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I want to share something I've been working on for a little while now - as the title says, it's 8v ITBs. I know, I know...we've been down this road before. But I have an interest in them and wanted to design something fun.
So a little background information...I originally started working on some layouts that would utilize the RHD 45mm DCOE throttles. While this would have worked, I didn't like how the DCOE throttles are spaced at 90mm centers and the 944 cylinder head is 122mm centers. I think this works well for the 16v motors that have spark plug access through the valve cover, but on the 8v it just didn't seem like the greatest solution by partially blocking access to two of the spark plugs. So I abandoned this route.
Next I thought about simply using Jenvey SF style throttles. They're proven, they're reasonably priced, they look good and they have a nice catalog of parts to put a system together. I wanted to create a system that utilized the stock throttle cable with a center pull arrangement, so I developed my own linkage and laid out a few concepts to analyze various manufacturing processes. I then 3D printed some parts to check for basic fitment in the engine bay.
Open trumpets. Looks cool, but filtered air is important.
Fully CNC'd plenum with removable plenum spacer for changing internal volume by 0.5L. Center inlet.
Cast and machined plenum with nice, smooth internal radii.
3D printed test assembly. Shown here with an earlier (and not great) version of the center throttle linkage.
Naturally I found a few things that didn't fit exactly how I thought and I discovered some things I didn't like about the overall design. This got me thinking...why not take this to another level and design my own throttle bodies? I decided to make them out of billet aluminum and keep them to 22mm in length. This allows for better fitment of the surrounding components with decent length ram tubes and a good size plenum. The throttle bodies were designed to be modular, so they will work with any cylinder spacing from 88mm and larger. This means they'd be great for use on VW 5-cylinder engines, Porsche flat sixes and even BMW straight sixes as well.
Short throttle bodies with 60mm long ram tubes connected to a machined plenum base.
Center pull throttle linkage, center feed intake plenum.
Throttle layshaft with rigid connecting links and individual throttle return springs.
I have a few different plenum concepts as I'm not sure what I ultimately want to go with. This will be partly dependent on what fits best in the car once the inlet manifold and throttles are actually produced. I've always liked the look of the center feed like BMW used on their M cars back in the day. It packages pretty nicely and lends itself well to being machined from billet and then welded together. I'd love to do some cast components, but it looks like going the machined/welded route is likely the best for small runs. TBD.
So ultimately I wanted to share this with you folks to see if there is any interest in the 944 community for some 8v assemblies like this. It would be nice to produce 8-10 sets of parts (or more) that would include the following:
Inlet manifold (machined and welded)
Throttle bodies (Black anodized, ball bearings and lip seals on throttle shafts, AL2024 throttle plates, 1215 steel throttle shafts with zinc plate)
Custom linkage (Individual throttle return springs and throttle linkage arms with bushings)
Intake plenum with ram tube extensions
This is kind of a feeler before I have anything machined. Naturally, I would prototype on my own to make sure everything worked first. Let me know your thoughts on the overall design, if you'd ever consider buying something like this and if so, how much is something like this worth. I have my own thoughts on pricing based on quoting parts, but curious how that aligns with the market.
I forgot to add some additional information to the first post so I'll add it here.
The throttle bodies are 45mm diameter. The inlet manifold utilizes flanges that have a taper in the throat to create a nice transition between the throttle body and the mating pipe elbow. There are 1/8"NPT threaded ports in the bottom of the inlet manifold flanges to accommodate various fittings to be used for the brake booster and for a vacuum signal for a MAP sensor. I started to put together a few vacuum collector concepts but haven't finalized anything yet.
The machined base of the intake plenum contains velocity stacks that are raised off the surface to improve airflow and increase air velocity. I have yet to add some additional ports to the intake plenum for the idle air control valve and for an IAT sensor, but these will be included.
The throttle bodies are currently set-up to accommodate a TPS on the right or left hand side, hence why there is a throttle shaft extension on both sides. I need to create an adapter so the stock NA TPS can be mounted to the back side (near the firewall) and I also plan to create an adapter to mount a BMW-style TPS on the front side. The machining currently on the side of the throttle bodies is 2x M4 x 32.5mm, standard to many aftermarket throttles and factory throttle position sensors.
The throttle arms on the layshaft are adjustable for precision alignment and there are individual airflow adjustment screws on top of each throttle for proper synchronizing. Idle adjustment is performed at the main throttle pull after the throttles are synchronized.
The system is designed to work with a stock fuel rail or aftermarket. I'd like to make some basic billet fuel rail assemblies similar to what is shown in the images, likely with 3/8"NPT fittings on either end so the end user can install whatever fittings they'd like.
I think that covers some of the stuff I previously forgot. Cheers.
