engine acceleration goes flat at around 4500rpm

General Topics for configuring, operating and tuning the Megajolt. Also see the <a href="http://www.autosportlabs.net/MJLJ_V4_Operation_Guide">Operation Guide</a>

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blakep82
Posts: 7
Joined: Mon Aug 01, 2011 8:44 pm

engine acceleration goes flat at around 4500rpm

Post by blakep82 »

Hi, i hope someone might have some suggestions for me here, the car stars, runs and drives, it was on DCOE 40s, and when i took it to a track day it would pull fine (not setting the world alight, but was reasonable for a car that hadn't yet had the carbs set up) accelerated, pulled fine up to 4500rpm, then just wouldn't go further.

the rev limit on the edis was set to 6500, and there were no other limiter on the engine.

figured this was maybe to do with the 40s. i know they were on the edge of their limit, so today fitted a pair of 45s, these came 2nd hand, already jetted for a race tuned c20xe, and i fitted them to my mostly standard x20xev (carbs and big race exhaust are the only mods) expecting it to rev beautifully, if not maybe a little rich, chokes a little too big etc, for my engine.

it runs a little nice, bit lean, but its got 38mm chokes, and i think 36mm is what i need, took a quick drive, 4500rpm and flat again.

can only be down to the ignition. i've got a trigger wheel on the crank pully, theres a 1mm deep recess in the 5mm thick trigger to ensure it centres on the pulley, and is bolted down.

the crank sensor bracket is 5mm thick, and bolted to a now redundant 2 holes for a power steering pump cast into the block. whats important is the sensor doesn't face the edge of the timing wheel, but points at the back of it, and the timing wheel has slots laser cut out of it, so any lateral vibration of the sensor should be an issue, and the gap between the sensor and wheel is the thickness of a steel rule. so under 1mm, and they do no touch.

i've got an omex limiter set to 6500rpm, and the megajolt one set to the same.

the rpm reading on the tuner software and on the rev counter do not drop out, it just gets to 4500rpm under load, or 5000rpm with no load (free revving)

I'm lost. given the result is the same with both sets of carbs, it's got to be electric? but how?

any ideas guys?

Thanks

blakep82
Posts: 7
Joined: Mon Aug 01, 2011 8:44 pm

Post by blakep82 »

this was a hand drawn, not to scale (the holes arent as square as the pic suggests) drawing of what i've got and the sensor points the the back of it, at 90 degrees to the wheel. sorry about the size of it

Image

vr6
Posts: 11
Joined: Sun Dec 14, 2008 8:29 pm
Location: eindhoven, nedtherlands

Re: engine acceleration goes flat at around 4500rpm

Post by vr6 »

Oddly enough , im experiencing the same problem . Ive had megajolt installed for ore than 12 years and after destroying the original unit ( my own fault ) i replaced it with the newer model.
I have a vw 2.0 abf long block bored to 83mm with wossnar high compression pistons , a 1.8 16v head ( cr 11.9 measured @ 225 psi ) 276 camshafts, groupe A exhaust with 4 branch manifold, dcoe 45.

The previous engine was a abf block with a diesel crank ( 95.5mm ) with standard but modified pistons to achieve a similar cr , and this reved to 7000 no problem but i changed the block to make more rpm with less vibration. Now this engine has almoast 6000 km on the clock and still goes flat at 5k , theses no more , and my plugs are quite sooty. If i stay under 4k the plugs have a good color and the engine pulls well but nothing like it used too, then reaching about 5k the power is gone.. it looks like its flooding but on a rolling road the lambda shows huge unburned fuel, and the rev counted in the car falls away from 5200 like i hit the limiter. It looks like the ignition gets weaker with higher rpm until finally the spark is too weak to ignite the mixture.

It looks like the crank sensor input is failing with higher rpm ( it need to pulse faster ) and is not capable potentially due to age , it is mounted well, im going to replace it and the shield cabel and see if there is an improvement.

brentp
Site Admin
Posts: 5956
Joined: Wed Jan 24, 2007 6:36 am

Re: engine acceleration goes flat at around 4500rpm

Post by brentp »

Checking the VR sensor output using an oscilloscope is a good idea to see if there's a harmonic around that RPM interfering with the signal. Double check the bracket rigidity and the gap between sensor and trigger wheel as well.

vr6
Posts: 11
Joined: Sun Dec 14, 2008 8:29 pm
Location: eindhoven, nedtherlands

Re: engine acceleration goes flat at around 4500rpm

Post by vr6 »

Some feedback..

I changed the vr sensor and the cable , it was a shielded cable but not from one piece and i significantly improved the mounting bracket , it has solved the problem. I also noted that the tip of the old sensor was touching the trigger wheel at some point as the surface was damaged and there was metal exposed. I suspect that due to harmonic vibration at 5k rpm the sensor was making contact with the trigger wheel and shorting out the signal causing the megajolt to revert to the edis signal default of 10 degrees instead of 36 btdc, this would certainly account for the drop of in power,

On a side note my rev counter still drops away at 5200 rpm but i suspect that this is due to one or both of the diodes going bad and not being able to block the signal from the coil pack and effectively shorting them self's out, might account for a weak spark ?? ill look into that.

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