The reason that you have not got an answer to your question is that there is not enough information. One has to know the eccentricity of the cam but then, as the cam moves along the follower it has a variable ratio from the eccentricity to the lift on the push rod. Therefore, first we need to know what lift you actually get and then it is a trivial matter to gear this up to 0.5". This can be calculated by taking the length each side of the rocker pivot, ratio 1:1 near enough with standard item, and then working out how the leverage ratio needs to be changed. Divide the larger dimension by the smaller dimension and that will give you the ratio. What ratio you need will determine how much you have to move the pivot. If you really can't do it then let us know what lift you are getting and the rest of us will let you know how much to move the pivot.
Offset ET28 hole in ET25 for more valve lift
- Thread starter johnmead
- Start date
I am only interested in max lift. Since you are not familiar with the MK5 cams, why even respond. Since the rockers are 1:1 the max lift at the valve is the same as the cam and that is 11mm on the intake and 10.5 on the exhaust. I gave these numbers in previous emails.
I can calculate the change if offset to give me what I want on my K&E sliderule. What I am looking for is information from anyone who has already done this and what their experience was.
Since you have obviously not done this, why reply?
John Mead
Oh Dear! How do I respond? I could use the Mad Max form of diplomacy, which you seem to favour, or the Magnetoman form of trying to explain things as clearly as possible. Your first posting did not mention Terry’s Mk5 cams and asked what offset gave what lift. That is a calculation which is so trivial that it does not need a computer or slide rule. A pencil and paper should suffice. I responded in posting 2 that you could be in for problems doing this with Vincent cams and once it was clear that you were using cams with radiused followers I moved on in posting 4 and pointed out that the exact ratio is not 1:1 but is variable as the point of contact at the lower side of the fork at the end of the rocker moves outwards. Greg, posting 6, Roy, posting 7 and I, posting 8, all pointed out that the potential problem is not the top of the lower guide but rather the bottom of the upper guide. From your posting 9 it seems that you had not understood that potential problem. In my posting 11 I mentioned that the eccentricity of the cam and the variable leverage ratio with the Vincent cam and follower geometry. If you measure the base circle of your cam and then the distance from the base circle to the top of the cam lobe you will find that that difference is much less than the lift of the valve. Similarly, because of the variable ratio of the lever action of the rocker as it tilts one needs to know exactly what lift one is getting, not what one expects. You seem to be altering the geometry of the valve stem to rocker and that is going to make a difference so before anyone can calculate what ratio is needed on the rocker one needs to know exactly what lift you are getting, not what you expect.
You have assumed that I have no experience of ratioed rockers. That is not so. I have two sets of rockers with different offsets to give different lifts and have helped a friend who has raced for years with ratioed rockers. We do know what happens and it can be made to work reliably but it absolutely no good telling you what offsets were used as it depends upon the cams, geometry etc. I repeat, the calculation of the offset for the pivot point of the rocker is so trivial, and so dependent upon what one is starting off with, that there is no point giving you a value.
Finally you state in posting 12 that you are only interested in max lift. Let me tell you a story. Several years ago a racing chum had some special cams made that had an extra 40 thou on their noses. The extra was nicely blended in and looked fine. We did the lift profile and I did the calculations for the velocity, acceleration and so on. The value for the acceleration worked out to be about a factor of seven higher that what is normally regarded as the maximum allowable. My chum went back to the cam man and was told not to worry; there were six sets of these cams out there with no problems. A race or two later valve heads pulled off and wrecked a lot of the engine. You want to go from 11 mm to 12.7 mm, an increase of only 15% but now go away and calculate what that does to the acceleration. I think that you might be surprised. I think that you will get away with it but as you point out I do not have measurements of Terry's Mk 5 so the ball is in your court.
