carburetors vs CIS

[svandamme]

Originally posted by dd74
Lousy, actually! Because at the time, Porsche needed something a bit more (gas) efficient than the paltry mileage of MFI. How's that for reality?

milage on sports cars is not important ...
if it weren't efficient , they wouldn't have used it on the racers..

[ianc]

I've seen full CIS systems going for $500 or $600 bucks. Do you have any idea how much they'll be in five or six years as parts become rare. Yes? No?

And you're maybe thinking that there'll be a bull market on Webbers in the next five years? So that maybe you'll be able to walk into NAPA in five years and suddently get Webber 40 parts more easily or more cheaply than Bosch CIS parts?

To each their own what anyone drives, how they drive it and how much a PITA they want their car to be.

True. If the (slightly) better throttle response of carbs is worth more to you than all the disadvantages (including not being able to get your car SMOGGED), then go for it.

ianc

[dd74]

Originally posted by ianc
If Porsche thought they could be faster or more fuel-efficient (both count you know) with carbs, they would have been running them.

They weren't; end of story.

ianc

Follow along, Ian. I was making an argument for the CIS you so worship.

In many cases, I agree with you that CIS is less...labor intensive. Mine, for instances, starts up no matter how warm or cold it's been.

But I'm never going to discount carbs as having the potential to raise some horsepower in an engine, and, to an extent, have some fuel economy too if set up optimally.

[ianc]

Follow along, Ian. I was making an argument for the CIS you so worship.

I don't worship it. The DME kicks ass all over it.

But, with an SC you have two choices: carbs or EFI. With Tony's kit costing about half what carbs do and allowing you complete control and adjustability over the entire fuel system (and spark to come), carbs just don't make any sense. Any mod that differentiates carbs from CIS can equally well be taken advantage of by EFI. The difference is that instead of endlessly rejetting six carburettors, you'll be tweaking a computer mapping, and getting more power and economy while you do it.

ianc

[afterburn 549]

For 3000.00 you can install a SDS sytem which includes double twin ignition,not shuch a bad way to go ,and real simple like it say (Simple Digital System) check out a dealer in your area.
You will wonder why every one is so romantic over prehistoric carbs.............once upon a time the only way we could get a fual charge in a cylinder was a ventry...........................................

[Superman]

Ian and Afterburn are correct. Both carbs and CIS take a back seat to EFI-type systems. Soon (if not already), the only reason to use Webers will be nostalgia. They are not more efficient than EFI, and the cost is/will be more for carbs. And a bigger PITA.

And again, with all due respect to those few who think they see more hp with carbs than CIS (dd, and whatever dyno data he's referring to, as well as Noah, I think), carbs WILL NOT produce more hp than a healthy CIS system. Period. I am certain. I believe Wayne has stated this also. There is just no way that carbs can either meter or atomize fuel like injectors that are under 60 psi of fuel pressure.

But as I said, carbs look cooler, sound cooler and FEEL like 50 more hp because of the throttle response. Somebody above minimized the throttle response difference. Don't. Anybody who has removed a CIS system and placed a well-tuned set of Webers or PMO's on a Porsche will SWEAR by carbs. The throttle response difference is colossal.

But again, at a constant engine speed (like a peak torque location which usually is somewhere around 5500 on the stock 3-liter), carbs will be dropping virtually droplets of fuel into the engine, while CIS will be spraying a FOG of fuel.

Sure, the debate will rage on. And carb users will SWEAR by carbs. (I would rather have carbs, frankly. Again, the throttle response factor is colossal. They are WAY more fun, sound better, look cooler and weigh less). And some will continue to believe carbs make more power. Because of this throttle response 'feel' and the occasional dyno result which is not reliable for whatever reason (perhaps the CIS system was not running properly before removal). Aside from these dyno anomalies, dyno after dyno result when done properly will show that the best carbs can do is to maybe match CIS peak torque. Ask Bruce Anderson if you like.

Here's another data point: Bruce Anderson found 240 hp by putting 98mm pistons onto 3-liter engines. That makes them a short-stroke 3.2. Folks who think a stock 3-liter can make 240 hp are dreaming. I'm not tryhing to offend anyone, but fantastic hp numbers can be found in many places. For realistic hp numbers, go to trusted, experienced sources. Sounds like some folks should read, or re-read Bruce Anderson's book. He's been the technical guru for PCA for decades.

