View Full Version : Intake length tuning and custom manifolds

10th May 2011, 13:01
I am in the research phase of my next project and intake runner length is one of the many variables that will come into consideration. My thoughts on how to present this information is very convoluted, so please bear with me. While there is a long and interesting thread on wave tuning, My research indicates that the y manifold diffuses the waves to such a degree that the usual formula's are meaningless. While there is a difference of opinions on why, longer manifolds after the y do have an affect on powerband. In theory the extensions are essentially an increase in plenum volume. The larger the plenum the more diffusion of wave intensity. One potential benefit here is reduced effects of reversion waves on carb performance. This provides an additional factor to consider in addition to wave tuning. In general terms, one of the reasons for a plenum is to diffuse the waves before they reach the carb. Short runner, large plenum manifolds are generally high rpm oriented manifolds. Long runner, small plenum manifolds are generally for lower rpm. In the Sportster engine additional intake length after the y often improves bottom end power which contradicts the general rule. This is either as a result of reduced reversion and /or the result of altered wave influence.

10th May 2011, 13:20
There is indication that separate runner dual carb manifolds on these engines have huge promise. Mike Roland built a dual runner Buell that had a carb on the right and another on the left. He experimented with many combinations and eventually the bike made power in the high 120's in 1200cc form. A switch back to a single carb with the same cams, exhaust, and compression resulted in about 105hp. While the later combinations became increasingly wild, the bike made 115 hp with .497 bolt in cams and pump gas tolerant compression ratios. I have never seen a 1200cc sportster make this kind of power with those cams and mild compression. While the bike in question had billet heads which complicates comparison, The bike was nothing special as a single carb bike. I am also told that Don Tilley developed a Pro Thunder bike that made 115hp plus in dual carb form and that the same engine was in the 105hp range with a single carb. It is important to consider that many of the Roadrace Buells have used very mild cams which limit upper hp. From what i am told the XB Buell grind aka SE.551 is a very popular roadrace cam and this mild cam is comparable in many ways to the .497 cam that Roland started with.

10th May 2011, 13:29
It is possible to fabricate separate runner manifolds with non XR style heads. While the room is quite limited, it is possible to run one manifold thru the v to the left and one out the right side as Roland did. This design manifold has been tried on the bigger displacement drag bikes, but the taller cylinders provide more room. One potential problem with long runner manifolds, especially ones that run horizontal, is fuel falling out of suspension although Roland never mentioned this as an issue. EFI would allow the runner to be dry and is a potential way to deal with that issue.

10th May 2011, 13:36
Another potential issue in a manifold of this design is the effect of vibration/harmonics on carb performance. PW Morris built a manifold along these lines for an AHDRA bike and abandoned his research because of vibration induced carb issues. Apparently this issue shows up in automotive tunnel rams and is usually solved with rubber spacers between the manifold and the carb. Please note that XR manifolds always have some rubber in the system. Once again EFI would not have these issues.

10th May 2011, 13:54
Preliminary research indicates that a crossover or balance tube near the end of the manifold, right before the carb would diffuse the waves to some degree before they hit the carb and provide the benefits if an IR manifold while at the same time reducing some of the issues that are possible when the waves start to talk back. The crossover can be rubber, but the ideal size is unknown.

10th May 2011, 14:05
Another potential benefit is the potential for more precise fuel mixtures between the front and back cylinders. I started a thread about this issue here. http://xlforum.net/vbportal/forums/showthread.php?t=1089548

10th May 2011, 14:10

A set of STD dual carb heads may be your answer. Like you, I have heard they are not that hot but, nobody has any dyno results on them.

I have seen no articles on porting them out and the results. They may perform rather lack luster in stock form but, really respond to some good port and valve modifications. :wonderlan

May be worth a shot.:)

The other side of the coin is this: Making dual fabricated manifolds to come out the regular side. This will require the following in order to work because of the sportsters narrower V.

