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SuzukiSavage.com
/cgi-bin/YaBB.pl General Category >> Rubber Side Down! >> Carburetor Comparison /cgi-bin/YaBB.pl?num=1552860080 Message started by DragBikeMike on 03/17/19 at 15:01:15 |
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Title: Carburetor Comparison Post by DragBikeMike on 03/17/19 at 15:01:15 Power potential is limited by air flow. It’s easy to get more fuel into the engine, but a lot more challenging to get more air in. On a naturally aspirated engine, there is only about 14.7 psi available to force air through the intake tract and into the cylinder. The air must work its way through the air-box and filter assembly, the carburetor, the intake port and the intake valves. Each one of these components takes its toll on the flow. I wanted to test the entire intake tract using three different carburetors to get a feel for the power potential of each carburetor. The contenders are the stock Mikuni constant velocity carburetor, a 40mm Kehin constant velocity carburetor (ala Big Twin Harley), and an S&S Super E carburetor (same pedigree). The Mikuni is billed as a 40mm carb, but if you examine the carb closely you will note that the bore in way of the throttle plate is 40mm, but the venturi is actually oval (40mm x 27.4mm). That works out to a cross-sectional area of 984.6mm squared, which is only equivalent to the cross-sectional area of a 35.4mm circle. So, the stock carburetor is really a 35mm carb, not a 40mm. It would be nice if one of you guys/gals could double check my calculation and confirm its accuracy. Is a 40mm x 27.4mm oval cross section equivalent to a 35.4mm circular cross section? |
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Title: Re: Carburetor Comparison Post by DragBikeMike on 03/17/19 at 15:02:22 The Kehin and the S&S are true 40mm carburetors. The Kehin is sophisticated while the S&S is a model of simplicity. |
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Title: Re: Carburetor Comparison Post by DragBikeMike on 03/17/19 at 15:03:33 In the interest of full disclosure: I intend to install the S&S on my bike. First, I like the idea of having a bike that’s different, and it will be cool to have the only Savage with a big S&S mixer. Second, it flows well. Third, it was never meant to be installed on a 40 cubic inch single so it will be fun and challenging to get it to work correctly. I will do my best to remain impartial on this comparison. |
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Title: Re: Carburetor Comparison Post by DragBikeMike on 03/17/19 at 15:04:43 Back in January I posted some flow numbers for our stock cylinder head, outlined some mods to my budget flow bench, and reported that the little bench couldn’t pull 10” H2O over the entire range of intake valve lifts. At that time, my testing was limited by the flow bench capacity, so I was doing declining pressure drop tests and converting the data to 10” H2O using the tables provided by DTec. Since January, I have beefed up the little bench so I can pull a full 15” H2O test pressure on both the intake and exhaust ports. The bench is working very well. The added capacity was the result of adding two more vacuum cleaners, so I now have the benefit of three vacuums worth of suction. My wife and family think I’m insane, and I’m a real hit with the neighbors too. |
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Title: Re: Carburetor Comparison Post by DragBikeMike on 03/17/19 at 15:06:14 I did tests on the three carbs with no airbox attached, and also tested them with an airbox configured for maximum flow. Let’s face it, not much point in installing a high-performance carb and then smothering it with a low-performance airbox. The high-performance airbox configuration was the same as that outlined on my previous airbox flow tests (K&N filter with the airbox plastic cover removed, decorative tin in place). So, a maximum flow run looked like this. |
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Title: Re: Carburetor Comparison Post by DragBikeMike on 03/17/19 at 15:07:13 But a real-world flow test of the entire intake tract looked like this. I want to give a BIG shout-out to “Playhard67” for donating this airbox assembly to the flow test project. The tests would not be possible without the mule airbox as I am unwilling to rip that sucker out of my fully operational scooter. A BIG Mahalo to “Playhard67”. |
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Title: Re: Carburetor Comparison Post by DragBikeMike on 03/17/19 at 15:07:50 As I mentioned in my earlier post, there is a formula floating around that predicts horsepower based on intake flow measured at 10” H20, so I tested each configuration at 10” (actual, no conversion). I wanted to try and determine if that formula is anywhere close to accurate. The formula is: .43 (CFM)=HP. The CFM value is measured @ 10” H2O. I have now found this formula in two high performance publications and “Fast650” has also come up with the same formula. The formula assumes that all other aspects of the engine are optimized. The compression ratio, exhaust system, quench, ignition timing and advance curve, cam lift and timing, combustion chamber geometry, etc. The formula must also assume fuel octane rating is appropriate for the BMEP, so 100+ octane gasoline would become part of the mix. If any of you are familiar with this formula, it would be great if you could share your experience regarding its accuracy. |
