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Twin VS Single Turbo...
OK, I've been seeing a lot of posts about people going with Twin Turbo setups for monstrous HP.
Heck, I'm even considering it myself down the track, however, my understanding was that a Twin Turbo is better because it offers better spool-up, with lower top end HP, whereas a big single has more lag, but gives better top end HP. Now, if a twin setup can flow high 700HP BUT reaches max boost around 4K rpm, then what's the point? Wouldn't you save costs and just go with a big single ball bearing setup which reaches max boost at 4K anyway and still gives you high 700's HP? Is going TT merely for "prestige" or bragging rights, but performs the same as a big single? Discuss and educate!
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Merv '89 911 Turbo Cab Protomotive MAP ECU, Twin Plugged Heads, GT2-EVO CAMs, 3.3L fully finned P&C's, ARP fasteners, C2T head gaskets, Titanium Retainers, Turbo spec valves, springs & guides, 964 splash valves, GT35R BB turbo, GSF Stainless Headers, Magnaflow Exhaust, Full bay Intercooler, TiAL 46mm w/gate, TiAL 50mm BOV, Apexi AVC-R EBC, SPEC Stage3+ Clutch kit, Crane CDI Ignition ![]() |
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As I understand it a twin turbo setup is usually a twin sequential turbo with the 1st smaller turbo kicking in at a low rev. then a second larger turbo kicks in at a mid range rev. This creates a smoother torque/HP curve and the larger power can better be put down to the asphalt.
A single turbo will have a more aggressive acceleration and more on off power (Incl turbo lag). My small practical experience with driving each is that the 2001 TT had a doscile streetable feel about it and it did'nt really feel the same as the 79 930 which I tried- Chalk and Cheese to coin a phrase. - But then again I am only a novice in the turbo dept compared to others here! rgds Ben
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Hi Ben. All modern TT 911's (or should I say 993tt's and 996tt's) run parallel turbo's, not sequential (like the 959 did). These are generally smaller turbo's matched to the engine for streetability (i.e: minimal lag), but top end HP is typically limited to 450 or so HP.
I'm referring to those TT setups that generate upwards of 650+ HP with larger turbos/compressors. Obviously two larger turbo's will still result in full boost being reached higher up in the RPM band, around ~4K). The question is, why bother with twins if you still get full boost at that point, when you can go one big single and save on plumbing costs etc...
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Merv '89 911 Turbo Cab Protomotive MAP ECU, Twin Plugged Heads, GT2-EVO CAMs, 3.3L fully finned P&C's, ARP fasteners, C2T head gaskets, Titanium Retainers, Turbo spec valves, springs & guides, 964 splash valves, GT35R BB turbo, GSF Stainless Headers, Magnaflow Exhaust, Full bay Intercooler, TiAL 46mm w/gate, TiAL 50mm BOV, Apexi AVC-R EBC, SPEC Stage3+ Clutch kit, Crane CDI Ignition ![]() |
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It's two-pronged problem (it actually has more "prongs" buth these two are the biggest
![]() to state an example: A well made BB single turbo might give you same boost treshold (boost treshold: a point where manifold-pressure becomes positive) and even extra HP on the top (due to better efficiency) but it won't spool up as fast diring short spurts trought rev-band beacuse it's heavier. If you dyno your car on static bench where you load engine on every RPM point and keep it there you will find your boost treshold accurately. But if you do a quick start in 1:st gear your big turbo just won't have time to spin up before your zing trough your power-range. Remember, those things spin at 120.000 RPM + , there is substantial amount of energy stored in them. If you drive your 930 at 60MPH in 4:th and floor it wou might ask yourself "what's the fuss all about" as you will get boost quite quickly already at 3000RPM compared to time it takes to reach 120 MPH or whatever. But if you start from standstill you'll just reach full boost when it's time to shift up. All of these things affect amount of turbo lag: - Boost treshold (primary a function of turbochargers aerodynamic/pump efficiency, engine volume and amount of ignition advance) - Impeller inertia (function of shaft mass) - Bearing drag (BB or oil-bearing? Oil-bearing has higher parasitic losses but is sturdier) - Amount of ignition advance (dumped ignition will send more heat into turbo, more heat -> more gases -> more spoolup Get the picture?
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Thank you for your time, Last edited by beepbeep; 08-11-2004 at 03:32 AM.. |
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Crotchety Old Bastard
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Well, there just isn't much left to say.
How 'bout "I got two tubos so there must be twice the power as one".
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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds '78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8 |
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The way I see it let's say you have two engines. One is a 3.0L turbo making 700HP and the other is a 7.0L N/A making 700HP. The 7.0L will almost always perform better with respect to throttle response and available torque than a turbo engine. Super or Turbo charging makes the engine "act" larger by forcing more air in but there are problems with that like parasitic drag, turbo lag, complexity etc. The idea of a twin turbo setup is to get the motor to perform more like the larger engine by reducing turbo lag and building up low rpm response. If the engine were being built for a specific type of racing where a single turbo could be kept on boost at all times it should perform equal to a large engine, the problem lies in the rpm range required for street driving. Low end torque is especially handy on the street and in higher HP turbo setups it takes twins to even approach the lowend of a large engine.
I've been fortunate enough to have gone from a small engine to a large engine with many of my cars in the past. My 912 is one such example, it had a carbed 2.0 911 motor when I bought it and the car had ZERO torque, or so it seemed. I then switched to a carbed 3.0SC motor, keeping the same trans and exhaust and the car is a whole different animal. The lowend of the larger motor allows me to start in 2nd if I choose, jump on the throttle anywhere above 1200rpm and pull smoothly etc. That's the idea of twin turbos, make it feel less like a turbo and more like a larger motor. There are other things to consider as well, twins can be very efficient because they can be located nearer the exhaust ports than a single in a banked engine like the 911 and therefore pickup more energy than if they were farther away like a single has to be mounted. Hope this helps a little. ![]()
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A single turbo setup will also generate more heat making you more prone to detonation. Two turbos would mean the load is split at the same psi.
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If you have two smaller turbo's set to 18psi each, wouldn't they not create the same heat as one larger turbo running that same boost level?
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Merv '89 911 Turbo Cab Protomotive MAP ECU, Twin Plugged Heads, GT2-EVO CAMs, 3.3L fully finned P&C's, ARP fasteners, C2T head gaskets, Titanium Retainers, Turbo spec valves, springs & guides, 964 splash valves, GT35R BB turbo, GSF Stainless Headers, Magnaflow Exhaust, Full bay Intercooler, TiAL 46mm w/gate, TiAL 50mm BOV, Apexi AVC-R EBC, SPEC Stage3+ Clutch kit, Crane CDI Ignition ![]() |
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Quote:
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Thank you for your time, |
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