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931 intake mods - which throttle body for EFI?

 
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924RACR  



Joined: 29 Jul 2001
Posts: 8018
Location: Royal Oak, MI, USA

PostPosted: Fri Apr 04, 2008 3:11 am    Post subject: 931 intake mods - which throttle body for EFI? Reply with quote

Due to some off-board developments, it looks like I can probably really get my EFI conversion kick-started this spring. This is the '82 931 DD car, which will get a 951 IC installed and converted over to a programmable EFI system.

Since we'll be ditching all CIS stuff and adding an IC, the stock intake manifold and throttle body setup is no longer optimal - clearly a revised intake with the TB on the front end, closest to and pointed at the IC outlet will be a much cleaner setup.

Plus we'll want a larger TB, I think (target is 250-300hp) and one that comes with a TPS would be a lot easier to deal with - gonna need that anyway!

So what's the consensus out there WRT good, inexpensive, reliable throttle bodies? Rustang 5.Slow is an easy though, but I do dislike Fords on sheer principle, even without looking at the data. Maybe GM 3.8L V6? They seem to work well at least up to 300hp... or maybe I should look at base Corvette (404hp now)?
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ideola  



Joined: 01 Oct 2004
Posts: 15426
Location: Woodstock IL

PostPosted: Fri Apr 04, 2008 3:26 am    Post subject: Reply with quote

I'm planning to switch to the Rustang 5.0 TB when I make the leap to EFI. AFAIK, it has a square bolt pattern (or pretty close to it) and a 65mm throat. It should be an easy swap out with my custom intake setup. I haven't evaluated any other options at this point.

BTW, Vaughan, let me know if you're interested in having my welder do up an intake like mine. One of the nice things about my setup is that it does not require relocation of ANY of the under hood components, it preserves runner length, and should improve airflow across the runners due to the diffuser plenum. Almost all of the other alternatives I evaluated required relocation of the alt and/or the coolant tank. It also positions the TB directly where the 951 IC outlet would be, with probably less than a 12" hard pipe connection.
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15 981 GTS | 04 955 Turbo | 88 924S SE | 82 931 (x2) | 81 937 | 81 Euro 931 | 81 Weissach | 80 US 928 | 80 US 931 '941' | 80 US 931 | 80 931 GTR | 79 Sebring | 78 D-Prod (x2) | 78 Poli-Form | 78 Limited Edition | 77 Martini
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endwrench  



Joined: 07 Dec 2002
Posts: 1629
Location: Victor, Montana

PostPosted: Fri Apr 04, 2008 3:40 am    Post subject: Reply with quote

Not sure what would be "best" but you should take a look at the TB's on the Chevy Vortec. They are 65mm but they shroud the edge a little to smooth out tip-in problems wich can be a problem with big TB's. They also have the TPS and a stepper style idle control solinoid built in. This will work well with MS2 And you can get rid of some of the extra plumbing associated with the stock set-up. All the Vortec TB's are the same size even on the V6 though it has heavier shrouding.

Todd
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'79 924NA. Rebuilt 9.5:1, MSDS header, Mega Squirt Injection, MJLJ-EDIS Ignition, 1.6L Whipple Charger and Intercooler, 10lbs Boost, 944 Trans, Custom HD Clutch.
"simsport" said....superchargers are better than turbos its official!....
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flosho  



Joined: 01 Jul 2004
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Location: Eau Claire, Wisconsin

PostPosted: Fri Apr 04, 2008 6:41 am    Post subject: Reply with quote

Ideola, could you PM me some details about your welder, I wouldn't mind a replicated manifold similar to yours if your welder is ok with building another... I was actually going to email you about it earlier just haven't gotten around to it.

Alot of people use the mustang 5.0 TB because they have the TPS right in and its pretty common to find cheap.
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emoore924  



Joined: 13 Apr 2004
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PostPosted: Fri Apr 04, 2008 1:18 pm    Post subject: Reply with quote

Look below for the chart that goes with this discussion...

//snip//

Purpose: This study set out to determine which of five throttle bodies (TBs) would be most suitable for a 924 turbo (931) intercooler upgrade. "Most suitable" meant the largest area available to flow air into the motor. Since some of the TBs had one throat and some had two, a standardized measurement needed to be found so that the two types of TB could be compared with each another. This comparison measurement is the "Single Throat Equivalent Diameter" or STED value for each TB. Simply put, the STED is a number representing the diameter of all throats combined as if the TB only had one round throat. The TBs can then be compared.

Warning: In deference to all you fluid dynamics engineering purists out there, this is NOT intended to be an exhaustive exercise in the flow characteristics of 924 TBs. Rather, it is a primitive, ham-fisted, arithmetically-challenged look at trying to figure out which TB might be the biggest dog on the block. Given what TBs might be generally available to the amateur turbo enthusiast out there, it helps the reader focus on which TB might be the best pick for someone looking for a little more flow for their horsepower-challenged 931 or something that might provide a little more hp potential if you're tuning. So please don't rain facts on my parade. I'm blissfully ignorant and would like to stay that way. If you can live with that premise, please read on.

