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Old 11-25-2007, 01:32 AM   #1
Fast4525
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Default PMO throttle bodies. Anyone try them?

Have any 964 or 993 owners tried the PMO individual throttle body set up?

http://www.clewett.com/products/throtbod/throttle.htm

ITB are know for having amazing breathing capabilities and for super quick throttle response, plus they sound great! It seems to me that these ITBs, combined with a light flywheel, cams and custom engine management could make for a nice 300hp 3.6L

Thoughts?
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Old 11-25-2007, 03:01 AM   #2
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What's the point? For less money/hassle, and better drivability you can go with the Vitesse modication. Similar power. What's not to like?
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Old 11-25-2007, 09:45 AM   #3
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I have used a similar setup as that, PMO manifolds with ITBs. The ITBs I used look the same as Cletwett is using, but I cannot be certain they are exactly the same. Setup works well, but you need to size the manifolds and ITBs to the engine for best results. It should be noted that the setup is not ideal in that the length of the outer cylinder manifolds is longer than the middle and the engine will have slight cylinder to cylinder variations measureable on an engine dyno with EGT probes. However, Porsche has been using manifolds like this and carbs for years. The setup I've had experience with worked great when tuned with a MoTeC ECU. Good inexpensive solution for ITBs. Not the best, but a good price/performance value.

Noah, what do you mean "better drivability" have you driven a car with ITBs? If so, what were the drivability issues you were experiencing?
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Old 11-25-2007, 10:33 AM   #4
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Me to want to know more about this butterfly’s.
Like, why the guys at Porsche motorsport in Weissach didn’t chose ITB on the RSR cars.
What is the difference / benefits of Single throttle body vs. butterflies
I guess the single throttle body like the one on the picture from FVD
http://shop.fvd.de/?VID=431548&VCD=7...Body=&CarYear=
produces higher velocity and better torque at low rpm. And at higher rpm the second chamber open (ore closes) and we get a flatter torque / hp curve?

So, if this is true; and I take off the stock plastic intake and put on ITB. I will lose power at low rpm. And gain at high rpm?

I am absolutely no expert in the field, this is just qualified guessing, please fill in with correct information.
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Old 11-25-2007, 10:52 AM   #5
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The 964 and 993 RSRs (engine type M64/04) all had individual throttle bodies. They were low mounted ITBs, not high mounted ITBs like the picture you posted. Porsche also fed the throttle bodies through a common "manifold" which was suppose to provide a more broad torque curve.

There are many different ways to design an engine, but it needs to be designed as a system of cofunctioning parts in order for it to be optimized. Individual throttle bodies can provide very broad torque, however, they are generally pared with camshafts designed with long duration for high engine speed operation. That engine combination won't necessarily give you low end torque.

Here is an example of two engines of 3.8l displacement and individual throttle bodies. The solid lines are my engine running 93 octane fuel, 12:1 compression, with camshafts with a duration near 320 degrees and high mounted individual throttle bodies without a common plenum. The dotted lines are a PMNA sprint engine running 110 leaded, 12.5:1, with low mounted slide valves and the factory common "manifold". Big difference with "area under the curve".


Click the image to open in full size.
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Old 11-25-2007, 11:15 AM   #6
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Quote:
Originally Posted by Geoffrey View Post
Noah, what do you mean "better drivability" have you driven a car with ITBs? If so, what were the drivability issues you were experiencing?
Geoffrey,

When Todd and I were in discussion and considering the options, this was his concern for a street car.
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Old 11-25-2007, 11:19 AM   #7
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Noah, it is all in how you design the engine...ITBs can provide great drivability if used properly with the right components. I did ITBs on a 3.0l with pistons, cams, and headers as a street car and it was wonderful AND got great fuel mileage compared to the stock system.
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Old 11-25-2007, 11:31 AM   #8
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"ITBs can provide great drivability if used properly with the right components"

and the right components are?
Geoffrey, if you are willing to shear the information, please post cam spec and links to ITB suppliers.
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Old 11-25-2007, 11:39 AM   #9
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I don't know what the right components are, you haven't given any indication to what displacement the engine is, what fuel you will run, what the application is -street, race, track, dual purpose, what your budget is, what you are starting with as a base, do you need heat, is noise an issue, hydraulic or mechanical rockers, what original components do you want to retain, what do you want replaced, etc. You can't design an engine over the Internet by picking from a list of components someone has posted.

