Cis to Carbs...documented power increase

[sbmackie]

Hello,
Does anyone have a before/after dyno comparing a cis to carbs conversion? I've read B.A's book; my 82 SC has the AC removed and SSI's to an M and K 2 to 1. I have a dyno showing three consistant runs the highest being (at the wheels) 167 HP. Assuming a 15% drive line loss, that makes 192 HP at the crank. Published number on the US based '82 per the owner's manual is 172, so just the exhaust and pulling the AC is 20HP. But, I've also seen "somewhere" that the real HP of the SC of all US based vintages is 180, for a 12 HP gain, which seems more reasonable to me.
So: dyno proof of HP gain adding JUST CARBS (no cams, etc)
Thanks
Yes I'm budgeting 'improvement$" as time and $$ permit.

[rattlsnak]

Don't do it..

A). The minimal gain of carbs only is not worth the cost.
B). It will not run as good as your CIS does on a primarily street driven car.
C). It will bump you into a Modified AX class where you be either with 3.2 cars or higher.

[Uncle Enzo]

.....but it sounds so sexy.

[tirwin]

I wouldn't do it either.

If you were going to go to the trouble and not change the cams and consider other mods it would be a waste to me. If you wanted to get rid of CIS, go the EFI route. Carbs is the wrong direction in my opinion. And remember that the late 3.0 motors like ours are intake constrained.

[Ayles]

Originally Posted by Uncle Enzo

.....but it sounds so sexy.

Looks good too! Impractical, but awesome. Doing the conversion right is definitely a can of worms.

http://forums.pelicanparts.com/porsche-911-technical-forum/754044-show-me-your-carburated-sc.html

[VFR750]

Well yes they do. And Bruce Anderson already provided the data.

Us made 172 SAE hp which is the same as 180 DIN. Different methods, different accessories on the engine. Both on engine dynos.

My CIS made 154.5 hp at the hub. So that is pretty close to 172 SAE crankshaft hp.

I did not dyno my carb conversion. Butt dyno definately says more power.

Most importantly it pulls very strong past 6000 rpm. Whereas the CIS was loosing steam past 5500.

One de instructor asked if I had done a 3.2 conversion or cams. It pulled so well

All I know is I gained ~5 mph at the end of the LRP main straight. 20-25 Hp is probably the increase.

[unclebilly]

Ummm, you are missing something pretty important.

Not all Dynos are equal. It appears that the Dyno that Porsche used Bach in the days to get the HP ratings for our cars was a lot more generous than any Dyno my cars has been on.

Some Dynos are quite stingy.

If you are looking for high HP numbers that's one thing but if you want HP for a race car to improve lap times that is quite another.

I have yanked CIS off of 2 cars to build race engines and have gone to carbs. CIS is all about fuel economy, Carbs give better throttle response, better tuning at the top end, and are simpler to troubleshoot. Oh and they are 20-30 lbs lighter.

CIS is better for cold starts - carbs have no choke.

I have run both Webers and Zeniths and I prefer Zeniths with custom throttle bodies.

One more point: CIS engines have Pistons that won't let you run aggressive cams. To change the cams, you have to do Pistons. CIS won't run right with aggressive cams due to reverberations in the intake. This is why you won't find many data points on engines with just carbs.

On the chump 911, we did carbs and distributoress ignition on an otherwise stock 2.7 and it was way different than a stock CIS 2.7. We did this for simplicity and ease of tuning.

[RarlyL8]

Did a carb swap years ago on a stock engine that we had already installed SSI exchangers. Changed the world for that engine. Added 10HP, felt like 50HP. Instant throttle response, extremely cool start up sound, like an F1 engine, when starting in the winter below 32°F. Fuel mileage plummeted, I didn't care. All that said, do not do this if you don't have experience tuning carburetors, they are finicky and require fiddling to get them optimized.

