QUESTION
I would like to install a crankcase vapour catch can on my freshly built Lotus twin cam engine in my Elan Sprint
I have a catch can on my Westfield with a Black Top 4AG E Toyota motor.
This is plumbed into the line between the head and the inlet so there is a vacuum created to draw the fumes out of the head.
Doing the same set up with the Lotus should have the same effect and not cause back pressure but I’ll heed your suggestion and see what happens.
Can you please supply a right-angle grommet for the outlet in the cylinder head, as long as it fits the Sprint motor.
D.C. - Queensland, Australia
ANSWER
Unfortunately your Black Top 4AGE engine is nothing like the Ford/Lotus twin cam engine
The Lotus twin cam engine is unusual as the crankcase is constantly pressurised
Unlike other Ford or Toyota engines there is no crankcase scavenging via the inlet plenum
Lotus have taken advantage of this design and optimised power by fitting twin 40 DCOE carburettors to the inlet runners (integral manifold)
However, there is provision for vacuum ports on No 1 and No 4 inlet runner
No 1 inlet runner is usually connected to the vacuum operated headlamp pods and experiences occasional operation
No 4 inlet runner is usually connected to the vacuum reservoir within the remote brake servo and operates on application of the brake pedal
Both of these ports enter the inlet runner between the throttle butterfly valve and the back of the inlet valve
Unlike a fuel injected engine with an oxygen sensor, carburettors cannot compensate for air being sucked in (vacuum) behind the throttle butterfly
In the case of the Lotus twin cam engine, the air being taken into the engine is metered on the atmospheric side of the throttle butterfly.
If you are going to effectively scavenge the crankcase then you will need a constant vacuum and these two mentioned ports are the only source
Tapping into one of these ports to derive a constant vacuum is not a good idea as it would cause the relevant cylinder fuel/air mixture to run super lean
I have looked at this design limitation for many years and have concluded that an external vacuum pump would be the best option
In modern day engines the crankcase is constantly scavenged and the vapours are returned to the engine via the intake plenum to be ingested and burnt off
What do you hope to achieve by attempting to scavenge the crankcase of a freshly built engine that will have little or no piston ring leakage (blow-by)
From experience, I fully understand the problems of excessive crankcase fumes contaminating and diluting fuel entering the engine (and the possibility of promoting detonation)
Excessively worn engines with significant crankcase fumes can contain hot oil vapour and if returned to the engine can cause smoke at the tail pipe (only if a mineral engine oil is used)
If you have concerns about your recently built Lotus twin cam engine and blow-by issues then there is a simple test you can carry out
Remove whatever is in the cylinder head breather hole, start the engine, hold a paper tissue in front of the breather hole and blip the throttle several times
If you have significant amounts of crankcase contaminants on the paper tissue then I recommend you carry out a cylinder leak-down test to establish piston ring leakage
A well built engine with the appropriate piston and rings you might expect leakage of 1 to 3%
A high mileage, well worn engine might show 25% leakage of more
A high leakage reading has a significant affect on compression ratio (decrease) and from experience makes starting an engine difficult
5 years ago I had an S4 Elan here and compression test showed pressure readings as low as 90 to 95 psi
And yes this engine had significant crankcase fumes which caused the spark-plugs to foul in a short space of time
I have been involved with Lotus cars since 1978 and during that time been fortunate to own various Lotus cars
During this time I have built numerous Lotus twin cam engines including turbocharged engines with charge coolers
With regular road going twin cam engines I have never found it necessary to install an oil catch can but I am always keen to learn something new
Copyright © 2025 Lotus Marques
I would like to install a crankcase vapour catch can on my freshly built Lotus twin cam engine in my Elan Sprint
I have a catch can on my Westfield with a Black Top 4AG E Toyota motor.
This is plumbed into the line between the head and the inlet so there is a vacuum created to draw the fumes out of the head.
Doing the same set up with the Lotus should have the same effect and not cause back pressure but I’ll heed your suggestion and see what happens.
Can you please supply a right-angle grommet for the outlet in the cylinder head, as long as it fits the Sprint motor.
D.C. - Queensland, Australia
ANSWER
Unfortunately your Black Top 4AGE engine is nothing like the Ford/Lotus twin cam engine
The Lotus twin cam engine is unusual as the crankcase is constantly pressurised
Unlike other Ford or Toyota engines there is no crankcase scavenging via the inlet plenum
Lotus have taken advantage of this design and optimised power by fitting twin 40 DCOE carburettors to the inlet runners (integral manifold)
However, there is provision for vacuum ports on No 1 and No 4 inlet runner
No 1 inlet runner is usually connected to the vacuum operated headlamp pods and experiences occasional operation
No 4 inlet runner is usually connected to the vacuum reservoir within the remote brake servo and operates on application of the brake pedal
Both of these ports enter the inlet runner between the throttle butterfly valve and the back of the inlet valve
Unlike a fuel injected engine with an oxygen sensor, carburettors cannot compensate for air being sucked in (vacuum) behind the throttle butterfly
In the case of the Lotus twin cam engine, the air being taken into the engine is metered on the atmospheric side of the throttle butterfly.
If you are going to effectively scavenge the crankcase then you will need a constant vacuum and these two mentioned ports are the only source
Tapping into one of these ports to derive a constant vacuum is not a good idea as it would cause the relevant cylinder fuel/air mixture to run super lean
I have looked at this design limitation for many years and have concluded that an external vacuum pump would be the best option
In modern day engines the crankcase is constantly scavenged and the vapours are returned to the engine via the intake plenum to be ingested and burnt off
What do you hope to achieve by attempting to scavenge the crankcase of a freshly built engine that will have little or no piston ring leakage (blow-by)
From experience, I fully understand the problems of excessive crankcase fumes contaminating and diluting fuel entering the engine (and the possibility of promoting detonation)
Excessively worn engines with significant crankcase fumes can contain hot oil vapour and if returned to the engine can cause smoke at the tail pipe (only if a mineral engine oil is used)
If you have concerns about your recently built Lotus twin cam engine and blow-by issues then there is a simple test you can carry out
Remove whatever is in the cylinder head breather hole, start the engine, hold a paper tissue in front of the breather hole and blip the throttle several times
If you have significant amounts of crankcase contaminants on the paper tissue then I recommend you carry out a cylinder leak-down test to establish piston ring leakage
A well built engine with the appropriate piston and rings you might expect leakage of 1 to 3%
A high mileage, well worn engine might show 25% leakage of more
A high leakage reading has a significant affect on compression ratio (decrease) and from experience makes starting an engine difficult
5 years ago I had an S4 Elan here and compression test showed pressure readings as low as 90 to 95 psi
And yes this engine had significant crankcase fumes which caused the spark-plugs to foul in a short space of time
I have been involved with Lotus cars since 1978 and during that time been fortunate to own various Lotus cars
During this time I have built numerous Lotus twin cam engines including turbocharged engines with charge coolers
With regular road going twin cam engines I have never found it necessary to install an oil catch can but I am always keen to learn something new
Copyright © 2025 Lotus Marques
