240Z RPM Limit

[Lotus]

Hey I just recently purchased a '72 240Z. It has had some work done to it, like a rebuilt top end, electronic distributer, and DGV Redline carburettors. My question is, what is the maximum SAFE RPM I can rev it to? I know it yellow lines at 6k and redlines at 7k. Just wondering, I really don't want to blow it up, the whole lack of rev limiter thing scares me.

[PanzerAce]

The consensus seems to be to impose a redline for yourself at 7500rpm. Going above that alot apparently is a good way to start breaking things.

[Austin240Z]

I put an MSD 6AL on my Z and had the redline at 7K. My Z had a stock cam and SU's, power died off before it hit 7, so there's no point in revving it up any higher.

[evildky]

when I was running carbs and an L24 I upgraded to electronic distributor and ran to over 8k, granted there was no real power to be had up that high but it took it and didn't break anything, I think the valves kinda float and keep it from reving any higher

[TrumpetRhapsody]

You DO NOT want your valves to float. You're lucky nothing bad happened. I've heard horror stories of bent valves from going over 7500 on a stock valvetrain.

[Austin240Z]

You DO NOT want your valves to float. You're lucky nothing bad happened. I've heard horror stories of bent valves from going over 7500 on a stock valvetrain.

I second that, valve float is BAAAAD, if you've ever bent a valve (I'm at 7) it's not something that makes you happy.

[evildky]

mine was a primary autodross car at the time, I did it on a regular basis and when I replaced the engien with the turbo it was still running strong and upon teardown looked fine, that head is not on a buds car, IIRCthe L24 piston height at tdc is still negetive of the deck

[Poindexter]

My "new" floor mats, the ones that made my accelerator pedal stick, the same ones that I burned to a crisp in a metal drum a bit earlier today in a fit of retribution, caused my Z to accelerate out of control the other day before I got on the key and switched it off. It pegged the tach above the 8000 mark for about a second. Boy it sounded fantastic, before it just about caused me heart-failure! My mechanic- who has seen and worked on every Japanese car since the mid-60's told me not to worry, that the float would have de-powered the engine enough at the top end to keep it from really revving to infinity. He has seen the same thing happen to another customers' 1970 Z where the owner froze for 30 seconds while the engine screamed. No spun bearings. No blown engine. Exactly like what happened to my car, their existing tappet noise just got a touch louder. Whether or not one could blow the engine if you held the pedal to the floor for a few seconds is open for debate.

 

Do Z's in fact have pistons that will touch the valves if they get out of time? I forget what that's called- anyone?

 

My legs were shaking for half an hour afterwards- as I really thought I had blown the engine. So perhaps a healthy Z is kind of like a Laverda or Ducati- no real rev limit? I'd still prefer some kind of cut-of!

[PanzerAce]

 

Do Z's in fact have pistons that will touch the valves if they get out of time? I forget what that's called- anyone?

 

That is what would be called an interference engine, and no, I don't know if the L is one, or for that matter how much lift you would have to turn it into one.

[rudypoochris]

When I received my 240z, it had a hole through the block and was missing a conrod. So no, floating the valves is not a good rev limiter.

[Poindexter]

Is there actually such a thing as a rev limiter? A real product not a work-around.

 

Somehow I would imagine that it might be an easier task if one were running an electronic ignition- which sort of ties in with my next research project- whether to install a Pertronix Ignitor.

 

I never want to live through another "L24 goes wild" incident again!

[rudypoochris]

Is there actually such a thing as a rev limiter? A real product not a work-around.

 

Somehow I would imagine that it might be an easier task if one were running an electronic ignition- which sort of ties in with my next research project- whether to install a Pertronix Ignitor.

 

I never want to live through another "L24 goes wild" incident again!

 

Well all ignition is "electronic" at least in some sense. Yes, there is such a thing as a rev limiter. Multiple companies make them. Typically they use different chips to limit the max RPM by cutting spark if you exceed it. MSD would be one company that sells such devices.

[Poindexter]

Well all ignition is "electronic" at least in some sense. Yes, there is such a thing as a rev limiter. Multiple companies make them. Typically they use different chips to limit the max RPM by cutting spark if you exceed it. MSD would be one company that sells such devices.

