My racecar (190E 2.3-16) was converted from Bosch Kjet to twin Weber DCOE carbs. As part of the conversion, we had to swap out the high-pressure fuel pump for something more carb-friendly. The new pump isn't cooperating with the old Bosch fuel pump relay.
I can bypass the relay, but then I'd lose the rev limiter. On these old Benzes, the rev limit is handled by the fuel pump relay/module/thingy.
Let's say I switch to a generic fuel pump relay that's connected to a switch on the dash. Is there a way to add a rev-limiter to such a setup?
An aftermarket rev limiter wired into the tach lead off the coil.
Why won't the pump work with the relay? Pump should be single speed, so one hot connector, one ground. The relay should only have one output to the fuel pump. If the system is for a multi speed pump, then look into changing the relay to a single speed and running the fuel pump on max. Although it sounds like you already have a a low psi carb pump. So you should just need the high speed side of the relay from the benz.
I just realized I've been looking at this all wrong.
Does a fuel-cut rev limiter even work with carbs? Since there's fuel in the float bowls, I suppose it would continue to run for a while even after fuel supply is cut off.
So, I guess I need to be looking for a way to cut spark instead ... right?
LanEvo wrote:
I just realized I've been looking at this all wrong.
Does a fuel-cut rev limiter even work with carbs? Since there's fuel in the float bowls, I suppose it would continue to run for a while even after fuel supply is cut off.
So, I guess I need to be looking for a way to cut spark instead ... right?
Correct. Cutting the pump with a carb is dangerous, it'll just cause lean spikes that could do damage. You need a spark cut limiter.
you can go old school with a gas pedal stop. aka a bolt that doesnt allow full throttle.
We ran an MSD box with a limiter. It has little chips you plugged in to set it and it cut spark.
rslifkin wrote:
LanEvo wrote:
I just realized I've been looking at this all wrong.
Does a fuel-cut rev limiter even work with carbs? Since there's fuel in the float bowls, I suppose it would continue to run for a while even after fuel supply is cut off.
So, I guess I need to be looking for a way to cut spark instead ... right?
Correct. Cutting the pump with a carb is dangerous, it'll just cause lean spikes that could do damage. You need a spark cut limiter.
This. Of course a spark cut limiter has its own downsides (increased ring wear, oil dilution, any cheap mufflers blowing up like balloons over time) but it's the only kind you can use with a carb, unless that carb has an e-throttle by some bizarre chance.
My 4AGE has a spark cut limiter set 200rpm above the fuel cut limiter to prevent overshoots, which it absolutely can do if you rev the engine out of gear. I'm going to add some spark retard to create a soft limiter to reduce the chance of any more mechanical damage to the bottom end.
Most spark cut limiters don't cut the spark completely, they cut random cylinders to keep the engine at the chosen rpm point. You shouldn't get oil dilution, ring wear or blown up mufflers, unless you ride the limiter for a considerable amount of time.
GameboyRMH wrote:
This. Of course a spark cut limiter has its own downsides (increased ring wear, oil dilution, any cheap mufflers blowing up like balloons over time) but it's the only kind you can use with a carb, unless that carb has an e-throttle by some bizarre chance.
My 4AGE has a spark cut limiter set 200rpm above the fuel cut limiter to prevent overshoots, which it absolutely can do if you rev the engine out of gear. I'm going to add some spark retard to create a soft limiter to reduce the chance of any more mechanical damage to the bottom end.
Just how much time are you planning on spending on the limiter?
If you want to go really low tech, and if the distributor will accommodate it, Bosch made (makes?) a rev-limiting rotor with a weighted spring cut-off. I've got a 6500rpm one in my 2002.
Several versions are available: http://www.pacificcustoms.com/bosch04024.html
GameboyRMH wrote:
This. Of course a spark cut limiter has its own downsides (increased ring wear, oil dilution, any cheap mufflers blowing up like balloons over time) but it's the only kind you can use with a carb, unless that carb has an e-throttle by some bizarre chance.
My 4AGE has a spark cut limiter set 200rpm above the fuel cut limiter to prevent overshoots, which it absolutely can do if you rev the engine out of gear. I'm going to add some spark retard to create a soft limiter to reduce the chance of any more mechanical damage to the bottom end.
How does a rev limiter cause ring wear?
z31maniac wrote:
GameboyRMH wrote:
This. Of course a spark cut limiter has its own downsides (increased ring wear, oil dilution, any cheap mufflers blowing up like balloons over time) but it's the only kind you can use with a carb, unless that carb has an e-throttle by some bizarre chance.
My 4AGE has a spark cut limiter set 200rpm above the fuel cut limiter to prevent overshoots, which it absolutely can do if you rev the engine out of gear. I'm going to add some spark retard to create a soft limiter to reduce the chance of any more mechanical damage to the bottom end.
How does a rev limiter cause ring wear?
With a carburetted engine, no spark, still drawing fuel, raw gasoline in the cylinders washes them down.
I wouldn't want a rev limiter that works by reducing fuel. MSD box on the ignition is what I would use. My RX7 has an MSD box, but I don't run a rev limiter - just my right foot and my wallet.
gearheadE30 wrote:
GameboyRMH wrote:
This. Of course a spark cut limiter has its own downsides (increased ring wear, oil dilution, any cheap mufflers blowing up like balloons over time) but it's the only kind you can use with a carb, unless that carb has an e-throttle by some bizarre chance.
My 4AGE has a spark cut limiter set 200rpm above the fuel cut limiter to prevent overshoots, which it absolutely can do if you rev the engine out of gear. I'm going to add some spark retard to create a soft limiter to reduce the chance of any more mechanical damage to the bottom end.
Just how much time are you planning on spending on the limiter?
Hardly any, but I'm a bit paranoid now. If I wreck another 4AGE crank (and/or rod) I don't know if I could get a replacement a second time.
In reply to MrChaos:
That's silly. Racing, full throttle is it.
Install hydraulic lifters.
wspohn
HalfDork
10/5/16 5:14 p.m.
The MSD limiter is a soft limiter and works well. Use them on the race cars.
