch your cruse RPM and peak MPH. The right net gear ratio can wake these things up.
Old 06-02-2009, 06:36 AM
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Dougs trans gears are HX (18:33) First
QU (24:30) Second
SP (28:25) Stock Third
VM (31:22) Fourth

This car will pull like crazy and does, never falling off boost. At 7,000 RPM his first gear takes him to 75 mph, Second to about 90, third is 134 and in top gear he's looking at 200 which the car will do. I think the best think for Doug may be to lower his first gear. Funny thing is looking at the engine spec sheet, we built this engine to rev to 8,600 and will easily live for a long time shifting at 7,600
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Old 06-02-2009, 07:09 AM
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Fred

You really owe it to yourself to study several Turbo books. I recommend a friend’s book TURBO by Jay K. Miller and most of MAXIMUM BOOST by Corky Bell.

A/R is simply turbine housing Area over Radius as a constant (same for Compressor). A 2” diameter turbine wheel will have a family of housings ranging from say .75 to 1.5 A/R as can a 10” diameter turbine wheel.

As it applies to this discussion, if your turbo in question has good top end power but lacks the bottom end and the existing housing is say 1.1A/R then a switch out to one of lower A/R (.8A/R) is advisable. However, if the turbo in question is already below the midrange (efficiency sweet spot) in available A/R then there is the consideration that the low end is not greatly improved while the top end takes a noticeable hit due to higher exhaust backpressure. In this scenario, consideration of a different turbo with smaller turbine wheel and corresponding family of housings is advisable.

My comment is based on a family of housing engineered for the turbine wheel in question. If the turbo is an aftermarket mix of larger wheels shoved into existing housings you have efficiency effects that defy the trend I just gave you. See there are limits to Power and RPM range in wastegated/single turbo installations and everyone in their search for power tries to tweak the system to do something that is inherently impossible for it to do.

Interesting though some are now coming to the conclusion that maybe 400HP and 5 speeds is just as quick (maybe not as fast) and the total system cost is less than say 500 Plus HP and 4 speeds. RUF came to this conclusion, when (?) in the early to mid 80s.

There are turbo systems to improve power throughout a wider RPM range, they are supercharger/ turbo systems, staged twin turbos and VNT turbo systems.

Last edited by copbait73; 06-02-2009 at 07:47 AM..
Old 06-02-2009, 07:22 AM
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Fred

You really owe it to yourself to study several Turbo books. I recommend a friend’s book TURBO by Jay K. Miller and most of MAXIMUM BOOST by Corky Bell.
Miller's book rocks - I like Bell's book, too.

It may sound nutty, but there is AMAZING info on the forums of other marques, too. NASIOC (Subaru forum) has sick, sick, sick info on turbo learning. Those guys are nuts and given there are so many cars, a lot more feet on the ground to try to figure this stuff out.

A good start: http://forums.nasioc.com/forums/showthread.php?t=533787

I found this especially useful for things we don't normally see on a 930 like twin scroll turbos and meth injection.
Old 06-02-2009, 08:21 AM
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Originally Posted by gtu935 View Post
Dougs trans gears are HX (18:33) First
QU (24:30) Second
SP (28:25) Stock Third
VM (31:22) Fourth

This car will pull like crazy and does, never falling off boost. At 7,000 RPM his first gear takes him to 75 mph, Second to about 90, third is 134 and in top gear he's looking at 200 which the car will do. I think the best think for Doug may be to lower his first gear. Funny thing is looking at the engine spec sheet, we built this engine to rev to 8,600 and will easily live for a long time shifting at 7,600
I am not an expert but it sounds like there might be a little bit of disconnect from what is wanted and the design goals. Communications is important in any relationship.

Is being able to go 200mph more important than off the line drivability. Is 165 mph fast enough if it gets much better low end performance?

The gear spacings look a bit interesting. 75 mph first gear split, thin a very short 15 mph split in second, then a 44mph split in third, and a then a 77mph split to top speed.

If first can be lowered with out drooping out of the power-band that could be a benefit.

There might be a significant improvement in boost onset and full boost if the turbine housing size is changed to a trim that will make boost 500 to 1000rpm sooner. Why 4k instead of say 3200rpm for full boost? Is this to support the HP after 7k?

Why do we want to make power to 7600rpm or more? Are the cams made for this. Is it possible that a different set of cams might move the power band down 500-1000rpm for more usable power.

I suspect that if it just pulled like stink to 7k with the right gears and it should be brutal and still drivable in traffic.

The best.
Old 06-02-2009, 12:40 PM
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gears

It's not the speed but the RPM drop between gears that you need to look at.
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Old 06-02-2009, 01:27 PM
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