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Title: Re: Carburetor Comparison Post by DragBikeMike on 03/17/19 at 15:10:36 So, let the games begin. I sucked the guts out of all three intake tracts. As mentioned earlier, all carbs were tested on the cylinder head with no airbox, and also with the airbox (setup for maximum street performance). I tested each carb on a box-stock cylinder head, and a modified cylinder head (The HammerHead). The CFM values were obtained using 15” H2O test pressure on the carb with no airbox, and 10” H2O with the airbox installed. I can’t explain why I didn’t test the airbox configuration at 15” too. All I can say is that I probably got fixated on the formula. All the tests were repeated several times to ensure accuracy. Box Stock Head Mikuni: 145 CFM @ 15” (no airbox) 113 CFM @ 10” (airbox installed) Kehin: 155 CFM @ 15” (no airbox) 122 CFM @ 10” (airbox installed) S&S Super-E: 159 CFM @ 15” (no airbox) 124 CFM @ 10” (airbox installed) So far, the S&S is looking pretty good as far as WOT operation is concerned. It shows about a 10% improvement over the stock carb. The Kehin looks good too with about 8% more flow than stock. These improvements over stock are about the same with or without the airbox. Modified Hammer Head Mikuni: 159 CFM @ 15” (no airbox) 117 CFM @ 10” (airbox installed) Kehin: 180 CFM @ 15” (no airbox) 128 CFM @ 10” (airbox installed) S&S Super-E: 191 CFM @ 15” (no airbox) 134 CFM @ 10” (airbox installed) As the cylinder head flow capability improves, the S&S & Kehin flow advantages are increasing. The S&S now flows 20% more than stock without the airbox and 14% more through the airbox. The Kehin comes in at 13% better than stock without the airbox, and 9% through the airbox. The spread is increasing. I think that shows that the larger carbs really start to shine once the cylinder head is modified. |
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Title: Re: Carburetor Comparison Post by DragBikeMike on 03/17/19 at 15:11:23 If we look at the performance potential using the formula, the stock carburetor on a stock cylinder head and modified airbox has the potential to support about 49 HP. That sounds optimistic. I can’t accept that number. Who knows, maybe with 14:1 compression, .035” quench, dual plugs, optimized ignition timing, a finely tuned straight pipe, an aggressive cam and VP C16 fuel (or maybe some of their oxygenated fuel), 49 ponies off the stock head and carb are achievable. Seems unlikely to me. What do you folks think? The formula predicts that the S&S carb has the potential to support about 58 HP on an engine with a high-flow head. If you consider that the engine in question would be built to the hilt, I guess it’s feasible, but still optimistic. There are a lot of high-power single-cylinder machines on the market these days. Many approach 70 HP, but they have huge valves and sophisticated engine management systems. Our cylinder heads have valves that are appropriate for a 400cc engine, not a 650cc, and our engine management system consists of a fixed-timing ignition system (with no rev limiter) and a carburetor design circa the 1940s. |
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Title: Re: Carburetor Comparison Post by DragBikeMike on 03/17/19 at 15:12:18 During initial testing on the CV carbs (Mikuni & Kehin), I noted that the slides on those carbs do not lift fully at 10” H2O. Under normal operating conditions, the engine speed would continue to increase, resulting in higher venturi velocity, resulting in the slide rising. My tests were performed at a fixed pressure differential, so the slides stopped rising once 10” H2O was established. In order to be fair to the two CV mixers, I took the slide springs out so that they would fully open at 10” H2O. I’m not sure if this was the right approach. Maybe I should have left the springs in place (of course their flow numbers would be in the toilet). An interesting point here is that when I started testing with the airbox, the Mikuni flowed better than the Kehin. The reduced flow through the airbox really affected the Kehin and at 10” the slide would hardly lift. I found that interesting. Can one of you shed some light on this? Since we all know that higher air flow should result in more power, let’s focus on the actual flow rather than trying to predict horsepower using a formula. On a stock engine, the Kehin outperforms the stock Mikuni by 8%, and the S&S outperforms the stock Mikuni by 10%. On an engine with a high flow head (and airbox), the Kehin outperforms the stock Mikuni by 9%, and the S&S outperforms the stock Mikuni by 14%. As cylinder head flow is increased by port work, these larger carbs really start to stand out. |
