And Now a new possibility for carburetion

[David Jackson]

I had a chat with a very knowledgeable man at Patrick Racing; perhaps you know him; Nigel is his name.
I started by asking about velocity stacks and wound up discussing their dual 42 Mikuni set up on a 15" or so long manifold which has two apparent virtues: 1) it moves the carburetion system out of the area my knee wants to be in! 2) some experts have talked about the ideal intake length for HD motors (and you and I have mentioned this in passing on this forum) being 14" or 15" or some such number; which this set up is.

The single chamber plenum set up with a sort of progressive carb opening makes theoretical sense to me (but remember that am just an ignorant layman here) as the single chamber means no abrupt changes of direction for the air; and the progressive set up (as Nigel explained) gives small throttle diameter for low speed operation and large (42 + 42 + 84 to me) area for WOT operation. Nigel felt that gains of 8 lbs/ft of torque in the mid range and 8hp were possible with this set up compared to my single 51 mm CV; along with more positive throttle response. Nigel made the point that the intake manifold I am using might be theoretically best at 11,000 RPM; and that would be moving my HD parts around rather rapidly!

Your thoughts on that carb set up?

Regards;

David Jackson

[Mike]

David, somehow this one got past me, sorry for late reply. Yes I've talked to Nigel in the past. He is correct in that the shorter the intake track length the higher the functional rpm (actually the other way round). The progressive setup works well and I believe makes for an easier pull. I like the plenum setup vs. unequal independant runners. I also believe a plenum helps absorb reversion and crossflow (although segregated runners would have no crossflow). I still have reservations about the inequality of the short side radius into the intake ports. Intuitively it would seem the air flow would favour the rear cylinder but maybe the longer run and more serpantine path makes up the difference. Would like to see flow bench testing through manifold and ports. Not sure about the hp claim, I would be more conservative at maybe 4. That will be more a function of the motor's potential.
Regards, Mike

[David Jackson]

Hi Mike;

Thanks for the reply. I was concerned that I had "used up" my quota of space and your time and energy! Thanks again for your reply.

When I mentioned Patrick Racing and the carb setup he told me about on the Harley Tech Talk I got nothing but negative impressions from the members there; though I fear most of them lack your scientific knowledge, experience with all sorts of carburetion set ups, and general HD savvy.

Nigel's set up is somewhat pricey at $1775 ++; and though he says he comes up with the right jets and so on, right out of the box, that is a little hard to believe; though not impossible I suppose. One person who has had experience with the Patrick Racing set up said it really did not do anywhere nearly as well as hoped for and that with the carbs hanging way out in the breeze finding bracketry which would hold up was a problem too.

After hearing all those negatives I kind of put that idea out of my mind; though I still think it should work out in theory. Have you had any practical experience with the Patrick Racing set up?

(My cases are being machined today and should be on the way for the baffles next week.)

Regards;

David Jackson

[Mike]

I've had some experiance with dual, forward facing Mikuni's, Webers, Quicksilvers and Dellorto's but not specifically with the Patrick setup. All I can say is that the stuff making hp on dyno shootouts is rarely those setups. We are starting to see more dual straight out carbs but again all the realistate problems kick in (more so in EFI). Bob Wood also has some forward stuff and I have no doubt they work very well. How about dual 51 CV's (Hee, Hee I know I'll get you with that one)? No worry's about using up my time on this forum, this info is a much for you as for the rest of those viewing. It's educated and inquiring questions like yours that keeps us on our toes.
Rgards, Mike

[David Jackson]

Thanks for the kind words Mike.

Yes two 51s; I saw that on Bobby W's site some time ago, as well as the single 51. I may have mentioned, not sure, that I asked him about the single 51 forward mounted carb. He said that with the things he is doing now it was not needed. He would make me a manifold though; for the paltry sum of only $2,300! I am still in shock about that one. (Maybe there is something wrong with my ears and the price is only $230.)