With my questions and responses, I hope I'm contributing to a fuller understanding and a more thorough discussion of Lotus' original question about rev limits, necessarily including any related control systems- with all of these various elements occurring in this one thread- and that should include all electronic and mechanical means, from line-item options to "standard" electronic ignition "kits" from main-line companies, like MSD and Pertronix, to any stand-alone "black box" mods, on through to perhaps some alterations to stock Datsun electronics, like Lotus might need.

 

This is a solution that could benefit many Z owners. Anything out there that could keep an engine safe from this type of damage, ruining a potentially quite-rare engine, one that is becoming more rare every day, and in the blink of an eye, is IMO a good thing.

 

Much of my research seems to show that the simplest, cheapest and therefore probably my best way to integrate EI to my 2/71, to be a Pertronix Ignitor, so I'll contact them on the subject of rev limits- unless someone has any direct experience. The MSD gear looks like it will cost much more for a full system, but it might be easier to implement a true rev limit, complicating matters somewhat. If I end up not liking the result, trashing the Pertronix stuff and returning to an "as delivered" condition won't cost me as much.

 

I'm also following the original line because I will also certainly need to deal with modifying a "stock" Z electronic dizzy sometime in the near future, because I'm already designing my *next* Z, which will be completely unfettered by any need to be stock- which is the specific mission of my current 2/71 model, so the *next one* will be a start out with a buckets of horsepower engine in a stripped-down, lightweight 240 shell.

[rudypoochris]

The best rev limiter really is your foot. There just really isn't any reason to wring a stock Z out that far. Even a full race/modified engine should be designed to have diminishing returns before its red line. That isn't to say rev limiters are useless in the least. It does mean that in MOST cases though, one could just lift off the gas...

[Austin240Z]

That is what would be called an interference engine, and no, I don't know if the L is one, or for that matter how much lift you would have to turn it into one.

I'll promise you it is. I have valve shaped impact marks on 4 of 6 pistons to prove it.

 

 

Luckily, the valves dont come down into the cylinder too far, so it doesnt pose a huge problem unless your cam stops moving and your crank doesnt (That's what happened to me).

 

My mechanic- who has seen and worked on every Japanese car since the mid-60's told me not to worry, that the float would have de-powered the engine enough at the top end to keep it from really revving to infinity. He has seen the same thing happen to another customers' 1970 Z where the owner froze for 30 seconds while the engine screamed. No spun bearings. No blown engine. Exactly like what happened to my car, their existing tappet noise just got a touch louder. Whether or not one could blow the engine if you held the pedal to the floor for a few seconds is open for debate.

 

What your mechanic said is somewhat true, valve float will de-power an engine so it wont rev off into infinity, but it will cause a hell of a lot of un-needed stress on your engine. It's really not something you want to happen. Not to mention that unless you've had your crank balanced, it's gonna probably want to go off in about a dozen different directions once you get up to a high enough RPM, causing a lot of stress on your bottom end as well.

 

Back to the valve float and it being a BAD thing. You should look at it this way. When you have valve float, the valve stays open, but your cam and rocker arm are still moving.

 

Why is it bad? because excessive space is being created between either the valve and rocker arm, the rocker arm and cam, or both. This is going to put a lot of undue wear and stress on these parts shortening their lifespan and possibly causing failure. The best way to think of it is picturing a hammer... If you take a hammer and try to drive a nail into a board, if you try to swing it less than an inch from the nail it is not going to get you much force on the nail, so instead you swing the hammer a few feet to get more force on that nail.

When it comes to the spacing between the cam, rocker arm, and valve, there is only a certain amount of space that you want to have. Any less will cause too much friction, and too much spacing will cause too much wear from impact force.

 

The best way I can put it is, when you rev it up that high and create valve float, it's like taking a hammer to the your valve train for no reason.