Almost all of the camshafts I use are ones that I have custom made to the individual engine's requirements ie RPM range, displacement, compression ratio, head flow data with intake and exhaust. Even if I did post camshaft information, it might be meaningless to your application.

I'm not witholding information here, anyone here knows I post information freely.
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Old 11-25-2007, 01:38 PM   #10
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The PMO throttle body set up looks to me to be a reasonable solution for someone looking to update an old Weber carburettor installation with engine management, however it does nothing to address the fundamental design flaw of the turns in the outer ports of the manifold which will result in each cylinder running at differing volumetric efficiencies throughout the rpm range of the engine. For new installations the better choice would clearly be a system with 6 identical straight manifolds.

Regarding Geoffreys comparison of his engine against the PMNA, the key difference here is that the PMNA engine would have been built with the RSR heads that have overlarge intake ports & matching large bore slide throttles and it is the port size which is responsible for the lack of low end torque of that engine.

So, how do you select the optimum manifold for any given engine?
1. In an ideal world, the intake port of any engine should be absolutely straight right upt to the back of the valve. In the case of the 911, there is a port turn in the head which cannot easily be altered, so accepting this compromise means that the best we can do is to use a perfectly straight intake manifold/throttle body/air horn assembly.
2. The ideal port tapers from the air horn all the way down to the minimum port area within the head. A tapering port increases charge density as the air moves down the tract, however too steep an angle and the wall friction overcomes the advantage of the design. If you are running stock heads with large ports (e.g. 993 or 993RS) there is precious little that you can do to put them right, so it can be useful to minimise the area at the bottom of the intake manifold rather than in the head.
3. The open area of the air horn (i.e. intake runner entrance area) should be of similar area to the maximum curtain area of the valve (PI x diameter x max lift), otherwise the pressure gradient between the inlet & outlet of the system could favour reverse flow.
4. The length of the manifold is normally tuned to match the resonant pulses of the intake system. The resonant length varies with cam duration and port areas and are usually optimised around the fourth, third or second harmonic lengths of the intake tract (think notes of an organ pipe) . The 2nd harmonic has the strongest pulse strength but requires the greatest length so most systems aim for the 3rd harmonic so that it can be fitted into the available space. Most 911 ITB's tune on the 3rd harmonic at peak power, the advantage being that the length is closer to the 4th & 5th's hence why you see more peaks in the torque curve below peak power.
5. The last factor affecting the design of the ITB is thus cam timing. In a nutshell, if you are going to fit a high velocity ITB system to your engine, the maximum benefit will occur when you have selected the cam timing that takes maximum advantage of the increased velocity in the intake system - which naturally means running a longer duration than stock.

Get all the parameters right, map the engine correctly and an ITB system can run as well on race cams as a standard engine on a stock manifold. My 4.0 litre engine, like Geoffrey's 3.8, will start on zero throttle from cold, idle at 900rpm, pull 3rd gear on the idle map and pull cleanly at full throttle from idle to redline.
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Old 11-25-2007, 02:34 PM   #11
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Colin,

Some interesting information. Thanks for sharing it.

As to the OP's thought: "ITB are know for having amazing breathing capabilities and for super quick throttle response, plus they sound great! It seems to me that these ITBs, combined with a light flywheel, cams and custom engine management could make for a nice 300hp 3.6L"

Looking only at cost, how do the monies compare - MAF vs. ITB's - if tuning an engine to ~300?

