What the process to convert from CIS to PMO??

[charlie.tango]

Hi,

Been contemplating switching out my CIS to PMO carb or EFI. In searching on here, it looks like there hasn't been a write up from start to finish? There was a how to for removing the CIS (Removing CIS with engine in the car - the writeup), but no step by step for the PMO side of things? Is it because it is straight forward?

Does the PMO kit provide instructions on the process? Just curious to gauge to see if this is something that is doable without needing to drop the engine and how much effort it will take to do the actual conversion?

Thanks!

[Coblue]

It's very straightforward, you take the old intake manifold off, put the PMO manifold on, and everything bolts to the manifold or the carb bodies.

Tuning them? THAT'S a different story

[HarryD]

I don’t know the conversion process but a more important question is why are you converting? If you are seeking a performance bump, you need to also replace your cams and possibly your pistons.

If you go with carbs, In return you will get poorer fuel mileage, a harder to start car etc. a decent EFI can address these issues but the cost is not cheap.

[jess p]

I’m contemplating the same thing for my 82 SC. Our host sells them and you give them the spec of your engine and they will spec the Pmo carbs for you. It’s recommended to re curve the distributor for the carbs. Plug the Cis return line to the fuel tank, buy an appropriate fuel pump for the carbs.
I just purchased new euro spec P/c from our host. Yes they are Cis pistons but will work just fine. I am backdating to SSI’s. I’m told the distributor will work as it is but a recurve will enable a higher rev. I’m looking at the cost of reviving my old Cis unit. New fuel lines, injectors etc etc. To me from what I’ve priced I can get new PMO’s. My Cis unit has sat for over 20 years. Is the fuel distributor still usable after sitting for 20 years, don’t know. I live in a warm climate ( Phoenix) so cold start is not an issue.

[charlie.tango]

When I had the engine rebuilt a couple three years back, I had sport cams similar to the 964 grind placed in. The reason I'm contemplating a swap is that my 911 is something I bought to work on as a way to get some me time. The other reason is that I heard the sound that a 911 with PMOs can make and sounds incredible.

Originally Posted by HarryD

I don’t know the conversion process but a more important question is why are you converting? If you are seeking a performance bump, you need to also replace your cams and possibly your pistons.

If you go with carbs, In return you will get poorer fuel mileage, a harder to start car etc. a decent EFI can address these issues but the cost is not cheap.

[charlie.tango]

How did you handle FPR and fuel lines? Did you use your stock fuel pump too? Did the PMO provide instructions on which fittings to use?

Originally Posted by Coblue

It's very straightforward, you take the old intake manifold off, put the PMO manifold on, and everything bolts to the manifold or the carb bodies.

Tuning them? THAT'S a different story

[Coblue]

I replaced the fuel pump and lines with what they recommended I use, used their regulator, and it was fairly easy to do , except for running the new fuel lines through the tunnel. I eventually replaced the gas tank as well but that was mostly due to contamination.
As I recall, they did recommend fittings. I don't recall what they are though!

[The Watson]

Highly recommend the swap. I did on my ‘77. PMO supplies a lot of good info to help get it all correct. Fuel line fittings are available from BAT (British American Transfer).

[PeteKz]

The CIS FPR is built into the Fuel Distributor, so you cannot use it for either the ITBs or carbs. You will need to check with PMO on whether a new FPR comes with the kits, or you buy it separately. You can reuse the fuel pump for ITB's, but it pumps at a lot higher pressure than carbs need. Carbs typically use fuel pressures of 5 psi, not 75 psi), so follow PMO recommendations on what pump and FPR to use for the carbs.

[snbush67]

You don’t need to change your cams. PMO’s work great with CIS pistons, but performance can be gained with cams.
You don’t need to change your fuel pump or plug the return line, PMO has a regulator that bleeds of pressure.
Tuned properly you can get excellent fuel mileage, up to or even better than 26mpg.
Tuned properly you can start without issue, cold or warm.
The stock distributor curve works fine with carburetors and you can hook up the vacuum from a secret port on the PMO’s.
You can change them out without a full engine drop, a partial drop is recommended.
Plug and play… yes, but it’s not plug and perfect. Perfect tuning will require an expert.

[The Watson]

Right, no need for FPR or new pump with PMOs. Great kit.

[icarp]

I often hear that the cis fuel pump is just fine to use with a carb set up.
I do NOT agree, it makes 77 psi , the ideal pressure for most carbs is less than 4 psi. Oh that's ok I'll just run a fuel pressure regulator , yikes !! , ok a proper FPR like the PMO, will happily cut 65 plus % or more with great efficiency , this is the regulated return type .
This the ONLY type I would ever use. This regulator is ESSENTIAL on a PMO- Weber set up.
But asking the system to regulate down to 4% of the standard pump pressure is not fair to the float system in a carb. You are just asking for trouble . What is aeration or entrained air? For the most part the fact that PMO says you can run a cis pump with their carbs is just for marketing reasons .
Saving $$$$ . lower cost .
I say the right part for the job . High volume low pressure pumps are the correct pumps to use , $65 from jegs or summit .
The red plastic housing Eddelbrock is what I put on snbush67's car . It produces 4-6 psi.
The PMO regulator is set to 3 psi and no problems , plan for the outcomes ,
Positive outcomes.
Or do what you like
Just my thoughts
Ian

https://www.jegs.com/i/Edelbrock/350/17303/10002/-1?gclid=CjwKCAiAuOieBhAIEiwAgjCvcqHiPPT8ZtiS87RBLm SLnolRPlpw9oPWDq8P_6rx45qs8iI_W0bm0xoCfsUQAvD_BwE#

[GG Allin]

