40mm or 46mm carbs?

[kellzey]

3.0 Short block with early 3.0 heads (39 mm intake ports).
Single Plug
Mahle 98 mm (3.2L) 9.5:1 P&Cs, optimized for carbed engine
Elgin Modified S Cams
Electromotive Ignition
Headers and hi-flow/sport exhaust

Should I run 40mm or 46mm carbs?

[Adam]

General consenus is that 46mm carbs are a little too big for a street driven 3.0.

If you're going to race the car (dedicated track car), then 46mm might be the way to go. I'm sure Chris Streit will weigh in with some advice...

Good Luck, keep us posted!

[cstreit]

I guess i couldn't dissapoint...

As Adam said, this completely depends on what you're using the car for...

40's will give you good driveability and decent mid-range power.
46's will give more top end horsepower.

If you plan on mostly street and some track, 40's are definitely the way to go.. You'll have good street performance and better oomph out of the corners out on the track without having to spin the motor wildly.

If you are planning on a track only car and have the modifications to support higher RPM's (7k-8k) in his motor, you'll be able to get more out of the 46's...

If you do decide the 46's, make sure to modify the venturi size to get the proper flow through the metering circuit... 36mm venturi's are a good place to start. 46 's will be peaky (ie sluggish under 4000RPM)

Sounds like a fun motor!

Chris

[tholyoak]

Last week I was talking to Richard at PMO. I wanted his opinion on what size carbs to use with my 3.0L. Traditionally it is stated that with webers go with the 40's due to the poor idle circuts. I wanted his opinion on whether this carries over to the PMO carbs with the better idle circuts since I was buying a set of 40 PMO carbs and wanted to make sure I wasn't making a mistake.
His comments were that basically with the PMOs it is recommended to use the 46s not the 40s. The 40s will still work fine but the 46s are a better setup.
So I guess it also depends on if the carbs you are using are Webers or the redesigned PMOs. I went ahead and bought the used set of PMOs since it was a great deal.

You also might want to rethink the electromotive ignition since I have read that there are problems with plug fowling with that setup and carbs.


Todd

[cstreit]

TOdd,

I'm curious about your comment on the electromotive system..

WHere did you hear this and what is the justification?

WHat alternatives are presented for twin plug systems? SInce Bosch no longer makes the 12 point disctributor, the only one I've seen offered would be a modified 993 twin distributor setup (with the failing belt) at a significant machining and parts cost! ($2000+) for this...

[tholyoak]

Chris

My comments were based upon Steve Weiner's statements on the Rennlist board and his website, which contains more info than you can imagine on performance upgrades, it is at

http://www.easystreet.com/~porsche/2.html

I had an electromotive system on my SC after a rebuild, still single plug, and found it no better than the stock distributor setup. I switched to an MSD box with the stock distributor and found much better results, allowing me to open up the plug gaps to 0.045" for better throttle response. This is with CIS, when I get the carbs on I'm hoping for similar results.

I didn't think using the 964 distributor was too much more than the electromotive setup. My understanding was to use the drive gear particular to your engine, and put the guts from the 3.0 distributor in the 964 head to use the magnetic trigger to switch dual Bosch or MSD boxes.
The belts aren't a big problem with the twin distributor if you install the vent kit and like any other belt replace it at specific intervals. I'm pretty sure this is the route I will go when I switch to twin plug in the next year.

Todd

[cstreit]

THe cost I got for the distributor, dual MSD boxes, machining, etc. was about 2250.... Electromotive setup is about $1500...

[CamB]

Chris

Is that a brand new distributor??? There is no way it should cost that much (IMHO). MSD boxes are cheap from Summit etc. you have one donor distributor.

I'd shop around a little. It just sounds too much!

[CamB]

Went and found some other info I remember discussing on Rennlist:

http://forums.rennlist.com/cgi-bin/rennforums/ultimatebb.cgi?ubb=get_topic&f=1&t=003449&p=

It doesn't sound $2000+ worth.

There is also a smart way to get heat to SSIs with the twin distributor (using a 993 turbo air guide). This is unlikely to be an issue for you Chris

[TimT]

I still wonder why Steve at Rennsport is so anti electromotive, I respect his knowledge and opinions, but it seems like taking a step backward to keep using disributors ( no way will I pay $2000 for a RSR 12 plug cap). It seems like most major manufacturers are using crank fired direct igniton instead of distributors. Many major race teams use engine management with ignition controls built in and coil packs to fire the plugs, the signal is taken from a craank trigger.

I have seen dozens if not a hundred cars running electromotive with carbs. Havent heard widespread rumours of plug fouling and driveability problems.

Racer are a really stingy bunch, Im sure all those club racers and pro teams that use electromotive want a good reliable ignition, so they can spend money on consumables (tires, gas, beer)

Also if your electomotive drops a coil while you are at the track, the local pepboys, autozone or parts house most probably has a "gm" coil in stock that will work

[CamB]

Yeah, I know what you are saying Tim.

I wish someone with heaps of money would try both!

FWIW, I have pretty basic EFI running a 3.2 with S cams on my '69. It has a 964 twin-plug dizzy and two modern Bosch coils and "ignition modules". It doesn't have a CD system, and I felt the disconcerting "lost a cylinder" feeling after getting tyres and alignment. It spent too long idling, basically (5+ minutes in my drive, 5 minute traffic-snarled drive to tyre shop, plus whatever they did then 5 mins more traffic). It fixed itself after 5 mins on the freeway but I think this highlights that it can happen.

I either need hotter plugs or to suck it up and buy 2 MSDs.

The problem may go away when the engine is better run in too.

[cstreit]

Well the quote I got from RennSport was over $2k...

That's over $500 I could spend on other goodies including those snazzy GE-80 German billet cams! woohoo