Cis to Carbs...documented power increase

[James Brown]

forgot a pic!

[tcsracing1]

so rebuilding CIS is more affordable/practical route as opposed to PMO carbs?

[2.7RS]

Originally Posted by RarlyL8

You don't do carbs for the $/hp investment, you do them for the driving experience. The fun factor is worth every penny.

Excellent advice

[snbush67]

Originally Posted by tcsracing1

so rebuilding CIS is more affordable/practical route as opposed to PMO carbs?

That really depends on if all of the components are there and working properly. If you just need to clean it and refresh some rubber pieces than yes. If your air box is cracked, wur is out of spec, fuel distributor needs rebuilt, etc. it can add up quickly.

[75 911s]

Originally Posted by tcsracing1

so rebuilding CIS is more affordable/practical route as opposed to PMO carbs?

I have done both. I think the CIS rebuild ran me about 2k, that included new injectors, sleeves, new hoses, new plugs, cap, rotor, and wires as well as a dizzy rebuild.

I recently went to carbs, I don't know exactly, but I think I'm up to about 6k for the conversion. Probably more.

The difference is night and day for my 2.7. in terms of excitement, thrill and just outright fun.

Stock motor with SSI, 2/2 muffler

CIS wasn't perfect, it had tuning issues all the time. I spent 2k at shops over the years dialing it and having it tuned as well as countless hours tuning it on my own. Still it was more reliable from a "get in, turn the key and don't worry" standpoint.

The reason for that is that the carb install is DIY and all new to me, so my confidence that it wont burst into flames isn't as high as It should be.

I also think I need to do more, I have my CIS pump regulated, but I think it's causing issues because the pressure is still too high with the regulator wide open. So I might swap to a low PSI pump. Which means changing some lines etc, probably another $200 job.

I went with vintage webers, and it was a ton of work to bring them around. Even now I'm having a leak, I think it's boiling on the left side after it sits, so I have more trouble shooting to do. The slightest leak and it stinks up the whole garage, but the second I drive it, all is forgotten.

It's a weekend thrill toy for most of us and even with my worn out 2.7 (160k miles) it is really really fun to drive with the carbs.

It also looks SOOOOOO much better than the CIS.

[VFR750]

Rebuilding the CIS system
Is probably the cheapest option

I just bought a fuel divider rebuild kit for $90 and a warm up regulator rebuild kit for $75. Both from a company in Germany. Quality looks good.

Planning for the someday going back to CIS.

ITMT, enjoying 3.0 carbed with DC40 cam. Seriously fun and quick. Cams make the difference. Maybe this spring I’ll dyno it.

[pmax]

Another factor to take into account besides initial cost is that CIS runs cleaner with more precise fuel management especially the lambda variant of it. I believe that's a big factor contributing to the longer life as in time between rebuilds of the 3.0 engines.

[911 SLANT]

I love my carb setup. Much more torque at bottom end. Sounds great when you open up the 46mm PMOs. I'm running them on a 2.8. PMOs alone were a little over 5,000. Like others said. Don't do it if your not all in with Cams,recurve, pistons,Exhaust, Duel band AFG. Numbers really start to climb up.

[irl]

Originally Posted by 911 SLANT

I love my carb setup. Much more torque at bottom end. Sounds great when you open up the 46mm PMOs. I'm running them on a 2.8. PMOs alone were a little over 5,000. Like others said. Don't do it if your not all in with Cams,recurve, pistons,Exhaust, Duel band AFG. Numbers really start to climb up.

Pretty, what headers are those??

[GH85Carrera]

You might gain a few HP at wide open throttle. You will loose driveability thought every other mode.

I am no expert, just a long time Porsche owner. In the 45 years of driving a Porsche I have seen many carb conversions from 914s to 911s and not one yet has idled, properly, or run properly under acceleration. From way to rich to way to lean and fuel starvation in hard corners.

I suspect some carb guru out there has one that runs really well, but on a cold day has to fiddle with it to start and run, or on a really hot day it will not start, and has flat spots in the power band.

So unless you are doing top speed runs and WOT is the only place you want power stick to fuel injection.

Just my opinion, you opinion may vary.

[ajenks144]

Everyone makes interesting points. To me it comes down to personal preference, drivability, and cost.

I think I'm going to go with carbs, but my situation is different. I have a 1970 911 project that I am just starting on. Kind of a "barn find". Car hasn't run in over 20 years. It came with a 2.7 engine, but never hooked up. So, my choices are wire up and debug the current CIS system, or install a set of Weber carbs which I already own from another project.

For me, I am willing to trade the drivability and tuning issues for the look and sounds of the carbs, given my starting point. If I had a solid working CIS 2.7, I might just leave it alone.

