Engine size with Dellorto DRLA 40?

[el mecco]

Not long ago I bought myself a beetle with dual Dellorto DRLA 40. I wonder what displacement this carburetors can be used on. Any thoughts? Can it bee used on bigger displacement than 1915 for use in a street car?

The case is already equipped with 94mm cylinders and pistons.

Frode

[viNce]

I used a pair on my old 2074 bus engine (FK43 Steve Tims Stage 1 heads) and they worked great with 36 vents.

[MC Dyno Don]

Answer: Any size engine combo will accommodate these carbs/the carbs will provide a smooth bottom end good gas mileage/recommended for smaller cam, heads, and exhaust for best results....  good luck..!

[RIP356]

I ran a pair in a split van, 2007cc with Fred Simpson heads, put 100 hp at the wheels
and was still making power at 100mph
Dave

[Bruce]

I use 40 DRLAs with the stock 34mm vents on my 2 liter long distance road trip engine.  Although 45s would likely make more top end hp, I wanted bottom end driveability.  With the small carbs it is almost like a modern FI car.

[Zach Gomulka]

If the carbs are too small for the displacement will the engine be difficult to tune?

[modnrod]

Small carbs can sometimes mask velocity issues, not-very-precise part-throttle A/F and increase atomisation, so most times small carbs are easier to tune.

For a streeter, good throttle response is much more preferable to that last ounce of top end HP I think.

[hotrodsurplus]

I wonder what displacement this carburetors can be used on.

The answer lies in the venturi size and engine tune. They will run great on a 2.7 provided you run very large venturis and short cam duration (slow engine speed). As modnrod noted, carburetors slightly too small for the application are more favorable than carburetors slightly too large.

Here's another more complicated way of looking at it. Carburetors slightly too small for the application will sacrifice a little bit of high-speed performance but will preserve performance at normal engine speeds where you drive 99 percent of the time. Carburetors too large for the application may increase high-speed performance but will sacrifice performance at the normal engine speeds where you drive everyday. Velocity, not gross flow, is the key to a car with good manners and useful performance.

Others may be able to pinpoint good displacement/cam/head combinations if you list the venturi diameter.

[el mecco]

Thank you for giving me good answers! Even if this will make my planning harder than I was expecting.  Now I have more possibilities than I believed.

Frode

[Roger D]

Bugger, this is the first topic I've read on this site [new boy] and I've just brought twin 40's to go on my standard 1600. Been told I can choke them down. Hope it will run o.k. as it's my daily driver.

[Bruce]

Roger, better would be to try to find some 36 DRLAs.  Many will say 40s are fine for a stock 1600, but IMO they're too big.

[hotrodsurplus]

Roger, better would be to try to find some 36 DRLAs.  Many will say 40s are fine for a stock 1600, but IMO they're too big.

I agree that 36s would be better for a 1600; however, for the most part the body size is irrelevant so long as he uses a 28mm venturi. After all, a Kadud is a 40mm bore with a 28mm venturi and that's considered an ideal 1600 carburetor.

Now the issue is whether the 40s have 28mm venturis. Most Dells came with 30s or I think 32s and 36s came with 28s. In that sense the 36s would make better carburetors than the 40s; however, there's a chance someone swapped 28mm venturis in there. If the 40s have the small venturis then they'd do the trick just fine.

If the 40s have 30 or 32 vents then they'd definitely be excessive for a stock-cammed/stock-head 1600. Unfortunately 28mm venturis are not inexpensive. Roger, if you need to buy smaller venturis you might be better off doing as Bruce suggested and getting a pair of 36s and selling the 40s. Then if you step up to a larger engine one day you can find a buyer for the 36s.

Not to take this too far off of the topic but a 1600 with twin carburetors and a header wakes up even more if you flycut that quench-killing step from the combustion chamber. You'll have to shorten the pushrods but that's no biggie (get the Bill Fisher book to learn now). That engine will snap hard and still run 87-octane fuel if you tune it properly.

[Zach Gomulka]

Would the engine behave any differently with 36's or 40's if the venturi was the same?