I believe you, Geoffrey, and others have achieved this using a MAF with stock cams?
Most everyone that has gone this route, speaks of the improved throttle response. Are the ITB's another step up, as I assume?
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Old 11-25-2007, 06:09 PM   #12
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Quote:
Originally Posted by sten gr View Post
Me to want to know more about this butterfly’s.
Like, why the guys at Porsche motorsport in Weissach didn’t chose ITB on the RSR cars.
What is the difference / benefits of Single throttle body vs. butterflies
I guess the single throttle body like the one on the picture from FVD
http://shop.fvd.de/?VID=431548&VCD=7...Body=&CarYear=
produces higher velocity and better torque at low rpm. And at higher rpm the second chamber open (ore closes) and we get a flatter torque / hp curve?

So, if this is true; and I take off the stock plastic intake and put on ITB. I will lose power at low rpm. And gain at high rpm?

I am absolutely no expert in the field, this is just qualified guessing, please fill in with correct information.
The answers to your questions lie in the options chosen by an engine designer when he/she begins to outline an engine for a specific performance requirement. The choice of using a resonance-flap plenum manifolds (with or without individual throttles) or conventional ITB's reflect the two methods of filling the cylinder during the inducion cycle. You can either use the Porsche method and fill the cylinder at a slow gas speed through large ports, or you can use my preferred method and fill the cylinder at a high gas speed through small ports with lower flow numbers.

My take on this choice is that cylinder filling (more correctly volumetric efficiency) is directly proportional to intake flow whereas it is proportional to the square of the intake velocity, therefore if you sacrifice flow to gain velocity up to a finite limit you will fill the cylinder more efficiently - and hence make more power. Smaller ports also have the advantage that they spread the torque across a broader rev range, pretty much a win-win choice. The only justification (that I can deduce) for larger port heads in production engines is that by necessity you match them with short overlap cams that are good for emissions, but this does not explain why Porsche went even larger when they built their race engines.

So, the answer to your question is that manifold choice reflects the intake port size and camshaft choice and that Porsche Motorsport style resonance flap manifolds are simply a "band-aid" to solve the problem of running with cylinder heads that have oversize ports which by inference also means that you will not be doing yourself any favours by running large-bore straight-stack ITB's on the same engine.
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Old 11-25-2007, 06:40 PM   #13
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Thank you Colin
Does this mean that 9M heads are more similar to 964 than 993 in port size?

I don’t want to change to much on my 964 RS engine, but until I have saved a bit more and can start to put to getter a full race engine, new cam`s ITB and a remap could pas.
It is especially the slow corners I need more power, but maybe another 2 and 3 gear would do the trick?
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Old 11-25-2007, 06:43 PM   #14
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Is this one on ebay?

http://cgi.ebay.com/ebaymotors/Porsc...spagenameZWDVW
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Old 11-25-2007, 08:42 PM   #15
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Quote:
Originally Posted by sten gr View Post
Thank you Colin
Does this mean that 9M heads are more similar to 964 than 993 in port size?

I don’t want to change to much on my 964 RS engine, but until I have saved a bit more and can start to put to getter a full race engine, new cam`s ITB and a remap could pas.
It is especially the slow corners I need more power, but maybe another 2 and 3 gear would do the trick?
The 9m heads are one on their own, having a very straight high velocity port design in comparison with any production Porsche head. That said, the 964 inlet port has the straightest and smallest area of port between the 964, 993 & 993RS, so it runs the 9m head closest in performance even though it is not perfect.

In my view the biggest difference you could do for your standard RS engine to improve low end torque and response would be to fit the 9m Motec system and headers.

Oh, and here's an example of the torque spread of a 9m high velocity port on our 4.0 litre engine with the production 9m billet head and ITB set up against a standard GT3 engine with it's resonance flap intake & variable cam timing:
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DoppelSpeedMeister - Porsche Club Great Britain Speed Champion 2006 & 2007 (Class 1 winner 2005 - 2008)
1995 993RS CS Speed Yellow 9m 450bhp n/a
1990 964 C4 Project, Guards Red (work in progress)
1997 9m93GT2 Lightwweight (work in progress)
1995 9m93 Speedster S (work in progress)
1986 911 LHD non-sunroof G50 Carrera Coupe mit 993 engine
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Old 11-25-2007, 08:42 PM
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