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Title: Re: Carburetor Comparison Post by DragBikeMike on 03/17/19 at 15:14:17 So, what’s the bottom line? The S&S flows better than the Mikuni or the Kehin. Whether or not it will make more horsepower remains to be seen. It will be difficult to set up and get tuned correctly. The manifold was a bit hard to fabricate, but it’s done and fits perfectly. It’s clearly not as sophisticated as the metric mixer, but hope rings eternal. The Kehin is more sophisticated than the S&S, and it fits nicely (except the throttle cables are on the right side of the carb). Some preliminary investigating into parts for a Kehin 40mm used on the KX650 and KLR650 shows that the emulsion tube is different, and the main jet is way smaller and uses a different thread series. It also uses a different needle jet and jet needle. The emulsion tube and needle jet for the Kawasaki are expensive, so in addition to acquiring a Harley Kehin, I’m willing to bet one would have to purchase the emulsion tube, needle jet, jet needle, etc. for a Kawasaki (or similar Kehin) to get that carb to work on a 40-inch single. Do any of you have anything to share about the 40mm Kehin CV carb used on Harley Sportsters and Big Twins? Has anyone out there mounted one of these carbs on an S40, and if so, how did it work? I would really like to test a Mikuni 36 & 38mm VM, and a 40mm Mikuni HSR, and any other carb that has potential for installation on the LS650. If any of you have an old jalopy carb you would be willing to loan for testing, I will be more than happy to suck its guts out. I’m sure the info would be beneficial to all the forum members. Drop me a PM if you are interested. Flow testing and modification on the HammerHead are going well. I have been able to achieve very good improvements without increasing intake port volume. “Fast650” has been instrumental in that progress and has done a great job of mentoring and keeping me on track (and of course he donated the HammerHead). He’s my sounding board. I’m finished with the intake and am preparing to move to the exhaust (an obvious challenge). Now that I can suck a full 15” on both ports, things should move along better. Once I am finished with the cylinder head flow tests and mods, I will share all the data with you along with a detailed map of the modified ports (intake currently about 195 CFM @ 15” H2O). |
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Title: Re: Carburetor Comparison Post by Dave on 03/17/19 at 15:59:13 It will be interesting to see how smooth you can get the bike to run in the real world with such a big carb. The "old school" part of my brain remembers that big bore carbs have issues with low speed jetting....as the air is moving so slow through the venturi at low rpm - it therefore gives very little "signal" or "vacuum" and the jetting is therefore difficult. This is one reason the 2 and 4 barrel carbs where created....the small venturi(s) could more accurately provide the low speed mixture, and the bigger venturi(s) would open when needed. As you mentioned - the Savage motor will likely have a very tough time making 50 HP. As you have tested and discovered - the quench area and exhaust port are not set up well for making HP....we have a lawn mower engine in some respects....it is really not made to be a performance engine. I think 40 HP is not too difficult to obtain with bolt on parts and a bit of porting. |
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Title: Re: Carburetor Comparison Post by SpamyToo on 03/17/19 at 16:19:02 Looks like you having way too much fun. Kudos for the work involved on that. I just wanted to throw in that my brother had the S&S carb on his bike for about 2 years and finally gave up on it. His motor was hopped up a bit with a cam and what not, I dont think he put the effort into making it work that you are, but after some amount of time he just didnt want to deal with the S&S. It was a back firing and gas dumping machine. In addition that he could sell the carb for a lot of money and just use a Keihin that ran great and didnt need constant attention. I think your chances of success appear to be much better. |
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Title: Re: Carburetor Comparison Post by LANCER on 03/17/19 at 17:55:26 I can send you a 36 & 38mm VM and an Edlebrock Quicksilver to test. Send me a PM with address again please. |
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Title: Re: Carburetor Comparison Post by Fast 650 on 03/18/19 at 11:23:55 13151A66646367570 wrote:
I think the bore to stroke ratio will be the main factor that limits power. More RPM=more power, and that long stroke places the limit at around 7000 RPM for acceptable engine life. You can spin it faster but you can start measuring time between top end rebuilds with a stopwatch then. Rings don't last long at those speeds, the heat anneals them and the ring seal goes away in no time. For the Savage, it will take boost or nitro to make the really big numbers unless you destroke it and increase the bore to keep the displacement similar. |