[hotrodsurplus]

Would the engine behave any differently with 36's or 40's if the venturi was the same?

It shouldn't and in my experience, it doesn't. But for disclosure's sake I've never run at the extreme ends of the spectrum.

Engineers optimize the passages within a carburetor to run best within a venturi range. Exceeding that range may make the carburetor meter fuel improperly. For example, you can run 28mm venturis in 48mm bodies; however, it may not behave properly.

The same goes for running venturis that are near the butterfly diameter. A venturi is nothing more than a pressure-differential generator. Necking down the air passage increases the velocity through the venturi and the air pressure reduces to below atmospheric pressure below the smallest diameter of the venturi. Pressure exerted on the fuel in the float bowl pushes the fuel into that negative-pressure area just below the smallest diameter of the venturi. So the venturi has to be sufficiently smaller than the throat diameter if it's to increase velocity/reduce pressure on the air passing through it.

[AntLockyer]

So the venturi has to be sufficiently smaller than the throat diameter if it's to increase velocity/reduce pressure on the air passing through it.

I've got a pair of 40s that were supposedly bored out to fit larger vents. Those on a small valved 1914 were amazing for driving everywhere.

[Zach Gomulka]

Would the engine behave any differently with 36's or 40's if the venturi was the same?

It shouldn't and in my experience, it doesn't. But for disclosure's sake I've never run at the extreme ends of the spectrum.

Engineers optimize the passages within a carburetor to run best within a venturi range. Exceeding that range may make the carburetor meter fuel improperly. For example, you can run 28mm venturis in 48mm bodies; however, it may not behave properly.

The same goes for running venturis that are near the butterfly diameter. A venturi is nothing more than a pressure-differential generator. Necking down the air passage increases the velocity through the venturi and the air pressure reduces to below atmospheric pressure below the smallest diameter of the venturi. Pressure exerted on the fuel in the float bowl pushes the fuel into that negative-pressure area just below the smallest diameter of the venturi. So the venturi has to be sufficiently smaller than the throat diameter if it's to increase velocity/reduce pressure on the air passing through it.

According to Solex back in the 60's, the venturi should be 80% of the throttle plate. See my thread in the Pure Racing section.

[Sepi]

Would the engine behave any differently with 36's or 40's if the venturi was the same?

It shouldn't and in my experience, it doesn't. But for disclosure's sake I've never run at the extreme ends of the spectrum.

Engineers optimize the passages within a carburetor to run best within a venturi range. Exceeding that range may make the carburetor meter fuel improperly. For example, you can run 28mm venturis in 48mm bodies; however, it may not behave properly.

The same goes for running venturis that are near the butterfly diameter. A venturi is nothing more than a pressure-differential generator. Necking down the air passage increases the velocity through the venturi and the air pressure reduces to below atmospheric pressure below the smallest diameter of the venturi. Pressure exerted on the fuel in the float bowl pushes the fuel into that negative-pressure area just below the smallest diameter of the venturi. So the venturi has to be sufficiently smaller than the throat diameter if it's to increase velocity/reduce pressure on the air passing through it.

According to Solex back in the 60's, the venturi should be 80% of the throttle plate. See my thread in the Pure Racing section.

So my new 2332 engine with DRD L7 heads 37,5 + 42 mm valves with 45 DRLA with 36 venturies would be quite ideal...?

[hotrodsurplus]

So my new 2332 engine with DRD L7 heads 37,5 + 42 mm valves with 45 DRLA with 36 venturies would be quite ideal...?

we're getting a bit off topic but on paper the diameter/throttle-bore ratio is within the ideal range. Another set of parameters would determine if your engine needs a 36mm venturi but that's for someone with more experience to answer. To answer that question you would have to know the displacement and the port-flow capacity at various valve-lift figures (those figures would determine the cam timing as well). 