 

 

*Edit* Not my best explanation ever, but I havent slept in over 48 hours, so that's the best you're gonna get for now *Edit*

[Poindexter]

I know exactly how you feel, Austin240Z. And you not only made perfect sense, but again I learned something new from these forums- and I really appreciate that. I also stay up for an extra day or more, now and again, just so in case an emergency or other critical situation presents itself and requires that I maintain peak alertness for an extended period of time, I've been there before, and know exactly what to expect from myself. My Dad, who was a Navy officer, taught me this, as I have inherited his love of the sea, and often sail offshore where any nightmare you might imagine can occur, and if you wait long enough- it will. Sweat more in practice so you will be less likely to bleed in reality.

 

That out of the way, in my case a bad decision to use ill-fitting floor mats that ended up slipping and interfered with my gas pedal almost blew up my engine, and there are other similar circumstances- like a binding throttle cable- that go beyond the ability of a right foot to deal with.

 

Much like the saying that the best firearm safety is "between the ears" or in the trigger finger- yet accidents still occur, the best safety for such an irrevocably destructive happening like a terminal over-rev, would still be a well-designed, fail-safe part that works transparently and as close to 100% reliability as is achievable. I agree that not every over-revving event results in an explosion, but I'd bet that *at least* they can break or weaken existing parts, reduce the overall engine's lifespan and hasten the demise of a perfectly fine piece of machinery.

 

I'd hope that my well-trained foot would always "kick-in" ( ) but why not have a cut-off that would always work, better and faster, and more reliably, hanging in the background to prevent these events from happening- regardless of cause, and taking the weaknesses inherent in all people out of the equation? Seems like there should be one, and one that has no negative affects on a running engine, is invisible until needed and deploys in time to prevent a disaster. Then add an espresso machine and it would be PERFECT!

[johnc]

A rev limiter needs to be in the ignition box or the ECU, not the distributor trigger. Most MSD, Crane, Mallory, and other aftermatket ignition boxes have adjustable rev limiters built in. All aftermarket ECUs have provisions for a rev limiter.

 

Running a L6 engine with stock valve springs above 7,000 rpm will reduce the rating of the springs. The springs get softer by some small amount. ITS racers, who have to run the stock valve springs, tend to replace the springs a couple times a season of they are regularly running the engine over 7,000 rpm on the track.

 

Also, a rev limiter does not save you from a bad downshift.

[Tony D]

Also, a rev limiter does not save you from a bad downshift.

 

Guilty.

Pegged my tach in my 75 Fairlady Z on a stupid downshift, plastic fan bent the tips back far enough to touch the radiator and cut it...

 

The old Ford Capris were an engine that was notorious (In the USA) for having a 'rev limiter', when in fact it was simply restrictive carburretion. You could flatfoot it, and you were lucky to get 5000rpms out of the Kent Mill. But put a Rochester 500CFM 2-Barrel on it, or the European Weber IDF induction on it, and it would twist the tach all the way 'round!

 

We have enough induction to be bad for us in free-rev. Including the Analog EFI setups as well. I used to take L-Engines to the scrapyard to die, and we would brick the throttle and take bets on how long before it went 'boom'... Skylines mostly. An occasional Gloria and Cedric as well...

 

They are indeed interference engines. The valves will hit. Things can go bad ranging from simply nicking the piston crowns, to a bent valve and broken Valve Guide, to a broken piston and resulting catastrophic failure.

 

Usually float will 'bounce' the valve and nick the piston crown, likely bend the valve if it happens more than a couple of times. But if you get the timing incorrect and it's mechanically held open when that piston is coming up, things can go bad quickly.

 

Just since we are talking about rev limits, the 'light flashes' on this one at 9500, and that's not the limit...

 

[Soundmasterg]

One other thing that is bad about valve float is that part of the cooling of the valve is accomplished by having it bang into the seat as it closes. Without that happening, then the valve gets red hot and is much more susceptible to breaking. It happened on a friend's volvo that had misadjusted valves and he over-revved it and broke the valve and spring. Woops.

 

greg

[Lotus]

Thanks for all the info, I was afraid to take it past 4k when I first got it. My other vehicle is an 80's toyota truck and it will explode if I rev it past 6k lol. I noticed that these engines tend to wind out pretty well and don't pull that hard in the lower RPMs. I'm used to truck engines like I said so I'm not used to winding it out that much haha