carburetors vs CIS

[randywebb]

I just use the other thing -- not the UniSyn - had one of those for years tho.

Maybe I should hook up a manifold of 6 Hg tubes like the old Fcar mechanics did...

[OZCarrera3]

I’m enjoying this thread and have nothing to contribute except this ‘left of field’ question, which is probably a stupid one. My question is, until recently almost most Japanese high performance sports bikes still used carbs in preference to EFI, why is this so? The relative efficiency of bike engines sees them produce far higher hp for their engine size than any road going Porsche. I know they don’t have to worry about torque and drivability to the same extent as car manufacturers, but what’s the deal? It can’t be weight/bulk because some bike manufacturers such as Ducati and Triumph run EFI.

Can anyone clarify this?

[ianc]

Dunno really, but at a guess?

Carbs are probably much cheaper to produce. Do motorcycles have to pass smog? Cold running issues are not so important (as you mention) because people generally don't ride them in the snow or rain. Economy is not such a factor either I wouldn't think.

ianc

[OZCarrera3]

Given that sports bikes are made with some pretty high tech materials (light alloys and carbon fibre) and are very performance focused, I don't thisnk the cost comes into the equation. As for smog, here in Australia they certainly do (as well as noise and other regs) so I can't see why they wouldn't in the States, particularly somewhere like California with the strictest emmissions anywhere.

[Eagledriver]

Randy,

I love your comment the BA says there is no HP increase with carbs. Of course you have no page or book references to back that up. Here I'll quote from the first edition 911 performance handbook page 133-134 on 3 litre 911 SC performance mods.

"The early style exhaust system can be installed on any of the 911 SC engines for a sizable performance gain of seventeen to twenty-two horsepower, depending on the year of the engine and the dynamometer the engine was tested on."

"The conversion to carburetors is good for an additional ten horsepower for a total increase of around thirty horsepower when combined with the exhaust change."

Nowhere have I sayed that carbs alone are good for all the power increase.

If you agree that the average stock SC will have 190 at the flywheel in good condition the you get to 220 easily using the above information. This happens to be the number I gave early in this post for carbs and early exhaust.

If you add race exhaust and 800 dollars worth of dyno tuning you get the other numbers I quote.

-Andy

[dd74]

Well, to be honest with everyone on this thread, I wouldn't know quite what to expect from a 3.0 engine with "just" Webers slapped on it. If it gains horsepower, great. If it doesn't, too bad. Frankly, I wouldn't want to be the $3K mule in that instance.

But considering the complexity of the carbs vs. CIS, I'm not altogether certain it is that much of a deal.

Personally, I've found my CIS to exceed my expectation for an FI system. And though I like to brag about my car having very little if no electronics on it, when the time comes to rebuild the engine, I'll most likely move forward than backward as far as induction systems are concerned.

Truth is carbs are "cool sounding" I guess, and look cool for the most part. But it is cooler to have a car that starts no matter what climate a person finds themselves in.

Dubious about the horsepower ratings on those carb-equipped 3.0s? Hell yes, I am. I only offer up that thread with the dyno results as a point of argument, particularly against authorities like BA. One way or the other, horsepower be dammed - in my opinion, nothing beats reliability.

Of course, I trust some of you will remind me of these words when I find myself in a "senior moment" within the Pelican classifieds, asking if anyone has a pair of Webers for sale.

[randywebb]

The quote is not from the book - it is from one of his magazine articles. I unfortunately do not recall which. I only work hard to retain citations for science or law, where someone pays my hourly rate.

Also, don't make the mistake of subtracting the the numbers in the book, which involved an extractor type exhaust, and trying to use that on a stock exhaust engine...

[ianc]

If you agree that the average stock SC will have 190 at the flywheel in good condition

That's really pushing it. I've read in the range of 172 - 180, and that's crank HP.

The original poster (although he seems to have buggered off somewhere) was asking about adding carbs to his otherwise stock SC. Without other mods, you just aren't getting anywhere by doing that. IMO, you're taking a big step back.

