Story and photos by Jeff Smith
Carburetor companies hate big camshafts. If you ask, they’ll tell you they get more tech questions about how to make a street engine work with a big cam than almost any other issue. The other big issue – and in America it’s all about being big – is that street camshafts are often chosen as much for their lumpy idle quality as for their power potential. So now that we’ve established that it’s not uncommon to find lots of street engines with a very large carburetor bolted to an engine with a cam with too much duration.
Rather than preach conservatism to our assembled choir of street rodders, the next best thing is to make this big cam selection run as cleanly as possible whether on the street or idling through a cruise night. The most common issues associated with a long-duration cam on the street are a very rich fuel mixture, mild to outright annoying hesitation under light acceleration, an extreme bog under hard acceleration, extremely poor fuel mileage, fouled spark plugs and in general very poor throttle response. If any of these descriptions fit your street car, then we will suggest some simple fixes that should help your car run better.
This is actually a fairly complex subject so we won’t be able to get into all the solutions and we’re going to break this story into two parts. For this story, we’ll hit a couple of simple checks and fixes that can help. If there is enough interest in this subject, we can return with more advanced recommendations that will also help. But for now, we’ll stick with the quick and easy fixes. And just to make it interesting, we’ll start not with the carburetor but instead with the ignition.
Timing the Spark
One of the biggest problems with big camshafts is when the cam is added to an engine with insufficient compression. In most cam catalogs, the larger camshaft descriptions include a compression ratio recommendation. For example, COMP’s Xtreme Energy hydraulic flat tappet XE294H cam with 250/256 degrees of duration at 0.050 and 0.588/0.593 inch lift and a 110-degrees lobe separation angle (LSA), recommends no less than 10.5:1 compression when using this cam. This recommendation is often ignored when selecting this cam but it actually is there for a very good reason. As intake duration increases, the intake closing point occurs much later. If you think about it, the engine cannot begin to compress the intake charge until the intake valve is closed. So the engine’s effective compression will be much lower than its static compression ratio because the intake valve closes later. This results in reduced cylinder pressure—especially at low engine speeds. That’s why after a big cam is installed in your engine, it seems like the engine feels sluggish at lower engine speeds.
Proof of this is to perform a cranking compression test before and after the cam swap. What you will find is that the big new cam has reduced the cranking pressure by perhaps 10-psi. The engine has not lost ring seal – the later closing intake valve prevents the engine from creating cylinder pressure until the intake valve closes. The best way to maintain a cranking compression of around 180 to 190-psi with a long-duration cam is to increase the static compression ratio. But that requires different pistons, or a smaller chamber cylinder head. Unfortunately, both of these approaches are expensive and demand quite a bit of effort. But there is a way to gain back some of that lost power through tuning.
Most stock engines use relatively conservative initial timing numbers. Even the late ’60s performance engines only spec’d single digit initial timing numbers. The initial timing spec for a 1969 SS 396 Chevelle engine equipped with a solid lifter camshaft and a 750 cfm Holley carburetor was a paltry 4-degrees BTDC. But this engine also had 11:1 compression and a cranking compression of 160-psi. A more modern street performance engine may run something closer to 10 to 12 degrees of initial timing. The problem today is that with an engine with a big camshaft and low static compression of around 8.5:1, it needs more initial timing.
Let’s assume for example, that we have a 454 Big Block Chevy that came out of a truck that’s now in between the fenderwells of your street Chevelle and the engine now has our previously mentioned XE294H camshaft, but the engine only has an 8.5:1 static compression ratio. In this case, we should bump the initial timing up to something closer to 15 to 18-degrees of initial timing. Starting the spark sooner in the combustion process will help the engine’s low-speed throttle response. However, this will probably put the total timing (initial plus mechanical advance) far too advanced once all the mechanical advance has been added in. Let’s say that the distributor’s mechanical advance adds 26-degrees of total advance. Adding 15 + 26 = 41 degrees of total ignition advance. Most street engines today run best with 34 to 38-degrees of total timing. In this case, the easiest thing is to limit the initial timing to 10-degrees – making the total 10 + 26 = 36-degrees.
Unfortunately, this doesn’t help our low-speed drivability problem so we have to go inside the distributor to limit the total mechanical advance. If you are lucky enough to be using an MSD distributor, then this step will be easy. MSD includes several ‘stop bushings’ that can be used to limit the mechanical advance. In this case, we’d like to limit the mechanical advance to 21-degrees. This will allow us to set the initial timing at 15-degrees for a total timing setting of 36-degrees. MSD supplies six different bushings: black – 18 deg.; purple-19 deg.; blue – 21 deg.;green-23 deg.; silver 25 deg.; and red – 28 deg. These numbers indicate the total amount of centrifugal advance available from the distributor. Let’s take the blue with 21 degrees, which gives us 15 degrees for initial timing. Assuming our advance curve starts at around 1,800 RPM, we now have roughly five more degrees of initial timing at lower engine speeds. If we also change the springs in the distributor so the curve is all in by 2,400 RPM, then this should also improve throttle response.
If the engine still seems sluggish or slow to accept throttle, we can try one more major change. This requires a distributor with vacuum advance. In most factory applications, the vacuum canister is connected to the ported vacuum outlet on the carburetor. This eliminates vacuum advance at idle. But with a really big cam and low static compression, you may need as much as 20 degrees of initial timing, which is difficult to achieve because you have to limit the mechanical advance to only 16 degrees. Instead, try hooking the vacuum advance hose to straight manifold vacuum. If your engine idles at only about 8 to 10 inches of manifold vacuum, this may only add 5 or perhaps 10 degrees of timing at idle, but that may improve throttle response enough to warrant the change. It’s a simple trick but very few tuners think to use it. Now you know more than they do.
At this point we must also point out the obvious that adding over 10-degrees of ignition timing to a street engine may create issues with low-speed detonation. Add a few degrees of timing at a time and then perform a road test. Finalize the initial/mechanical/vacuum advance curve with the usual caution to stay away from detonation. If the engine rattles, pull the timing back or slow the rate of advance to help eliminate detonation. Also remember that a cold inlet air system is very helpful in reducing an engine’s tendency to rattle—especially at part throttle. Even a 10-to-15 degree reduction in inlet air temperature is significant.
With the ignition timing tweaked and tuned, we’ll move on to the carburetor in part two of the story.
Fuel Air Spark Technology (FAST) 901/260-3278
Holley Performance Products 270/781-9741
Innovate Motorsports 949/502-8400