Hi,
I've been reading this forum for a couple of years (its been very helpful) but this is my first post. I have a 13b S5 street ported engine. It came with a RB manifold and Weber 48 IDA. Actuators, air pump, AC were removed. It was used in a sand rail so low end torque and fuel economy were not important. My plan is to put this engine in a Porsche 912 for street driving. I want low end torque (as close to stock as possible) and better fuel economy. Top end RPM & HP would be nice but not so critical (the Porsche transmission was designed for a 146hp engine maxing out at 6,500 RPMs, I don't want to destroy the transmission). That is the best low end torque and fuel economy using a carb. FI would be nice but I'm doing this on a budget.

My plan is to reinstall the air pump and actuators (to improve low end torque).

Ditch the weber 48 ida carb and replace it with a RB holley intake and Edelbrock AVS2 500 or 650 4bbl carb (to improve fuel economy). I'm not sure which size is better suited? 500 maybe a bit small while 650 too big. I would be interested to hear from anyone that has used a 650 CFM carb on a 13b streetport engine & high flowing exhaust.

This week I read about the VDI intake system on the 13b S5 (the system used to adjust runner length between long and short. Long intake runners improving low end torque, short runners to improve high end HP). It got me thinking about the effects on low end torque from RB Holley intake, which is short. So I though about some how using the stock S5 VDI intake with a 4bbl carb. But I don't think this is possible? If I am correct, the VDI intake needs the cars computer to control it (which I don't have).

So I did some research and found this article about trumpet length (the trumpet between carb and air cleaner): Blog - Emerald Adjustable Length Intake Trumpet Development

Like intake length, a longer trumpet increases low end torque (see last graph in link above). What if I fabricated an adapter for the 4bbl carb, from the carb inlet to air cleaner that separates the air intake for the primary and secondary. For the primary I would use a long tube, say 20" long. This would improve low end torque at lower RPM's. For the secondary I would use a very short tube, to improve high end HP at high RPM's. Basically the 4bbl carb would have two air cleaners connected to the carb with a long tube feeding the primary and a short tube feeding the secondary. At low RPMs just the longer tube would feed air. At high RPMs both the long and short tubes would feed air. Would this crudely replicate the effect of a stock VDI intake?

 

this can be a big topic, but i'll try to keep it short.

basically to run the 6ports and VDI you need the hardware, IE the factory intakes.

you could try to run a carb with the factory intakes, but the runners are so long, and make so many turns, it would probably be pretty bad.

basically any aftermarket computer would run the factory hardware, and be a big upgrade

 

Right. After I posted I found out I need the lower factory intake to operate the actuators. And if I go VDI I need the factory the upper intake. What I am thinking now is either:-

Keep the lower factory intake. Fabricate an adapter from the lower intake to the base of the 4bbl carb. Fabricate an adapter from the top of the carb to allow two separate air intakes (long for primary, short for secondary) to replicate the effect of VDI. The result would be less bends than the factory upper intake.

or

Keep both the lower and upper factory intake (keeping actuators and VDI). Fabricate an adapter for the 4bbl carb. Use one regular air intake.

I will use the factory air pump but do not have an ECU. I know the VDI needs a signal from the ECU so I need to find a way to activate the VDI solenoid without it. Has anyone done this?

The actuators don't require the ECU, just the air pump right?

 

If you're set on staying carb and retaining the 6 port actuators, RB makes an adapter for the OEM 6-port lower manifold to accept a Weber 45 or 48 DCOE (side draft) carb. This would give you the longer runner length you're after, and allow you to retain the aux port actuators, but there won't be any provisions for the VDI system. The primary and secondary ports will be linked to the same upper runner (1 per rotor). I think this would be a much better setup than trying to retain the entire factory manifold, where you would likely run into issues with the fuel falling out of suspension and just wetting the runners.