[IS300]

would someone explain ... simply the differences and cost to upgrade from..
CIS to:

EFI
MFI
PMO

I have searched...
thanks anthony

[Joe Bob]

Unless you upgrade the cams, don't do it. MPG goes into the dumper and the costs don't justify the benefit.

[VFR750]

Expect to spend between $5000 to $8000 to convert from CIS to something else.

Silly you say?

You will need for a carb conversion from CIS:
New oil lines to clear the SSI
SSI exhaust (to take fullest advantage of the carbs)
Carbs with manifolds
Fuel pump or regulator
Recurved distributor (or a 78-79 will do)
Fuel lines and filters
Plumb a new vacuum circuit
A whole slew of idle jets, main jets, air correction jets
A wideband O2 sensor for tuning it yourself
Backdated heater ducts (blower system doesn't fit)
Replumb the evaporative system
A new breather tank system

And your Time. Add a lot more if you pay someone to do it for you

All this stuff adds up and it isn't cheap.

$5k

even cheap Zeniths get expensive fast if you rebuild them properly with new bushings for the shafts.

Efi and mfi probably more expensive. Mfi more because you will have to invent a space cam that works for a 3.0 (or buy a rare race one)

Efi
Electronics and tuning in top of all the same stuff as the carbs.

[juanbenae]

Originally Posted by RarlyL8

Did a carb swap years ago on a stock engine that we had already installed SSI exchangers. Changed the world for that engine. Added 10HP, felt like 50HP. Instant throttle response, extremely cool start up sound, like an F1 engine, when starting in the winter below 32°F. Fuel mileage plummeted, I didn't care. All that said, do not do this if you don't have experience tuning carburetors, they are finicky and require fiddling to get them optimized.

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

[VFR750]

Ok, that may have come across a little snarky, but the truth remains.

If you really want to do a conversion you have to treat it like a very big project with a lot of seemingly unrelated actions and parts

The list I gave you took me several months to create, and buy the various parts. I incrementally did things in preparation, like backdating the heater ducts while still running CIS. Same with the o2 sensor. Deleting the cruise control and a/c.

Then I solved all of the problems like fuel lines and breather tank setup before I jumped in

The day arrived, yanked out the CIS. Bolted in the carbs, ran the fuel and vacuum circuits, added the fuel regulator and filters and added the breather tank. Took a few days.

Then started up. Adjusted timing and then spent the next several weeks tuning.

I posted a lot of information here. Search "Zenith TIN".

Most of that info applies to any carb installation.

Hope that helps.

[tirwin]

VFR750, he already has SSIs so he won't have that cost, but your point is well taken.

Scott, I am not claiming to have the right answer, but I did some napkin math last year. Modifying a 3.0 for more HP gain seems to quickly reach a point of diminishing financial return. You have pretty much done all the low hanging fruit - backdate heat, SSIs, sport exhaust, lighten the car, etc.

At the end of the day, the telling metric is $ per unit of HP gain.

I suspect that there are 3 options:

A) a somewhat constrained 3.0 modification whether that's carbs, EFI, etc. (but not going totally crazy)

B) turbocharge the 3.0 (it has been done)

C) a 3.6 swap

My best guess is if you were to rank them in terms of $ per hp gain it would be B, C and then A. At some point you cross a threshold where the cost of modifying a 3.0 is more expensive than just doing the 3.6 conversion. You should talk to Uwanna about his 3.6 transplant and check it out.

There are pros/cons of each. I started 'sharpening the pencil' on this last fall and got distracted by work and other stuff. All I'm saying is that I would strongly encourage you to lay out the options and figure out the costs and risks and let the math speak for itself.

[dfhtrhjn]

I have two 911s both with the same 2.7S engine. My race car is carbed with the exhausted punched out, and my other is stock. The carbs make the engine feel out of this world compared to CIS. I get about 1k RPM wider power band and instant throttle response with carbs on the same engine. The CIS gets me about 28 MPG HW, and the carbs get me about 15MPG HW, but it's worth it.