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Title: Re: Carburetor Comparison Post by DragBikeMike on 03/18/19 at 15:45:05 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. |
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Title: Re: Carburetor Comparison Post by SpamyToo on 03/18/19 at 18:01:18 Ill see what I can find out. I know he bought the bike with the carb installed. It came from an older guy who like tinkering and did pretty cool machine work. He made the intake himself and it was a nice looking billet setup. My brother might actually still have it. I think he ended up going with a Keihin PWK38. Many of our dirt bikes run Keihin, so we already have a plethora of jets to work with for these. |
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Title: Re: Carburetor Comparison Post by Fast 650 on 03/18/19 at 23:43:00 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. |
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Title: Re: Carburetor Comparison Post by Dave on 03/19/19 at 04:11:57 7B7D720E0C0B0F3F0 wrote:
I have a mental problem wrapping my head around this. When mounted on a twin, the carb has twice as much air flowing through it at idle (or any other chosen rpm).......even though only 1 cylinder at a time is using the air. |
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Title: Re: Carburetor Comparison Post by Armen on 03/19/19 at 04:49:10 I love this :-) |
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Title: Re: Carburetor Comparison Post by LANCER on 03/19/19 at 08:53:16 Yes, HD engines can and do make 50hp/cyl but they also have a relatively straight exhaust port too. That is a difference that affects exhaust velocity/volume, but do you know how much ? The DR650 (90-95) makes 46hp stock according to Suzuki while the LS650 makes 29-30hp. The DR has a little more cam lift, a single tube header a bit larger than the LS, the carb may flow a bit more fuel, and the exhaust ports on the DR are smooth and almost straight compared to the corners, bridge and outer ring on the LS. Other than the exhaust ports, is that enough to raise the peak HP by 50% ? Doesn’t seem like it to me. It is just my opinion but I think the exhaust ports are the primary reason for the power difference. The DR and LS were built simultaneously so the differences were selected for a reason by the factory. Why not just use one engine ? It’s cheaper. Thoughts ? |
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Title: Re: Carburetor Comparison Post by DragBikeMike on 03/19/19 at 14:35:33 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? |
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Title: Re: Carburetor Comparison Post by batman on 03/19/19 at 15:00:29 I see Lancer's points ,but if the S-40 had been given a DR motor , It would have lost it's draw as a mild starter bike, and who would have traded up, even to an S-50, with a bike that would already have easily done the ton? I think using the 40 mm carbs to be a mistake, use of the 36mmVM carb has shown increased Hp ,due to higher fuel/air velocity, the 40mm carbs may not except at WOT. More air /fuel does equal more HP , but only if the flows are balanced , intake and exhaust ,and exhaust is our problem .The fact that the stock carb's venturi is about equal to 36mm, only encourages me to keep using the stock CV (with mods). |
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Title: Re: Carburetor Comparison Post by LANCER on 03/20/19 at 12:23:25 "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. |
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Title: Re: Carburetor Comparison Post by Dave on 03/20/19 at 14:39:02 Well as weird as it might sound for Suzuki to build 2 different 650cc single cylinder engines.....BMW did just the opposite. They built an 800cc twin....and offered it in different power outputs, and called it a 650, 700 and 800! https://en.wikipedia.org/wiki/BMW_F_series_parallel-twin |
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Title: Re: Carburetor Comparison Post by batman on 03/21/19 at 14:34:37 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 . |
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Title: Re: Carburetor Comparison Post by playhard67 on 03/21/19 at 16:42:42 I have nothing to add to this.....my head is hurting. But, looks like an extreme amount of thought and testing is going into this process. I am sure, in the end, you will achieve what you are looking for. |
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Title: Re: Carburetor Comparison Post by hotrod on 03/21/19 at 22:05:15 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. |
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Title: Re: Carburetor Comparison Post by DragBikeMike on 03/21/19 at 23:35:14 Hotrod: The DellOrto sounds like a good match. It's not a constant velocity carb but it is a variable venturi. Has a smooth bore and no choke. The size is right for sweet street performance. Keep us in the loop. Let us know how the fit-up goes and any problems you encounter (manifold, cables, slide apertures, jetting, etc.). More options to choose from has to be a bonus. I know someone else who has his eyes on a DellOrto. Not sure if I'm spellin that right. |
