Reversion happens when exhaust gases flow backwards and contaminate the intake charge. (It is the opposite of The Scavenging Effect.)
What causes it?
Backpressure in the exhaust system causes reversion. It gets worse with excessive Valve Overlap and improper Valve Timing.
Performance camshafts take advantage of scavenging at high rpm. If the exhaust system is too restrictive for the cam, reversion can occur.
Reversion usually happens at low and mid rpm. Remember, air will always try to move from high pressure to low pressure. So, at low rpm, during valve overlap, exhaust gases can flow backwards due to:
- Low pressure in the intake manifold (vacuum)
- High pressure in the exhaust system (backpressure)
- The upward motion of the piston
How does it affect performance?
Leftover exhaust in the cylinder means less room for fresh air and fuel. Less clean air and fuel equals less power. At idle and low rpm, reversion will also cause:
- Lower intake vacuum
- Poor idle quality
How is it identified?
In carbureted engines, fuel may be seen “standing” above the carb. The fuel can also get pushed up into the air cleaner. If your air cleaner is wet and smells like fuel, reversion is likely to blame.
Another indicator is exhaust soot in the intake runner. Look for discoloration in the cylinder heads and intake manifold.
How do you prevent it?
Some valve overlap is needed to maximize the scavenging effect. So, all high-performance engines will have some reversion, especially at low rpm. However, it can be minimized.
Improve Your Exhaust Flow: A stock exhaust can have 8 psi (or more) of backpressure at 6,000 rpm. Log-style manifolds can cause exhaust from one cylinder to be sucked into another. Upgrade your exhaust system with a set of Headers and a free-flowing Exhaust Kit.
Pick the Right Parts: Long Duration cams with lots of Overlap make more power at high rpm. However, the intake, heads, and exhaust must be upgraded to allow for proper airflow. Mismatched components will make the engine run worse, not better.
Warning: DO NOT port match the cylinder head to the headers. Many headers have ports that are larger than the ports on the head. This design helps resist the reverse flow of exhaust.