Engine Back Pressure??
Thread Starter
Drifting
Joined: Jan 2002
Posts: 1,645
Likes: 11
From: Schaumburg IL
Engine Back Pressure??
Has anyone heard of a term called "reversion" as related to a bbc.
Issue is that I have a Q-Jet that drips from the boosters, in fact I have 3 Q-Jets that drip. I have had one of them worked on by a known expert and the problem persists. Fuel pressure is from a stock fuel pump so I don't think it is too high - however, I am installing both a gauge and regulator.
The effect of this issue is the intake runners and head ports are wet with gas and an oil mixture if you pull the carb and/or intake manifold. I have tried blocking off the PCV line with no effect. There are white clouds of smoke when reviing the engine.
Now the real kicker - if I install a Holley Avenger 770cfm, after about an hour of run time the ports and runners dry out and the smoke almost - not totally stops.
I have been told from some engine builders that I may have "reversion" going on. OK, what is this, how do I identify it and how do you fix it??
Issue is that I have a Q-Jet that drips from the boosters, in fact I have 3 Q-Jets that drip. I have had one of them worked on by a known expert and the problem persists. Fuel pressure is from a stock fuel pump so I don't think it is too high - however, I am installing both a gauge and regulator.
The effect of this issue is the intake runners and head ports are wet with gas and an oil mixture if you pull the carb and/or intake manifold. I have tried blocking off the PCV line with no effect. There are white clouds of smoke when reviing the engine.
Now the real kicker - if I install a Holley Avenger 770cfm, after about an hour of run time the ports and runners dry out and the smoke almost - not totally stops.
I have been told from some engine builders that I may have "reversion" going on. OK, what is this, how do I identify it and how do you fix it??
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
Reversion occurs to some degree in all engines. On the intake side, the flow reverses up the intake from a late-closing intake valve. You'll typically see it more pronounced in engines with high-duration on the cam. You can also experience some internal EGR from high-overlap cams where some exhaust gas is introduced through the early stages of the cylinder intake cycle. The evidence is sooting on the venturi and booster. It's not normally something that requires attention. It just is and we spray the carb down from time to time with carb cleaner to make it pretty again.
If the boosters are dripping fuel at idle, there's a problem in the fuel system. Trying to solve this by chasing a theoretical ghost like reversion will be as effective as changing brake pads.
If the boosters are dripping fuel at idle, there's a problem in the fuel system. Trying to solve this by chasing a theoretical ghost like reversion will be as effective as changing brake pads.
Le Mans Master
Joined: Nov 2008
Posts: 5,691
Likes: 12
From: columbus oh
Reversion occurs to some degree in all engines. On the intake side, the flow reverses up the intake from a late-closing intake valve. You'll typically see it more pronounced in engines with high-duration on the cam. You can also experience some internal EGR from high-overlap cams where some exhaust gas is introduced through the early stages of the cylinder intake cycle. The evidence is sooting on the venturi and booster. It's not normally something that requires attention. It just is and we spray the carb down from time to time with carb cleaner to make it pretty again.
If the boosters are dripping fuel at idle, there's a problem in the fuel system. Trying to solve this by chasing a theoretical ghost like reversion will be as effective as changing brake pads.
If the boosters are dripping fuel at idle, there's a problem in the fuel system. Trying to solve this by chasing a theoretical ghost like reversion will be as effective as changing brake pads.
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
Le Mans Master
Joined: Jan 2006
Posts: 6,506
Likes: 139
From: Butler Pa
I have been told from some engine builders that I may have "reversion" going on. OK, what is this, how do I identify it and how do you fix it??
Race Director
Joined: Mar 2006
Posts: 14,112
Likes: 28
From: Florida
Qjets run fine on as little as 3psi and you are right in checking even the pressure of a stock pump.
Run as little consistant pressure as possible without starving the carb at wot. You need flow, not pressure.
Run as little consistant pressure as possible without starving the carb at wot. You need flow, not pressure.
Thread Starter
Drifting
Joined: Jan 2002
Posts: 1,645
Likes: 11
From: Schaumburg IL
I'll try to answer all questions
1. I use the standard Q-Jet gasket for the carb 1/4 thick etc. This is a 73 so the alum plate is not used in that year.
2. I think the fuel pressure may be too much
3. I agree if the Q-Jet is still dripping then the Holley is a solution.
4. The cam is Crane 272 or 218/218 at .050 and .500 lift with LSA of 112. Not very radical.
thanks
JP
1. I use the standard Q-Jet gasket for the carb 1/4 thick etc. This is a 73 so the alum plate is not used in that year.
2. I think the fuel pressure may be too much
3. I agree if the Q-Jet is still dripping then the Holley is a solution.
4. The cam is Crane 272 or 218/218 at .050 and .500 lift with LSA of 112. Not very radical.
thanks
JP
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Thread Starter
Drifting
Joined: Jan 2002
Posts: 1,645
Likes: 11
From: Schaumburg IL
Thanks for the idea on the return line. There is one but as you say, it may be kinked or plugged. That is a later project as I am into changing the intake gaskets and adjusting the valves.
