[Fot] Six cylinder cams--some real facts
Larry Young
cartravel at pobox.com
Thu Nov 12 15:15:42 MST 2015
Are you referring to the thread I started on Spitfire cams? I agree
completely, there is a lack of factual information. A few years ago, I
had great intentions of developing 6 cylinder cams with an approach
similar to the one I took for TR3/4 cams
(http://tildentechnologies.com). One of the problems is that in order
to select a cam, you really need accurate and complete numbers, both
seat-to-seat and at 0.050. The problem with seat-to-seat numbers is:
(1) there usually wrong by at least 10 degrees (usually but not always
understated) and (2) they don't tell you as much about peak power as the
numbers at 0.050. The problem with seat-to-seat numbers has gotten so
bad it is now called "advertised" duration. What is that? Quoting
numbers at 0.050 seems to be an American thing (I think started by
Harvey Crane). British cam makers either don't have those numbers are
they will not release them. I will never buy a cam without 0.050
numbers. If we all took that stand, then they would have to provide
them. Given that, the only way to proceed is to select cams with known
characteristics and measure the numbers. Comparable dyno data would be
nice, but none of us are likely to get this the way Kas did. The
exercise of measuring the cam can often be revealing, e.g. (1) valve
lash that opens and closes the cam on the high velocity flank causing
seat wear and valve bounce and (2) lift rates (velocity not
acceleration) that will cause running off the edge of the lifter. I
agree that larger lifters could allow a better design. Also, the lobes
could be much larger if you run without cam bearings, which could also
allow a better design curve as well as a stronger and stiffer camshaft.
I don't agree completely with your logic about a 0.490 lift limit. If
the cam gives 0.490 valve lift, that will occur for zero degrees of
duration. To have the valve open to 0.490 for any appreciable period,
the peak lift would be much greater than 0.490. Unless there are
clearance issues, you would not want a cam that goes up to 0.490 and
abruptly stops due to inertia considerations and valve train vibrational
problems (valve float, etc.).
We have debated the issue of flat versus crowned lifters several times.
I still maintain crowned lifters is the industry standard method. I
know that Ford has used it for many years. A properly crowned lifter
correctly matched to a tapered lobe was first introduced to compensate
for the imperfect alignment you mentioned. It will also produce better
lifter rotation.
Just getting the names of cams that work is not going to be of much
help. You really need the full set of numbers to see trends in what
works and what doesn't work. I have an old Cam Doctor set up on an
ancient DOS PC (one with card slots). It is a PITA to use, but beats
measuring by hand. I once requested people send me some good cam
examples, but Bob Lang was the only one that sent any. Unfortunately,
they didn't sound that interesting, so I still have them and haven't yet
measured them. Sorry Bob, at least I haven't forgotten them. The only
cam I've measured is a BP270, which is a good street cam (it's actually
260 duration). I'd love to have detailed measurements on a 150hp PI cam
and some of Kas's race cams. I wouldn't mind getting back to this, but
I'm more interested in street cams now that I am out of racing. I could
easily help with some design work, but don't want to get back into
producing and selling. Several years ago, I started a spreadsheet
similar to the one I did on TR3/4 cams and could post it for download at
my website if anyone is interested. It is not really adequate due to
the lack of complete numbers on many cams.
- Larry
On 11/12/2015 1:35 AM, Enquiries Road & Track wrote:
> there seems to be very little factual info in the recent thread and
> i'm hoping to fix this by starting this new thread and challenge other
> racers to share some some real info
>
> i previously offered the comment that cams cant be considered in
> isolation and a race engine is quite different to a street engine,
> with few practical compromises
>
> it would probably help everyone if interested people listed what 6
> cylinder cams they have used/are using along with a few other comments
> and what their thoughts are. of course, every cam grinder/vendor
> espouses the virtues of their own product, but only on forums like
> this can we get any real world experiences.
>
> I'll start with some experiences gleaned over 40 years with these
> engines...