Methodology: Three categories of TB were included in the study. The first was a TB taken from a mid-80's Audi 5000. These are known to fit the 924 and (at one point in time) were readily available. The second type of TB was a rare aftermarket Weber single throat TB. And the third grouping encompassed three Porsche factory TB's, two from turbo installations, and the last one was from a non-turbo, or "normally aspirated" 924 (a.k.a. "n/a") of unknown vintage.

To determine a proxy for the relative flow potential of each TB, the area of the TB needed to be calculated. To do this, the ID of each throat was measured using a needle caliper. The measurement was made of the narrowest part of the TB, where constriction would be greatest. The measurement of the primary throat was recorded in the "diameter" column of the table below (column B). The area of the primary opening was then calculated (column C). From this result, an adjustment was made to account for the thickness of the throttle plate and pivot rod (column D), and a net area calculated (column E). In the case where there was a primary and a secondary throat, the secondary was then measured and similar metrics calculated for it (columns F-I). These two areas were then combined and the equivalent diameter resulting from that area or combined areas was calculated (column L). This final result, or equivalent diameter, is the STED which appears in the column M in the table below.

Further explanation is probably in order regarding the attempt to account for the width of the throttle plate and pivot bar (which ultimately effects the STED). The actual size of these obstructions varied slightly from TB to TB but generally they were about 3/16 thick, so this constant was used to reduce the effective area of the throat. Not being a thermodynamicist or fluid dynamics engineer, the real need for this adjustment is unknown, but it is included anyway. Realitly of course, probably lies somewhere between the result without the adjustment and the result with the adjustment. Both results are included for your convenience of course and may be used as bookends to bracket best case/worst case outcomes.

Findings: The Audi TB has the greatest area, and therefore the best STED, of all the TB's evaluated -- coming out slightly ahead of the Weber and easily besting all of the Porsche factory units. The dual throat Audi unit exceeded the single throat Weber area by an adjusted 0.19 sq/in, or 5% more area. Based on these calculations, the Audi unit at 2.307 STED should perform like a single-throat TB somewhere in the 2.5-2.3 inch range, slightly overlapping the Weber area figures, but besting the Weber's 2.252 STED.

When the Audi TB is compared to the stock 1980 dual-throat unit, the Audi TB exceeds the stock specifications by a more significant margin. The adjusted area numbers for the Audi TB versus the Porsche 1980 931dual-throat unit are 4.826 sq. in. versus 3.890, or about 25% better area for the Audi unit (that's almost 1 sq. in.!). And, when the STEDs are compared, the Audi still comes out better at an adjusted 2.307 STED versus 2.051 for the factory unit, or a 12.5% difference in diameter -- worth paying attention to.

It should be noted that ALL of the difference in area (and therefore the STED) between the Audi and the 1980 stock TB lies in the diameter of the primary throat. Interestingly, the secondaries of the Audi and the stock unit are exactly the same (please see note 1 in the table below).

The Audi and the Weber units both exceed the specs for all of the stock units, but don't discount the factory units just yet. At first glance, the stock dual throat unit from 1980 seems to just edge out the single-throat unit from the 1981 or 1982 model years. However, once one adjusts the dual throat figure to accommodate two throttle plates and two pivot rods in the throats rather than just one in the later unit, the two factory turbo TBs essentially tie one another. Was there some kind of factory improvement in the later years throwing an advantege to the single-throat? Who knows. There is no way to tell from this study (perhaps our fluid dynamicists could chime in here).

As might be expected, the stock n/a TB brought up the rear, banking just an adjusted 2.518 adjusted area figure and a 1.791 STED. Perhaps adequate for a unmodified n/a car, but not the direction one would go in for a turbo, or even an n/a car with light work done.

Conclusion: If building up a performance 931 and more flow is desired, the Audi unit has the largest STED. The Audi unit should therefore should be able to support the highest flow and therefore more HP than the Weber or any of the stock units.

From a practicality perspective, for the early 1979 and 1980 turbo cars, the Audi TB is the clear choice. It is an easy fit to the existing components and offers significantly improved flow potential over the stock factory unit. Clearly no brainer territory. But if you're building up a 1981 or 1982 931 and you want to retain the round intake charge tube, TP sensors and manifold (rather than backdate to the ovoid charge tube and manifold that the earlier cars have and that the Audi piece would fit), that means the Weber might be the way to go for later applications -- if you can find one. If you choose the Weber over the Audi, you'd give up a little flow potential, but you should be able to fit the round charge tube and intake manifold to the round Weber with much less headache than switching in the ovoid Audi part. Keep in mind that you will still have to make some minor modifications due to the difference in ODs between the Weber and the stock round TB -- the Weber is bigger, and you'll have to add the TP switches, but these modifications are not insurmountable.

Bottom line: Pick the TB that matches the shape of your current charge tube and manifold setup. Frankly, even though the STED figures for the Audi or the Weber are a little different but not much, they're markedly better than the factory setup. So, the upgrade is easy if you can find the TB that matches your current setup (round or oval...)

