13 replies to this topic

[RMVR53]

ok call me an idot, glutten for punishment, or what ever. I'm making one more attempt to breath life back into the XT. The 40DCOE isn't going to work (but it looked REALLY cool) so I'm toying now with the idea of a pair of 32/36 weber's or if the esteemed panel thinks thats too much air/fuel I'm also considering a pair of Holly Bug sprays or just a pair of 34 Solex's (one barrel off a Bug). It's a dual port head remember...

Bill

[rxleone]

Have a look at this build - I know he used Sub4 heads but you might get some info there?


Good luck dude.

[RMVR53]

anyone else?...I'm doin SOMETHING with it this weekend!

Bill

[monstaru]

32/36's are a little to much as a pair.one of them does fine.cheers, brian

[NoahDL88]

I'd look into what the Bug/VW crowd uses for their dual carb setup, that would be close to the right CFM that you need.

Remember, going too big is just as bad as going too small.

[IASUBE]

Dang it.. wrote a long post on why you should do it... some how it dissapeared... ain't got time to re write the whole thing, bottom line I think you should do it.. it won't be too much carb for it... just my op.
Iasube

[3eyedwagon]

Cubic inch displacement X Maximum RPM X Volumetric Efficiency (a street engine is about 80%) / 3456 = CFM needed


350 CID x 6600 RPM X 0.80 volumetric efficiency / 3456 = 534 cfm needed.

That is the math for a basic small block chevy in semi-stock form, and what it needs carb wise to be efficient.

Do the math on your combination, and decide for yourself.

And just for the record; most people tend to over carb the bejeebus out of their motors.

And another thing; A K&N air filter, and a set of expensive spark plugs does not make your engine a "race" engine. The volumetric efficiency doesn't increase to 100% by putting some flowmaster stickers on the window. And pretending the motor will spin another 3000 rpm more than it realisticly will, or will normally see isn't going to benefit you either. Use the formula honestly for best results.

[IASUBE]

Ok, yes it's true, that is the generaly used math for carb sizing..
But there is a lot more going on than just "CFM" when it comes to carbs...
manifold layout and design, multi-cylinder vs. IR set-ups, different cfm ratings for style carbs...etc. everything come into play, sometimes in ways we wouldn't expect... one quick example; GM offered basicly one size carb, the Q-jet on everything from a 250 cid 6 cly to their 454 cid big block. that really doesn't fit the above math very well, and before anyone jumps on "they made a bunch of different size Q-jets" they really didn't, go check.
If you're really interested, Smith and Morrison's book, "Scientific Design of Exhaust and Intake Systems" is a great primer to get you started.
IASUBE

[RMVR53]

can't remember the site not but I caculated the CFM needed at 170. A Holly bugspray is 200 so it will work as a single but I have to fab a manifold or attachit somehow to a turbo manifold but the problem I see there is that manifold is not designed to have fuel flow through it...only air. I have yet to find the CFM rating of a Solex 30, 32 or 34 PICT but I'm guessing they are in the 100-150 range so a pair might not over-carb the engine and I can make an adapter plate that sits right on top of the spider waterjacket manifold...no 'manifold'. We shall see!!!

Bill

[monstaru]

regardless, i have actually carbed an EJ motor.i can;t tell you a number to beat hell.....
but i can tell you, that a 32/36 is plenty of carb if you run bigger jets.you could actually choke out the engine way rich, before you got to the end of the size range.
i know, because i tried it.jerry swears up and down that his big carb motor has more power.it is possible,but i think it would be a very fine line ....

dual 34's would be a little overkill, but the thing to keep in mind is mounting.when you have synchronize it ieasier to have them in line than to have some crazy setup.....the less complicated the better.

downdrafts will definitely work out better in the long run.i have a set of sidedrafts currently, and they are looking at being fairly difficult to setup properly, althought they would look ************in killer.

see if friends of friends and such have something you can kinda mock up...

cheers, brian

[spikedbeetle]

I used a pair of single barrel 40mm carbs on my EA81. They seem to work great. I could have done a couple things different to make it better. I also found this video online of a suby motor with a lot of carb and dual port heads.

I say go for it!

Video:

http://www.streetfir...beetl_92098.htm

Mine:

[RMVR53]

Picked up a pair of 34 PICT's today. Gonna use both as cores to set up the manifold adapter plates. One looks like I can put a kit in it and I'll be good to go. The other I'll have to replace with a 'real' one as its missing a few things (acc pump housing, low idle solenoid, etc). Never did find the CFM for these but did find CFM for a bugspray (200-300 depending on series) and most VW Guru's in the area thought the 34's were in the 150ish range so a pair should work out. I don't want to kill gas mileage! My next plan is to set up the linkage. Thinking about snagging the center pivot link from a V12 Jag. Should do the trick for both movement and length of rods. (every car should have some Lucas part on it!!)

Bill

[spikedbeetle]

I am excited to see what kind of power difference you notice.

Maybe you menyioned it already, but what is your current plan for the manifold or manifolds? I had thought if I were to do this all over again. I would probably buy a couple of aluminum blocks from an online metal supplier. I found a place that would sell a 3" x 3" cube for $30 or so. I would have the intake part drilled at a machine shop. Then I would drill and tap everything else into them.

150 sounds about right. I used 3eyedwagon's calculations. My carbs came out to 165 at 5000 rpm.

Can't wait to see what you come up with!

[RMVR53]

I'm not really concerned with power...gain or loss...at least I hope not much loss. Going from MPFI to carb I am anticipating loosing both power and MPG and hope not much of both. Yeah if I put a pair of Weber's or something like that I would expect a significant power gain but MPG goes all to heck...don't want that. "Manifolds" are going to be short and sweet. The guy making them for me is waiting for me to tell him how long (height) they can be first so I need to figure that out. I'm guessing about 2" right now sitting on top of the spider water jacket. His plan is a pair of 3/8" aluminum plates milled/matched to the carb and manifold ports with a Siamesed aluminum pipe to match the size of each. Either that or a milled block. He hasn't decided yet. We have discussed the block route from the standpoint of his making some money on the side knocking these out on his CNC if this works. He figures the hot rod Subie guys and dune buggy crowd might like them

Bill