Lotus Elan

Got Any Carburetion Books?

PostPost by: type26owner » Fri Apr 16, 2004 2:59 pm

I'm in need of help to find any technical information on two specific areas of carburetion. First is an empiral method to determine the optimal size of venturis. Two is to try and confirm or not the validity of the emulsion viscosity theory I've come up with. Someone knowledgable must have discussed these subjects in a book or article probably back in fifties or sixties. Could you look through your personal librarys and just post titles, authors and dates for me please so I can track them down. Even better yet spend a few minutes and scan the index and look for any chapters which might apply so I don't end up chasing my tail. My spare time is very valuable to me. I'm coming up with very few hits using web search engines. I already have ALL the Weber related books. Here's one I just finished which my father had in his library but it didn't help me that much.
'Scientific design of exhaust and intake systems' Philip H. Smith 1968

TIA,
Keith
type26owner
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 1616
Joined: 18 Sep 2003

PostPost by: steveww » Fri Apr 16, 2004 3:20 pm

I have been following your quest through your various posts. Still looking for the Holy Grail ;)

As you mention books I have just read a very interesting book all about ignition systems. I had not really appreciated what was going on in a dizzy before. People spend a lot of time and effort messing with carbs, manifolds, air filters, flowing heads etc then just go with the standard dizzy and timing. The engine should be treated as a whole (that really should be "hole" - where all the money goes :D ) You should have a look at the ignition settings as well as you continue your quest to the perfect running twinc.

BTW: I have checked on the SU HIF44 will fit in place of the Strombergs. The SU is only 5mm (3/16") taller than the Stromberg. Just looking in to linkages now.
User avatar
steveww
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 1838
Joined: 18 Sep 2003
Location: Northamptonshire, England

PostPost by: type26owner » Fri Apr 16, 2004 7:27 pm

Steve,
I have been following your quest through your various posts. Still looking for the Holy Grail

It's only 'voodoo' or 'holy grail' stuff because none of the books written on the subject actually explain all the info needed to actually tune a carbie. I've got to conclude the only reason they would not do so is because they didn't know it either! So the lesson here is just because someone has written a book does not guarantee they are knowledgable or correct.

This is just another engineering problem to first comprehend and then possibly solve in a way to make it user friendly. Piece of cake! ;) Please help me with the literature search. I don't want to have to duplicate work that's already been done.
Regards,
Keith
type26owner
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 1616
Joined: 18 Sep 2003

PostPost by: type26owner » Sat Apr 17, 2004 3:08 pm

Geez, realized since I'm being thrifty with my spending I actually need a mathematical way to derive the best size of venturi first. Minimal input from the Weber Manual still makes it a crapshoot. Then apply any empircal testing method. No wonder a well done tuneup on a six carbie engine can cost $5k+ by the old guessing method. :rolleyes:
Keith
type26owner
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 1616
Joined: 18 Sep 2003

PostPost by: rgh0 » Sun Apr 18, 2004 5:00 am

Keith

I have a Borg Warner Australia publication from 1976 which you may not have, titled "A guide to tuning Weber carburettors" which has a series of tables in it for selection of choke sizes.

The relevant section for a 4 cylinder sports car with 400cc per cylinder and a carb barrel per cylinder is 2x 40DCOE with chokes as follows

standard - 30mm
high performance - 33 mm
competition - 36mm

These seem pretty consistent with normal Elan practice over lots of years.


Rohan
In God I trust.... All others please bring data
User avatar
rgh0
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 7162
Joined: 22 Sep 2003
Location: Melbourne, Australia

PostPost by: gjz30075 » Sun Apr 18, 2004 7:00 pm

Keith,
I'm sure you've seen the John Passini books on Webers entitled "Theory" and "Tuning and Maint." It seems to have more 'depth' than the others but, again, lots of empirical data.

Greg Z.
Greg Z
45/0243K Sprint
36/5727 pre airflow coupe
User avatar
gjz30075
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 2828
Joined: 12 Sep 2003
Location: Roswell, Georgia, USA

PostPost by: steveww » Sun Apr 18, 2004 8:40 pm

Not related to your Weber mission but.... I was just reading an article about A series tuning and the guy fitted an 8 port head then used Motorbike carbs, 4 of them one for each cyclinder. The carbs he used were Keihin (sp?) so I had a quick Google and found out they do them in up to 40mm choke size. They are also Constant Vacuum type carbs, like Stromberg / SU only they have acceleration pumps.

So throw those old Weber in the trash and bolt on 4 bike carbs ;)
User avatar
steveww
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 1838
Joined: 18 Sep 2003
Location: Northamptonshire, England

PostPost by: tdafforn » Mon Apr 19, 2004 9:02 am

A guy I new in a past life did just this sort of conversion on a Triumph Spitfire..
Still ended up with lots of fiddling to remove "flatspots" and the like..
Looked really cool though
Tim
1972 +2S130
User avatar
tdafforn
Fourth Gear
Fourth Gear
 
Posts: 867
Joined: 12 Sep 2003
Location: Kenilworth Warwks.