Weld up the intake port openings and re-position them to make them "curve". This will help the air flow because of the sharp bends needed in the manifolds. You will also need to re-locate the mounting holes. (at least one anyways)

You can use FI and weld in bungs or just use carbs. Some narrow carbs like Delortos or Mikunis would work side by side

This set up gives you an ideal intake runner length of 6 inches or so.

You would be making your own set of hybrid heads! It may just work. You can experiment on some old 1200 heads. They are cheap.

Good luck my friend!

10th May 2011, 14:32
There was a set of STD XR style heads on Ebay a month or so ago. I considered them but passed. Their use would require different exhaust and cams since the intake and exhaust ports are in different locations. I believe Tilley' last generation of Pro Thunder bikes used conventional style heads and dual carbs. The rear cylinder heads manifold goes left and the front goes right. Morris and Roland were along the same lines. The use of conventional heads provide more options if the I/R set up does not work for some reason and allows for back to back comparisons between I/R and Y based designs. I have considered running both carbs out the right side but the runner lengths that Roland found desirable are not practical in that situation. I have not seen the manifolds used by Tilley or Roland but have seen those used by Paul Morris. His had the same basic radius on both cylinders. They were as gradual as space would allow and the carbs ended up roughly 90 degrees from the starting point where the manifold met the head. The more curves in the design, the more fuel falling out of suspension becomes an issue. The XR style head provides a straighter shot for sure.

10th May 2011, 14:43
Im interested. I think you're onto something.

10th May 2011, 14:48
If you were to build a duel intake with about 18" runners the advantages would be huge. The problem is that there are all sorts of hidden EFI tables that would be totally wrong.

Megasquirt may be the only realistic option at that point. This would open an entire new world of tuning on one of these motors.


10th May 2011, 14:58
There are large plenum y manifolds in use in EFI etups for the Big Twins that run two throttle bodies out the right side. These designs are also popular with drag racers using carbs. These drag race setups have been around for years and those that have used them claim substantial power increases over more conventional y manifolds. I am not sure how a design of this type would work in the lower RPM ranges as drag racers have different needs. My guess is they would be less than ideal. A true I/R manifold could be tuned to work at street or roadrace practical rpms. The last development of the Roland engine was as a Pro Thunder race engine. It was very different than the cylinder heads down but the top end, heads, manifolds, carbs, exhaust was pulled from the smaller bike. It had S&S cases, special crank and rods, big cylinders and a long list of very expensive upgrades. I have not seen Dyno pulls from this engine but it is reasonable to assume that it made more power than the much milder 1200.

10th May 2011, 15:15
What about something like this? Not 18 inch runners but certainly dual. http://i.ebayimg.com/00/$(KGrHqJ,!kwE2JCl3OOkBNlg!uYlVw~~_3.JPG They make this manifold as well. It does not address the each side part of your requirement but it is nothing a little aluminum might solve. http://i.ebayimg.com/00/$(KGrHqYOKkIE238iyBm2BNviQjL-PQ~~_3.JPG

10th May 2011, 15:21
Hi Andy. Thanks for your input. An 18 inch inlet length may be optimum but does not appear practical for packaging reasons. Roland worked with 9 and 11 inch intake lengths and this is probably the practical upper limit. There are several reasons why EFI makes more sense in this application but its use will complicate this project beyond my comfort zone. Despite this, EFI is being considered. For the record, information you have posted on various public forums have showed up in my research. While you are considered the resident EFI guru here, my research indicates your experience in these matters far exceeds what most on this forum realize. I read everything you post on this forum and pay attention to what you say with great detail. Steve

10th May 2011, 15:25
Interesting Ralph. Tell me what you know about the system. Is such a system available? The manifold is what interests me most, but the whole system is interesting. Educate me please.

10th May 2011, 15:27
I just saw the dual carb y manifold. Reminds me of the large plenum dual carb manifolds that came with some of the 4 valve stuff. That one would be easy to make, but not a I/R setup. Still interesting. Steve

10th May 2011, 15:35
Dude, that is nuts. Never seen an S&S like that. Ha.

