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| Cam Timing https://slantsix.org/forum/viewtopic.php?t=38163 |
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| Author: | aypee 6 [ Mon Nov 30, 2009 10:59 pm ] |
| Post subject: | Cam Timing |
Hi everyone This is my first post on this site, although I have been reading it for quite some time. This is what I have done to my engine so far, and would like some feedback if I have done things right or wrong 225 .060 over ported and polished head 1.7 Int 1.44 Exh .100 off for 42cc .080 off deck comp 10.8 to 1 gear drive (noisy) aussiespeed hurricane 4brl intake 425 4brl holley 245 @ .050 cam .325 lift at cam, .487 at valve in opens 36 in closes 78 ex opens 70 ex closes 40 cam installed at 113deg 3000 stall converter in 904 3.23 rear engine runs on LPG only, but am thinking going back to 98. What are your thoughts, and does my cam timing sound right. Thanks in advance. |
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| Author: | Joshie225 [ Mon Nov 30, 2009 11:26 pm ] |
| Post subject: | |
With the listed valve events the cam is 294/290° duration and is ground on 108° lobe centers, but is also ground 3° retarded putting the intake centerline at 111°. I would advance the cam to 102° intake centerline. With the cam as retarded as it is your are giving up a lot of low and mid range cylinder filling (and torque) due to the late closing intake valve. Other than that everything looks darn good. |
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| Author: | aypee 6 [ Tue Dec 01, 2009 2:20 am ] |
| Post subject: | Cam Timing |
Thanks Joshie225 for your reply, I am only new to this engine building game and thought I would give my slanty a little more poke for the street. When I first installed the cam I set it straight up at 111deg and advanced it 2deg to give me 113deg which I thought would give me more monifold vacuum, if advancing cam to 102deg, would this lower manifold vacuum or have I got everything mixed up! Also timing is locked at 27deg with no vacuum advance. |
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| Author: | Joshie225 [ Tue Dec 01, 2009 3:35 am ] |
| Post subject: | |
The relative cam position is expressed in degrees after top dead center that the intake valve is fully open. Advancing the cam means fewer crankshaft degrees after TDC that the intake valve is fully open. Therefore a smaller number is more advanced than a larger number. Closing the intake valve late by retarding cam timing allows the piston to push mixture back out the intake valves at low engine speeds. Also, compression cannot start until both valves are closed so a late closing intake valve reduces the running or dynamic compression ratio. Less mixture and lower compression kills low speed performance. I would run both mechanical and vacuum advance. With that much cam duration you're looking at 15-20° initial timing and enough more mechanical for right about 30° total timing. Once the mechanical advance is squared away you can add some vacuum advance to pick up economy and light throttle performance. |
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