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L28 and DCOE sizing 40's or 45's?

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Hi Guys,


So i have started the work on the engine, and after much deliberation, i may have the budget to guy a new set of webers! :D


The problem is, what size do i go! As i have been researching, there seems to be staunch support on both side, and thought i would ask for some user opinions.


My car will rarely see a track, and will mostly be used as a casual cruiser. So leaning towards 40's.


But as i said, is this the way you guys recon i should be going?!


Thanks for your help guys! :)


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Agree with JM. If you read Passini's books on Weber Tuning you find that the ideal Main Venturi size is 80 to 85% of the main bore size. For all out track racing you can go to 90%, but at a big loss in drive-ability. 80% is what most street cars and Autocross cars should be aiming for and in fact what most " universal " DCOE's come equipped with . Same for Mikuni's and Dellorto's. This assumes a cylinder size of 350 to 400cc. It's all about Booster signal strength and Booster signal depends on the pressure drop created by the air speeding up to pass through the smaller venturi. The carbs are engineered around the 80 to 85% Venturi to Main bore sizing.


40 mm bore size x 80% = 32 mm


45 mm bore size x 80% = 36 mm



A lot of guys start with a 40 mm Bore carb. Find it's weak on the top end,  so they install larger Venturi's such as 36's and the carb turns into a sloppy mess on the bottom end. The reason is that they've gone out of the recommended ranges of Bore size to Venturi size. However, if you started out with a 45 DCOE equipped with 36mm Venturi's you are smack in the design range of the carb to bore size. You woul have better top end and lose minimal if any driveabilty over a 40 DCOE with 32 venturi's.


The important thing is the PERCENTAGE of pressure differential created by air passing through the Venturi in relation to main Bore size. That creates a strong and stable Booster signal.


On an L-24 I would consider nothing smaller than Triple 42's. L-26 and larger, 45 DCOE or 44 Mikuni's all the way.

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So, silly question, but does altitude come in to play at all for this part of the discussion? As in someone like me starting out at 6k ft elevation wanting to drive nearby mountain roads up to 9-10k ft elevation, would you still put a 45 on a L28 and it not be too much carb. This is a general question, I know there are many factors involved.

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Not a silly question at all. In fact a very good question that requires some hard thinking.


IMHO, Altitude should not make any difference in choosing carburetor BORE sizing. Remember it is the pressure differential between the bore sizes of the main venturi and the main bore that creates Booster signal and regulates the air/fuel curve. The total amount of air ( dependant on air density ) does not affect Booster flow curve all that much. As far as carburation is concerned, all that you may want to do is perhaps run the main jet a size smaller if the altitude change exceeds 5,000 feet. . But carburators are self compensating to an cetain extent. 4,000 feet in elevation change can easily be handled by a well tuned carb. Even a stock carburator. The air/fuel ratio will get a bit richer... but the actual air/fuel curve should remain the same.


Racers running Pikes Peak still run as big a carburetor as possible. You still have to get as much air to the engine as possible. It becomes even more critical as air density increases. " Choking" down a carb  or running a smaller carb is the last thing you want to do.


What they will do is build extra compression into the motor. It is not uncommon for Pikes Peak motors to be built with 15  to 1 or higher compression ratios. Pikes Peak start line is at mile & and is at 4,720 ft above Sea level. The top of the course finishes at 14,110 feet. Jack Roush built some hillclimb motors in the 1990's  that could not be run at Sea level because the CR was so high. They had to be started at elevations higher than 4,000 feet or thye would detonate themselves to pieces.


If you live in a high altitude area, such as Denver, consider building an engine with a higher CR. of course that means that you will have issues with detonation if you travel to Sea level . It's all a good excuse to build a nice Turbo engine....

Edited by Chickenman

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Remember, your "budget" will get beat up. You'll probably end up changing E-tubes, jets and chokes to tune it properly. 6 chokes...that's $150 bucks right there. If you get the right ones. Jets, you need to buy 6 each and may not be the ones that work best.

But, its worth it. Nothing better than triples. FWIW, I've got my 40s with 36mm chokes on my stroker and pulls hard to redline but wish I would have bought 45s

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