Rev Limiting for FI ?
- Thread starter powerband
- Start date
The rev limiter keeps the engine from exceeding a specified rpm by eliminating spark to various cylinders and is meant to be on only for short periods of time.
In order to keep an engine at a certain rpm under load you must kill enough power so it no longer accelerates.
What is it you want to do?
In order to keep an engine at a certain rpm under load you must kill enough power so it no longer accelerates.
What is it you want to do?
I'm new to modern high performance ign. control devices and have seen the car shows/races with Forced Induction cars holding RPM's at their optimum boost levels for launch. Obviously with Fuel Injection there is a means to limit fuel and RPM with the computer boxes, how / is this possible with carbureted Blow-thru setups?.
thanks
thanks
Yes you can control RPM and boost with a Blow-Thru Carburetor set-up
In order to get launch boost with a turbocharger in time the engine needs to be at WOT and lots of fuel so the rev limiter in the ignition system is the only thing holding the rpm.
Secondly the wastegate along with the boost controller limits the boost.
Here is one of our 7 second 1/4 mile runs at Pacific Racway.
The upper line is RPM and the lower line is Boost.
The first section from -8 to -3 seconds is where the driver is foot braking into the staging lights.
The rev limiter is set to 3400 rpm and the throttle is wide open and the boost controller is set to 9 lbs of boost.
Notice that at 3400 rpm and WOT the boost doesn't go much over a few lbs.
From -3 to 0 (Launch) is where the car is staged and the tranbrake is set so now we can raise the rev limiter to 4800 rpm and we get 9 lbs of boost just in time for launch.
Once the car starts down the track the boost controller allows boost to build till it gets to 25 lbs and it holds it there.
This particular run was 7.03 @ 198 mph shifting at 8000 rpm.
[image]https://www.dropbox.com/s/atqqgrkmzqlasam/Seattle%20Boost.jpg?raw=1[/image]
Here is the same car at Boise ID so you can see the staging and full run.
It's launching at 12 lbs of boost, 35lbs max boost and shifting at 8500 rpm.
https://www.youtube.com/watch?v=cGX-3u_huvc
That was 10 years ago.
Now they set the transbrake before staging and bump the transbrake to get into the bottom staging light.
This way you have more time at a higher staging rpm and can get a higher launch boost sooner.
This can be done with a Blow-Thru Carb or EFI set-up
In order to get launch boost with a turbocharger in time the engine needs to be at WOT and lots of fuel so the rev limiter in the ignition system is the only thing holding the rpm.
Secondly the wastegate along with the boost controller limits the boost.
Here is one of our 7 second 1/4 mile runs at Pacific Racway.
The upper line is RPM and the lower line is Boost.
The first section from -8 to -3 seconds is where the driver is foot braking into the staging lights.
The rev limiter is set to 3400 rpm and the throttle is wide open and the boost controller is set to 9 lbs of boost.
Notice that at 3400 rpm and WOT the boost doesn't go much over a few lbs.
From -3 to 0 (Launch) is where the car is staged and the tranbrake is set so now we can raise the rev limiter to 4800 rpm and we get 9 lbs of boost just in time for launch.
Once the car starts down the track the boost controller allows boost to build till it gets to 25 lbs and it holds it there.
This particular run was 7.03 @ 198 mph shifting at 8000 rpm.
[image]https://www.dropbox.com/s/atqqgrkmzqlasam/Seattle%20Boost.jpg?raw=1[/image]
Here is the same car at Boise ID so you can see the staging and full run.
It's launching at 12 lbs of boost, 35lbs max boost and shifting at 8500 rpm.
https://www.youtube.com/watch?v=cGX-3u_huvc
That was 10 years ago.
Now they set the transbrake before staging and bump the transbrake to get into the bottom staging light.
This way you have more time at a higher staging rpm and can get a higher launch boost sooner.
This can be done with a Blow-Thru Carb or EFI set-up
62Cometman
Well-known member
The setup you are referring to is call 2 Step system, similarly related to that is something call anti-lag. 2 Step is just a form of ignition rev limiting at a lower RPM for launching and anti lag actually changes fuel timing so more fuel actually leaves the chamber and burns in the exhaust to more quickly spool the turbo. Pmuller is right that you do not want to hold ignition based rev limiting for extensive periods of time as they are basically a controlled miss in the engine. Transbrakes work in a different way without harming the engine. However since you usally run a manual transmission the other 2 options would likely work best for you. Also to limit boost an electronic boost controller can limit boost in any gear so you only make so much in 1 st and 2nd then full boost in 3-5. All of which make you parts last longer if its gonna be drag raced. In either case good luck! 
Are you looking at doing something with your Vortech V2 250 six and T5 tranny?
I've done a few dozen launches with the NA 61 Comet 250 tri-Power / T5, and experimented with different launch RPM's simply with the Tach. I'd like to be able to hold the revs steady (and fairly safe). Also want to stage the SC'd 250 Maverick but am concerned about over-revving at the launch before dropping the clutch. SC was pushing @ 5-8 PSI, now with changed SC's drive pulley it can easily push 20 lbs. The Vortech / 250 has a BOV on the pressure tube to Carb I can calibrate to desired boost.
Before going the MSD BTM [$$] , I've been researching other methods with the developing device options. The (Pertronix) inexpensive HEI module with onboard Rev limiter got my attention as apossible development tool ...
https://www.summitracing.com/parts/pnx-d72000/overview/
haev fun
I've done a few dozen launches with the NA 61 Comet 250 tri-Power / T5, and experimented with different launch RPM's simply with the Tach. I'd like to be able to hold the revs steady (and fairly safe). Also want to stage the SC'd 250 Maverick but am concerned about over-revving at the launch before dropping the clutch. SC was pushing @ 5-8 PSI, now with changed SC's drive pulley it can easily push 20 lbs. The Vortech / 250 has a BOV on the pressure tube to Carb I can calibrate to desired boost.