The DanST stuff is okay, but I don't particularly care for their 8v kit. I don't like how it's only offered in an open trumpet configuration (I'm developing a plenum version for better behavior on the street at lower RPM) and I don't like how the fuel rail is tucked underneath the throttle bodies instead of using the stock location. The stock location should provide better fuel economy and require fewer additional changes. Not knocking the DanST kits at all - just not my preference.
So ultimately I wanted to share this with you folks to see if there is any interest in the 944 community for some 8v assemblies like this. It would be nice to produce 8-10 sets of parts (or more) that would include the following:
Inlet manifold (machined and welded)
Throttle bodies (Black anodized, ball bearings and lip seals on throttle shafts, AL2024 throttle plates, 1215 steel throttle shafts with zinc plate)
Custom linkage (Individual throttle return springs and throttle linkage arms with bushings)
Intake plenum with ram tube extensions
This is kind of a feeler before I have anything machined. Naturally, I would prototype on my own to make sure everything worked first. Let me know your thoughts on the overall design, if you'd ever consider buying something like this and if so, how much is something like this worth. I have my own thoughts on pricing based on quoting parts, but curious how that aligns with the market.
Have a great Saturday.
JRP
I know it's a totally different engine and not relevant data, but I'd definitely buy one for a 3.0/S2. If it ever comes to that and you need a car to prototype on, I'd happily lend you mine.
I know it's a totally different engine and not relevant data, but I'd definitely buy one for a 3.0/S2. If it ever comes to that and you need a car to prototype on, I'd happily lend you mine.
I'd love to eventually see this expand to other engines, so I appreciate the kind words. We'll see what happens!
I'd love to eventually see this expand to other engines, so I appreciate the kind words. We'll see what happens!
My I ask what your mechanical background is? Would you be open to designing a bespoke system? Maybe there are enough of us 16v'ers who would do a group buy. I'm within driving distance of you, and as I mentioned, I could provide the car.
Additionally, you mentioned a plenum vs straight filters for drivability's sake - what's the disadvantage to a street car with filters?
My I ask what your mechanical background is? Would you be open to designing a bespoke system? Maybe there are enough of us 16v'ers who would do a group buy. I'm within driving distance of you, and as I mentioned, I could provide the car.
Additionally, you mentioned a plenum vs straight filters for drivability's sake - what's the disadvantage to a street car with filters?
I'm a mechanical engineer with 15 years experience in the pump industry. I got my start with centrifugal fire pumps, but for the last 5+ years have been working with swing piston and diaphragm pumps. Kind of like working on simplified, miniature engines.
I would definitely be interested in developing a 16v system - I started with the 8v since that's what I currently own and I didn't see any other complete 8v systems I liked. I designed the throttles such that the same throttle body can be machined open from 40mm to 54mm I.D. (naturally with different throttle plates). From my understanding (re: understanding and research - not personal experience) 54mm should be large enough for a pretty spunky 3.0L 16v engine. I very much appreciate the offer of providing a test car, as I don't have access to a 16v car otherwise.
The plenum provides advantages and disadvantages depending on the application. The advantage of the plenum is it allows for use of an AFM/MAF sensor on the plenum inlet (re: can utilize stock engine management) and it also isn't as twitchy on initial throttle tip-in. You can absolutely use standalone engine management with a plenum (and should if you want to maximize performance) but certainly don't have to for a street application. Take a look at what BMW did with their E30/E36(Euro)/E46/E90 M3s and their E28/E34/E39/E60 M5s. The disadvantage of the plenum is that a really spicy race engine will be starved for air at high RPM unless the plenum is quite large, which is why you often see race applications use a simple filter set-up and short intake trumpets.
I appreciate the feedback thus far. Please keep it coming!
No one should be encouraged to run an AFM on an ITB engine.
It sucks enough on a stock 944 engine and would defeat the purpose of the ITBs.
MAF or MAP only, FTW
Ha, agreed. Definitely not recommended these days since it is restrictive and there's better tech out there, but it was how they did it back in the 80s.
E30 M3 engine bay
E28 M5 engine bay
I'm sure the charts are out there somewhere, but it would be interesting to see the performance difference on an ITB engine with a a MAF versus an AFM and no other changes. Back to back comparisons are always fun.
No MAP for ITB cars - they make very little vacuum at light- medium load. You will end up in a situation where the engine make 0psi MAP at 30% to 100% throttle, but has very different airlfow, so it will either be lean at WOT or very rich at part throttle. The best method is alpha-n with baro correction. MAF works too if the ECU supports it, but you lose some of the response that ITB do so well.