The factory turbo TBs aren't completely useless however. They might be adequate upgrades for someone with a factory n/a setup looking for a little more flow -- maybe you're running a cam in a P&P'd head or something. In that case, consider that the factory n/a TB is very much alone at the bottom of the list. That being said, it probably wouldn't hurt to upgrade your n/a setup to the turbo TB if you can find one of the turbo units cheaply and easily. I doubt that fitment would be difficult, so if you're considering an upgrade to either of the stock turbo setups, other than shape, it would probably be worth considering. For a n/a upgrade, I'm not sure I'd spend a lot of time or money looking for an even bigger Weber or Audi unit but that's up to you and your machinist.

Combining with an intercooler setup: Whether you're using the Audi TB or the Weber, a good size for the intake/intercooler pipe should should be in the range of 2.25-2.50 inches. This choice aligns well with the calculated STED of either TB and supports the flow rate calculations necessary to generate up to around 250 HP with the proper turbo and intercooler configuration. Do your own calculations, but unless you're upgrading something else in the system (significantly!), this pipe size should handle the flow necessary to generate all the HP you can.

For my project, I've used 2.25 pipe for three reasons. First, a readily available 2.25 cast aluminum elbow neatly fits the outlet of my K26 turbo so I can eliminate the short factory charge tube and start the pipe run to the inlet of the intercooler with a 90 degree bend that closely follows the frame rail towards the front of the car. So no monkeying around to hook the turbo outlet to the rest of the intercooler system. That's great. Second, the inlet and outlet of the 4X6 PWR a/w intercooler I am using are 2.25. So that size pipe fits the i/c too. Last, the input to the factory charge tube is 2.5", again, making an easy transition.

Good luck with your project! Let's keep those 924 turbos on the road!!!


Last edited by emoore924 on Fri Apr 04, 2008 2:08 pm; edited 7 times in total
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Min  



Joined: 04 Nov 2002
Posts: 2365
Location: Vernon, British Columbia, Canada

PostPosted: Fri Apr 04, 2008 1:35 pm    Post subject: Reply with quote

I was pleased the the rustang 5.0 throttle bodies size/features. I'm going to be using one. Lots of nice upgrade options later if you need a bigger one down the road too.

Min
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ideola  



Joined: 01 Oct 2004
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Location: Woodstock IL

PostPosted: Fri Apr 04, 2008 1:44 pm    Post subject: Reply with quote

Here is Eric's chart:


Along with the downloadable excel file.
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15 981 GTS | 04 955 Turbo | 88 924S SE | 82 931 (x2) | 81 937 | 81 Euro 931 | 81 Weissach | 80 US 928 | 80 US 931 '941' | 80 US 931 | 80 931 GTR | 79 Sebring | 78 D-Prod (x2) | 78 Poli-Form | 78 Limited Edition | 77 Martini
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924RACR  



Joined: 29 Jul 2001
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Location: Royal Oak, MI, USA

PostPosted: Fri Apr 04, 2008 10:16 pm    Post subject: Reply with quote

Yeah, I definitely will have to check out that intake. Have any pics, installed or otherwise? Email them over, if you would...
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Vaughan Scott
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'79 924 #77 ITB racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype
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ideola  



Joined: 01 Oct 2004
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Location: Woodstock IL

PostPosted: Fri Apr 04, 2008 10:38 pm    Post subject: Reply with quote

Vaughan
You can see the entire development process of my intake in my picasaweb album, starting here.

Here is a photo of the finished product, ready for powder coating and installation.


...along with a link to my thread here on the board.

FYI, it's a long way from getting installed on the car, so if you want to borrow it for a week or so to take a close look at it and compare it to your setup, I'd be happy to drop it by on a Sunday or Wednesday when I'm passing thru RO. FYI, this particular unit was made using a SI manifold because I wanted the larger port for the original TB to install a bank of fuel enrichment injectors. As you can see in the pix, the relocated TB will be a SI. I tried to come up with an approach that would allow me to have a single mounting plate that could accommodate both a SII TB as well as the Rustang 5.0 TB, but the "overhang" of the throttle linkage on the SII TB prevents a larger mounting plate required by the Rustang unit.
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924RACR  



Joined: 29 Jul 2001
Posts: 8018
Location: Royal Oak, MI, USA

PostPosted: Sat Apr 05, 2008 1:09 am    Post subject: Reply with quote

Thanks, those pics help a lot for reference - sadly, I have no real good pics of the engine bay, etc, for such visualization and discussion!

I think we'll go with a completely custom plenum, canted over to where you have the TB placed...
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Min  



Joined: 04 Nov 2002
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Location: Vernon, British Columbia, Canada

PostPosted: Sat Apr 05, 2008 2:05 am    Post subject: Reply with quote

924RACR wrote:
I think we'll go with a completely custom plenum, canted over to where you have the TB placed...


Thats the route I would pick also.

Min
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leadfoot  



Joined: 11 Dec 2002
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Location: gOLD cOAST Australia

PostPosted: Tue Apr 08, 2008 7:32 pm    Post subject: Reply with quote

Something like this... Very sensitive on the tip in, although it was supercharged and use to run about 3psi in closed loop through the recirculating plumb back BOV.
I would suggest a slightly larger plenum if your streeting it though.
From memory this is 80mm piping...
Stuart


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