Link to manifold:
Sidedraft Carburetor Upper Manifold Section for 86-92 13B 6-Port - Racing Beat

Without an ECU, you could use a generic RPM switch to trigger the solenoid which controls the aux port actuators. Assuming you're running a distributor, you should be able to attach the switch signal input to the negative side of your trailing coil to get the appropriate RPM reading.

 

Thank you Kellie. I did look at the RB DCOE intake however I want to try and use a 4bbl carb so in can get some better fuel economy.

That's great information about the RPM switch. To clarify would I need a switch for the lower intake 5/6th port actuators if I don't use the ECU? I thought the lower intake actuators just use the air pump pressure to work?

I understand I would need a RPM switch to control the VDI solenoid without the ECU, if I keep the VDI. Thinking it would be better not to use the VDI intake due to the curves. Just use the lower intake so only one 90 degree bend.

Sorry for all the questions. I wish I had stock engine so I could see how everything operates.

 

this would end up really tall, and due to the long intake runners with many bends, it would probably work really badly

 

Noted. I think I will use just the lower factory intake and use seperate long and short intakes from the carb to air filter. I have some height in the engine bay and will use a gentle bend for the long intake.

Attached is a rough sketch.

 

VDI specifically changes the shortest path from front to rear rotor. The open adapter would act like a constantly open VDI.
I'd say stick with the stock lower and get the 6-port actuators working. Electric conversion with RPM switch writeup here: https://www.rotaryresurrection.com/2...elec6port.html

 

There was a thread about VDI once before and RXSpeed16's post jogged my memory. It is a misconception that "the VDI system is used to adjust runner length between long and short". As the page from the FSM shows it does not. The path the intake air takes remains the same. The VDI only changes the path of the pressure wave.
It looks like what you're doing is something different from what the VDI does.

 

Thank you both for that information. The factory VDI is more complex than I thought. I had read about pressure waves. Trying to figure out their effect is going to be too difficult (many dyno runs with different set ups, a lot of trial and error).

I will still give the custom separate long and short intakes a go as per my diagram. This wont replicate the factory VDI but it should give an increase in low end torque. When just the primary carb jets are used air can only pass through the long intake. Once the secondary jets open the short intake will draw air (along with the primary long intake).

The reason for open adapter below the carb is that I had read people have had better results with open phenolic spacers on edelbrock 4 bbl carbs compared to 2/3/4 way spacers.

 

if you have an open spacer below the carb, that is going to remove any tuning you did in front of the carb.

if you want to run the VDI, you should just run the factory intakes with an aftermarket ecu.

the 48 IDA is a good setup too, it would easily make more high end power than the VDI would

 

I did think about that. My understanding is that without an open spacer the primary and secondary jets are directed to separate intake ports. Using an open spacer means primary and secondary can go to all intake ports. I know that dosent sound very scientific. I guess the air is pulled by the intake with the most suction and path of least resistance. The only thing I know is that people have posted that the open spacer has given better results with the Edelbrock. I can always separate them if it dosent work.

The effect of the long and short intake shouldn't be effected either way. The air can only pass through the long intake at low RPMs regardless of whether the adapter below the carb is open or separated.

 

the American V8's all have an open plenum under the carb, and so all cylinders pull from the plenum (a dual plane intake splits the engine into banks or something weird), and the flow through the carb is pretty steady. the carbs are jetted for this kind of application.

the default rotary setup is each barrel of the carb goes to one port, its like an ITB. the flow is not steady, it has a lot of pulses.

if you put a carb that is jetted for an open plenum on an ITB intake, it will be way off, and the easy way to fix it is the open spacer. you could also re-jet the carb but this is hard, and time consuming, and its more expensive than fuel injection anyways, so why bother?

Mazda has a few SAE papers on intake design, basically they did all the hard work all you need to do is read em

 

Thank you for the information. Stand alone fuel injection would be nice but the more I read about it the more I get confused. What I brought was a 13b street ported S5 motor that is already converted to a weber 2bbl carb. It's going in a porsche 912 so there is no wiring loom.