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Title: Re: Carburetor Comparison Post by hotrod on 03/22/19 at 04:47:51 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. |
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Title: Re: Carburetor Comparison Post by Fast 650 on 03/22/19 at 11:19:01 2E2D38212D2278744C0 wrote:
The cylinders are 45 degrees apart, but the firing order is most definitely not 45 degrees too. If it was, the second cylinder would be coming up on the compression stroke 45 degrees behind the first cylinder. Try kicking one sometime with the ignition off to see for yourself. Bring it up on a compression stroke, let the kickstarter ratchet back to the top so you can get a full kick from it, and give it all you've got then. No matter how hard you kick it it will only go thump one time for each kick. You will never spin the engine fast enough for the second cylinder to make a compression stroke with one kick. |
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Title: Re: Carburetor Comparison Post by batman on 03/22/19 at 12:13:53 You are of course correct. I had that thought sometime after ,that I was wrong about the firing order. Shows my lack of never owning a Harley !!! I still think I'm right concerning the piston speed . |
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Title: Re: Carburetor Comparison Post by DragBikeMike on 03/22/19 at 16:22:30 Hotrod: I bet the install is almost identical to the Mikuni VM. If the DellOrto is the particular design I have in mind it's pretty much the same as a VM. Has a round slide and the cable comes in through the top and hooks into a slot in the slide. It should be easy to install that type of carb. Good luck with your project and keep us informed. |
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Title: Re: Carburetor Comparison Post by Fast 650 on 03/23/19 at 10:09:29 2221342D212E7478400 wrote:
Maybe. The 80 inch Harley does have a much longer stroke, but the Sportster is very similar at 3.8125 inches. A common budget performance mod used to be replacing the Sportster carb with one for a big twin. Sportsters were either 36mm or 38mm depending on the year, while the big twin was 40 mm. So the velocity difference may not have as much effect as you are thinking. |
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Title: Re: Carburetor Comparison Post by DragBikeMike on 03/23/19 at 16:07:34 I suspect that the LS engine will run at approximately twice the speed of the Harley to achieve similar jet circuit transitions. For instance, out of the box, the S&S utilizes a .040" airbleed which brings the main jet on-line at about 3000 rpm. With that same .040" airbleed, the LS engine would probably have to run about 5000 or 6000 rpm to bring the main jet into play. So I'm guessing I will need to use a smaller airbleed on the LS. Finding just the right jet combination to allow the carb to transition from the low speed circuit to the high speed circuit will be tricky. I may have to try unorthodox stuff like increasing or decreasing the number of holes in the emulsion tubes. The Harley is pretty much a 5500 rpm engine until you really start to breathe on it. The S&S E runs very well on a 5500 rpm Harley. Our engines are 6500 rpm engines and I'm looking to increase that to about 7000. That's the redline on the DR so I think the valve train will be good to 7000. That should make application of the S&S carb a little easier, but still difficult (at least for me). I don't think it will be a cakewalk. I think it will most certainly be much more difficult than adapting a Constant Velocity or Variable Venturi carburetor. The fixed venturi will be a big challenge. If I don't get it to work properly at least I will learn a few things. |
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Title: Re: Carburetor Comparison Post by Dave on 03/25/19 at 05:17:55 I just remembered something from my past. I built a 3.5 liter six cylinder motor that had 46mm Weber carbs - and it had one venturi for each cylinder. It wasn't hard to get jetted as others had been down that path and created a jig for drilling additional fuel metering holes, and it ran really well and was smooth - however those huge throttle plates made it difficult to cruise at a steady speed. The engine made a lot of power and needed very small throttle openings for normal traffic, and any movement of throttle pedal made a big change in speed....either opening or closing....it seemed you were either speeding up or slowing down - a steady speed was difficult to maintain. It could be a low HP 650cc engine and a 40mm carb won't suffer from that same issue.....but it could be that a steady cruise speed might be harder to control with a large bore carb. It was difficult to hold a steady speed....or make small changes in speed. The engine made a lot of power and at normal traffic speeds re was 580cc per cylinder and just a small amount of movement of the throttle made a big difference in the air flow to the |
