need suggestions on tripple carbs.

[Dtsnlvrs]

Basically All I am saying is the only advantage to running 45's over 40's is if you need very large venturi's, and you dont need very large venturi's unless you are making a ton of HP at a very high rpm...BTW a very high rpm for a nissan I-6 is 8k rpm and it decreases with an increase in stroke. Anyone who builds engines will tell you that HP is made in the head....I seriously doubt that his head is ported in a manner that would require the use of 45's....if it is I will stand corrected, but the amount of porting I am talking about would cost as much if not more than the engine itself. A large enough cam may need 45's, but that is negated because of the enormous stroke of the LD28 crank. Rebello does know engines, so does Sunbelt, so does Honsowetz, and so does Gene Crowe...and a little known company in England called Janspeed. (They used to do some really trick heads). Oh I better just get off this soap box before I get a headache. BTW the 40's and 45's cost the same.

[Phred]

FWIW

I just set up a new TWM manifold with 45 weber's. All it took to match the 45mm throttle bore to the manifold was a small radius blended into the runner. On the head side, the manifold was opened up very slightly to 36.5mm which is the size of my intake ports. the manifold is doweled to the head for proper alignment. The nice round holes and radius were done with a milling mach., and then the runners were smoothed up. The manifold in stock form had a 42.5mm bore. The manifold runner length is only 4 in. and has a nice smooth taper down to the port dia. To maintain the velocity, and the lower torque, keep the head port dia. as small as practical. This setup is for a .040 over F54 with a stroker crank.

Phred

[Phred]

This is the head side.

[2ManyZs]

Phred, so that people can decide which is better for their engine, do you have some kind of baseline which would help people choose whether the 40's or 45's would suit their engines?

Such as a minimum CR, valve size, head preference and cam specs?

I think we all know that 40's are probably too much for a bone stock engine, but is there a set of parameters that should be met before you can go from 40's to 45's?

[Dreco]

well, when I talked with my engine builder, the discussion was wether to get 45's or 48's!!:classic: From my understanding, the head porting that was done to mine was pretty extensive. Cost me about 1100 dollars. If I go with this set up, I'm also assuming I'll have to install a better fuel pump. I guess an inline electric feul pump would sufice, or should I just get rid of the mechanical on for a high flow electric one.

[Dreco]

If I get 45's, which ones to I look for ?? I saw a code of 45DCOE, but it also ended in 13,14,15. what does the last number mean??

[Dtsnlvrs]

Call Top End Performance, www.racetep.com they have complete weber kits, manifolds, linkage, fuel pumps, halmeters etc. BTW specify to them that the manifold and the mount kit need to be for 45's as well. They will help ya' out.....also BTW I do stand corrected......if you have that much head porting you could use the 45's....just be carefull not to overrev the motor, as it is gonna be a free spinner at the top end.

[Phred]

Atten. 2ManyZ's

There is a formula I have around here someplace for computing throttle bore size. It uses displacement, RPM, and Volumetric effieciency, ( an educated guess unless actual #'s can be measured on a dyno) The RPM used is the max used in your particular application. which is tough to figure for a street car. The formula will give you the right size for say, 7000 RPM. But what about drivability at lower RPM. So, you have to use the SWAG method and experience to size carbs to an engine. An engine such as I'm building or the one Dreco has will produce more HP on the top end with 45's or 48's, but if you were to drive the same engine on the street, you'd need 40's to 45's to have some driveability in the lower RPM ranges. Thats why the 40's are so popular. but for best HP with high volumetric #'s. you need the carbs that will flow enough to match the capability of a prepared head and cam. That means bigger carbs. Also, the bigger carbs require proper jetting to get most out of them. Thats the great thing about Weber's ( or Mikiuni/Solex). You can adjust idle, main, and air jets, and choke size (venturi) all independently. For a race engine, this is critical to get max HP without damage from a lean condition. This is where the dyno is indespensible. Using fuel flow #'s and exhaust gas temp., proper jetting can be made quickly, or you can spend all day the track and still not get it right. Lots of things to consider, Thats what makes it fun. And expensive.

Phred

[Dtsnlvrs]

These are the formula I have

Throttle bore = (sq root of (dicplacement per cyl in cc X max rpm / 1000)) X 0.82

Main Venturi = (sq root of (dicplacement per cyl in cc X max rpm / 1000)) X 0.65

Main Jet = main venturi X 0.04

Air corrector = Main jet + 0.20

These of course are "begining" numbers...final tuning and cam selection will refine the process

[Dreco]

So If I get 45's I guess the next thing I need to do is figure out which 45DCOE I need, looks like there are different kinds, depending on what jetting size your need.

[LanceM]

Looking in my Haynes tech book on Webbers the # after the DCOE is a series # but dosen't really seem to make any sence... They are lumped together and apart, and the only referance to the series # is to accelerator pump stroke and float level settings and they don't go in number order smaller to bigger, other than that there is no mention of series # for any other time in the manual.

For sure sounds like time to call an expert to make sure you order the right thing.