PostPost by: marcfuller » Tue Apr 20, 2004 12:00 am

Don't trash them - the Weber DCOE is respected trick for the 80ci Harley V-Twins. Here's a link to a conversion option -
<a href='http://www.sturgisswapmeet.com/options.cfm?prodname=0&prodid=4159&catid=4&subcatid=383' target='_blank'>Dual Throat Forward Facing DCOE </a>

Bike guys want car carbs, car guys want bike carbs ... and the grass seems greener...
-Marc '66 Elan DHC (36/6025)
http://www.lotuselan.us
marcfuller
Third Gear
Third Gear
 
Posts: 254
Joined: 14 Sep 2003
Location: Monument, Colorado

PostPost by: steveww » Tue Apr 20, 2004 8:50 am

I think no matter which carb you use there is always an ammount of magic involved in getting them set up properly. Due to the very dynamic nature of the way engines run and their fuel and ignition requirements almost having an infinite range. Satisfying those needs with a pure mechanical system, carb jets and sprung weights, you are always going to end up with compromises which give rise to hesitation, flat spots etc. This is of course why modern cars all use a computer to control ignition and fuel, even so there are still compromises. I replaced the original Bosch system on my Porsche with a Motec system and got a significant power increase.

The fact is the Lotus is now 30+ years old and no ammount of ignition and fuel tuning using the original systems is going to make it run like a modern car. Personally I like the characteristics of old cars, the fact that you have to Drive them and not have a computer do it all for you. I have taken the new Porsche 996 Turbo round the race track. Yes it is very quick but ultimately it is rather boring to drive because you don't have to drive it, the computer does. I have so much more fun when taking the old Elan round the track because I have to really drive it, OK my lap times are a lot slow but my smile is much wider.

If you want an engine that will take WOT at 1500 rpm got for mapped fuel injection and loose the character. Keep carbs and a dizzy and enjoy its character.

Putting my soap box away now :D
User avatar
steveww
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 1838
Joined: 18 Sep 2003
Location: Northamptonshire, England

PostPost by: type26owner » Tue Apr 20, 2004 1:33 pm

Hey Steve,
I respect your opinions but I'm not drinking the Kool-Aid...... please stop trying.
Regards,
Keith
type26owner
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 1616
Joined: 18 Sep 2003

PostPost by: steveww » Tue Apr 20, 2004 2:51 pm

I dont get the Kool Aid comment :huh:

I hope you do not think that I was having a go at you :unsure:

Your desire to find a better way to do things and not take the regular path sounds very much like myself. I am sure you have gained much knowledge on the set up of Webers that these days very few people have with the dominance of mapped fuel injection. Perhaps one day when you have it licked you should write your own book or in the spirit of like minded enthusiasts publish it for free on this site.
User avatar
steveww
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 1838
Joined: 18 Sep 2003
Location: Northamptonshire, England

PostPost by: TomR » Wed Apr 21, 2004 7:57 pm

To my mind, there is one real compromise involved here. The pressure drop, related to the square of the flow through the venturi, costs engine performance, so a big (or no) venturi is best. Conversely, that same pressure drop is needed to draw fuel from the float bowl, and the flow of fuel is related to the square root of the pressure drop imposed by the venturi. If you don't have sufficient pressure drop, the fuel flow is insensitive to variations in air flow and tuning is difficult.

For WOT tuning, a big venturi will give adequate pressure drop to give good fuel control, but that same venturi will not give enough pressure drop to allow a stable operation at part throttle and vice versa. The mathematical formulations for determining the venturi pressure drop and fuel or air jet flows are in any undergraduate fluid mechanics text. Unfortunately, similar formulations for pressure drop vs. flow for air-fuel mixtures in the emulsion tubes require more a specialized graduate two-phase flow text. This information was severely undeveloped before 1965 or so, so you won't find much in old engine texts. Finally, vaporization of the fuel in the manifold is affected by venturi pressure drop also at WOT and this may affect your compromise, especially if fuel economy is important.

Your mission, should you decide to accept it, is to figure out what is the best compromise for your application. The books never tell you that.

Tom Radcliff
TomR
Second Gear
Second Gear
 
Posts: 102
Joined: 19 Sep 2003
Location: Vernon, CT US

PostPost by: type26owner » Thu Apr 22, 2004 1:48 pm

Carburetor Performance: How to Tune & Modify (Motorbooks Powertech Series)
by Forbes Aird, Malcolm Elston (Contributor)

Is anyone familiar with this book? Does it discuss the emulsion viscosity effect in any detail? It's quite pricey from Amazon at $36 in good used condition so I'm leery whether it's really worth the cost.

Keith
p.s. Going to tackle my son's Holley next on his RoadRunner. Same issues!
type26owner
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 1616
Joined: 18 Sep 2003

Total Online:

Users browsing this forum: No registered users and 8 guests