With carbs you can easily tune for power, you can run the hottest plugs on the market, burn tons of fuel, advance your timing a little, upgrade cams if you want, go dual plug if you want and really make use of it all.

All that begin said, if I were to do it all over, I'd go efi, motec, Rothsport, etc. Carbs are probably the most fun, but modern race tuneable fuel injection is better in every way.

[will hung]

Originally Posted by tirwin

VFR750, he already has SSIs so he won't have that cost, but your point is well taken.

Scott, I am not claiming to have the right answer, but I did some napkin math last year. Modifying a 3.0 for more HP gain seems to quickly reach a point of diminishing financial return. You have pretty much done all the low hanging fruit - backdate heat, SSIs, sport exhaust, lighten the car, etc.

At the end of the day, the telling metric is $ per unit of HP gain.

I suspect that there are 3 options:

A) a somewhat constrained 3.0 modification whether that's carbs, EFI, etc. (but not going totally crazy)

B) turbocharge the 3.0 (it has been done)

C) a 3.6 swap

My best guess is if you were to rank them in terms of $ per hp gain it would be B, C and then A. At some point you cross a threshold where the cost of modifying a 3.0 is more expensive than just doing the 3.6 conversion. You should talk to Uwanna about his 3.6 transplant and check it out.

There are pros/cons of each. I started 'sharpening the pencil' on this last fall and got distracted by work and other stuff. All I'm saying is that I would strongly encourage you to lay out the options and figure out the costs and risks and let the math speak for itself.

Until the 3.6 needs rebuilt...

You can do an EFI swap fairly reasonably if you stay disciplined and avoid project creep.

[RarlyL8]

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

[Matt Monson]

Originally Posted by dfhtrhjn

I have two 911s both with the same 2.7S engine. My race car is carbed with the exhausted punched out, and my other is stock. The carbs make the engine feel out of this world compared to CIS. I get about 1k RPM wider power band and instant throttle response with carbs on the same engine. The CIS gets me about 28 MPG HW, and the carbs get me about 15MPG HW, but it's worth it.

With carbs you can easily tune for power, you can run the hottest plugs on the market, burn tons of fuel, advance your timing a little, upgrade cams if you want, go dual plug if you want and really make use of it all.

I think this kind of nails it. People fixate on peak gains because that's what manufacturers of parts do. They try to suck you in with this much extra HP. The carbs don't give you a ton of extra HP. But they give you more torque. They increase the area under the curve substantially. And they do it sooner(lower rpms) with excellent throttle response.

The OP autocrosses his car. I personally think his "lap" times could improve substantially with a carb conversion.

[RSBob]

Both of mine have '82 3.0s. One has Webbers and the other CIS. The carbs look sexy and give better response, but not night and day. By comparison the Webbers get horrible MPG which a friend calls a "controlled fuel leak". The other issue with carbs is that your garage will reek of gas unless you put in a perpetually running exhaust fan like I did. Would I go carbs again for the intake sound and looks? Not again.

[James Brown]

Originally Posted by RarlyL8

Did a carb swap years ago on a stock engine that we had already installed SSI exchangers. Changed the world for that engine. Added 10HP, felt like 50HP. Instant throttle response, extremely cool start up sound, like an F1 engine, when starting in the winter below 32°F. Fuel mileage plummeted, I didn't care. All that said, do not do this if you don't have experience tuning carburetors, they are finicky and require fiddling to get them optimized.

did the same on a SC, Zeniths with new ventures, SSI, M&K, built dissy with recurve. (the recurve is the best bang for your buck on HP you can do!) result is instantaneous throttle response butt dyno off the chart, M&K and carb intake sound worth the price, easy!

cost, PMO manifolds, linkage parts, air filters-$ 800
used carbs, jet kits, rebuild gaskets, custom made venturies-$ 600
dissy recurve/rebuild-$ 250
fuel hose, fittings, fuel regulator- 300
total parts +- $2000 not counting your SSI, M&K, back date heat stuff