Oldmics (2/1/2010)
For sake of discussion, If a blower cam is retarded (when using a blower setup), would the engine benefit if switched over to carbs and advancing the cam?Depends strictly on the camshaft and exactly where or how it’s installed. Although the answer should be yes, there’s not a clear cut answer to this unless specific details about the camshaft itself are known.
Let's see if I can condense some basic knowledge regarding camshafts to just a few paragraphs. As a general rule, advancing the camshaft increases low end torque by increasing the dynamic compression ratio. The intake valve closing value on the cam is a major player in regards to the dynamic compression ratio and exactly where the intake valve closes after BDC is a determining factor for the cranking compression. As the cam is advanced, the intake valve closes earlier or closer to BDC thus allowing additional cylinder volume for the cranking compression to increase which translates to an increase in torque at the lower rpms. Cranking compression doesn't start until the intake valve is closed. The sooner it closes, the higher the cranking compression.
Assuming the camshaft is optimum for the engine, then advancing it moves the torque band to a lower rpm while retarding the cam moves the torque band to a higher rpm. This is a tradeoff as when the performance peaks are shifted in the rpm band, they fall off in other parts of the band. If the camshaft is too big to begin with (too much duration), then advancing it can help to restore some of the misplaced low end torque. If the camshaft is too small then retarding it can move the rpm band up somewhat while also lowering the cranking compression at the same time. But if a camshaft must be advanced or retarded a significant amount to regain performance in the rpm range that’s desired, then another camshaft should be considered. This is where talking to your cam manufacturer for a recommendation will get you closer to what you really need than just arbitrarily picking a camshaft out of the catalogue.
Keep in mind that advancing or retarding the camshaft moves the relationship of the valves in regards to the pistons. Advancing the camshaft brings the intake valve closer to the piston while giving the exhaust valve more clearance. Likewise, retarding the camshaft brings the exhaust valve closer to the piston while moving the intake valve further away. The rule of thumb I use is 0.025” change in valve to piston clearance for each 4° the camshaft is moved.
And then there’s lobe centerline. If the lobe centerline is tight (small value) then the torque will be peakish as it will come on strong when it does start but will also leave in a hurry. If the lobe centerline is wider, the torque will start coming in earlier but will not be as high or peaked and will leave later. Intake manifold vacuum is typically better with a wide lobe centerline cam grind versus one that has narrower lobe centers. Camshafts that are ground on tight or close centerlines will have more overlap at TDC and hence, more rumble or lope due to the inefficiencies this causes at lower rpms. This same overlap is a player in the manifold vacuum as the vacuum decreases as the overlap increases. That’s because the open exhaust valve essentially causes a vacuum leak and thus reduces the vacuum. At higher rpms, this same overlap promotes scavenging by having the suction created by exhaust flow in the exhaust helping to draw in the intake charge. Too much scavenging is also not a good thing so there’s a fine line here. I'll add that too much overlap as a result of tighter lobe centerlines on a supercharged or turbocharged application can have part of the boost going out the exhaust and not adequately filling the cylinder as intended.
And let's not forget the lift. If the heads are ported, then they are expected to handle more total valve lift which translates to more topend power. More lift if done in conjuction with heads that flow well will allow a larger intake charge to fill the cylinder on each intake stroke. This simply translates to more power by lieu of increased combustion pressure without having to actually increase the compression ratio. In a drag car, this ulitmately means more mph. If the camshaft lobes have aggressive opening ramps by way of a small advertised duration and a high duration at 0.050”, then you can have the best of both worlds in having good low end torque as well as improved performance at the higher rpms. But this comes at a cost and it’s usually at the expense of increased lobe and lifter wear as well as requiring stiffer valve springs. Because of the wear factors involved, more lift is typically accompanied by an increase in duration. This is where roller camshafts shine as you can get the best of both worlds without the wear associated with a flat tappet camshaft when trying to use camshafts with very aggressive profiles.
The Isky camshaft for last years EMC Y entry had the following specs.
282° Int / 286° Exh advertised duration
254° Int / 258° Exh duration at 0.050”
Ground on 109° lobe centers
Installed at 107° intake lobe centerline (2° advanced)
0.370” Int / 0.376” Exh lobe lift
Dove 1.6:1 aluminum roller rockers.
Best performance was with valve lash at 0.016” hot on both valves.
165 lbs cranking compression with 10.1:1 static cr and 8.0:1 dynamic cr.
Good overall performance up to 6500 rpms on 91 octane fuel and has good idle characteristics with decent manifold vacuum. But don’t look for this camshaft to work well in a run of the mill stock headed Y.
In a nutshell, the camshaft must be selected on the basis of the application to which it’s being fitted. There is not a single camshaft that can be considered a ‘fit all’ for any modified engine. A camshaft that’s best suited for a blower application will not be the best fit for a normally aspirated application but almost any camshaft will start up and run regardless of its misapplication.
Lorena, Texas (South of Waco)