A Little Knowledge is a Dangerous Thing, I think some learned soul once said; and that's where I am really. I was directed a site with some formulas by an HTT user I think; you probably have heard them all before, long ago; and they become old hat to you but are a little of a back to basics refresher to me. One formula relates to carburetor size (but I can't find the cfm rating for various carbs which I might have access to for my bike) (these formulas came from Dolmetsch Engine Development); it is: "to find the carb size at 7000 RPM double the cubic inches. If my engine is going to be 113 cubic inches then I need a 226 cfm carb at 7,000 rpm. Well 7,000 rpm is somewhat unrealistic for my engine but 6,000 might not be though I don't know what the formula would be for that engine speed. (Obviously there are other variables but that little formula gives a bit of an idea. I would suspect that two 51mm carbs might be overkill; though maybe with my 264 cam still climbing in power output the two would mesh at 8,000 rpm or so ... ha ha)

Back to intake tract length; has anyone ever experimented with some sort of a still air box between the carb and intake ports on a V Twin?

I note that you say that the HP shootouts are not won by bikes with forward mounted carbs. Could the formula be mistaken? Or is there some other factor which takes precedence at those HP shootouts? I have never been to one and so have no real idea what wins or even what gets entered.

Another interesting formula apparently is derived from Smokey Yunick's work. It relates to head flow requirements at 8,000 rpm and can be adjusted from there. "The formula uses the volume of one cylinder in cubic inches times 5.5 to arrive at the required air flow for 8000 rpm." "If I only want to turn 6,000 rpm then I multiply the results of his formula by 6000/8000 to get the required number." So ... my motor, 113 cubic inches displacement (Is that the same as volume? I am not absolutely certain.) Half of that is 56.5 cu. in. ... times 5.5 equals 310.75 times (6,000 divided by 8,000 or ...) .75 equals 233 cubic feet per minute.

Dometch has another formula relating cfm to horsepower; head flow relating to horsepower "We use the rule of thumb of 2.2 cfm per horsepower. It gives us a reference point at the very least." If we use that formula then 233 divided by 2.2 gives 106; which seems a little weaker than what people get from Harleys with good parts in them.

There are more of these formulas: piston speed is an interesting one. With the 4 3/8 stroke my 113" motor has, about 5,000 rpm is the safe limit yet many motors go faster than that. Another is a rod length to stroke ratio, and I don't know the rod length of the 4 3/8 flywheel set up. Supposedly 1.85 is an ideal. Another is the bore/stoke ratio; with 90% being held out as an ideal.

All this is great fun to learn about and all; once in a while though a dose of cold reality hits me. I sell real estate for a living and just yesterday was talking to a young man who is a potential buyer; the talk got to bikes and all he would even consider riding on the street any more is a Harley. He used to ride a highly modified Oriental 4 cylinder, 600 ccs I think he said (that's less than 40 cubic inches!, for those who don't think in ccs) and was timed at Sears Point at 202 mph and he had more throttle to go. Can you imagine? (I think he got badly injured on his little rocket ship somehow, that's why only a Harley now, more sane speeds and more sensible riding.)

Regards;

David Jackson

[Mike]

The H-D world is kinda weird in that they never give cfm's for carbs yet that is all the cars guys go by. We've run the numbers and sometimes still do but we actually take a somewhat simpler approach. We decide what our HP target/potential is for any given combo. Knowing that, we cause the head (or choose heads) to flow the required cfm. That number comes from both the math and practical/dyno experiance. Then an intake system is choosen that does not reduce this flow when installed on the head and checked on the flow bench (slightly larger to account for dynamics with a running engine, the math will always give a smaller value than what is really required for the same reasons). We know, again from dyno and flow bench work what these numbers are and generally what is required given all the other restraints to hp in the average H-D. When we're in unkown territory we'll simply go to the flow bench and confirm the new combo. Ultimatly the dyno will also confirm (or not). This is somewhat oversimplified, but is how we came up with the venturi vs hp numbers for our throttlebody. It is funny but we are now starting to hit hp numbers, on street motors, for which we do not have enough carburetion to support (at least in over the counter stuff). We just ran similar testing on the SE 50mm t-body and discovered that the actual on head flow numbers we not nearly as high as we had predicted which was confirmed by dyno numbers that were less than expected. Once we had the flow numbers, the dyno numbers made perfect sense (and confrimed our suspicions). Walters technologies did a bunch of testing on plenums, still air boxes and ram air. Not sure how it all worked out because their last prostock bike would have a variety of setups over a race weekend.
The rod ratio for a TC with the 4.375 stroke is 1.75:1 using a 7.667 rod length. Closer to 2:1 is better and Smokey says to use the longest rod you can physically get in. We have a 2.41 ratio in our AMA racer and may have gone too far. Piston dwell at TDC/BDC is more than most cams are designed for (we have yet to build a custom cam, but should). Most people look at rod ratio's and stroke as a function of motor life. The shorter the ratio, the greater the side loading of the piston into the cylinder wall. The longer the stroke the higher the piston speed. These both relate directly to expected engine life.
The bore/stroke ratio seems yet to be determined as evidenced by the import bikes being all over the map on this, especially on the off road single and two cylinder stuff.
Good resource literature for this stuff (as applies to H-Ds) is "The V-twin Tuners Handbook" vols I and II. "Power Secrets" by our pal Smokey is also a great read.
Regards, Mike