On another note, make sure your linkage allows you to adjust each throttle independently. ITB are super sensitive at low throttle angles, and it run poorly at idle and low RPM if they are not all at the same base position, and if they don't all start to open at the exact same time. The best way to do this is to have them fully closed at idle and idle on the IACV, or for each to have it's own adjustable idle stop and a linkage with as little lash as possible.
With your plenum design, make sure the walls are not too close to the trumpets. A bad plenum will cost a ton of power, and a good one will make a lot of power.
For development, consider making it modular enough you can try a few runner lengths. You can do your resonance calculations based on the cam, but you will find that it probably needs some tweaking to do what you want.
You can also use a 944 S lower AOS with the top from any other AOS to delete the filler cap, then just fill through the cam tower to gain some space for the plenum.
No MAP for ITB cars - they make very little vacuum at light- medium load. You will end up in a situation where the engine make 0psi MAP at 30% to 100% throttle, but has very different airflow, so it will either be lean at WOT or very rich at part throttle. The best method is alpha-n with baro correction. MAF works too if the ECU supports it, but you lose some of the response that ITB do so well.
For my prototype assembly I plan to just run using a MAF and the stock engine management. Eventually I'd like to put together my own EMS, but don't want the additional variable of the new EMS while trying to prove out the TB's.
Originally Posted by 962 kid
On another note, make sure your linkage allows you to adjust each throttle independently. ITB are super sensitive at low throttle angles, and it run poorly at idle and low RPM if they are not all at the same base position, and if they don't all start to open at the exact same time. The best way to do this is to have them fully closed at idle and idle on the IACV, or for each to have it's own adjustable idle stop and a linkage with as little lash as possible.
The throttles each have adjustability on the linkage arms. The throttle arm that is fixed to the individual throttle shaft is connected via a d-profile arrangement so there is no adjustment here. The adjustment comes from the lower throttle arm connected to the common layshaft. It can be moved to any position necessary to compensate for slight differences in angles of the upper throttle arm due to machining variations and tolerances. The link arms are connected via precision shoulder screws and bushings to eliminate as much slop as possible.
The airflow through each throttle can be adjusted via the cone point set screw/nut on the top of the throttle body. It's just an air bypass like a stock throttle body. This hole you see in the upper right hand corner of the throttle body is covered by the inlet manifold flange when the throttle is mounted. The intent is to make sure all the throttles are completely closed (the return springs will force them into this position) then measure the airflow on each throttle, synchronizing them with the airflow adjustment screw.
Originally Posted by 962 kid
With your plenum design, make sure the walls are not too close to the trumpets. A bad plenum will cost a ton of power, and a good one will make a lot of power.
This will largely be dictated by what fits under the hood, so it won't be completely optimized. The intent isn't to make this the highest performing system out there, since that would be more dedicated to a race application. I designed it to be a highly-functioning street system that could look almost stock-ish. Because I made it modular, the inlet manifold and throttle bodies could be used with any number of different plenums or open trumpet arrangements.
Originally Posted by 962 kid
For development, consider making it modular enough you can try a few runner lengths. You can do your resonance calculations based on the cam, but you will find that it probably needs some tweaking to do what you want.
Totally. Right now I have the ram tube extensions at 60mm so I can still fit a decent sized plenum. I can experiment with something shorter or longer and have considered experimenting with different volume plenums as well to see how the power band shifts up and down the rev range.
Originally Posted by 962 kid
You can also use a 944 S lower AOS with the top from any other AOS to delete the filler cap, then just fill through the cam tower to gain some space for the plenum.
I'm actually wondering if I can fit the complete 944S AOS and still keep the filler neck. Any idea if the filler neck clears the rear intake runner on a NA manifold? I know the S manifold has a sharper bend here. It might be something I just need to purchase and test fit. I could possibly "tweak" the rear intake runner a bit for clearance if it wasn't off by too much.
Hard to make a money argument on M cars, but I bet they did that due to cost still.
hotwire mafs existed in the early 80s and were in production (the Porsche 928 got it in 1985) but the AFM probably cost more.
But the increased responsiveness of the engine with a MAF or MAP is unbelievable vs the AFM. I did a MAF conversion 10 years ago and was blown away. Made more peak power and way more power under the curve. I’ve got dyno sheets around here somewhere.
AFMs are hot garbage.
Originally Posted by JRP944
Ha, agreed. Definitely not recommended these days since it is restrictive and there's better tech out there, but it was how they did it back in the 80s.
E30 M3 engine bay
E28 M5 engine bay
I'm sure the charts are out there somewhere, but it would be interesting to see the performance difference on an ITB engine with a a MAF versus an AFM and no other changes. Back to back comparisons are always fun.
01-21-2023, 05:18 PM
944 8v ITB Development
