Valve Overlap  |

Hydrogen Hot Rod Parts

What is valve overlap?

Definition & Description

Overlap is when the intake and exhaust valves are open at the same time. It is expressed in degrees of crankshaft rotation.

Not all camshafts have overlap. If they do, it happens at the end of the exhaust stroke and the beginning of the intake stroke.


(Hint: If your intake valve opens before TDC and/or your exhaust valve closes after TDC, you may have overlap.)

  • Altering the cam with a decreased valve overlap decreases torque at the lean limit.

  • THC and the CH4 reductions are approximately 41% for the HCNG fuel with reduced valve overlap.

  • The NOx from the camshaft with decreased valve overlap is higher than that of original camshaft.

How is it measured?

Overlap is not usually listed on the cam card. However, it is easy to calculate if you know the opening and closing points of the valves. The formula is:

Exhaust Closing Point + Intake Opening Point = Overlap

For example, this camshaft has the following opening and closing points:

The exhaust valve closes AFTER TDC, but the intake valve opens BEFORE TDC. In this example, there will be overlap.

8° + 13° = 21° of overlap

Keep in mind the opening and closing events are listed at 0.050 in. of Valve Lift. So, the calculated overlap is at 0.050 in. True overlap will be slightly more.

Can I adjust the amount of overlap?

You can't adjust the overlap on your camshaft. You will need a new cam. For more overlap, you need more Duration and/or less Lobe Separation. For less overlap, you need less duration and/or more lobe separation.

How does it affect performance?

At high rpm, overlap takes advantage of the Scavenging Effect. However, at idle and low rpm, overlap will produce low vacuum and a rough idle. This is great for a race car, but not so much for other vehicles.

Any camshaft with a significant amount of overlap is not recommended for street-driven cars. The engine will have poor low-end power. Low vacuum also makes for a very hard brake pedal. Overlap also contributes to Exhaust Reversion, which isn't good.

just replaced the regular cam in a 500 cu in 8.2 L Caddy motor to a cam that has NO INTAKE STROKE, NOW THE MOTOR IS TWO STROKES ON ALL 8 CYLINDERS. THE INTAKE IS SEALED OFF.


But HHO has its own oxidizer and needs no intake. Nitro needs no intake.....and the Photon Reaction patent pending by Dave Parker, contains its own oxidizer too.


Such a motor can go beneath the sea or into space. No Intake and No exhaust. Fired by light. A limited chain reaction that is a million times overunity and needs no additional fuel.

4. Conclusions

The effects of the VO on the full load performance and emission characteristics of an HCNG engine were investigated. The torque values and emissions resulting from modifying the camshaft, by decreasing the VO duration, were examined for a heavy-duty natural gas. The performance, implementing an operational strategy change, was also evaluated to determine whether the engine’s specifications were satisfied.

The results from the experiments are summarized as follows:


The camshaft with a reduced VO duration has lower torque values at the lean-burn limit because of the decrease in volumetric efficiency. The decrease in exhaust gas energy also significantly decreases the boost pressure and torque values.


The increase in residual gas, caused by the reduced VO duration, shortens the ignition delay through the addition of thermal energy to the intake mixture. On the other hand, it decreases the speed of the oxidation kinetics, which is closely related to the combustion speed.


The trapping of the unburned mixture and burned gas, caused by closing the exhaust valve early, is the main mechanism of the THC and CH4 emission reduction. The THC and the CH4 emission reduction is approximately 41% for the HCNG fuel.


The level of the NOx emissions from the camshaft with a reduced VO duration is slightly higher than or comparable to the emissions from the original camshaft. This is because of the additional thermal energy of the residual gas.



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