[Jim Ratto]

if the motor is STOCK stock, meaning 4-dowelled crank, hair-clip rockers, stock valve springs, no full flow, no sump etc... then I doubt you want to twist more than 5K much at all. Don't set your carb tune up for best preformance up @ 5K+ or you'll be compromising (the way it runs) where you drive the car 90% of the time and if you wind it out to justify the big vents, you'll beat the motor up and maybe fling a rocker off.
I'd say 30mm max for vent size. When I bought my 36DRLAs new in 1988 they had 30's in them. I ran them on an 87 x 69 with bolt together shafts, swivel feet, HD springs/retainers/keepers, Engle VZ25 and 69mm Rimco CW crank. I should have run a 1.5qt sump too as I had the warning light on often.
A "reinforced" 1600 or 1641 with good valvetrain, dual carbs and extractor makes for a really really fun driver. maybe a set of 1.25 rockers with adjuster scew @ pushrod end, good HD springs/retainers, ground keepers, nice pushrods, chop flywheel down and do a good extensive tune on it. Next level would be to CW the crank and 8 dowel it and then port heads.
have fun

Jim R.

[hotrodsurplus]

I'd say 30mm max for vent size.

I agree though even 30mm might be a bit too ambitious. History certainly bears it out. Porsche used 24mm venturis on its 1600 Normal engines. Those make 60 DIN/70 SAE horsepower. I don't think you can wring much more than 70hp from an otherwise-stock dual-port VW. It would probably take at least rockers, some compression, and definitely a good exhaust system if not some port cleanup to make much more than 70.

The 28mm venturis let the Porsche 1600 Super engines make 70 DIN/85 SAE horsepower. A dual-port 1600 can match that but with more aggressive cam timing and head work which would also require stiffer valve springs, stronger pushrods, and a counterweighted crank. The 30mm venturis would likely put a more aggressively built engine in the upper 90 SAE hp range. In fact Porsche used 32mm venturis in 40mm Solex PII carburetors to make the Super 90 (90 DIN/~105 SAE horsepower).

An engine cannot produce the potential horsepower that a too-large venturi offers and a too-large venturi cannot produce the velocity that a lesser-horsepower engine requires for drive quality at slower speeds. A slightly undersized venturi will sacrifice top-end speed but at least it will build lots of happy velocity everywhere else. A too-large venturi on the other hand really is the worst of both worlds.

I'm not saying you need 24mm venturis for a stock engine (they're seldom available anyway). 26s would likely be the cock for dolly but are rare. The 28s are probably a bit too big for most 1600s which makes the 30s a bit excessive. At least in my flawed logic anyway. They'd work for sure but probably at a cost at other engine speeds. Again, we're talking stock 1600s.

[Jim Ratto]

I'd say 30mm max for vent size.

I agree though even 30mm might be a bit too ambitious. History certainly bears it out. Porsche used 24mm venturis on its 1600 Normal engines. Those make 60 DIN/70 SAE horsepower. I don't think you can wring much more than 70hp from an otherwise-stock dual-port VW. It would probably take at least rockers, some compression, and definitely a good exhaust system if not some port cleanup to make much more than 70.

The 28mm venturis let the Porsche 1600 Super engines make 70 DIN/85 SAE horsepower. A dual-port 1600 can match that but with more aggressive cam timing and head work which would also require stiffer valve springs, stronger pushrods, and a counterweighted crank. The 30mm venturis would likely put a more aggressively built engine in the upper 90 SAE hp range. In fact Porsche used 32mm venturis in 40mm Solex PII carburetors to make the Super 90 (90 DIN/~105 SAE horsepower).

An engine cannot produce the potential horsepower that a too-large venturi offers and a too-large venturi cannot produce the velocity that a lesser-horsepower engine requires for drive quality at slower speeds. A slightly undersized venturi will sacrifice top-end speed but at least it will build lots of happy velocity everywhere else. A too-large venturi on the other hand really is the worst of both worlds.

I'm not saying you need 24mm venturis for a stock engine (they're seldom available anyway). 26s would likely be the cock for dolly but are rare. The 28s are probably a bit too big for most 1600s which makes the 30s a bit excessive. At least in my flawed logic anyway. They'd work for sure but probably at a cost at other engine speeds. Again, we're talking stock 1600s.

makes you wonder what kind of rpm the little Type 692 Carrera "GT" 4 cam 1600 "came on" at with their 38mm chokes in 40 DCM Webers!!

