TheSamba.com :: View topic - maximum displacement for dellorto 40 DRLA ?

Mon Mar 02, 2015 9:41 am

Hey guys, a few questions that I'm having trouble figuring out myself. Any help if you are able?, thanks!
I am planning a bus engine around some parts that I already have. They are:
ported and polished 040 heads
case cut for 92, stroker clearanced
Dellorto 40 DRLA carbs
I plan to use some thick wall 92 cylinders, and a stroker crank, with full weight flywheel.
Not decided on the cam yet, the few I am considering are low rpm torque cams, will run around 9:1 compression
RPM will be below 5k

My gut feeling is that the 40 dells will be fine for a 2180, and that I should go with an 82 stroke crankshaft. Maybe they'll be fine for a bus engine turning lower rpm, but at what point are they too small? But how do I calculate the relationship between what a carb will flow, and what the displacement needs at X rpm? I spent some time searching, but the answer still isn't clear..

Mon Mar 02, 2015 10:44 am

I think it will be fine also forthe 2180. The variables is the valve size and cam selection. 40 valves light p/p and a mild cam is np. I have heard of folks using dell 40s on 2332s.

Mon Mar 02, 2015 10:45 am

What valve sizes?

For carb sizing I use a formula that I found in a Solex book from the 60's, Selection and Tuning of the Carburetor (I think, don't have it in front of me right now). Take the square root of the displacement (per cylinder) x rpm, and multiply that by .82. So for a 2180:
2180/4= 545cc (per cylinder)
545•5 (5 for 5,000rpm) = 2725
Square root of 2725 = 52.2
52.2•.82 = 42.8

By their theory, you go to the next smallest size carburetor available. So if a 42.8 butterfly is ideal, go to a 40mm (or a 42 DCNF!)

It's not always right on, and many people have proven smaller and larger carbs work well, but it will get you in the ballpark.

Mon Mar 02, 2015 10:54 am

Weber made a chart in one of thier books for vent size to peak hp rpm and single cyl displacement.

For the rpm and displacement you are going to use a 34mm vent will be fine.

If you ran a bigger cam and heads it would be too small. To undercarb an engine all you will lose is top end.

Dan

Mon Mar 02, 2015 11:35 am

I run 40's on my 2275 w/FK65 cam. 236@ .050
34 vents, 55 idles, 140 mains, 200 air

The cam supports power to about 5,000 rpm.
I have plenty of carb for the cam/engine size.
Throttle response is rediculous (good).

I built the engine with a goal of being a mtn climber.
In my 63' ragtop I can take two people from the valley
floor to Shaver Lake at 5,200 foot elevation in 3rd and
4th gear. Never got below 3,000 rpm, even on some
12% grade sections. Very few aircooled cars are seen
climbing this grade and for good reason.

Mon Mar 02, 2015 11:38 am

If the cylinder heads have stock size valve they would reach their limit a good deal earlier than the carbs will. 40 Dells will support 170 hp on a good day. 160 is "easy"
T

Mon Mar 02, 2015 11:40 am

Equally important is to have the carbs match the heads. Of course one can't flow more than the other! The larger the engine the lower the rpms will be it's happy place.

IMO 40 DRLA are perfect match for some good 40x35.5 heads like Tims stage 1. Can support 140hp possibly up to 160hp.

Mon Mar 02, 2015 1:01 pm

Thanks all! This is what I was hoping for..
The heads are CB 044 40x35 with a mild port and polish
I was going to use a smaller crank, but then happened on these carbs and well you know what happens when you start thinking. More is better right?
82 it is

Mon Mar 02, 2015 1:33 pm

yes they will work fine on a 2276.even on a 2387.when your out for max power&max rpm then you need biger crabs.

Mon Mar 02, 2015 1:40 pm

mark tucker wrote: yes they will work fine on a 2276.even on a 2387.when your out for max power&max rpm then you need biger crabs.

I agree with Mark, if you want a mild performance and medium RPM's then the work good. I ran a pair of 40 DRLA's on my 2332 while I wated for my new carbs and was surprised how well it ran up to 5K :wink:

Mon Mar 02, 2015 6:41 pm

Should be asking how much power will they support, not what displacement.