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Cut spark instead of fuel at rev limiter?

crazybiker

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It also seems to be a highly contentious topic about how running-in should actually be done. Manufacturers tend to say "take it easy", many mechanics claim that a good flogging helps the piston rings bed in against the liners better or some such.

That said I'd still follow whatever the manufacturer recommendations are simply because I assume they know best having designed the thing.



As have I. I've heard with boat motors aswell that your meant to do the same thing. But to me, it just doesn't seem normal, so I always take care running in a motor.
 

Matt_vp

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When you say it "can" be made to work, is it any trickier than wiring one into a supra or silvia?

No not really, depend on what type of commodore.

I seem to recall seeing a video of an engine assembly line, where one of the steps after it's been fully assembled was to rev it at high RPM for about a minute? I've also heard from numerous places that modern engines are pre-broken in by the time you get hold of the car, and that the break-in period is much less crucial than it used to be in the past.

It also seems to be a highly contentious topic about how running-in should actually be done. Manufacturers tend to say "take it easy", many mechanics claim that a good flogging helps the piston rings bed in against the liners better or some such.

That said I'd still follow whatever the manufacturer recommendations are simply because I assume they know best having designed the thing.

Mechanic tell ya to flog it because it means more work for them.

Maybe that inital 1 min of high revs beds every thing in but if the engine breaks during that minute what happens. They throw it in the bin and start again, if your engine blows up in a new car, what happens. It goes to manufacturer and they plug there computer into it. Check the top rpm, how many times the limiter has been hit, top speed ect. Then decide if they want to give you a new engine or if u will be paying for it. Seen it happen to a mate in a new falcon at then end of 08.

Seen blokes with dirt bikes say the same thing about flogging it, and others run it in properly, the ones that got flogged had a very short life compared to the ones that are properly run in.

Im no mechanic, but do work in the metal industry and can see the need to run large chuncks of alloy with lots of moving parts made out of various steels in.

If you cut the spark but not the fuel supply, won't you be dumping fuel down the exhaust and ruining your cats? Assuming your running cat.

If you have modified your car enough to need a spark cut limiter, im sure you would have steel or empty cats in your car.

All in all i think spark cut is better than fuel cut on highly modified cars or cars running FI with high boost.
Matt
 
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greenfoam

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They do run new motors at high revs as soon as they come off the production line to make sure they wont break. But I allways wonder how many have cracks and *almost* broke after that effort :p. But I guess the factory have it worked out how long you need to run them and how fast for any flaws to break loose
 

Lost_Pete

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Based on some work we did on Subarus.

Fuel cut is much easier on your engine than ignition cut. Assuming your engine has multiport fuel injection.

If you cut the fuel the cylinder just pumps air. The O2 sensor will report a lean exhaust but it is really only air, not a lean mixture.

If you cut the ignition you fill the exhaust with raw fuel so that when you come off the rev limiter you will get a backfire in the exhaust. The raw fuel can damage your cats and the backfire does not help either. In addition the backfire can stress your valves as it tries to push the exhaust valve open. We had some Subarus dislodge their valve shims when they tried using ignition cut.

Of course if you run carbs or single point injection then you are limited in your choice of rev limiters.
 

danja

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A little off-topic, but still interesting - a video of an engine assembly line. They don't show it being run-in, but they do mention it at then end. Perhaps more interesting is that the engine looks like it might actually be the HFV6?!

[YOUTUBE]<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/WPxr2DvWUsM&hl=en_US&fs=1&"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/WPxr2DvWUsM&hl=en_US&fs=1&" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>[/YOUTUBE]

Another episode from that show where they show the block itself being made:

[YOUTUBE]<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/NQe4G0Q9qiU&hl=en_US&fs=1&"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/NQe4G0Q9qiU&hl=en_US&fs=1&" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>[/YOUTUBE]
 

Immortality

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Based on some work we did on Subarus.

Fuel cut is much easier on your engine than ignition cut. Assuming your engine has multiport fuel injection.

If you cut the fuel the cylinder just pumps air. The O2 sensor will report a lean exhaust but it is really only air, not a lean mixture.

If you cut the ignition you fill the exhaust with raw fuel so that when you come off the rev limiter you will get a backfire in the exhaust. The raw fuel can damage your cats and the backfire does not help either. In addition the backfire can stress your valves as it tries to push the exhaust valve open. We had some Subarus dislodge their valve shims when they tried using ignition cut.

Of course if you run carbs or single point injection then you are limited in your choice of rev limiters.

you make a good point. however aftermarket ignition systems like MSD alter the cylinder that the ignition is dropped on to prevent the cylinder loading up and backfiring. i can rev mine out to the rev limiter and it will hold the revs, do that with a factory fuel cut system and it feels like you hit a brick wall. rather violent and can't be good for the engine either
 

crazybiker

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do that with a factory fuel cut system and it feels like you hit a brick wall. rather violent and can't be good for the engine either


...and the rest of the driveline with that constant jerking.
 

Lost_Pete

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The fuel cut algorithm I wrote cycled cylinders and as the revs increased cut 1, then 2, then 4 cylinders to give a softer feel to the cut.

I would agree that a total fuel cut is like hitting a brick wall!

The VN V6 rev limiter must be different as it appears to have a soft limiter then a hard limiter....or at least when I've hit it!
 

crazybiker

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The rev limiter in mine is so hard, that if you bounce off it a few times, it almost feels like whiplash.
 
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