Andy

[mgatepi]

I did PMO's, cams, MSD, distributor and SSI's
Sounds amazing and love the throttle response! And the look is pretty sexy!
What I hate are the cold starts.....pops and pops and then smoothes out.
Is it worth the $$$$ I have in it....... maybe

[RetroSC]

Just want to state, so the OP doesn't get too excited, a stock 3.0 w carbs will not in a million years make 206HP at the wheels that's 230+ crank HP. I'm not saying yours didn't make that (Incredibly generous dyno), but a 230HP 3.0 has cams, pistons, etc, to get to that number, no way around it.

You might pick up 10HP on a stock motor, awesome throttle response and sound, and a nice tidy engine bay, but not 50HP!!

Originally Posted by juanbenae

I too went from CIS to weber 40mm carbs on a early 3.0L with ssi's and it was night & day. I would disregard all the suggestions of little HP gain because in my case it was night & day difference. during the dyno tune the car did 206 at the wheels and as rarly states it seemed like more. did not dyno the car prior to the carbs being placed so I assume it was somewhere near the declared by Porsche 180hp. unless your concerns are maintaining value with a unmolested car then fine. if you are looking to wake the car up, go for carbs.

t

[911 SLANT]

Originally Posted by irl

Pretty, what headers are those??

Brian's M&K RSR Headers

[75 911s]

Originally Posted by GH85Carrera

You might gain a few HP at wide open throttle. You will loose driveability thought every other mode.

So unless you are doing top speed runs and WOT is the only place you want power stick to fuel injection.

I do neither of those and I love my carbs. My 45 year old CIS was just as finicky as how you describe carbs; Idle issue, gutless, and hot start problems.

[banda]

I went from CIS to Weber 40s on my 78SC motor with stock CIS pistons. Installed high rise 46mm PMO intakes with webcam 20/21's. K&N Hats with SSI's/Dansk sport muffler stock ignition box and distributor. The difference is night and day it pulls hard to 7k and feels like a different car. However all the mods work in harmony. Would I do it again? heck yes. It took me a few years to collect the parts all used, I spent about 3k but that was five years 5 ago.

[930cabman]

Originally Posted by 75 911s

I do neither of those and I love my carbs. My 45 year old CIS was just as finicky as how you describe carbs; Idle issue, gutless, and hot start problems.

In the same boat with my '1975 2.7 fresh engine rebuild. I am sure when new the Bosch FI was the best all around system, but after 1/2 century chasing down issues are a PITA

In the past I have converted a 1985 Alfa spider, a couple 914's from Bosch FI to Webers and have always had luck

[ahh911]

In general I just struggle with the idea that modern carbs yield better performance than modern EFI. Please correct me if I'm wrong, but even nascar has been running EFI for decades, and I'd imagine every other motorsport. If this is just an assumption, my apologies. So what makes it different on our cars? Is it variable valve timing or variable dynamic intake runners that is lacking? Could be that.

Phil

Edit: I took the initiative to look up nascar fuel injection, just to see if my statements were off the mark or not. Wiki had the following statement which I think is powerful in favour of injection, mechanical at least, I was worried that homoligation was why injection wasn't used, but that's not the case. Anyhow, if true, injection was banned because it cleaned house, according to wiki. In 2011 injection was finally allowed in the premier series. so.

wiki: ( I can only assume it's true?)
History
During the 1957 NASCAR Grand National season, president of NASCAR, Bill France, Sr., immediately banned fuel injection from NASCAR. This ruling was in result to the dominance of the 1957 Chevrolet. General Motors did not offer many 1957 Chevrolet stock productions with mechanical fuel injection. However, enough were produced to allow it into NASCAR competition due to homologation rules. This ban on fuel injection would continue into the 21st century.

[930cabman]

Originally Posted by ahh911

In general I just struggle with the idea that modern carbs yield better performance than modern EFI. Please correct me if I'm wrong, but even nascar has been running EFI for decades, and I'd imagine every other motorsport. If this is just an assumption, my apologies. So what makes it different on our cars? Is it variable valve timing or variable dynamic intake runners that is lacking? Could be that.

Phil

Edit: I took the initiative to look up nascar fuel injection, just to see if my statements were off the mark or not. Wiki had the following statement which I think is powerful in favour of injection, mechanical at least, I was worried that homoligation was why injection wasn't used, but that's not the case. Anyhow, if true, injection was banned because it cleaned house, according to wiki. In 2011 injection was finally allowed in the premier series. so.

wiki: ( I can only assume it's true?)
History
During the 1957 NASCAR Grand National season, president of NASCAR, Bill France, Sr., immediately banned fuel injection from NASCAR. This ruling was in result to the dominance of the 1957 Chevrolet. General Motors did not offer many 1957 Chevrolet stock productions with mechanical fuel injection. However, enough were produced to allow it into NASCAR competition due to homologation rules. This ban on fuel injection would continue into the 21st century.

The stock Bosch FI on my 1975 911S I am sure performed well for many years, but being a carb guy I found a set of Weber 40 IDTP carbs. Fooling with the various items included with the Bosch system and the lack of a local specialist is forcing my decision, I am changing. Engine is fresh and looking at also changing exhaust. Having heat is a nice luxury, but does have a cost. Billy Boat? TIA