And again, like folks say, if you make engine mods like hot cams, then CIS is going to fail. And if you do that, EFI or some similar system is still going to outperform carbs. One more time: CIS systems run at about 60 psi. MFI ran at more like 200 psi. EFI is probably similar. Frankly, MFI systems have throttle response that leaves carbs in the ditch. Or maybe you guys are still inclined to disagree with folks like Bruce Anderson and the entire Porsche factory racing team.

[Superman]

Again, not wanting to insult anyone, but think about what you're saying. You're suggesting that Porsche left SIXTY hp on the table when they made the SC model for six years. Porsche. Does that seem reasonable?

And if someone is going to suggest this is simply because of emissions compliance, you'd be partly correct. Carbs are not efficient enough to always provide the optimum, even flow of fuel. The reason they cannot pass emissions is because the air/fuel ratio cannot be held constant enough to pass. From one millisecond to the next, carbs deliver different amounts of fuel under same conditions. The are simply sloppy.

When a bunch of us got together and dyno'd our stock 911's The most torque we found at the rear wheels was 184. That equates to perhaps 205 hp. And we had a Carrera there (which was not the winner, but should have been). Porsche specifies 180. 240 is FANTASY, unless you open the engine.

[turbo6bar]

Originally posted by ianc
If the (slightly) better throttle response of carbs is worth more to you than all the disadvantages (including not being able to get your car SMOGGED), then go for it.

ianc

If smog is a concern, isn't CIS the only option? No tbitz Megasquirt system or aftermarket EFI...

I've read online discussions about carburetor venturis atomizing fuel extremely well. Of course, this only happens when the velocity reaches a threshold. DIY-EFI mailing list covered this type of discussion.

As far as carbs producing more HP than CIS, I think it is certainly possible. After all, doesn't Tony's EFI kit make more HP than stock CIS? On top of that Tony's EFI uses lower fuel pressure than stock CIS. Obviously there is some restriction in the CIS setup.
Just the way I see things.
jurgen

[ckissick]

I bought my 911E with Webers, then put them on my 3.0 replacement engine. I don't know about HP, except what I've been told. The guys down at Jerry Woods like my setup, and they say it's faster than normal. But I'll have to dyno the car someday.

Anyway, I just wanted to say how well the engine runs with the Webers. It was very easy to jet and adjust, and always runs great. And if my oil breather hose gasket ever leaks, it's a 10-minute job.

I've never driven a 911 with fuel injection, so I can't compare on performance. As for throttle response, all I know is when I give it just a little gas, the car immediately leaps forward and pushes me back in the seat. And the sound!

I'd like to try a CIS early (light-weight) 3.0L 911 someday so both of us can compare. Anyone? I'm near Half Moon Bay.

[ianc]

If smog is a concern, isn't CIS the only option?

Maybe. I bet I could sneak a stealth EFI installation past Ahmed down at the Olympic station. The numbers would certainly pass. Carbs wouldn't quite cut it there.

After all, doesn't Tony's EFI kit make more HP than stock CIS? On top of that Tony's EFI uses lower fuel pressure than stock CIS.

It does. I believe he claimed 23 HP. CIS operates at higher pressures compared to L-Jet derived injection systems like DME though. Fuel pressure does not necessarily equate good atomization. What's important is always metering the correct amount of fuel under every condition based on as many data points as you can. Carbs don't have feedback sensors and can't properly adjust the flow of fuel under varying conditions. This only makes them acceptable for relatively steady state operations like the track.

ianc

[Eagledriver]

ianc,

I agree that we don't agree or disagree on the benefits and drawbacks of Carbs vs CIS.

For those who doubt my power claims all I can say is that 3 of these engines that I mentioned have been dynoed at Jerry Woods enterprises witnessed by Rich Walton and Thomas Jameson (both of whom are on this forum but will proabaly not waste there time jumping into this thread). The engines were on the dyno to fine tune the jetting and ignition timing.

The reason the carbs make so much power is the lack of pressure drop on the way to the intake port. CIS intake starts to choke the engine at high RPM.