10th May 2011, 15:59
A guy that goes by ultrarider has messed with a duel intake on his big twin. You may want to PM him on the subject. He is a good friend of mine.

He has converted old Harleys that had Morelli EFI to closed loop and ABS...

He is doing a home built dyno just for something fun to play with in his spare time at home. The guy is good.


10th May 2011, 16:03
I'll check him out. Thanks Andy

10th May 2011, 23:10
Here is a photo of the manifolds that Paul Morris was experimenting with on an AHDRA bike for an idea of what I have in mind.

10th May 2011, 23:13
Here is a link to the Mike Roland Buell which is what got me thinking. Notice the overlay between the dyno pulls of the twin carb vs dual carb dyno pulls and make note that the single carb bike makes considerably more power until around 5k. http://www.visi.com/~moperfserv/buell.htm

10th May 2011, 23:23
I run a dual carb setup on my 05 1200. I believe it is an old idea built from an intake based off an old "branch" setup, with dual 30 mm mikuni round slides, I used my stock y manifold, and had to add a 1" yost spacer to clear the heads. Sorry to report, but as of right now, I do not have any dyno reports, but will soon.

11th May 2011, 00:06
Post what you can, photo's, dyno pulls, or comments and thanks for the contribution. Steve

26th April 2012, 17:54
Most pratical way to dual carb (IR)?

Anyone fab an intake to fit stock heads? 2 carbs same side.

Cut and weld a stock rear head ? 1 carb each side

How about 2v holley down from the top? thru tank

26th April 2012, 18:21
A manifold or pair, straight up might make sense, but would create a ton of packaging problems and I have never been impressed with car carbs on bikes.

The key problem in all these I/R set ups is packaging issues. P. Morris's set up was neat but his engine is very tall so it gave him more room to work between the cylinders. He gave up on it because harmonics messed with fuel delivery at high rpm. The shaking buzzed the floats up and down and the bowls ran dry, the longer the manifolds were, the worse it got.

He went to a y manifold with a huge plenum and two carbs. You loose much of the benefit if a true I/R system doing that, but if it did not work better he would not have gone that route.

Roland's Buell was set up very much the same way as Morris's but the Buell ran a pair of Rolands heads and I think the intake location may have been moved slightly to make for more room- not an option for most of us. Roland was involved in some way with an 88 inch sportster that had a comparable intake system, but those heads they were using were produced in very small numbers and are not available.

The I/R stuff has incredibly potential but the key question becomes is it worth all the trouble to package it and develop it to the point of it working?

26th April 2012, 19:09
The I/R stuff has incredibly potential but the key question becomes is it worth all the trouble to package it and develop it to the point of it working?[/QUOTE]

Sure it is, to win a trophy!

Could cut and weld the rear head to move intake port.
Looking @head now, need 1.625 id aluminum tube.

26th April 2012, 19:26
It sounds like you are getting itchen to get back into the dyno challenge stuff Charlie? Keep me in the loop, pm me any time if you want to bang some ideas around.

As close as I can figure it, the bikes that took full advantage of intake tuning were able to increase peak power by about 12 percent over the typical y manifold, short runner stuff. Two of the bikes I know of only made power in the 105 range with a single carb and short y manifold and the same package made a shade over 120 when fully tuned to take full advantage of the I/R and the related wave tuning. steve

26th April 2012, 20:27
Best way to test this is to make an ir intake from mandrel bent tube ($30). cut and fit tubes and maybe flatten the sides that touch (d shape). run 2 cv carbs. problem is how much flow will we loose @the turn into port. Taller motor (bt) would be easyer.

Smallest point on my y intake is 2 3/8 inch, motor is extra short.

Harley should have built a real kr/xr?1200 and cast 1 new head.

26th April 2012, 20:51
Here is a photo of the manifolds that Paul Morris was experimenting with on an AHDRA bike for an idea of what I have in mind.

Sorry for helping to dig up old info, but;

Why 2 compression release valves?