If you do want to make the other mods to get the extra power, then you have a choice: 3K$ for carbs, or 1600$ for EFI. Why anyone would choose carbs in that scenario given all the disadvantages they have over EFI I just can't figure...

ianc

[javadog]

Guys,

I have enjoyed this thread quite a bit but I'd like to add a few data points. The reason carbs can and will make more horsepower than CIS on a 911SC engine (or whatever) is simply because the intake plumbing on a CIS car won't flow as much air. Take a look at the path the intake air travels the next time you see one of these engines disassembled. It ain't pretty. The air flow sensor plate also offers a little more restriction too.

As for how much horsepower is added with carbs, that depends on the carb tuning. I once tuned an engine on a dyno and found that the optimum jetting for peak power didn't give a very nice power curve at lower rpm levels. Stating that adding carbs to an SC engine will add X horsepower is a little simplistic. Too many variables. Converting rear wheel horsepower to crank horsepower is also pretty pointless. The drivetrain losses aren't a constant percentage, even among similar models.

As for carbs having problems, let me say that if they are set up correctly to begin with and clean fuel is used, they are pretty much maintenance-free for a long time. I have in my stable a Weber-fueled 12 cylinder, which ought to be a nightmare, right? Six separate 2 barrel carbs, lots of linkages and a million things to go wrong. The last time the carbs were fiddled with was 1994. It runs fine at 100 degrees, or at 40. The jetting probably isn't optimum at either, but the old butt dyno notices no problems. Starts fine, hot or cold, with no chokes whatsoever. Gas Mileage? Who knows? I don't care. My CIS turbos get crap mileage too. It probably has a great deal to do with my right foot.

I also fool around with motorcycles when I get the time. Fuel injection has been used by BMW for a long time and the sportbikes started to get it about 1998 or so. To this day, the best carburetting bike I've owned has been a Yamaha R1, with carbs. Probably the best torque curve and most precise throttle control of any bike I've ridden. The newer injected stuff runs fine once you are into the throttle hard but cracking it off idle takes a careful hand as they are all a bit abrupt. As emissions standards for bikes lagged a long way behind that of cars, bike manufacturers got away with carbs for a long time and the carbs they used were a good deal more sophisticated than what cars used. Keep in mind, the usual Weber for a Porsche is a really old carb design. Now, the emissions laws are getting to the point that EFI is the best solution. Catalytic converters are on the horizon.

EFI is nice if you can program it well enough. From what I see, most installations don't get enough dyno time to do a good job of programming. There are a LOT of data points. If you've ever seen what a car manufacturer goes through to map an engine's software, and how long it takes to do a thorough job, you'll probably come to the conclusion that most custom aftermarket EFI systems will not be optimised very well. They may run, but... You might be able to buy an EFI system for $1,600 but you'll spend a fortune to program it, if you do it right.

Back to lurking now...
Jeff R.

[Eagledriver]

I work hard to retain citations for the Porsche 911. These cars are my passion. I've rebuilt 2 of my engines and helped others in our club rebuild theirs. I'm not in the habit of making rediculous claims I can't back up. I don't understand the reference to subtracting numbers from the book. We are comparing apples to apples.

Carbs alone according to BA produce 10 HP on a car with an early exhaust (pre 75).

I feel we have gotten off track here. The question is performance mods for a 911SC. I merely stated what one can expect from various mods and back up the numbers with dyno data and the 911 performance handbook. Others have done a good job of pointing out the drawbacks to carbs.

There are two guys in our spec class with CIS and both wish they had carbs because they can't get the power out of the CIS. This is why the CIS isn't used on factory race cars (except a couple of turbo varients).

EFI with separate throttle bodies for each cylinder is also just as powerful as carbs. We have a guy running that and his motor is around 240 HP (Stock cams, and CIS pistons).