[dynajohn]

I have watched this discussion on Harley Tech talk with some interest and amusement. Nigel Patrick is apparently not very well respected by the crowd at Harley Tech Talk. Perhaps they dont follow the suscess of his Yamaha Warrior drag racing program. At any rate I have an interest in this discussion because Tom Reiser (T-Man Performance) is using the Patrick dual Mickuni carbs and manifold as a basis for a new carb setup on a 124 inch twin cam he is building for me. It has been stated on Harley tech Talk that the Patrick setup uses a plenum manifold. I saw the Parick manifold after Tom had started modifications on it (cut it apart) and it is not a plenum or common runner design. The Patrick manifold (a Edelbrock casting) uses seperate runners internally for each carb all the way to the inlet ports. The carbs do have a very slight progressive action which frankly makes no sense to me in view of the seperate internal porting in the stock manifold. As delivered from Patrick the manifolding does not appear to be sufficient for a big inch Harley motor producing high HP. After Tom Reiser finishes with the Patrick manifold (extensive modifications) it will be a plenum design. Mike knows Tom Reiser and his abilities as a engine builder and constructor of special parts. This is somewhat of an experiment but I have faith that if this configuration can be made to work Tom has a good chance at suscess. Mike (I think) Tom has reached the conclusion that the streetable carburator sizes and manifolding available have become a limiting factor for high output Harleys. Of course there is always the big D S&S with 2 thunderjets but I don't consider this choice a true street carb you can ride everyday and not have to constantly fool with.

[Mike]

John, Welcome to our site. I must confess that I simply do not have enough time in the day to visit all the web forums going on and am sometimes ignorant of those discussions.
As far as the Patrick racing set-up, if anyone can make it better then TR's your man. Sometimes an already good product just needs a little help. Yes, Patrick has dominated the Prostar series for several years. I would also agree that the progressive setup does not make sense with a segregated runner. It really comes down to a realistate problem between the ports. We just do not want to get into major fabrication or frame mods to fix this problem so we are kind of stuck working with what we have. I believe TR will also agree that even the Zippers D3 is not enough for some of the street motors were seeing. We really do need something that is big enough yet user friendly (62mm Mikuni?). This is true of injection also, look at all of the 58mm t-bodies ending up on bikes.
Regards, Mike

[dynajohn]

There is a possible problem with the plenum manifold setup (like the Woods manifold or the Patrick Racing one). Bobby Woods told me his single carb 51 mm Cv setup was somewhat finniky about jetting when weather conditions change but that it did produce more power. He also told me the manifold sweats (condensed moisture on the outside of the manifold due to the manifold temperature being below the dew point of the air) in mild to moderate weather. The lower manifold temp. could cause gas to recondense inside the manifold and pool on the manifold floor. This could cause the erratic carburation. Cars don't experience this because of the high underhood temperatures and on current models direct port fuel injection so there is no gas to condense in the manifolding. In my opinion fuel injection is the way to go. It took a few years for the Harley aftermarket to develop the expertise to deal with fuel injection but the extra complexity is worth the ability to tune more precisely and automatically compensate for changing weather and altitude. If I could have I would have chosen a F.I. bike to modify for the street but they were not available in Dynas when I bought my bike. I am sure Tom would agree that even the Big S&S D is insufficent for some motors being built today. There is no truly good answer using the traditional Y manifolding with one or multiple carbs. Packaging will always limit performance results on a 45 degree V using stock or near stock frames. Seperate inlets and throttle bodys for each head theoretically would remove the limitation of the Y manifolding but require more complexity. I know theses motors are currently limited to around 1.30 HP/ CI at the wheel for reliable street applications. My last Kawasaki ZX-12 which I rode on the street and drag raced made 2.6 HP/ CI at the wheel. Obvously it was designed from the get go as a high performance engine which the HD is not. Frankly it is amasing that shops like you'rs and Toms can get the reliable HP out of this design. I will let you know how the custom Patrick system Tom is working on works out.

