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Throttle body sizing?

Just wondering what people's experiences have been with throttle body sizing on 930 motors? My 3.4L EFI motor has an OEM size 69mm 1995 993 TB and plastic intake manifold and Im wondering if this will be adequate to support my future target of 700whp? I noticed that some peoples cars have larger TBs, so for my dyno tuning this month I ordered a FVD 71mm overbored 993 TB just to see if that will make any difference. My motor currently puts down 600whp thru the 69mm 993 TB but Im thinking it may be a slight restriction at high revs? So much air passing thru there when you have a big turbo.

I see that a company called IPD makes big bore TBs and plenums for the 996 turbos and claims a 30hp increase in top end HP with the 75mm TB/plenum combo. What works for the turbo 996s might also work for my 930 motor?
Old 08-01-2008, 04:45 PM
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If I wanted to know about this I would have tried to take boost readings before the TB and after. If there is a difference you are losing power, if not then you are not.

Of course I have no idea how you would work out how much bhp you are losing if you did have a pressure drop...

Hopefully someone that knows will comment.
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1981 UK 930. G50/01 shortened, 964 3.8RS Fibreglass Body Kit, 18" Alloys 8.5" F & 10" R, 225's F & 285's R, Special Colour Metallic Blue Paint, FIA Sparco Evo's, A/C and Air Pump removed, Electronic Boost Controller, GHL Headers, Tial46 WG.
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Old 08-02-2008, 08:42 AM
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Boost pressure isn't going to tell you anything. You need to be able to moniter cfm of air flow.
Old 08-02-2008, 09:42 AM
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My thought I that I should measure the ID of my compressor outlet when I get it back and add maybe 5mm to compensate for the throttle plate? I see a lot of 930s here using 75mm TBs - should flow plenty but just wondering if there is an throttle response penalty? Has anyone here picked up HP by going to a larger TB with no other changes? Some of the 1000whp+ supras I see online are running 100mm TBs!
Old 08-02-2008, 12:00 PM
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Real-world experience trumps math any day but are comparitive restrictions in the system a starting point? The combined area of the intake ports are going to be about 75cm2 but the area of say an 80mm throttlebody is about 50cm2. That might imply a higher velocity through the throttlebody than the heads and lots of other variables but would that be a starting point to think the throttlebody might be a constriction?

I'm interested in the comments of the experienced here. I sourced an 80mm throttlebody in support of 500+hp on the track (partial throttle less important) but am always willing to second-guess my expensive purchases
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Old 08-02-2008, 01:45 PM
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My research showed keeping air velocity just below 300ft/s was optimum. By my calcs a stock 3.2 Carrera TB is a hair over 300 ft/s on my car. I've put big TB's on other cars with very little to no performance increase so I'm not a fan of big TB's, but I'm planning to go a little larger to see if it makes a difference.
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Old 08-02-2008, 03:51 PM
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I dont think a bigger TB will make a huge difference either. Im just going with a small 2mm increase for this round of dyno testing. If it helps I may consider going to 75mm later on.
Old 08-02-2008, 04:56 PM
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Don't you think that if you don't get a differential pressure either side of the tb at your max rpm and at wot your not losing any power?
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1981 UK 930. G50/01 shortened, 964 3.8RS Fibreglass Body Kit, 18" Alloys 8.5" F & 10" R, 225's F & 285's R, Special Colour Metallic Blue Paint, FIA Sparco Evo's, A/C and Air Pump removed, Electronic Boost Controller, GHL Headers, Tial46 WG.
Fitting - New service kit.
Needs Fitting - Innovate XD-16 Kit, Kokeln IC. Stephen's K27 HFS, EVO Intake Assy & his Modded USA Fuel Head.