-Andy

[Eagledriver]

I guess my initial post stating that carbs are a huge improvement was overstating things abit. On the track 10 HP is a huge improvement but on the street if you have a good exhaust already the improvement in actual power is not that large. As has been pointed out however the sound is worth about 50 HP!

-Andy

[randywebb]

It isn't just the sound - the engine will be more responsive.

re step back - carbs are forward in fun, maybe nil or not much in hp; and back in 1. emissions, 2. fuel economy, and 3. cold start ability &/or smoothness. I really think we all agree on this paragraph.

I've driven (and owned) 911's with carbs and w/CIS (both 2.7L engines) back to back over the same roads w/in a few minutes of each other. I tended to choose the carb'd 2.7L even tho it was a few hundred lbs. heavier than the CIS one -- for the reasons given above.

Now, if you really want to "do sumthin' " get a good EFI setup - it's the best of all worlds for right now. Soon we will be seeing some direct injection gas engines, and can argue about that.

[A Quiet Boom]

Hmmm,

Guess I'll chime in here. As has been stated CIS should work better than carbs due to better fuel atomization but in reality it does not due to the loss of airflow caused by the longer runners as well as sharing a common plenum. Carbs are IR (individual runner) systems and each cylinder can be tweaked for max performance, with their short port length they were made to produce HP, CIS on the other hand is more setup for torque with the long runners and common plenum.

Let's take a look at what's available for carbs on a typical american V8. The low end of the performance spectrum is a dual plane single carb manifold, good for mileage and torque. From there we move to the low rise single plane single carb intake, designed for street performance and mild track. Next up is a high rise single carb (think Edelbrock Victor series), the high rise straightens the route to the port and the larger common plenum gives needed capacity for higher rpm power. Next comes the tunnel ram which really straightens out the route to the port and allows dual carbs and a large plenum for max performance. At the top of the "food chain" are the fabricated sheet metal inakes the pro-stocker's use. These are true IR systems using twin Holley Dominator style carbs and no plenum, each barrel feeds one cylinder directly and massive HP can be obtained. Believe me if a common plenum long runner intake would make more HP the drag racers would be using it since that's all they care about.

IMHO the best choice for a naturally aspirated 911 would be an IR EFI system but those are hugely expensive. This leaves carbs which don't hafve to be $3000 BTW if you can do a little work yourself. On my 3.0 I'm running Zenith's off a 2.2, I modified them with 36mm venturis and drilled both the idle and main jets larger. Once tuned they drive and start very well, especially if you jet the idle a little on the rich side. On a cold morning I only need a couple of blocks to get the carbs working right which is a surprise for a design without a choke or enrichment circuit. Throttle response is instant and the carbs pull all the way to redline without nosing over. My total investmet was less than $500 including rebuilding both carbs. I did install and early style exhaust and an MSD system when doing my engine swap and I'm certain they both contribute to the performance of the car, particularily the MSD timing computer which allowed me to remove and pin both the mechanical and vacuum advance in the distributer and set the timing curve for best performance. Total cost for the MSD setup was around $600 and includes the rev limiter.

My future plans call for moving this motor to the track/street car and once I've finished a few projects I plan to make my own IR manifold/throttlebody setup for it using and after market controller. Unfortunately time constraints have kept me away from the P-cars for a awhile but I hope to get back to them this winter.

I did have one problem with the carbs that I feel I should share for everyone's benefit. Last fall winter came early and I didn't get the car into storage, then work picked up and it wound up sitting outside all winter. When I tried to start it up this spring I was having all sorts of trouble with the old gas and the fact that the car sat outside all winter so I had to use starting fluid to get it running. Well I had a back-fire in one cylinder that was strong enough to bend one of the butterflies and I only recently had the chance to get it running (I'm building a new house). Other than looking like hell the car runs great now though. Hopefully I can give her a good wash and polish this weekend.