[hotrodsurplus]

makes you wonder what kind of rpm the little Type 692 Carrera "GT" 4 cam 1600 "came on" at with their 38mm chokes in 40 DCM Webers!!

From www.356carrera.net/history_official.asp?cat=11

"factory men often went to 8.000rpm in the gears posting 0-60 times at 10.8seconds."

Wow. I guess that gets close to the answer.

Didn't they know that the venturi/throttle bore violated the .8 ratio? No wonder those engines just blew up. The generous Vasek Polak did all of those Carrera owners a huge favor by giving them Super 90 engines in exchange for their 'troublesome' Fuhrmans. What a saint he was to relieve them of such 'junk'. 

[Jim Ratto]

sorry to steer this thread way into foul ball territory. Seems Porsche used some "skewed" calculations to come up with some pretty wild venturi sizes, yes like the 40DCM's. A friend of mine was working on a pair of these Carrera carbs and I asked him "Are the venturies really 38mm?" and he said yes, they were basically profiled "sleeves" down the bores. The 2-liter 587 motor (in 904GTS and some of the hot hot hot 356's) ran a 40mm choke in 46IDM's. And the 906 sports-racer (the prototypes ran Bosch plunger injection) ran a 2-liter flat 6 (so 332ccm per cylinder! the equivalent Vw motor would be 1328cc!) ran 42mm vents!!
A glance at the jetting for these cars makes you scratch your head too... though I've learned- it doesn't work in a VW. 

[modnrod]

A glance at the jetting for these cars makes you scratch your head too... though I've learned- it doesn't work in a VW. 

It might if you held the little motor above 8000rpm down Mulsanne for a few minutes.........

[Brian Rogers]

WOOWHO in my pair of 40 Dells is a set of 28's and 34s. Now comes the question of jetting the 28's for a stockish 1600. what is recomened?

[hotrodsurplus]

WOOWHO in my pair of 40 Dells is a set of 28's and 34s. Now comes the question of jetting the 28's for a stockish 1600. what is recomened?

Wait, you mean that you have a set of four 28s and a set of four 34s right? Because if one carburetor has 28s and the other carburetor has 34s then you got problems.

a ton of things influence jetting but venturi size factors hugely. My 36s have 28mm venturis so the jetting should be pretty similar to yours:

28mm    venturi
9164.2    emulsion tube
122       main fuel jet
180       main air correction
55       idle

[Brian Rogers]

Yup 2 sets currently with 34s. No extra jets. Gotta get 'em for the 28s

[hotrodsurplus]

Yup 2 sets currently with 34s. No extra jets. Gotta get 'em for the 28s

Call John at Air Cooled. He has jets and will work with you on tuning.

[Brian Rogers]

How big can you go, engine size wise, with the 40 Dells? I think cam and valve and port size you would want to be conservative.

[Jim Ratto]

How big can you go, engine size wise, with the 40 Dells? I think cam and valve and port size you would want to be conservative.

You can build any size engine. But there's a relationship between the rpm you want it to twist and how much air it needs to ingest, restricted by intake cross section, inlcuding carburetor size. HP is a product of RPM, but for a good all around street motor, I think throttle response is more important than a number made on a dyno pull. You can make a very highly tuned motor that will widen everybody's eyes with big HP figure, but you might hate driving it. Even if you tuned all the burps and farts out of high hp/liter motor, you might grow tired of having to wind the thing to the moon to get it to clean up and drive off.

You can kind of err the valve timing around to "fool' the engine into thinking it has bigger carbs than it does, but doing that usually makes for a really fussy and bitchy motor. Think in terms of balance. Some factors of the motor kind of define what the other ingredients need to be. Displacement is always a factor that will help define what the rest of the motor needs to be/have. Big CC means you have big lungs that need to be filled- so to work with big cc- cam needs to be more overlap, intake runners need to flow more volume, same with carbs..again if you're looking for RPM and HP.
But a guy can run a 94 x 78 with 40mm carbs/34mm chokes and around 252 deg @ .050 with 40mm intake valve and make 150-155hp @ a reasonable 5000rpm.
Hope this helps