[Fot] Six cylinder cams

[fubog1]

Duncan and Larry have nailed it.
It's completely subjective, what is expected out of the engine, HP/torque, what is the operating range, and depends on so many other factors, ie cam profile and phasing, port & valve sizes, CR, intake and exhaust header & pipe, carbs etc etc.
Also compression calculations based solely on head thickness aren't really accurate, valve depth and head shape vary and affect volume.
A slight lump on top of the pistons into the chamber will get the compression up without making the head too thin, and won't interfere with flame travel if it's less than .050 or so and the edges are radiused/clearanced.
Another related point is that it's easy to get mechanics red-mist when selecting cam profiles. BTDT, several times actually. Be realistic. A true race engine never operates below 4-5K RPM.
If your working with a TRactor engine, forget about the high revs, take advantage of the stump-pulling (slight exaggeration) torque characteristics that it has.
Glen

 

 

 

-----Original Message-----
From: Duncan Charlton <duncan.charlton54 at gmail.com>
To: Larry Young <cartravel at pobox.com>
Cc: Mike Piggot (new) <mikeandjudypiggott at att.net>; fot <fot at autox.team.net>
Sent: Sat, Oct 31, 2015 4:52 pm
Subject: Re: [Fot] Six cylinder cams


Larry’s comments make me wonder the precise intended purpose of the engine
mentioned by the original poster.  “Track days” might mean different things to
different people.  Does P. J. want a good flexible power band, or does he need
an ultimate dyno horsepower figure?  Does the motor have the gear set to be able
to take advantage of an engine with very little low-end torque? How high among
his priorities is the ability to be able to drive the car back on the trailer at
the end of every track weekend?  Lack of sponsorship and cash prizes mean that I
must find the setup that maximizes the fun/cost ratio.

Duncan


> On Oct 31,
2015, at 1:34 PM, Larry Young <cartravel at pobox.com> wrote:
> 
> That is an
interesting article, but only speaks to one side of the issue.  I always say
that you want to pick an intake duration for an RPM range that you're willing
and able to turn. We all know what happens to a stock TR4 crank which sees
>6,000 on a regular basis.  Some guys want a cam good beyond 7,000, but they are
just not willing to push it that far. Nowadays I'm more of a street car guy, but
still like to have power.  I don't believe the choices should be to do nothing
or go all the way to race specs.  From 1968 on Triumphs were severely detuned to
pass emissions.  Compression ratio and duration and hence cam overlap at TDC
were reduced.  For example, the early TR250/6 had 193 degrees duration (measured
at 0.050), while the TR4 was about 215 and the early PI TR5/6 was 226. It seems
crazy to rebuild a street car of this era to the original specs.  Bumping the
compression ratio by a point and increasing the duration to something like
210-215 will make the engine the way it would have been without emission
restrictions.  I would not feel guilty about polluting the planet, since most of
these cars see few miles annually and most of the other pollution controls (e.g.
ignition retard) probably quit working years ago.
> - Larry
> 
> On 10/29/2015
7:25 AM, Duncan Charlton wrote:
>> Cam specs and static CR are interactive. 
This will give you a rundown if you are not already knowledgeable about this
subject:
>> 
>>
http://www.crankshaftcoalition.com/wiki/Cam_and_compression_ratio_compatibility
>>

>> Duncan
>> (Texas)
>> 
>> 
> 
>
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