Weber carbs with CIS
 

Have an 81 SC, plan to backdate exhaust with SSI's soon. Long term, engine rebuild is in the plan, motor runs strong at the moment, so it will be a year or so. I do plan to go with cams and P&C's to get more juice, as well as weber or PMO. My question is does it make sense to do the carbs now (with SSI). From what i have read, i think i can realize 15-20 HP from the SSI and another 10-15 HP with the carbs with stock 81 CIS. Any experience feedback greatly appreciated.
Thanks.

"...with the carbs with stock 81 CIS?" Do you mean "with carbs in place of stock 81 CIS?"

Yes, as replacement of stock CIS, thought this was a given.

Unless you have CIS problems then my vote would be to wait.

do you drive in the winter?

There are many threads in here about carbs on SC engines. There are many pro and cons to this conversion. With proper tuning and a recurved dizzy you can get 230-240 HP with your stock SC motor SSI exhaust and 46mm carbs. The down side is that your gas milage will suffer. (I get maybe 14MPG) and the car will be a little finicky when cold.

-Andy

if a street car i'd do 40s

a trick carb install etc costs big bucks and a lot of time

do it now and just retune them later

do PMOs and forget about all the imperfections of Webers

Concur Ronny/Eagle

thanks for the feedback to date, I am not so concerned with gas mileage, will not drive in the winter. I'll definitely go with carbs with the planned rebuild, so was just thinking if i can get 10-15 HP with the stock P/C and cam set up and SSI's in the meantime, would not be bad idea.

I run 46 Webers on a 3.0 liter with GE60 cams, 10.5:1 pistons in stock Mahle cylinders and twin plugs through an Arron Burnam distributor. This motor runs very well. If you're looking for something to do in your spare time, carbs are fun and easy. You won't see a big change in performance with the stock pistons and cams but the throttle response will go way up.

If you're going to go with a bigger cam during the re-build, don't go too small with the carb choice. I think you can make 46 work just fine in the mean time. William Knight @ knightrace is a wealth of information regarding set-up. Have fun with it.http://forums.pelicanparts.com/uploa...1305130917.jpg

"proper tuning" means new cam & new pistons

I'd just wait on the carbs - you may want to twin plug the heads & do other things that affect the jetting

be conservative on the size if you plan to drive street, rally, or AutoX

Can I ask a stupid question? I've seen people say that changing to carbs or MFI without a cam won't make a big change in performance, but will increase the throttle response. What does that mean? How can that be? If throttle response is better, then doesn't performance have to be better? There is zero lag time with my CIS. It's not like a turbo or some new Audi that seem to have to think for a second after hitting the gas pedal. I'm suspicious that what many people have said they notice isn't throttle at all, but sound - that is six throats opening up at once increases the volume instantly. There I can see the CIS is a little less loud - though still very 911.

Metered fuel versus a big old slug of fuel.....mpg goes in the dumper as well.

throttle response = transient change in motor rpm

"performance" is being used in terms of more hp, which is an equilibrial measurement of the motor's capacity to do work

I guarantee that your CIS does not have the throttle response of carbs

Why pay big bucks for carbs when you can fit an EFI setup with a common rail for less and have the advantage of electronically metered fuel.
Several threads describe similar conversions.
Carbs without correct cams are like soda without the fizz...
The CIS cams have almost zero overlap making them pretty useless for any upgraded power setup. It's overlap and high lift that gives the engine more breathing.
Having said that, if you fit S cams you will need pocketed pistons to clear the valves... but then you got all the power you want.
In my 2,7s I had 150bhp with CIS.
I fitted Weber 40IDAs, early S cams and JE 9.5 compression pistons, early type (2in2out) exhaust and hiked the power to 210bhp. Now I have excellent throttle response, torque and power of a Carrera RS.
Downside? Well it takes several tries to start, it doesn't keep idle when cold, fuel consumption is higher but who cares.... its a blast.http://forums.pelicanparts.com/uploa...1305490108.jpg

I guess it all depends on how you define performance. Your definition of throttle response is a rate of change to rpm. But, if RPM changes, so does acceleration - unless I guess you are in neutral or a sloshy automatic transmission. It sure seems to me that a faster rate of acceleration is higher performance.

And, as far as I understand it, the 72-73 T with mechanical injection had the same hp and acceleration figures as the 73.5 T with CIS. So, I would question if the 72-73 MI T, if it truly had the same hp and acceleration, then it would have the same throttle response.

I realize though that there are lots of variables between the cars, and that they probably didn't have the same hp or acceleration, despite the Porsche publicized figures.

Re: throttle response.
If you look at the CIS inlet tracts they are a good foot+ long from the throttle butterfly to the valves. And one butterfly serves the fuel/air mixture for more than one cylinder, plus whatever happens in that CIS box it ain't clean flow...
Now the carbs have butterflies at their bottom, only about 6 inches from the valves and they are served clean venturi pressurised air from the top of their bodies and trumpets.

You open the throttle, the butterflies turn, air is sucked in, it travels x distance before it enters the combustion chambers.
When revs are low and inlet air speed slower to have a shorter run means the mixture reaches the combat zone faster.
At high revs you cannot feel it but at lower it makes a good difference.
The effect can be described as a very faint turbo lag.

Please excuse oversimplification.

Good point. Only thing is, the CIS injectors stick right into the head. So, I don't know that there is any such delay.

Just to be clear, I agree completely that cars with MFI and carbs can be and usually are tuned to higher performance levels than CIS cars, and that they therefore have quicker throttle response. I'm not in any way trying to say my CIS T is anything other than it is, or some how more powerful than MFI or carb'd cars.

I'm just giving CIS a bit a defense here, where I think it gets blamed for more than it should. We all know that CIS cars come with lower compression and less aggressive cams. But, isn't that all because the CIS injection right into the head is in fact MORE efficient than MFI and carbs? And doesn't the CIS injection provide a WIDER power band than MFI or carbs? And, there are certainly less parts to wear out, and quite frankly, tend to "perform" better in stop and go than an S can ever dream of.

So, back to my earlier point, if you had a CIS T 2.4 and an MFI T 2.4 that in theory have the exact same HP and exact same acceleration numbers, then they can't have different throttle responses - or if they do, the CIS must catch up and accelerate quicker somewhere in that band to get to the same 0-60 number. But, I also don't believe Porsche is always honest with their published numbers.

I also stand by my original point that carbs and MFI create more instantaneous sound than the CIS cars - but not necessarily more acceleration (for otherwise equal cars). Heck, I have a 120 hp English car with dual side draft Webers that screams when I hit that gas. It exemplifies the phrase: it's more fun to drive a slow car fast, then a fast car slow. And my 73 CIS T is nowhere near as fast as my Boxster S. But it sure feels faster because of the sound of the 911 (even as a CIS)! And, personally, I think the 2.0 911s are the best sounding cars of them all.

Like I said, just defending CIS where it is fair to do so. SmileWavy

seems you are conflating several different phenomena. or figures of merit for IC engines, or at least Otto cycle ones

Internal combustion engine - Wikipedia, the free encyclopedia

section 8 may have something in it