Regarding Dave's comment on difficulty tuning:  That's why I view the project as fun and interesting.  I agree that tuning the large venturi carb will be challenging and I view it as a good chance to learn a thing or two.  The S&S was specifically designed for Harley big twins ranging from 57 through 80 cubic inches.  That works out to about 28 to 40 cubic inches per cylinder.  The carb only feeds one cylinder at a time so I think I have a decent chance at adapting it to the LS40.  If its a flop, all I have lost is the time I spend since I already have the carb.  I ran the specific Super-E on my Softail for many years.  It performed flawlessly, and I really saw no difference in response and drivability between the S&S and the stock Kehin.  Of course the overall flow through the carb is higher in the twin cylinder application, but the velocity when it's feeding only one of the two cylinders should be fairly close, especially if the twin is an 883.  I think it can work.  If I can successfully get the Super-E workin on the Savage we will have another mixer to choose from.

Hey SpammyToo: Please tell me more about your brother's experience.  Where did he get a manifold from?  Do you know if he played with the air bleeds or did he confine his efforts to the main jet and intermediate jet?  When he gave-up on the S&S, what sort of Kehin carb did he then install?  Was it a Harley take-off like the one I tested?  Was he able to tune it using available Harley jets?

Thanks for the offer Lancer.  I sent you a PM.

Fast650:  IMO, the stroke on the LS is pretty reasonable for what I want to achieve.  With a 3.7" stroke, the average piston speed at 6500 rpm is 4008 fpm, well below the accepted standard of 4500 fpm for a street engine.  At 7000 rpm the average piston speed is 4316 fpm.  We ought to be able to make 1.25 HP/cubic inch at 7000 rpm or less.  I think 50 HP is a reasonable objective.  I agree that if I decided to challenge one of these modern single-cylinder engines I would need to increase the redline dramatically.  With the current cylinder head, bore & stroke I think it would be impossible.  But a 50 HP Savage cruiser would be killer fun.

The Harley crowd routinely make 100 reliable HP with 80 inch Evolution Big Twins.  That's 50 HP per cylinder with a 4.25" stroke.  If a pushrod, two-valve EVO can make 1.25 HP per cubic inch, our overhead cam four-valve metric motor can make 1.25 HP per cubic inch.  We can do this.

I agree that 50 HP should be doable. What I meant was that if you are looking to make 70+ HP, that is when you will run into  the problem of needing way more than safe rpm or some boost pressure.

I love this

Looks like we have some very good comments.

"Spammy": thanks for looking into your brother's experience with the S&S and Kehin.  I'm sure it will be a big help.

"Fast 650": I think we are on the same page.  A friend of mine who races big 4-strokes in the desert told me that these modern big-inchers run up as high as 13K.  The LS geometry will never support that.  Those engines have huge 38mm valves and need a huge bore to accommodate the big valves.  And your absolutely right about the stroke, 3.7" is way to long for a 10K+ redline.  Even if I could pull that off I don't think it would be much fun to ride a 650cc single on the street with the engine buzzing around at 8 or 9K all the time.

"Dave":  I hope I don't screw up my response here.  I am not looking at the flow through the carburetor as "continuous".  It starts and stops.  If a Big Twin with 40 cubic inches per cylinder has a 40mm carburetor mounted, that carburetor is only feeding a 40 cubic inch cylinder on each intake stroke.  Granted, there are two intake strokes every 720 degrees, but only one at a time.  The velocity through the carb venturi is determined by volume that is being filled and also the piston speed (which I agree is higher on a Big Twin due to the 4.25" stroke).  The flow starts, and then stops, and then starts again, and then stops again.  So yes, total continuous flow is twice as much on the Big Twin, but the flow through the venturi during a single intake stroke is close to the same on both the twin and the single.

Look at it from another angle.  The carbureted Harley Evolution Big Twins were delivered with 40mm Kehin carburetors.  There were several popular replacement carbs, S&S E, Mikuni HS40, QuickSilver, Revtech, etc..  Those were all like 38 to 40mm carbs.  Most metric big-inch singles come with 40mm carbs (or carbs billed as 40mm like the Savage).  All of those engines have two things in common, 40 cubic inch cylinders and 40mm carbs.  The Big Twin has two of the cylinders, but only a single carburetor to feed them.  That carburetor just happens to be 40mm.  If the flow is double, why didn't the Motor Factory have to install a 56mm carburetor (about twice the cross section of a 40mm)?

