carb sizing question for 3.0 modified engine

[jmz]

I know Bruce Anderson will say that Weber 40's are the proper size for a 3.0 engine

However, I would rather hear from the pelican communitiy on what works for them 40s or 46's for a modified 3.0.

My engine is a 3.0 w/ Elgin Mod S cams (GE 60 equivalent), some port work and about 9.3:1 c.r.

[Formerly Steve Wilkinson]

I suspect 40s are right. I have an SC engine modified to 3.4 liters, S cams, twin ignition, about 11:1 compression and some other stuff, dynos to the estimated equivalent of 290 hp at the flywheel and I am using 46mm PMOs. Can't imagine you'd need them for what you list as mods.

Stephan

[jwetering]

I'm running 40 IDA on my Ge60 cam, 9.5:1 CR piston 3.0 litre. 46 are too big. I'm running 34 mm venturis...maybe they're 36, but I'm pretty sure they're 34. I've got my jetting info at home if you're interested. Carbs run great from 2000 rpm to redline nary a stutter, no dead spots. all good.

[jmz]

Here is what mine pulled at the wheel with the 40s.

run 1. max power 210.2 max torque 199.1
run 2 max power 210.7 max torque 197.9
run 3 max power 210.1 max torque 195.4

max torque was acheived at approx 4700 rpm
max power was acheived at approx 6100 rpm

For a refernce point the 993 that was on right before me pulled 240

[jmz]

Thanks Jasper.

I would like the settings. For what it's worth my car ran well with the 40s but they have really been acting up lately. #3 and #5 were dead holes with the 40s. I have tried blowing out the passages and cleaning but haven't had any luck. I put the 46's on and all six holes fired.. I haven't got them dialed in yet.

I have been running 36 venturis on the 40s and saw a noticable improvement over the 34's.

Jay

[Vipergrün]

Thread hijack in progress.....

Steve, what would it cost to build the motor you stated, assuming a solid core to start with? Sounds really nice.

[jpnovak]

Jay, do you have a dyno curve? I just bought some new pistons and will go with the mod_S cam too. I am trying to figure out how the area under the torque curve looks.

Do you have JEs or other pistons. I don't think CIS will clear that cam.

Mine will have 98mm RSR pistons that should measure out near 9.8:1 on the SC heads. I too wondered if the 40IDAs would be too small and have been researching MFI, EFI and larger carbs.

If you lost two cylinders I assume that it was a fuel passage blocked. Did you remove, disassemble and clean, before replacing with the 46s? Are you sure that the plug wires weren't the problem?

[jmz]

Jaime,

I was getting spark but no fuel. I did test though by putting new wires on the offending holes.

I am sure a fuel passage was blocked. I pulled jets and sprayed air and carb cleaner to no avail. ...I didn't get too deep into a carb re-build before throwing up my hands and putting on the 46's

I used some Mahle pistons that Andial had laying around and matched them to my cylinders (more or less). They are similar to an rsr piston w/ deep valve reliefs.

CIS pistons and Mod S / GE60 cams probably wouldn't groove together.

I've got a copy of the curve in my glove box. I am w/o a scanner right now but could fax it to you if you want to see it.

Jay

[Formerly Steve Wilkinson]

Wires in the holes is a major no-no.

Stephan

[jmz]

What are you talking about Steve?

I am talking about putting new plug wires to the corresponding plugs of the cylinders (holes) that aren't firing.

Jay

[jwetering]

ok jay here's my carb setup details:

F3 emulsion ubes
36 mm venturis
60 idles
155 mains
180 air corrector jets

These were recomended by Richard Parr at PMO, and Bruce Anderson more or less concurred. As mentioned earlier - this set up works for me.

Now about the lack of fuel at #3 and #5....I don't need to tell you to NOT drive the car like that. Theres are horror stories right here on this board of burned pistons etc caused by fuel starvation and lean running conditions - be very afraid of this.

Do not however be afraid of tearing those webers down and cleaning them up. Get one of those blow up diagrams and a few cans of carb cleaner/brake clean stuff. I used an ultrasonic bath with Simple Green followed by generous compressed air lashings myself, but I realize a lot of people wouildn't have access to one of those. Wayne's book has a chapter on weber rebuilding and it's more than enough detail to get the job done. Just make sure you put everything into little labelled baggies. Richard Parr at PMO will sell you weber rebuild kits for not crazy money.

I also modified my float bowls according to Bruce Anderson's little blurb in his performance handbook where you grind away the materials around the fuel pick up and install horizontal dams. This was done in an attempt to cure a severe stuttering problem I was having during slaloms. The bowl mods did not solve this problem however.

I did however JUST solve the problem, by undoing a modification made by a PO according to Richard Parr's instructions. Richard sells a jig which is used to drill vent holes in the carb tops which will dump boiling fuel into your carb throats rather than out of your bowl vents. A good idea in theory if you live in a climate where you might boil your fuel. BA mentions this mod in his book too. Well, I can tell you that during slaloming, fuel gets sloshed up and through these vent holes, and you get raw fuel down your throats, which causes a major stutter. Last thursday I JB welded these vent holes closed, and last sunday during our Whistler autocross, the problem had gone away. So I'm going to go ahead and not recomend that mod.

Good luck

edit - CIS pistons may *just* clear the bg cams, but the problem is that teh CIS piston isn't shaped right for to make the kind of power. I put JE pistons in my motor and as well as eing performance shaped, they are about 200 grams each lighter. 7000 rpm comes up pretty fast nowadays

[Wayne 962]

Back to the original question. 46s are too big for both a 3.0 and a 3.2 - UNLESS, you're running the 3.2 on the track where you're going to be at high RPM all of the time. Otherwise, it's just too much carb for the engine...

-Wayne

[jmz]

thanks all, I'll just quit being lazy and really clean the 40s. I should be able to get access to an u.s. cleaner.

jay