I have removed the Weber 40IDC 3A carbs for a rebuild.

They currently have 32mm venturies.

Will larger venturies give more top end power?

The engine is 2.7 from a '77 911. Internals (P/Cs, compression ratio, Cams) are unknown. Bursch Headers with megaphones (with removable baffles).

The car is 90% track. Weighs less than 2100lbs.


Thanks

Eddie

Yes you will get more top end, but you will give up somewhere else

I could be wrong (it happens every few years ) but IMHO 32's may be significantly too small for a 2.7. I have 32's on my 2.2S and its Webers are set up to factory (carbed) 2.0S specs. My car had larger chokes, 34mm IIRC or maybe 36's, previously and it had a nasty tendancy to bog at lower revs and break up (cavitation?) at high revs.

I hate to mention this but in this particular case you may be well-advised to search the Pelican 911BBS for "2.7 jetting" as I know this has been discussed before. There are tons of Weber-equipped 2.7RS clones running around, both street and track.

Also, you may want to contact Richard at PMO. He can provide jetting advice based on your specific application and can also supply any parts you may need.

Did you know if you spell carbs wrong you get crabs?

Sorry. I'll try to post something useful next time.

LMFAO...

I pulled the carbs apart tonight.

Current jetting:

Mains - 135
Air - 180
Idle - 55
Vents - 32
Emulsion - F26

I did a bit of research and found this is the recommended jetting for a stock 2.7 CIS motor.

I need to determine the specs of my motor. The previous owner was not much help.

Eddie

It's not a good idea to just change one aspect of your "crab"
set up.....unless you're chasing a specific problem.....too rich....too lean ...flat spots...yada.

Lemme ask a question.
Is your seat of the pants power curve lineal or does it have a distinct setyoubackintheseat bump around 3500-4000 rpms?

Well Maybe I'm and idiot (I doubt it) But CIS cars don't have carb jets. Maybe you need a 2.7 S or RS.

For something cammed like a 2.7S engine you will want to stick with about an 8mm difference between the carb and venturi size. You will loose velocity if you go any larger. Examples:

Have a customer with a 914-6 2.7 that had 40/36's and 208rwhp on the dyno. He is also running 39mm intake ports and R cams. The car didn't have much of anything on the bottom end, but didn't like to turn much past 6600rpm's either.

We tried a fresh set of 46's with 38 venturies, dailed stuff in. No huge HP gaines, but oh man! The power comes on much, much sooner and the 914 pulles like a raped ape to 7500rpm now!

Bigger is not always better.

For fun, I threw his old 40/36's on my 2.2, ran great, but when I put my 40/27.5's back on, now my 2.2 has gobs more torgue, but doesn't pull in the upper RPM's like it did with the 40/36's. You will loose torque as you increase the venturi size. I would stick with the 40/32 set you have.

Since the SCCA has stuck me with 34mm Venturis, I have done alot of testing with other venturis, 32-37.5

For the 2.7 S I would recommend a 34 it is a compromise on all fronts, but will retain the Power under the curve or PUC. with the sacrifice of only a couple HP up top. The data I have from testing indicates a 36 mm venturis over a 34 mm venturi is an increase of only 4 hp above 5500 rpm to a max of 7200 rpm.

34 mm is a very good choice. but your car is light so you can try 36mm, lap times will be the final indicator of proper sizing.

Erik

The power curve is fairly linear. The rev limiter is at about 6700rpm (distributor rotor limiter?).

3500-4000 seems to be the fat part of the power curve. The power seems to flatten out over 4500-5000.

I have access to a set of 36mm chokes. What jetting should I use with the 36s?

I know that compression ratio and cams are the major factors in determining jetting, but how much does do headers and mufflers affect jetting.

I have not had a chance to run the car with the baffles out of the megaphones. October 30 is the IRDC lapping day at Bremerton Raceway. If the rain holds off, I'll run the car with open megaphones.

Dan,

Are the 40/38s available for rent?

Thanks

Eddie

Probably, they are going on a cutomers 2.8 tonight to experiment. The 40/36's I have are known "Good" carbs. The 3 engines I have seen them on need only dailed in and away the cars went. Smooth. If you are not in a hurry, let this guy borrow them for a bit and I will ask the owner of the carbs if he doesn't mind letting them get tossed about a bit. I have known him for years so he would probably bee cool with it.

30th?? Dang, I might have to show up for that one myself.

Dan,

Oops!

I meant to ask if the 40/36 Webers are available for evaluation?

Thanks

Eddie

Hmm, I might have another set we could use though.... They are 40/36's as well.

I'll do the antipercolation modifications and the put the 40/32s back together with the current jetting. It will be a good baseline for comparison.

I'm not in a big hurry.

When I bought the car last January, it had only been run as an AX car for the last few years. It was suffering from neglect. The owner moved on to a 944 Turbo. The engine experienced a fair amount of low rpm popping and backfiring. After cleaning out the fuel tank, fuel filters, and idle jets, and dialing in the idle mixture, the car ran quite strong.

Two weeks ago I went to a Alfa Romeo lapping day where it rained very, very hard. I have K&N Rainshield air cleaner hats but no engine cover raintray. Before lunch, the engine started running like crap! Missing. stumbling and backfiring. Initially I though it was carburetor related (plugged idle jets or such), but I think spark plug wire arcing was the culprit. New wires are going in this weekend.

I knew the Webers needed a rebuild, so I pulled then apart last night. They were pretty dirty, but I have seen worse.

If you could come up with another set for comparison, that would be great!

The 30th should be lots of fun. I'll take over my motorhome and cook up lunch for anyone who shows up.

Eddie

Sounds like CIS cams to me. That being the case, there's nuthin' to be gained by winding the thing over 6-6.5K.....thus, the bigger chokes won't help (tho 34s might work)....and well may hurt the bottom.

The CIS cams have very little overlap and thus minimal cylinder scavenging.....which is what one needs for top end power in a NA motor.

JP,

What are the specs on your 2.7?

What are the differences between a 2.7 and a 2.7S? bigger ports and valves?



JP ... your engine compartment looks fabulous! (from the show us your engine thread)

Eddie

Hey Eddie,

Put those 40/36's on that guys 2.8 last night and holy !!! Big difference on his car. Althoug he has 37mm ports, mod E cams and 9.5/ratio so his engine was just eating up the fuel last night. We of course had to "Test" drive the new carbs. Anyway, the set we pulled off his car are 40/34's not 40/36's like I thought. Bummer, but if you want to try them out, I don't think he will care at all for a while. They are in pretty good shape and should be a bolt on and go sort of thing. Good enough to just fiddle around with. Up to you.

My 2.7 has 9.7:1 compression, Solex cams & very mild porting.
The "crab" set up is right out of Andersons book for his hot rod 2.7:

34mm venturi
F3 tubes
135 mains
145 air corr
60 idles.

Works for me.

He also did a comparo (dyno graph in the book) between
32,34,& 36 mm venturi and the difference is minimal.

The 2.7 CIS motors are basicly all the same...powerwise...165/175 hp depending on exhaust... with the exception of the 2.7L "normal" from 1974 ......small port , 8.0:1 compression, 150 hp.

Dan,

Thanks for the offer. I'd love to try out the 40/34s.

I've got the 40/32s ready to go back on so we can do a comparison.

Maybe this weekend?

Eddie