Hasse
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| posted on 4/1/07 at 12:02 PM |
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DIY rev limiter?
Hi,
I`ve been thinking about building a simple rev limiter for my X-flow GT engine, but know really nothing about how they normally operates.
My thought was to simply switch off ignition coil feed at about 6700rpm for a period of 1 sec, and switch on again.
So I expect to notice the switch off, and would then back off the throttle, and/or change gear.
So... comments please.
Would this work OK, or is there any major concerns?
/Hasse
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macnab
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| posted on 4/1/07 at 12:06 PM |
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This is not something I'd try and make, good luck.
My concern is that it didn't work and you wrecked the engine...
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smart51
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| posted on 4/1/07 at 12:08 PM |
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A second is a long time in the heat the moment. Why not switch it off if above 6700 and back on again below 6650? Otherwise, I'd say a quarter
of a second at a time is enough
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SeaBass
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| posted on 4/1/07 at 12:12 PM |
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What your describing is a hard cut limiter - ie ignition is cut to all cylinders for a set length of time to allow the engine to drop below the
critical rev limit.
You would be better using a microprocessor and writing the software to cut individual cylinders first just 1 cylinder then 2 and so on if revs are
still going up. This shouldn't be difficult to do as you will be counting ignition pulses to arrive at an RPM figure. This method is what is
referred to as soft cut.
Cheers
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SeaBass
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| posted on 4/1/07 at 12:53 PM |
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quote:
Originally posted by robocog
IIRC the magajolt and megasuirt soft rev limiter cycles which cylinder doesn't get a spark (I guess so you dont fuel soak plugs wrecking them or
wash the bores out)
Yup think this is true on the Emerald also...
[Edited on 4/1/07 by SeaBass]
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David Jenkins
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| posted on 4/1/07 at 01:50 PM |
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The megajolt retards the ignition 100rpm before the limit - that's what their website says, anyway!
I was warned off the rotor arm rev limiters - aparently their action can be a bit harsh, making the engine pop and bang violently.
Currently I have an Omex rev limiter (for the time being, until I fit the MJ unit) and that blocks every other spark, making a fairly painless limiter
- the engine won't rev any faster, but the only other symptom is that the engine sounds 'choked'. It picks up immediately once the
revs drop again.
David
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smart51
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| posted on 4/1/07 at 02:05 PM |
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A system that changes the ignition timing to give less power would be a lot kinder to your engine
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paulf
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| posted on 4/1/07 at 04:53 PM |
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The megajolt retards the ignition to 0 degrees 100 revs before the hard limit.
I regularly test mine when i get a bit carried away driving and it is very effective. An ignition cut would work but would be harsh and also likely to
damage the exhaust wnen the engine fires uop again.
Paul.
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Syd Bridge
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| posted on 4/1/07 at 05:17 PM |
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How do you lot think the rev limiters worked, up until everyone started to use pic's and engine management computers?
They earthed the coil, that's how!!!!
Wreck the engine? Get serious.  No moreso than when you turn the ignition off, or backoff and it overruns.
We've raced with an ignition cut rev limiter for ten years on the Mini's, and never a problem. (Well, except for the flames on the
backfire frightening a few people.)
This is very old, tried and tested, and very reliable analogue technology.
You don't have to do everything with a computer chip, you know?
Very doable with a modified Maplins rev light kit!!!!!!!!And been done, to boot! 
Cheers,
Syd.
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MikeRJ
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| posted on 4/1/07 at 05:23 PM |
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quote:
Originally posted by paulf
The megajolt retards the ignition to 0 degrees 100 revs before the hard limit.
I regularly test mine when i get a bit carried away driving and it is very effective. An ignition cut would work but would be harsh and also likely to
damage the exhaust wnen the engine fires uop again.
Paul.
AFAIK there is no hard cut rev limiter built into Megajolt. The EDIS module has no provision for this as the rev limiter on it's original
application worked by cutting fuel.
You can use one of the RPM activated outputs plus a transistor or relay to stop the sparks though.
[Edited on 4/1/07 by MikeRJ]
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BenB
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| posted on 4/1/07 at 05:51 PM |
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Yup. Hard cut limiter switches them all off for a while.
Soft cut gets rid of every other coil pulse. PIC would do the whole thing quite easily.
Non-PIC version could just use a frequency to voltage convertor, a voltage comparator, a binary counter and 4 flip-flops.... Cheap as chip but then
rev limiters aren't exactly expensive....
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paulf
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| posted on 4/1/07 at 06:01 PM |
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Thats correct the megajolt has a switched hard liimt output that can be used to pull in a relay to cut the ignition supply, it is set to be a hard
limit , the soft limit is programmed to operate 100rpm before hand . I have never used the hard limiter as the ignition retard works fine and I
considered it likely to damage the silencer if there are to many backfires, it still bangs and pops to a small extent on the soft limiter.
Paul.
quote:
Originally posted by MikeRJ
quote:
Originally posted by paulf
The megajolt retards the ignition to 0 degrees 100 revs before the hard limit.
I regularly test mine when i get a bit carried away driving and it is very effective. An ignition cut would work but would be harsh and also likely to
damage the exhaust wnen the engine fires uop again.
Paul.
AFAIK there is no hard cut rev limiter built into Megajolt. The EDIS module has no provision for this as the rev limiter on it's original
application worked by cutting fuel.
You can use one of the RPM activated outputs plus a transistor or relay to stop the sparks though.
[Edited on 4/1/07 by MikeRJ]
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NS Dev
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| posted on 5/1/07 at 10:00 AM |
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quote:
Originally posted by David Jenkins
The megajolt retards the ignition 100rpm before the limit - that's what their website says, anyway!
I was warned off the rotor arm rev limiters - aparently their action can be a bit harsh, making the engine pop and bang violently.
David
I can second that!!!!
The old 2.8i ford v6's used these!!!
Nearly spun my capri 2.8i special on the way to college one morning!! Sideways round one of the roundabouts near cannock on the A5 and it spun up a
bit more than it should, hit the limiter and bang, like hitting a brick wall, made the back end come round very quickly..........good job they had a
quickish rack and power steeing!!! Missed ym exit and had to go right round doh!!!
2.8 xr4x4 sierras are the same, quite violent in action, but simple enough! Don't suppose a 2.8 rotor arm fits a crossflow dizzy does it??
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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Hasse
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| posted on 5/1/07 at 10:51 AM |
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Thank you for all your replies!
Using a microprocessor and soft ware is not really an option. I´m to "old school" for that, and buying a rev limiter is leaving the
"locost theme", even if it not so expensive.
So, do you think using a frequency to voltage converter and a comparator with some hystereses, like 100 rpm, would work better than switching off for
a certain time? (Thought first of about 1 sec, but probably shorter)
"Hard cutting" by switch the coil to ground seems to be OK from your answers, or would it be better to disconnect the supply to the
ignition module? (Ford fiesta dizzy).
/Hasse
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Syd Bridge
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| posted on 5/1/07 at 12:27 PM |
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Hasse,
Check your u2u's for the solution that we used for the Mini's.
Syd.
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