Before going the MSD BTM [$$] , I've been researching other methods with the developing device options. The (Pertronix) inexpensive HEI module with onboard Rev limiter got my attention as apossible development tool ...
https://www.summitracing.com/parts/pnx-d72000/overview/
haev fun
The ignitions you are looking at have a set rev limiter that is always on and prevents the engine from overrevving and getting damaged.
As 62cometman was pointing out, you are looking for an ignition box that has a "2 step" rev limiter that can be switched on and off.
The newest one and least expensive is the PerTronix Digital HP Ignition Box 510. It has some very nice features and has more spark energy than the MSD counterpart.
The previous choice was the MSD 6AL-2
For a clutch set-up we use the Omron Z-15GQ-B7-K snap action switch.
This is the switch that activates the 2 step rev limiter.
https://www.dropbox.com/s/32v634bu8ughz ... 3.JPG?dl=0
It activates as soon as the plunger is depressed but the plunger can be depressed an additional 1/4" before it bottoms out.
The switch mounts to a bracket that attaches to the pedal assembly.
You get to be creative and design and install the bracket so the switch is activated by the clutch pedal arm.
The switch has a 1/2" threaded section so it's depth in the bracket can be adjusted.
It is important that the switch is adjusted so it de-activates as the clutch begins to load the engine.
At launch the throttle is wide open so you don't want the 2 step rev limiter to let the engine go untill the clutch is at least partially engaged.
For The SC'd Maverick 250:
The all in one unit that controls the timing curve and boost retard curve along with a 2 step rev limiter is the MSD 6530. I have used this box along with many of the street and racing MSD ignition systems.
All timing functions are programmable by laptop. There is a timing graph/map for the advance and one for the boost retard.
Once you get lines on map you can just grab points along the line with your curser and drag them to wherever you want to make changes.
Since the MSD 6530 is doing all the timing you simply lock the distributor's mechanical and vacuum advance and use it to trigger the MSD box.
As 62cometman was pointing out, you are looking for an ignition box that has a "2 step" rev limiter that can be switched on and off.
The newest one and least expensive is the PerTronix Digital HP Ignition Box 510. It has some very nice features and has more spark energy than the MSD counterpart.
The previous choice was the MSD 6AL-2
For a clutch set-up we use the Omron Z-15GQ-B7-K snap action switch.
This is the switch that activates the 2 step rev limiter.
https://www.dropbox.com/s/32v634bu8ughz ... 3.JPG?dl=0
It activates as soon as the plunger is depressed but the plunger can be depressed an additional 1/4" before it bottoms out.
The switch mounts to a bracket that attaches to the pedal assembly.
You get to be creative and design and install the bracket so the switch is activated by the clutch pedal arm.
The switch has a 1/2" threaded section so it's depth in the bracket can be adjusted.
It is important that the switch is adjusted so it de-activates as the clutch begins to load the engine.
At launch the throttle is wide open so you don't want the 2 step rev limiter to let the engine go untill the clutch is at least partially engaged.
For The SC'd Maverick 250:
The all in one unit that controls the timing curve and boost retard curve along with a 2 step rev limiter is the MSD 6530. I have used this box along with many of the street and racing MSD ignition systems.
All timing functions are programmable by laptop. There is a timing graph/map for the advance and one for the boost retard.
Once you get lines on map you can just grab points along the line with your curser and drag them to wherever you want to make changes.
Since the MSD 6530 is doing all the timing you simply lock the distributor's mechanical and vacuum advance and use it to trigger the MSD box.
Thanks for the replies,
I will probably try out the new Pertronix HEI's with rev limiter on the NA Comet 250. Still unclear whether this type control will hold revs but still deliver full power. (as in staging)
The SC'd Maverick 250 will need timing retard referenced to boost. I am getting detonation at the new higher boost levels and looking at options for 'when the snow melts'. Winter projects include rebuilding and installing a Cobra T5 in place of the Toploader 3OTT , a Posi unit for a 3.80 center and some drivability upgrades.
have fun
I will probably try out the new Pertronix HEI's with rev limiter on the NA Comet 250. Still unclear whether this type control will hold revs but still deliver full power. (as in staging)
The SC'd Maverick 250 will need timing retard referenced to boost. I am getting detonation at the new higher boost levels and looking at options for 'when the snow melts'. Winter projects include rebuilding and installing a Cobra T5 in place of the Toploader 3OTT , a Posi unit for a 3.80 center and some drivability upgrades.
have fun
powerband":1kt5dweo said:
Take the time to look at the MSD 6530 instructions and see if you have any questions.
https://documents.holley.com/6530.pdf
The way it works is the following:powerband":2fvj9dwv said:
You stage then push the clutch pedal in which activates the 2 step rev limiter by way of the clutch switch you installed.
You then hold your throttle wide open and the 2 step holds the engine at the rpm you set the 2 step at.
As you let the clutch out to launch, the clutch switch deactivates the 2 step just as the clutch is engaging and the car lauches at the full power available at that rpm.
The Pertronix 510 has both the tach output and the shift light output lines.
Having a large LED Shift light works nicely for both day and night racing.
It also uses the tach to verify the rev limiters rpm settings.
There is a tach calibration feature to check the tach's accuracy.
Here are the instructions for the Pertronix 510
https://www.jegs.com/InstallationInstru ... 51-510.pdf
May I suggest that before you purchase an ignition system, get the switch, mount it and get it adjusted as described in post #8
The switch is the key component in making this work.
If you have any questions please ask.