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Title: Re: Carburetor Comparison Post by DragBikeMike on 03/25/19 at 11:29:56 Thanks for sharing the info on the Weber setup Dave. That may turn out to be quite useful to me. Do you think that a change to the throttle linkage (less throttle plate movement per inch of peddle movement) might have helped. I could possibly fabricate an elliptical pully for the throttle shaft so that at the lower throttle openings the throttle plate movement to twist grip ratio would be less than at higher throttle openings. Do you think something like that might have improved conditions on your six-cylinder application. |
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Title: Re: Carburetor Comparison Post by Dave on 03/25/19 at 18:32:28 It might have helped to have some form of progressive throttle, with a snail shaped cable ramp.....the one is used was circular. |
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Title: Re: Carburetor Comparison Post by DragBikeMike on 04/07/19 at 17:18:55 Lancer loaned me three more carburetors to test. A BIG mahalo to Lancer for trusting me with his valuable mixers. The new candidates are a Mikuni 36VM, a Mikuni 38VM, and a QuiuckSilver 38mm. I tested the three new carburetors using the same process as the stock carb, Kehin 40, and S&S Super E. Here are the results of the tests: Box Stock Head Mikuni 36VM: 149 CFM @ 15” (no airbox) 109 CFM @ 10” (airbox installed) Mikuni 38VM: 153 CFM @ 15” (no airbox) 113 CFM @ 10” (airbox installed) QuickSilver 38: 157 CFM @ 15” (no airbox) 117 CFM @ 10” (airbox installed) Modified Hammer Head Mikuni 36VM: 172 CFM @ 15” (no airbox) 126 CFM @ 10” (airbox installed) Mikuni 38VM: 178 CFM @ 15” (no airbox) 130 CFM @ 10” (airbox installed) QuickSilver 38: 184 CFM @ 15” (no airbox) 132 CFM @ 10” (airbox installed) |
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Title: Re: Carburetor Comparison Post by DragBikeMike on 04/07/19 at 17:22:33 So let’s stack them all up so we can see how the six carburetors compare to each other. In the interest of brevity, I’m only going to arrange them based on the results recorded off the HammerHead tests since that’s the highest flows achieved. Modified Hammer Head Stock Mikuni CV: 159 CFM @ 15” (no airbox) 117 CFM @ 10” (airbox installed) Mikuni 36VM: 172 CFM @ 15” (no airbox) 126 CFM @ 10” (airbox installed) Mikuni 38VM: 178 CFM @ 15” (no airbox) 130 CFM @ 10” (airbox installed) Kehin 40 CV: 180 CFM @ 15” (no airbox) 128 CFM @ 10” (airbox installed) QuickSilver 38: 184 CFM @ 15” (no airbox) 132 CFM @ 10” (airbox installed) S&S Super-E: 191 CFM @ 15” (no airbox) 134 CFM @ 10” (airbox installed) |
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Title: Re: Carburetor Comparison Post by DragBikeMike on 04/07/19 at 17:24:45 To get a feel for what we could expect from an air filter/airbox change, I installed Lancer’s Uni Pod filter on the QuickSilver 38. The Uni filter flowed 134 CFM @ 10” H2O, an increase of 1.5%. That was with the red outer sock removed. When I installed the sock it flowed 132 CFM, the same as the modified airbox. Let’s look at the Mikuni VM setup. It looks to me like it would be an easy installation. |
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Title: Re: Carburetor Comparison Post by DragBikeMike on 04/07/19 at 17:29:18 That’s an impressive throat. A full 38mm with almost no impediments to airflow. I bet this mixer will bring out the beast in your savage. |
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Title: Re: Carburetor Comparison Post by DragBikeMike on 04/07/19 at 17:30:52 The VM36 is almost identical except for a slightly smaller throat. The QuickSilver also looks like an easy installation. The throttle cable might be a little tricky but nothing we can’t handle. |
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Title: Re: Carburetor Comparison Post by DragBikeMike on 04/07/19 at 17:31:55 The Uni Filter is an easy installation. Wish it had some weather protection. |
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Title: Re: Carburetor Comparison Post by DragBikeMike on 04/07/19 at 17:35:10 IMO, the three Mikuni carburetors should be very easy to setup for most applications. The 36mm & 38mm VM carburetors will provide a boost in performance, and the stock CV carburetor will probably provide the best mixing qualities for trouble-free everyday street use. The Kehin 40mm CV, the QuickSilver, and the S&S E are the flow kings, but I’m betting they will be difficult to setup. The 38VM is so close in terms of flow that only the most discerning performance seeker will be able to tell the difference. It looks like the other five contenders will have a very hard time outperforming the 38VM overall. Thanks again to Lancer for the loan. It allowed us to gather a lot more comparative data. Knowledge is power. Best regards, Mike |
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