[Mike]

H-D knows how to make hp, take a look at the new XBRR, 1.88 at the rear in a reasonbly reliable package! I look forward to hearing how things go.
Mike

[David Jackson]

I am more than likely the one who said the Patrick manifold is a single plenum design and that is what I thought I had heard from Nigel. I can't understand how I misunderstood him but Dyna John has seen a manifold cut in half and that it has runners. Strange as I thought the single plenum and progressive carburetion would make sense with a single plenum but not for runners. Nigel and I talked about it at some length; the idea being that one 42mm carb would do fine in low speed situations, such as traffic; and both carbs would be available for high speed needs. I once had a 427 Vette with 3 2s; and I believe that's how they worked.

David Jackson

[dynajohn]

Progressive action in multiple carbs does work with plenum manifolding. I had the Patrick carbs in my hands in TR's shop and actuated the linkage while looking at both slides from the inlets to the carbs. As delivered The lead carb only leads the 2nd carb by a slight amount. In fact you have to watch very closely to see the difference. The way the stock manifold is built internally with seperate runners you could not put much lead in either carb or one cylinder would be carrying more load. I did not pay close attention to the carb linkage setup so I don't know if it is adjustable to be able to increase the lead for one carb. I am sure TR will get into this when he sets them up. After the manifolding is modified to make it essentially a plenum we could use a true progressive carb setup.
The manifold and twin carbs as delivered are a beautiful setup to look at but I don't think they were designed for very high performance levels. To be fair to Nigel I don't think he designed this setup to outperform the best of the current high performance carb setups on the market but to give a mild increase in performance and deliver a custom look for the chopper crowd which after all is where most of the market is. At the performance level they will deliver as shipped I suspect you can get better performance at a much more reasonable cost if that is you're primary concern. With a opened up plenum manifold grafted to a larger port inlet manifold the story may be different. That is what TR is trying to find out on my motor. By the way Mike great site and a real service to the Harley performance community.

[Mike]

Thanks Man! It's all for you guy's!!! Oops.....and gals!!!!!!!!

[David Jackson]

I remember when a chopper was a chopper (40 years ago and a little more). It had a slightly extended front end to keep the front of the frame off the ground when going over bumps etc. and the frame lugs which were not needed for big tanks, and so on were chopped off; hence the name. The idea was that anything which did not need to be on the bike was not; and what was needed was supposed to be small and light. What these machines are truly which we call "choppers" nowadays, I am not sure; except for the fact that they are custom works of rolling sculpture, an art form actually, but not a true chopper to me.
The bobbers of today are closer by a bunch, though I think the term bobber predates the term chopper, but I am not sure.
I am not a rolling work of art rider; I just want a nice, reasonably quick cruiser. I like mid mount pegs, regular handlebars, and all the things which go along with a regular, ordinary, motorcycle; just like millions of others.
I really am suprised indeed that what Nigel told me about his carb setup is so obviously off the mark. Progressive carburetor action with single runners? And carbs that small? Something does not seem right at all. It will be interesting to learn more from DynaJohn about what gets done to his set up.
I wonder why Nigel felt compelled to spin me such a yarn, or alternately, if I have lost all my marbles!

Regards;

David Jackson

[Mike]

Marbles are like smoke, hard to get back in once out. Ever let the smoke out of your wiring harness?!!! The term Bobber does predate Chopper in that early bikers removed the hinged section of the rear fender (among other things) thus "bobbing" the bike. I have it on good authority this started during WWII when military bikes would crash requiring bent or offending parts to be removed just to get going again or removed in anticaption of same. In at least one case this included the brakes! Same thing when people started racing many of these same bikes after the war. Appologies, the time machine got me.
Yes, I too 'am ancious for the results. Dynajohn has the right man doing the work! The Nigel setup may work well when used with the rest of his combo. It could make sense that slightly laboring one cyl. makes for a softer, more progressive initial acceleration. But now I'm in the dangerous position of theorizing and guessing.
Regards, Mike