givemethebighammer
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| posted on 22/11/05 at 11:28 PM |
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cams repro or blank
I've been looking at cams again today, and came up with a couple of questions.
How can you reprofile an original cam to give better performance, surely it is not possible to machine an original cam to give more lift etc ?
In relation to the above people like Kent and Piper advertise cams produced from both repro and blank cams, same techical specs but with vastly
different prices.
or am I missing something ?
 
Rescued attachment piper.JPG
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Surrey Dave
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| posted on 22/11/05 at 11:41 PM |
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I think they reduce the base circle diameter which will give more lift, and also reshape the ramps to give faster opening..................
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Chippy
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| posted on 22/11/05 at 11:45 PM |
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I have used reprofiled cams in the past,(from Kent's), and they seemed fine, never had any problems with them.
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Ben_Copeland
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| posted on 23/11/05 at 08:57 AM |
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I was going to use repro one's too... saving money !!!
Sure they must be ok, not sure about racing tho
Ben
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MikeR
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| posted on 23/11/05 at 09:09 AM |
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it seems strange but they way i understand it in very simple terms they basically take some off the bottom of the egg shape. This has the effect of
making the pointy bit "more" pointy giving more lift.
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NS Dev
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| posted on 23/11/05 at 09:37 AM |
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Exactly right Mike. Take the bottom off the egg and move the follower up to compensate for the difference in base circle. This gives both more lift
and more duration, regrinding the lifting ramps of the cam can then reduce duration again for the same lift but with a harder valve acceleration.
simple to say, harder to achieve good results!
nowt wrong with reprofiled cams as long as they are done by people that know what they are doing!
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givemethebighammer
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| posted on 23/11/05 at 11:23 PM |
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So what you are saying Nat, is that you need to put new cam followers (or some kind of shims ?) in with the camshaft ? I presume people like Kent or
Piper supply these with the cams ?
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MikeR
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| posted on 23/11/05 at 11:26 PM |
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you should always use new followers with a new cam shaft ........ but i believe they are standard followers.
(see i said it took some getting your head round)
imagine a circle, now put a 5mm bump on it, the bump sticks out 5mm. Now make the circle smaller, how big is the bump, its still in the same place but
in relation to the circle its bigger now, hence the lift has increased.
(ok, ok, it makes sense to me! :p)
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NS Dev
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| posted on 24/11/05 at 12:57 AM |
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quote:
Originally posted by givemethebighammer
So what you are saying Nat, is that you need to put new cam followers (or some kind of shims ?) in with the camshaft ? I presume people like Kent or
Piper supply these with the cams ?
Right - that's another question!!
Firstly, not necessarily! Your followers are hydraulic ones, which will self adjust to the new base circle of the cam (assuming it is within the
limits of their adjusting range, but if bought from a reputable supplier it will be)
Secondly, as Mike says, you "should" always use new followers with new cams, but as long as they are not too badly scuffed, you can just
planish your existing followers and reuse them.
Just use some P240 wet and dry paper, stuck upside down onto a piece of flat plate glass with wd40 and covered in wd40 as well. Linish the followers
on this as per the photo's below (thanks to Dave Andrew's website!)
Before:
After:
Finally, you can convert engines with hydraulic followers over to "solid followers". These are just solid bits of metal that transfer
force from the cam to the valve. Pug 205 GTI engines for example (as well as many others!) use solid followers as std.
For these, you need to shim them as you suggest to get the correct valve clearance.
Unless you are going for really wild cams (which generally have to come from blanks to get enough metal in the shaft to get the profile right) then
you'll be sticking with hydraulic followers anyway.
[Edited on 24/11/05 by NS Dev]
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MikeR
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| posted on 24/11/05 at 06:21 PM |
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In my defence i'm used to my old crossflow engine and thats what i'm talking about.
Sorry for the confusion.
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