I used to find these numbers hard to believe too but now I have to compete against these engines with my 2.7 MFI powered engine and am getting spanked on the straightaways. My engine is no slouch an I measure 195 HP at the rear wheels with my G-tech (addmittedly a rough tool compared to a dyno). Keep in mind the G-tech doesn't measure wind resistance and these pulls are in 2nd gear (around 60mph). I like to think I have about 230 HP at the flywheel.

-Andy

[randywebb]

I don't know when the event above happened but BA is on record several times as saying no hp gain with just carbs. Maybe that has changed....

It makes little sense to me that carbs alone could yield significant add'l hp over the CIS intake for the reasons that S-man adroitly summarized (BTW, I read his comments to say "carbs WILL NOT produce more hp than a healthy CIS system [if both use the same cams, pistons, etc.]"

If there are engine dyno data showing that a _healthy_ CIS produced 40 or 60 hp less than _just_ a switch to carbs, I have sure missed it and would like to see it -- and the methodology. Maybe we have been deluded all these years - or maybe someone has found out something new - in a place no one ever thought to look. This sort of thing happens once in a while in science - but as we say "stong claims demand strong evidence." I can see that the pressure drop might be one factor, but I don't understand how the ultra-mild CIS cam could produce that much of an increment - it seems to be the limiting factor to me.

We can all agree that carbs are more responsive & look better. I think they are easier to troubleshoot - some have disagreed with that in the past. Maybe it's age-related, since I grew up with carbs. With CIS, you troubleshoot by throwing away parts that cost about $200 each until you find which one broke...

Also, you can repair carbs while broken down on a lonely highway... well, sometimes you can. Since I sometimes drive on roads where the next car might be hours or days later that can be useful.

It's great that we have some lower cost EFI alternatives...

[ianc]

With CIS, you troubleshoot by throwing away parts that cost about $200 each until you find which one broke...

That's the myth but not the reality. Each part has a diagnostic procedure used to establish its viability just like any other component. Many of them can easily be repaired or modified. Troubleshooting CIS is much easier to me than carbs. Most problems fall into common symptom areas and one can look at those components first. With carbs, who knows? One float valve not closing completely? Bit of a vacuum leak on one of the throttle valves? Incorrect jets? Take all the unknowns of one carburetor and multiply by 6, then throw in some linkage nightmares for grins. Oy!

ianc

[dd74]

I think this longish thread will add a little fire to this lively argument. As I stated in an above post, there was someone named Tony who dynoed his Weber-equipped stock SC engine. I failed to mention that the engine also had 20/21 cams. Regardless, the horsepower results of this engine can be found on page 6 of the thread:

Let's Play - Predict the HP Game!

I also take into account who of the Porsche lumanaries on this board had viable and original thoughts about this motor's results in lieu of the simple regurgitation of Bruce Anderson's findings...

[randywebb]

re: troubleshooting CIS

That's the exaggeration and often the reality. Unless you have a lot of specialized equipment...
Good pt. about the 6 barrels - really only 2 complete carbs of course -- then there are the 12 barrels on an old Fcar.
I will also admit that carbs are crude devices -- for more on this there is the book "Future Primitive" by John Zerzan, and the "Electronic Neanderthal" a wood-working site.

dd - BA's findings are often based on Jerry Wood's dyno work. nuf said?

[dd74]

Originally posted by randywebb

dd - BA's findings are often based on Jerry Wood's dyno work. nuf said?

Is that the same Jerry Woods who dynoed the engines Eagledriver mentions?

[ianc]

Looks like he got slightly less than 200 RWHP, which translates to about 235 crank HP.

Again, he's using a fresh rebuild, headers, a sport muffler, and cams; all the things carbs need to show any gain. Great, for the track. Wonder what it would have been with EFI?

ianc

[dd74]

Originally posted by ianc
Wonder what it would have been with EFI?

ianc

Good question. What's your guess, particularly with the TBitz setup? More hp?

[ianc]

That's the exaggeration and often the reality. Unless you have a lot of specialized equipment...

Fuel pressure tester and VOM is all you need. What do need to troubleshoot carbs? Besides the Unisyn of course.

ianc

[ianc]

What's your guess, particularly with the TBitz setup? More hp?

Maybe, although it would probably be close. Carbs can shine on the track under relatively steady state conditions. It's in cold running and day to day that they fall down IMHO. They just can't adjust to conditions the way FI can, and troubleshooting and parts are horrible!

ianc