>
> the cam lobes are rarely perfectly aligned with the lifter bores and
> vice versa. triumph 6 cylinder cam lobes are meant to be perfectly
> flat and so are the lifters. the rotation of the lifters comes from
> offsetting the lobes relative to the lifter centre. if you test this
> during disassembly/ assembly using engineers blue on your lifters,
> you will see what is going on and probably be horrified as to how bad
> some are. experiences with flogged out lifters and/or lobes often
> point to problems here. i have had occasions to move the cam to
> correct this (by changing the thrust plate fractionally), but its a
> trade-off and brings about other issues. some of the the turkish made
> cam blanks i've seen have poor control over lobe separation and the
> cam grinder cant correct this.
>
> the factory cam grind can be identified by the number of rings
> machined into the front bearing. when we could get factory blanks out
> the back door, they had no rings on them, but neither did the early
> 2000 cams. from sometime in mid70's, the cam 'stock" got thicker which
> is a good thing. early 'thin" cams (Mk1 GT6 etc) cannot be modified
> very much as the base circle encroaches on the stock . these days,
> radically welded up cams to fix this shortcoming are rare, so the
> problem has basically gone away by using new blanks.
>
> the oiling flats machined into the front and rear journals are
> perilously close to the end plates and some cam makers stuff this up.
> anyone assembling the engine should check this lest they end up with
> unexpectedly low oil pressure. a real problem if you have switched to
> a slightly thinner alloy front plate.
>
> most modified 6 cylinder heads stop flowing at about 0.490" valve lift
> (on inlet) so there is not much point in a cam/rocker system that
> opens the valve more then is needed. To choose a real 'race" cam, you
> really need this head flow info for your engine. without it you are
> just guessing.
>
> most race spec TR cams seem to rely on long durations, typically 300
> degrees plus (this is valve motion, not 050" duration which is more
> like 245 degrees). if you have a factory/Lucas PI engine, then cams
> like this are still quite street drivable with the injection properly
> set up. The factory TR5/6 PI cam is a struggle on carbies, bordering
> on horrible, because it produce so little vacuum at idle.
>
> The factory TR5/TR6 PI cam has valve duration of 280 degrees and it is
> "mild". This cam grind, which is commonly copied, is great on any PI
> street engine, especially if the compression is raised as far as you
> can get it with the fuel available to you. On 98RON fuel, i have run
> these at 12;1 with conventional (but modern) cast pistons. distributor
> re-curving is a must
>
> The factory 2500PI cam has valve duration of 260 degrees and works
> just fine with carbies. we used to fit these to US imported TR6's and
> raise the compression at same time get another 10-15HP.
>
> it is not possible to grind a modern aggressive long duration, high
> lift cam, on any original TR cam without an awful lot of welding,
> which typically bends the cam to a point of useless. it is hard to
> even find blanks that will do this , but they are out there.
>
> these modern design, aggressive race cams can't be run with a
> conventional triumph lifter. the lifter diameter needs to increase
> about 040" as a minimum to stop what is known as edge riding. The cam
> acceleration rate (how fast it can lift or close) is mostly governed
> by lifter diameter. oversized lifters require block modification which
> is not hard.
>
> For a race cam, the lobe will look like a house brick with some
> rounded edges. anything that looks like your original cam lobe,
> probably wont cut it in a race engine.
>
> I have run hundreds of modified 6 cyl engines without cam bearings and
> with zero issues. having said that, the latest cam grinds will benefit
> from the added support a block with bearings will give. also, if done
> properly, this should improve oil pressure by reducing losses from cam
> journals.
>
> A recent 2500 engine for a mostly track car............
> 98RON fuel, 12:1 comp, long exhaust primaries, highly modified head
> (intake flows 150cfm at 25"@0.550" lift)
> cam 308 intake valve duration, 250@ 050". max lift on intake 0.495"
> comment. sounds fantastic & pulls like an ox all the way to 6750
> (imposed) limit. almost too lazy. for a race engine. currently being
> changed to more aggressive cam
>
>
> Terry
>
>
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