[Fot] Camshaft recommendation

Tue Feb 11 18:23:07 MST 2020

Michael,

People often seem to overlook the main performance benefit of increasing the rocker ratio.  That is the increase in valve velocity.  The velocity of a flat tappet is limited by it's diameter. If you grind the cam lobe for too much velocity the contact patch will move out past the edge of the lifter and things will self destruct.  So tappet velocity is limited.  However, since tappet velocity times rocker ratio equals valve velocity, increasing the rocker ratio will increase valve velocity.

First decide what will be the optimum valve lift for your engine then divide by rocker ratio to determine the net cam lift needed to achieve that valve lift.  Yes, you can grind the cam with a large enough lobe to reach desired lift using stock rockers but you are limited in how fast you can open the valve.  Now if you grind the cam with less lift then use 1.65:1 rockers to reach that same desired valve lift you will be moving the valve about 13% faster. That means it will be open further in a given period of time.  Graph it out and you will see a big difference.  Valve velocity is a huge factor in performance.  

I can understand why someone who has tried to use high ratio rockers with a cam that was already maxing out the valve lift with stock rockers would decide that high ratio does not work.  Valve lift was already at max. Increasing it further may be a detriment.  If duration was also maxed out for reasonable torque then the increase in lift during the overlap period caused by the increase in rocker ratio may reduce the low end torque.  Properly applied with the right cam profile, high ratio rockers enable performance that is just not achievable with stock rockers.

Richard Good
Good Parts Inc

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On Tuesday, February 11, 2020 Michael Zbarsky via Fot <mzbarsky at yahoo.com> wrote:
Thanks everyone! Good stuff. I’m sticking to a redline of just over 6K. I’d also like to keep the 1.65 setup rather than invest in a 1.55 so keep the intel and suggestions coming. The GP3 is a definite contender. 
Mike 

On Monday, February 10, 2020, 4:07 PM, Geoff Byrne <gkbyrne at optusnet.com.au> wrote:

Ditch the 1.65 rockers no good for racingGeoff ByrneTR6 racer down under

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On 11 Feb 2020, at 12:52 am, van.mulders.marcel--- via Fot <fot at autox.team.net> wrote:

I also once bought a Goodparts 1.65 rocker roller assembly for a TR6 engine, but in the end I had to use a 1.55 roller rocker assembly, also from Goodparts, because, with the 1.65, it is difficult to find a race camshaft with a  lobe lift that is low enough. With lobes of 8mm or higher, I could not find valve springs to cope with the high valve lift. I bought a G5 camshaft from Schneider, they call it  the 320-F grind (duration is 268° at 0.050", timing is 57-83    87-53 , installation figure is then 103° :is that the same as the G5 grind?) with 0.340"/8.6mm cam lift and that makes for a  valve lift of 12.9mm with the 1.55 rocker arms. With 1.65 rockers , valve lift would be 14.2mm and I could not find valve springs for that lift. The valve springs I used are part no 281003-063.20 from Summit (Isky), 320lb/inch spring rate. They recommend 0.45"/11.43mm  (1.250" - 0.800"from closed to full open position, so it is about 1.4mm more in my engine, but there is no problem concerning  coil binding : the installed height is a 1mm more than 1.250 , the pressure in closed position is lower  of course, but still 34kgs (105kgs at full lift) an that seems to be enough till 7500 rpmRacetorations have a race cam with 7.6mm cam lift, probably that would work with 1.65 rocker arms. I still have the 1.65 rocker assembly, it is new and it could work with a road cam or any camshaft with a maximum of 8mm cam lift.Marcel
Van: "fot" <fot at autox.team.net>
Aan: "fot" <fot at autox.team.net>
Verzonden: Maandag 10 februari 2020 02:34:27
Onderwerp: [Fot] Camshaft recommendation

Hi all, I’m building a TR6 race engine and have a set of 1.65 ratio roller rockers from Goodparts. Any recommendations for a suitable camshaft? I will be putting in cam bearings. 
Thanks, Mike

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