You have assumed that I have no experience of ratioed rockers. That is not so. I have two sets of rockers with different offsets to give different lifts and have helped a friend who has raced for years with ratioed rockers. We do know what happens and it can be made to work reliably but it absolutely no good telling you what offsets were used as it depends upon the cams, geometry etc. I repeat, the calculation of the offset for the pivot point of the rocker is so trivial, and so dependent upon what one is starting off with, that there is no point giving you a value.
Finally you state in posting 12 that you are only interested in max lift. Let me tell you a story. Several years ago a racing chum had some special cams made that had an extra 40 thou on their noses. The extra was nicely blended in and looked fine. We did the lift profile and I did the calculations for the velocity, acceleration and so on. The value for the acceleration worked out to be about a factor of seven higher that what is normally regarded as the maximum allowable. My chum went back to the cam man and was told not to worry; there were six sets of these cams out there with no problems. A race or two later valve heads pulled off and wrecked a lot of the engine. You want to go from 11 mm to 12.7 mm, an increase of only 15% but now go away and calculate what that does to the acceleration. I think that you might be surprised. I think that you will get away with it but as you point out I do not have measurements of Terry's Mk 5 so the ball is in your court.
John, this may be of interest....When John Trease here in Australia was building the engine for Ian Boyd's Vinton racer, he redesigned everything possible in the head, and used 1.6:1 rockers. The load imposed on the cam and followers was so high that they wore quickly. So he changed to solid followers and more modern profile cams. Of course he also fitted a high pressure oil system, with slipper type big end bearings as well. I think from this info that the week link in the valve train as far as extra load is concerned would seem to be the cams and followers......this may not be an issue for your set up, but if the engine is to survive a long time, it maybe something to consider. By the way.....what cylinder heads are you using, are they originals, or the newer squish band chamber heads...?
Bruce,
I think that the Boyd machine has the same solid type of followers that an American V-8 hot rod would have. Many years ago I looked at installing solid Triumph lifters in a Comet. The advantage to this set-up is that it allows you to use a wide variety of cam lobes. In fact, you can buy cam lobes right off the shelf and weld them on your shaft. You can not do that with a Vincent cam lobe. When I had cams made for the racer I asked what the final timing turned out to be and the maker said he had no idea. All modern cam digital readout machines are designed to read solid (1:1) lifters, not lever lifters where the lift ratio changes.
David
David, most american v8's run with roller followers, wonderful things, but I cannot find enough space in the vin timing chest to get them in. Triumph followers have been used in vins already, Merv Hackett built a sidecar engine some while ago, it also had overhead rockers, went like stink until the I N A big-end let go and wrecked the G 50 Matchless piston = the head!
Greg these very fast Vincent engines heads modern days, check base of head to the top of the head still the same a mount of finns, radius lifters were used on picadors with no problems,
Thank you. That is the type of information I am looking for. Real world experience instead of pontificating by someone that likes to see himself in print.
I am using the new Terry Prince heads with squish, 92mm pistons and a 92mm press together crankshaft. With .500 lift the MK5 cams will have a max spring pressure of 257 lbs, which is not overly high. The beehive springs will coil bind at .850 so .500 lift will not be a problem in that area.
The MK5s make max HP point is 6800rpm and the crank is good for 9500rpm. Drive side mainshaft is 30mm with tapered Timken bearings.
I have plots of the MK5 cams that I did with a pair of dial gauges mounted in modified "D" breather caps along with a degree wheel and they are symmentrical for both .433 on the intake and .413 on the exhaust.
Do you have any contact information for John Trease as I would like to talk with him.
Both heads were flowed on a Flowmaster FL600 flowbench and the flow continued rising after reaching .500. Hence the question.
John Mead
Back in the 1960's HC cams in California sold a set of roller cams for Vincents. They used HD Sportster rollers on the end of lifters that mounted just like the Vincent ones. Profile was "Hot Sportster", but I do not have any info on lift and duration.
I tracked down the current owner of the HC business to Colorado and have spoke with him several times. He does not want to sell me the VIncent information and parts he has but I keep calling him a couple of times a year to BS and ask him if he is ready to sell me the stuff.
John Mead