I had never thought of putting 2 carbs on my bike but from what I understand, that and a longer intake manifold generate 12% increase in HP? Is this based on the stock carbs and would other high flow carbs perform better or does the fact you are now only feeding one cylinder negate the benefits of those high flow carbs?

My head hurts trying to follow some of this performance stuff, but the more I read, the more curious I am...

26th April 2012, 20:55
Best way to test this is to make an ir intake from mandrel bent tube ($30). cut and fit tubes and maybe flatten the sides that touch (d shape). run 2 cv carbs. problem is how much flow will we loose @the turn into port. Taller motor (bt) would be easyer.

Smallest point on my y intake is 2 3/8 inch, motor is extra short.

Harley should have built a real kr/xr?1200 and cast 1 new head.

Where are you taking that measurement?

Since I have an 89" Stroker motor I know the distance is longer (they had to fab one intake manifold out of 2).

26th April 2012, 21:00
closest point between head ports (on back side of manifold)

26th April 2012, 21:08
carb size; y manifold shares carb one 40 mm. IR intake wants 2 40 mm carbs or bigger.

1970 trans am racing, ford ran 2 dominators (1050-1250 cfm) on a IR intake Boss 302. Most people think 750 cfm is to much for a 302!

26th April 2012, 21:12
closest point between head ports (on back side of manifold)

Cool, I will check this tonight.

You guys have me thinking a totally different tangent now. Thanks dammit! lol!

I had intended on upgrading to a Mikuni 43mm in the near future. Not sure I can afford 2 and all the HW that goes along with it.

26th April 2012, 21:35
you need a 45 mm unless you can buy a 48mm

27th April 2012, 01:33
Started reading the link posted by maru about the roland buell, 128 hp and 103 tq back in 2001 with a 650 lift cam, killer heads, and 2 ss e carbs.

27th April 2012, 01:42
Mike Roland is a super smart man. I have a whole lot of respect for him. His work and some of the results he has achieved has made me think twice about about everything I thought I knew. That 128hp engine was only a 1200 and it only had moderate compression in most of its evolutions. The big cam was on the intake side only, the exhaust cams were stock Buell cams, comparable to an se .497. There was an incredible amount of dyno time put into that engine though, they were constantly messing with it to see what they could learn.

He built the engine in the XR1200 that qualified on the pole at Daytona this year. Has built and tuned Nitro bikes that have set world records in the quarter mile and has been involved in record setting speed trials bikes as well.

The heads from that Buell ended up on a Bonneville bike.

Screw Loose Dan
27th April 2012, 01:44
Maru - Not sure if you saw this thread (http://xlforum.net/vbportal/forums/showthread.php?t=1304686) and what chief was doing this winter. Not sure if he finished the project or not. And, I think he probably has different objectives then your project, but might give some ideas.

27th April 2012, 02:09
Yes Dan, I saw that thread. It gives you some idea what I was talking about when I mentioned packaging problems.

I know a lot of guys like the XR look, placement wise, and to some extent so do I. the problem is there are some real packaging issues with that arrangement and then you still have to ride the thing. I like what Roland did better, at least for a Buell. You can run off the rack exhaust and cams and there is potentially more room for a reasonably natural riding position on the bike.

One last thing, an inch here or and inch there seems to have the potential to change things quite a bit in terms of the effective powerband. Both Roland and Morris experimented with different lengths and I imagine Tilley did as well.

Screw Loose Dan
27th April 2012, 02:32
Agreed packaging is a bear. No way around that. Just like most things in life, full of compromise.

12th November 2014, 19:52
Does anybody have a picture of Mike Roland's Buell or the article ?

12th November 2014, 21:25
the camming on the sporty is pretty much open/shut and overlap is not so much. this works hand in hand with the short primaries. matter of fact, you will get better gains from exhaust mods than intakes. another thing is that flow rates with long or big primaries can cause problems. if the flow is lazy, the fuel will drop out and will cause problems. wot probably would sustain it but lower rpm would suffer. I liken this to a dual plane manifold vs a wide open tunnel ram. each has perks but you will be hard pressed to have your cake and eat it too!!!