Originally Posted by icarp

I often hear that the cis fuel pump is just fine to use with a carb set up.
I do NOT agree, it makes 77 psi , the ideal pressure for most carbs is less than 4 psi. Oh that's ok I'll just run a fuel pressure regulator , yikes !! , ok a proper FPR like the PMO, will happily cut 65 plus % or more with great efficiency , this is the regulated return type .
This the ONLY type I would ever use. This regulator is ESSENTIAL on a PMO- Weber set up.
But asking the system to regulate down to 4% of the standard pump pressure is not fair to the float system in a carb. You are just asking for trouble . What is aeration or entrained air? For the most part the fact that PMO says you can run a cis pump with their carbs is just for marketing reasons .
Saving $$$$ . lower cost .
I say the right part for the job . High volume low pressure pumps are the correct pumps to use , $65 from jegs or summit .
The red plastic housing Eddelbrock is what I put on snbush67's car . It produces 4-6 psi.
The PMO regulator is set to 3 psi and no problems , plan for the outcomes ,
Positive outcomes.
Or do what you like
Just my thoughts
Ian

This is all good to know. I've been running a stock '88 pump with my PMO's with no issues but I never really thought about it. While the car is down for major service, I might just change the pump.

In regards to harder to start, my engine barely makes a half a turn and fires right up. I just give it a little squirt before turning the key.

[charlie.tango]

Awesome, a lot of great info. I'm leaning towards carbs because despite the lower mileage I think any decent mechanic can do a "good" tune. I think going the EFI route you would need to source a tuner that is familiar with the ECU you decided to use?

[icarp]

Originally Posted by charlie.tango

Awesome, a lot of great info. I'm leaning towards carbs because despite the lower mileage I think any decent mechanic can do a "good" tune. I think going the EFI route you would need to source a tuner that is familiar with the ECU you decided to use?

Charlie, find the man who will jet your carbs and engine tune before you start .
He must be a 25year plus expert on carbs , quiz him on tools and AFR numbers he wants at what rpm and throttle position, elevation, Baro, air filter system, ports, cam, comp ratio, .

Find out first then start the process

I think there are fewer than 50 people in the country that are real carb experts
except for american v8 fans , though not the same system

Paul Abbott was our true champion , but he had to retire , maybe he can recommend someone.

most carbs are just tuned to an OK level , this is far from fantastic !!!!

Ian

[timmy2]

You’ll get a lot more “me” time with EFI by learning to tune it yourself!

[75 911s]

I really enjoyed the change up. I decided to do it in phases.

1. Phase 1, take the CIS off, add carbs (Stock 2.7) and use PMO regulator

I had a lot of issues, but most of them were dealing with vintage webers. New PMOs would be much easier.

My CIS fuel pump could be regulated no lower than 4.5psi ish. That seemed to be OK.

2. Phase 2 Swap to low pressure CIS pump. This was more involved for me as my car came with the fuel pump in the back (1975) so as part of this phase, I relocated it to the front like later cars.

I cover some of the headaches of converting in my build thread.

Some other things to consider:

- Breather setup
- Fuel filter, low pressure carb fuel pump, new lines etc.
- Ignition changes (you should change to a hotter fatter spark),
- I did an RS curve on my distributor. It's not cheap.
- linkage geometry, I had to change the throttle rod from the bell housing on the trans to carbs. I had to make the hole in the engine tin bigger. The CIS bell housing crank is different angles than the mfi/carb angles. Though not 100% necessary, getting the right one on there can help with WOT and linkage issues.

3. Phase 3 will be cam/piston change up. But that's full motor rebuild time.

[Dpmulvan]

Originally Posted by icarp

Charlie, find the man who will jet your carbs and engine tune before you start .
He must be a 25year plus expert on carbs , quiz him on tools and AFR numbers he wants at what rpm and throttle position, elevation, Baro, air filter system, ports, cam, comp ratio, .

Find out first then start the process

I think there are fewer than 50 people in the country that are real carb experts
except for american v8 fans , though not the same system

Paul Abbott was our true champion , but he had to retire , maybe he can recommend someone.

most carbs are just tuned to an OK level , this is far from fantastic !!!!

Ian

He must be a 25 year plus expert on carbs?? Quiz him on tools? Where do you come up with this stuff?? Have you ever owned a 911 with Webers? Have you ever torn down and rebuilt a set of Webers?? Ever tuned a set?
They’re carbs not space shuttle guidance systems. Shouldn’t be so quick to discourage someone. Carbs get a bad reputation because a lot of them are worn out and not properly restored, can’t tune crap carbs no matter how good you are. Yeh EFI is great if you have the money.

[MrBonus]

Originally Posted by charlie.tango

Awesome, a lot of great info. I'm leaning towards carbs because despite the lower mileage I think any decent mechanic can do a "good" tune. I think going the EFI route you would need to source a tuner that is familiar with the ECU you decided to use?

I've found more shops are comfortable with EFI than they are with carbs and, at least theoretically, once tuned should require no adjustment going forward.

[Showdown]

In addition, if you go with an ECU from a company like Haltec they can remotely access your car and ECU while running to run diagnostics and help tune it provided all the mechanicals are in order. Pretty cool.

In my search to find tuners who were familiar with Megasquirt I had no problem, vintage Porsches on the other hand were for some reason super intimidating and several tuners/shops said no.

WRT the conversion, the mechanicals are fairly straightforward; remove CIS, install ITBs. There isn't a set instruction manual as every car and owner are slightly different and will have to figure out where to put components, how to run hoses and wires all based on their wants/needs and those of the car.

The tuning is what gets most people but it's not all that hard once you start to understand the process and what you're trying to do and with what tools.