1983 UK 911 3.2 Carrera Sport Coupe. Black, Black Leather with Red Piping, Black Alloy Gear Knob, K&N Air Filter Element, Turbo Tie rods.
Needs Fitting - K&N CO Sensor, Round A/F Dial Gauge, Factory Short Shift Kit.

http://www.danasoft.com/sig/Iamnotanumber.jpg
Old 08-02-2008, 10:29 PM
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I guess in theory zero DP would be no restriction, but then it probably wouldn't work very well as a throttle.

Unless you're just going for big dyno numbers, it's power under the curve that's important not peak hp. If you're making another 20 hp at 7000 rpm but making 30 hp less at 5000 rpm, you'll probably be slower around a track.
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Old 08-03-2008, 07:14 AM
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Check out Jim2's awesome thread on his EFI conversion. Post #124 is the start of his throttle body fabrication.

930 EFI Conversion in process
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Old 08-03-2008, 08:09 AM
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Thanks Boostfix that is an excellent thread! Hmm so he went to a 76mm custom TB on his carrera manifold and measured a negligible pressure drop. Seems like that TB would be adequate for nearly any 930 motor. I have a good lathe and milling machine at home, but the FVD bored TB was pretty cheap so I didnt feel it was worth the bother. If that 2mm increase gains me anything, I can always bore it out even further on my own, provided there is sufficient meat left on the walls of the TB to do so. Or I could always make one completely from scratch like Jim2 did but thats a pretty time consuming proposition!

Last edited by GT2EvoGuy; 08-03-2008 at 07:58 PM..
Old 08-03-2008, 07:51 PM
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My two cents : It's a little hard too say specifically weather a larger throttle body will give total performance. You might gain a numerical but suffer or end up with poor throttle control.

Since your engine is already tuned and sitting at 600hp, I am going to be very cautious about what I say here. As there are a good many ways to reach 600hp and depending on how you approached such performance levels one should act with caution.

The way I like doing things ( personal pref )
1 : What is your turbochargers flow aprox as far as CFM's are concerned.
2 : What dose your throttle body + Manifold's flow charictor/CFM
3: What dose your cylinder head on flow on the intake side and exhaust.
4. What dose your exhaust/Turbo Manifold flow.

This is sort of a general rule of thumb, I don't see a 71mm throttle body jumping you up too 700hp. you might increase your performance soon rather than later or have minuet gains with out addressing the listed above. Then again I know very little to nothing about your specific engine.

Good luck with your project~
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Old 08-03-2008, 08:12 PM
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GT2Evo, if you encounter a drivability problem at low throttle openings simply alter the geometry of the throttle arm. Though I find mine to be more sensitive than the stock TB it's still got sufficient modulation down low.

My rationale for 76mm is to achieve the same cross section area as the ID of the tubing leading up to the TB while accounting for the lost area of the butterfly shaft. You certainly don't need to get this picky when the objective is simply to reduce inlet stream losses. Not sure which intake manifold you are dealing with but I had to open mine up, it was the same size opening at the original TB.
Old 08-05-2008, 07:09 AM
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My car has the accufab race throttle body on it. It is around 100mm at the opening and necks down to 80mm. It was delveloped for the mustang crowd. The renegade class was limited to a 75mm throttle body so they experimented with the velocity approach. Long story short it works.
Old 08-08-2008, 07:39 PM
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Holy sh** thats a huge throttle body

How much hp did you pick up with that TB over your previous setup may I ask? Im sure that 80mm would support nearly any power level that Im shooting for and then some.

For simplicity sake I will probably just try to bore out my 993 TB as far as I can if I find that the motor is getting choked off.

Im getting my GT4094R in a few days so my first order of business will be to measure the ID of the compressor outlet. Really not much sense in going more than a few mm larger than that to compensate for the throttle butterfly I would think?

Hopefully I will have a dyno sheet to post soon of my new turbo with the 71mm TB
Old 08-11-2008, 07:59 PM
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I dont have any before and after dyno numbers. Here is the latest on my throttle body. Everything on my car is v band now. If you need a fab guy in your area. I have a guy that is BEYOND GREAT!





Old 08-11-2008, 08:03 PM
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