[RoninLB]

Originally posted by randywebb

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.

the greater the induction drop from atmospheric the less atm psi the gas mix will be exposed to when atomizing. Kinda related to the balloon rising in the atmosphere. The resulting smaller vapor particles are turned to more energy in combustion. X amt of gas is turned into smaller or larger vapor particles, depending on the envelope. The more energy the higher the EGT, to a point. For example changing venturis is a major factor in atomization, among others. And just increasing gas volume beyond max power enlarges the vapor and lowering EGT.

I know of a 2.7 with a mild cam & carbs that pulls noticably stronger than a stock 3.2 during 3-4.5k rpm in 4th gear.. no testing done above that.
go figure.

[ianc]

Good post Boom. Good thread too.

ianc

[Superman]

More than a couple of turbo variances. All Turbos were CIS for a long time, even during the Motronic Carrera years. And there is reputedly no intake choking on 3-liter engines. Exhaust, yes. Intake, no. What some of you guys are saying is that you can bolt carbs on your stock 3-liters, and approach 3.6 hp.

Ah well. Carbs look and sound cooler, and feel like more hp. EFI beats everything, or can anyway, if properly mapped. Carbs, properly jetted work great. MFI, properly tuned works great. What's this whining about MFI? Didn't they make an MFI 3-liter? Then there's the SC/RS......What system did that have, and what are the hp numbers? As I recall, they are brisk, to say the least.

[turbo6bar]

Originally posted by Superman
And there is reputedly no intake choking on 3-liter engines. Exhaust, yes. Intake, no. What some of you guys are saying is that you can bolt carbs on your stock 3-liters, and approach 3.6 hp.

How do you explain the inferred HP gains on tbitz's Megasquirt system?
jurgen

[A Quiet Boom]

Comparing turbos to NA is apples to oranges, besides the turbos intake is a shorter runner system anyway. I've also heard of using a 3.2 intake for a performance gain on turbos. Forced induction will somewhat overcome resistance in the runners.

As for tbitz's kit, I'm not surprised at increased HP. After all you've removed the air door which in and of itself is a restriction.

I believe it is absolutely possible to get 3.6 like HP with a 3.0 and carbs but (and it's a biggie) a 3.0 will need to make the power higher in the rpm band to reach those numbers and thus sacrifice torque to some degree, assuming you equaled the HP of a 3.6 it's certainly make less torque and both the HP and torque curves will be higher and more peaky thus making for a less drivable engine on the street than a 3.6.

[Superman]

Originally posted by A Quiet Boom


I believe it is absolutely possible to get 3.6 like HP with a 3.0 and carbs but (and it's a biggie) a 3.0 will need to make the power higher in the rpm band to reach those numbers and thus sacrifice torque to some degree, assuming you equaled the HP of a 3.6 it's certainly make less torque and both the HP and torque curves will be higher and more peaky thus making for a less drivable engine on the street than a 3.6.

Yeah, you can do all this. But not with just carbs. These guys are suggesting that Porsche built such an inefficient fuel system (that meters fuel well enough to pass emissions) that you can just bolt carbs on and approach 3.6 power (with carbs which cannot meter fuel accurately enough to pass emissions). Makes little sense. But yeah, you can move torque and horsepower (a bull***** calculated number anyway) upward in the rpm range. You'll need cams for this.

[A Quiet Boom]

Supe,

I totally agree, carbs alone won't get you there. Any experienced engine builder will tell you you need to look at an engine as a system and improve airflow throughout the system. This is why intake, cams, and exhaust should be closely matched, even any port work done to the heads. I've seen guys spend big bucks porting V8 heads only to negate the effect with poor cam, intake or exhaust choice. I'm of the opinion that carbs are a waste of time without at least backdating the exhaust.

One thing I can't stand is when people talk absolute HP. Where in the RPM band is it, what's the curve look like, where and how much torque does the engine make, how flat is the torque curve? All these questions are important because none of our engines operate at peak power all the time, in fact a street car may only see peak power 2-3 percent of the time. Take a 3.6 and a 3.0 of equal HP and the 3.6 will outperform the 3.0 in most cases except possible the track assuming the 3.0 is geared to optimize the power curve.