Q&A / Tech

Ask Away! with Jeff Smith: Choosing an Optimal Camshaft and Ignition Timing for Double Hump Heads

This is directed to Jeff Smith – great article on ignition timing.  I have a few questions.  

My problem is I can’t seem to get the timing right with either the initial or top end total and my engine is slow to start.

I have a ‘67 Camaro with a 327 with double hump heads running stainless steel 2.02 values, springs rated for a .600 lift cam. Holley 750 DP with manual secondary, Holley blue electric fuel pump set at 7 psi. The ignition is an MSD 7A ignition with an MSD distributor sitting on an Assault Racing Polished dual plane aluminum intake manifold with a Muncie M22 trans and 3.73:1 rear gears. 

Cam specs are as follows:

  • Crane Cam 
  • 252/ 252 degrees duration – at 0.050-inch tappet lift
  • 0.521-inch valve lift

I advanced the cam 2 teeth (I have no idea why I did this 25 years ago). Should I set the cam/crank back to zero?

What should my initial and total timing be?  

Any help would be great.

Todd B.

Jeff Smith: Here is where supplying plenty of information with your question really helps diagnose a problem. Todd included his cam specs along with engine size, transmission, and gear ratio. From the description, one point immediately jumped out at me. We’ll get to his ignition timing questions but the more important item is that he mentioned advancing the cam by two teeth. Yikes!

If he had said he advanced the cam 2 degrees, that would be acceptable. But his description said two teeth! I sent him a return email and he verified that he advanced the camshaft two teeth on the cam gear. That’s a huge change.

Assuming he used a typical dual-row performance timing set, most are set up with a crank gear with 22 and a cam gear of 44 teeth. The cam gear will always be double the number of crank teeth because the cam must always spin at half engine speed. The cam that Todd selected is a pretty big camshaft with 252 degrees at 0.050 (308 degrees advertised duration).

All cam specs are listed in crankshaft degrees to make it easier to measure everything with a degree wheel. So 360 degrees / 22 teeth = 16.4 degrees per tooth on the crank gear. Since the cam gear has twice as many teeth, we can express that as 720 degrees / 44 = 16.4 degrees. So each tooth on the cam gear is worth 16.4 degrees.

This is a cam and crank gear layout just to put all this gear talk into perspective. The cam gear (top) will always have twice as many teeth as the crank gear (bottom) so that the cam spins at half engine speed.

This is a cam and crank gear layout just to put all this gear talk into perspective. The cam gear (top) will always have twice as many teeth as the crank gear (bottom) so that the cam spins at half engine speed.

When Todd advanced his cam gear by two teeth, he effectively moved the cam by 33 degrees! This is a huge change. By comparison, the better aftermarket timing sets allow you to move the cam usually no more than 8 degrees.

One way to look at what Todd has done is by looking at the position of the intake centerline (ICL). This cam has an intended ICL of 107 degrees after top dead center (ATDC) when installed “straight up” with the timing gear dots aligned and assuming everything is in the right place. By advancing the cam 33 degrees – this pushes the ICL from 107 to 74 degrees ATDC. According to COMP Cam’s Billy Godbold, 21st Century engines now use what is called variable valve technology (VVT) that “swings” – advances or retards – the camshaft based on engine speed and load. Bill says VVT can move the cam as much as 50 to 60 degrees which is a monstrous amount of movement. This movement is controlled by the engine computer and constantly adjusts the cam based on engine rpm and load.

Godbold says that COMP Cam testing reveals that advancing the cam to around 90 degrees ATDC helps these stock LS cams at low engine speeds. As rpm and load increases, the cam timing is retarded. Our previous math shows that the small-block Chevy cam is far in advance of that number by another 16 degrees (74 vs. 90 degrees ATDC)!

A very important issue with either advancing or retarding cams is that this directly affects piston-to-valve (P-V) clearance. When the cam is advanced, this brings the intake valve closer to the piston, decreasing the P-V clearance while increasing the exhaust valve clearance. So advancing the cam by 33 degrees should have radically tightened the P-V so much that the intake valves should have hit the pistons. But with a short stroke 327 that might be 0.025-inch below the deck (piston-to-deck clearance), it’s apparent the valves just barely miss.

A subsequent email from Todd also mentioned that his cranking compression was a mere 85 psi. This is another major clue that something is very wrong. A typical 9:1 compression 327 – even one equipped with a long-duration camshaft – should have at least 160 psi of cranking pressure and perhaps more. Todd’s 85 psi reveals that the cam timing is way off and there is only minimal cylinder pressure – which is why the engine is hard to start and runs so poorly.

So now that we’ve identified the real problem with Todd’s engine, we can make some suggestions. The Crane description for this cam says “Competition only – requires 12.0:1 minimum compression” among other suggestions. Assuming this is a street car, this cam offers too much duration by at least 15 to 20 degrees. Since the front half of the motor needs to be accessed to change the cam position, we’d recommend a complete cam swap.

A better cam – sticking with Crane – might be a Crane H-296-2 which specs out at 234/242 degrees at 0.050 with 0.473/0.488-inch lift. This cam is 18 degrees shorter and will run much better and offer much more power in the lower engine speeds where street engines spend all their time. If the loss of roughly 0.040-inch of valve lift is bothersome, I’d suggest adding 1.6:1 rockers to this smaller cam. That will pump the valve lift by almost exactly 0.030-inch with intake lift around 0.503-inch.

I would insist that Todd degree the cam to ensure it is installed correctly and once that is achieved, for ignition timing I’d recommend 15 to 16 degrees initial timing with a curve that achieves 40 degrees of total timing. This means the mechanical advance will need to create 24 degrees of mechanical advance.

Some may howl at that much total – but remember – this engine uses the old double-hump iron heads with casting numbers (I’m guessing) ending in 461 or 462. These were very inefficient chambers so they need the extra timing. Todd may discover that 42 degrees might make it run even better. These recommendations assume that the compression is probably not more than 9.5:1. Remember, this is a short stroke 327. Making compression with what we’ll assume are flat top pistons will be difficult with the 3.25-inch stroke. All this assumes that the engine won’t detonate with this much timing. If it does, then remove 3-4 degrees and try again.

These same timing figures will work for that larger cam although I would push the initial to perhaps 18 degrees initial. With that big cam and minimal compression, the engine will need a bunch of timing because it’s going to suffer from lots of exhaust dilution in the intake manifold – much like a huge dose of EGR.

I would also suggest inspecting the inside of the intake manifold and look for black, sooty residue in the interior of the manifold. If that exists, that’s exhaust dilution from a combination of a late closing intake valve and overlap. The best way to eliminate that is by running a shorter duration camshaft. There’s no way to fix that problem without changing camshafts.

Hope this helps!

Tags: , ,

2 Comments

  1. Very good read. Hit on a couple of things I didn’t even realize was an issue. The sooty intake to be more specific. I guess I need to reevaluate a few things. Thank you.

  2. Great information thanks

Leave a Reply

Your email address will not be published. Required fields are marked *

*

This site uses Akismet to reduce spam. Learn how your comment data is processed.