"Armen": Hey Buddy, you got me started on this.

"Lancer":  Your point regarding the LS vs DR is well taken.  On the surface, the engines look pretty close so why the big disparity in HP.  That's a secret I hope we can uncover.  I can see that the DR has a point higher compression ratio, and it has a larger bore and shorter stroke.  It may have a nice tight quench zone.   I don't know if they have the same size valves but I know they use the same valve springs so that's a pretty good indication that the valves are about the same weight.  Of course the nice straight exhaust port is a big bonus for the DR.  There certainly seems to be some sort of weird design decision that was made regarding the LS exhaust port.  If you think about it, manufacturing the head with that screwy dipsy-doodle exhaust port most likely costs a bit more.  What were they thinking?  I'm hoping I can come up with some simple modifications that will improve the flow significantly.  There's not a lot of options on the roof of the port.  That leaves the sides and the bottom of the port plus that ridiculous 1.3" outlet.  I can't wait to get started on that.  The saving grace is that we have about 120psi differential working for us on the exhaust, whereas there's only about 15psi differential across the intake.  Of course reducing the friction in the exhaust system will help the port flow better.  Its for sure a challenge.  Help!

Can you tell me anything specific regarding the 1995 DR650 valves?  Do you know exactly what size they are?  Is the DR650 carburetor a true 40mm or is it an oval venturi like the LS?  What is the ID of the DR header?  Does the DR have a tight quench or is it the grand canyon like the LS?

"Lancer":  Your point regarding the LS vs DR is well taken.  On the surface, the engines look pretty close so why the big disparity in HP.  That's a secret I hope we can uncover.  I can see that the DR has a point higher compression ratio, and it has a larger bore and shorter stroke.  It may have a nice tight quench zone.   I don't know if they have the same size valves but I know they use the same valve springs so that's a pretty good indication that the valves are about the same weight.  Of course the nice straight exhaust port is a big bonus for the DR.  There certainly seems to be some sort of weird design decision that was made regarding the LS exhaust port.  If you think about it, manufacturing the head with that screwy dipsy-doodle exhaust port most likely costs a bit more.  What were they thinking?  I'm hoping I can come up with some simple modifications that will improve the flow significantly.  There's not a lot of options on the roof of the port.  That leaves the sides and the bottom of the port plus that ridiculous 1.3" outlet.  I can't wait to get started on that.  The saving grace is that we have about 120psi differential working for us on the exhaust, whereas there's only about 15psi differential across the intake.  Of course reducing the friction in the exhaust system will help the port flow better.  Its for sure a challenge.  Help!

Can you tell me anything specific regarding the 1995 DR650 valves?  Do you know exactly what size they are?  Is the DR650 carburetor a true 40mm or is it an oval venturi like the LS?  What is the ID of the DR header?  Does the DR have a tight quench or is it the grand canyon like the LS?

The valve size in the DR 90-95' is the same as the LS, and the valve springs in both are the same as well as the rockers.
The DR carb is similar to the LS, it may have a bit larger effective bore and may have an accelerator pump.  It has been awhile since I played with one.
Stock bore on the DR is 95mm and as noted the CR is 9.5:1 with a flat top piston just like ours.

     

The SS carb was designed for Harley's ,  but not the S-40. no one has considered the main difference between the two, namely the stroke. The HD being a longer stroke /smaller bore will most certainly pull a higher carb vacuum,  and inertia of fuel mix entering the cylinder  due to its higher piston to cylinder wall,  speed.
         It's been said that the HD is feeding only one cylinder at a time ,but this  rings false. the two cylinders of the HD are only 45 degrees apart (as is the firing order) which means the intake strokes (and intake valve openings ) overlap, which may account for the need to run a larger carb.
       I'm not saying the SS carb won't work ,but I think it may be a bear to tune .

My attempt will be a 36mm Dellorto PHF carb I have laying around. The internal choke is jammed but may not be needed. I have different slides, and adjustments of the accelerator pump may be enough to make that a non issue. It will be all seat of the pants and gas consumption to see if this works out.

DBM : I have a Mikuni manifold that will fit. The throttle cable comes through the top of the carb so there may be a problem with tank clearance . We'll see. I don't want to have to right angle the carh through the side of the frame. Too much weight on the manifold,  and I don't like the look.