If I did EFI I guess the cheapest option would be to use a secondhand factory ITB with a standalone computer. Would the factory ITB be sufficient for a streetport with a flowing exhaust (no cats)? For daily driving use.

Approx how much will a standalone computer cost? And how much would aftermarket larger ITBs cost if required?

The posts I have read here on converting to EFI are several years old and were around $2k upwards. I can get a carb for $450, lower intake used for $75 and make an adapter for around $50 in materials. Total cost around $600. I know I'm sounding cheap. The reason being is that a water cooled engine will lower the cars value. Im not a porsche snob. I think the rotary is a great match as its fairly light weight and the low torque is suited to a 2,200lb car. But when I sell the car I will have to convert it back to the orginal engine. I'm not in the position to spends thousands on a rotary conversion just to end up removing it in the future.

 

why not keep the weber? its far superior to any of the 4 barrels when you mount them sideways.

with a carb you also need to factor the jets, by the time you get it jetted properly, your $600 will be closer to $1000...

 

look into a microsquirt...I don't think you could find a better bang for your buck as far as a efi solution that works on a 13b...you will have one output left over that you can use to drive the vdi or fan control or whatever .
https://www.diyautotune.com/product/...iring-harness/

just to add...obviously much more involved then a carb but with the 912, EFI would be simplified if you have the room to mount the microsquirt in the engine bay with a proper fuse/relay distribution block.

 

I forgot about the jet kit for the carb. The Edelbrock jet kit is pretty inexpensive though at $70. I know a lot of people recommend weber IDAs. The thing that puts me off them is that no one gets good MPGs, low teens is what I keep reading. I know its rotary but I would like to get mid twenties. Gas around here is close to $4 a gallon.

The Microsquirt looks very affoardable and has got me thinking about the whole EFI thing. There's plenty of room in the engine bay.

What is an inexpensive setup for the throttle body, injectors or ITBs? Around a few hundred dollars. I guess the cheapest options is a used factory intake & throttle body but it won't give an increase in performance over stock. The new aftermarket kits are probably going to be expensive. So that leaves used parts from other cars or motorbikes. I'm ok with this last option. It would be just helpful to know which parts/setup would be best for this (Microsquirt with a 13b S5 streetport). The information is probably here already. I just need to go through all the posts. It seems there are so many one off custom set ups using used parts it's confusing me.

 

cheapest EFI intakes are just stock ones, you could do it for under $50, you keep the 6 ports and VDI, and emissions if needed

#2 run the stock lower intake, use the RB upper Sidedraft Carburetor Upper Manifold Section for 86-92 13B 6-Port - Racing Beat and an efi throttle body. you keep 6 ports, and emissions if needed.

use the IDA intake, and IDA throttle body. no 6 ports, no vdi, no emissions.

for fuel, you can keep the primary rail and injectors from the stock setup for all of these, and or buy new EV14 injectors that are 800-1000c and only run those

 

Thank you j9fd3s. I read stock will limit hp. I would like to keep the lower intake for the actuators and use a RB upper intake.

I need to research posts here more on what throttle body to use and fuel rails. If the stock ones take advantage of the street port and flowing exhaust (no emissions as the car is pre 1967) then I will use these. Meaning if the stock parts don't restrict the gains from the street port and exhaust.

I'm custom making the exhaust by the way for flow and to be quiet. Two in to one equal length. After they collect around 2", to 2.25" ending around 2.5". No mufflers or presilencer. Several 90 or 45 degree turns as wide as possible. Have to do this as there is hardly any space. Total length 11'. 10' 6" of that will be enclosed in a box with kiln type insulation 2"+ thick. So the whole exhaust will essentially be one big muffler. Debating whether to even use perforated tube on the straight sections. I think the bends will help reflect the sound along with all the insulation.