Theres been alot of post of R's miss shifting and bending valves.....Im guess most of these guys are running on their stock ECUs. My ? is isnt the rev limiter suppose to save you from damage? Right now im running on a chipped P28 with unlimited rev, which is kinda scary. I miss shifted the other day also, and now my car kind of sputters during idle, but its not real bad though. It drives normal after about 2 minutes. My main question is, how do i know which valves to replace? Obviously i would the change the valves that physically look bent and dont seat properly, but what about ones that are bent so slightly that you cant see it?

 

No. The rev limit prevents you from exceeding a certain RPM in whichever gear you are accelerating it, it merely cuts fuel to stop the car from accelerating. When you put the car into a lower gear, say redline of 3rd into 2nd, the wheels are still traveleing the same speed as redline in 3rd gear, when you engage second gear you cause the main/countershaft to turn at a far higher RPM. Now when you release the clutch you *FORCE* the motor to turn well past "redline". This is a direct mechanical connection and no electronic safety device (like a fuel cut) will prevent this from happening.. at least no currently developed technology that I know of

Take the head off and look.


[Modified by Dave-ROR, 10:54 AM 7/10/2002]

 

rev limiter only works while you are IN gear and accelerating...

 

Take the car to a mechanic you can trust and ask him to remove the head and check for bent valves. I would be impressed if you could tell which ones were bent without removing the head.

 

Yea, what they said. No safety, that's why it usually isn't covered under warranty.

 

What you want is the SMG II from BMW. It won't let you downshift if the revs are still too high....

 

Yeah, they just spin main bearings

 

I thought it was the piston rod...

 

Is there any other way to see if the valves are bent besides LOOKING? What happends if it is only bent slightly and you cant even tell?

 

Do a leak down test, that will tell you where(if) you are leaking air through.

 

let me get this right...
so a rev limiter works.. for example, while you're on 2nd gear and did not shift to 3rd when the gear runs out, then the rev limiter will kick in and prevent the engine from going over 10k

but it does not work when you down shift from 5th to 2nd...

 

First, it's not a rev limiter per se, it's a fuel cut. All it does is cut fuel. Cutting fuel does not make an engine stop revving if the motor is being *forced* to turn by the wheels. When accelerating through a gear you hit that gears top speed, so the motor just sits and bounces against the fuel cutoff and the car is still going say 90mph, now when you shift from say 3rd to 2nd, the car is still moving at 90mph however the different gear ration now mechanically forces the motor to spin at 11,000rpm (or whatever it would be). At that point fuel cut or any other normal safety device doesn't mean a damned thing. An electronic fuel cutoff will never be ing control of a mechanical overrev, the clutch disengaging the tranny from the engine is the only safety "device" you'll have until the car slows down (under pretty impressive force )

 

Thats what i wanted to say but couldnt word it right.

 

I'm sure they've thought of it, but I'd like to know why they can't implement a gear lockout switch of some sort. Given certain rpms and speeds, certain gears would be locked out. Seems like a simple way to prevent over revs, but again I'm sure there must be something preventing them from doing this.

 

It's not a fuel cut-of it's an ignition cut off. If you cut your fuel at 8400 rpm you'd end up with nice big holes in your pistons. When you hit the rev limiter the engine misfires (via lack of spark) so it won't spin it's self any faster.

If you miss shift with a manual transmission (you're trying for 3rd into 4th but you pull it into 2nd instead) there nothing in the world that will save your motor other than luck.

If you do miss a shift like that your are being too abusive. Treat the shifter as if it's made of glass. Your diving will be much smoother and your transmission will thank you. Trying to bang though the gears doesn't make you any faster.

If you have any bent valves get them all replaced. There's not such thing as *a little* bent. It's either bent or straight, no grey area. If you're worried about the cost, you 'll be paying much more for labour than parts so just get all the parts replaced so you're only paying labour once. It's cheaper in the long run. (Especially if, in a few months, you drop one of the valves that you neglected to replace) That would be VERY expensive to repair.

Hope than helps.

 

it's definately a fuel cut.

 

I'd like to hear an explanation for this one.

 

Uhmmm no. It's a fuel cut. Misinformation on this forum is pretty common, don't help spread more.

-Dave, who like SGT would like to hear that explanation as well.

 

yea dude whats the deal with that?
its a fuel cut and if im correct fuel cut is 8600?

 

yes, it's a fuel cut. Think of it this way, 2 things can make your engine spin up. 1 is fuel, the other is momentum/inertia. When you try to overrev the engine using fuel, you will hit the fuel cut at around 8600 RPM, and your tach will bounce off it (zing zing zing).

When you spin up the engine using momentum, by going from say 3rd at redline to second, you're going to hit around 11,000 RPM no matter what, because fuel isn't what's spinning the engine up, the cars inertia is.

YOU are the best rev limiter in the car. Never count on gizmos to save your ***, drive smart. If you're letting out the clutch and the engine is zinging upwards faster than it should, stab that bastard back in.

I think HADA 11 is thinking that it must be an igniton cut becuase a fuel cut at 8400 RPM would be a severe "lean" condition, but i know for a fact that when you're coasting in the car, if you're not on the gas, the engine is using ZERO fuel. D and I had a good argument about this, and even though we found the answer, I never found out why this lack of fuel is different than the lean-run conditions that are bad....anyone have this info?

 

now thats a good answer.

 

I think (99% sure) it's technically 8400RPM redline and 8500RPM fuel cut..

No load?

 

On a J&S when you hit the rev limiter, it will light up and pull timing. The ECU stops injecting fuel, but there is still residual fuel on the sides of the port and runner. Usually at redline you are near max duty cycle, so the ECU is using almost the entire available time to inject fuel (aka, it is not just injecting when the intake valve is open)... so fuel gets all over the sides of the port. The next cycle, some fuel gets in, but not enough for airflow -- and since a lean condition burns hotter you get detonation. It quickly stops, but soon enough the motor has slowed to the point that the ecu returns fuel... and the cycle starts over...

Dustin

 

wickid

 

funny that someone posted this thread............about 4 months ago, i was being stupid and racing a z28 on the way home from work.
while on the second run, i missed shifted from 3rd gear to 2nd gear, ha and top it all off, i shifted at 9000 rpms.
anyways, i missed and the engine overreved like hell, pressed the clutch pedal and shut the car off and coast to the side road.

kinda cross my finger and crank the car and started fine but i had no clutch engagement. after realizing the engine was find (lucky me) and i kill a 10,000 mile type-r oem clutch.

lucky me, it turn out everything to be fine, hell, now i see why engine balancing can be important sometimes.

though, the pressure plate fell apart in pieces. the rivets that hold the plate together, let loose therefore, breaking most of the holder on the pressue plate.
i wish i had some pics but kinda wierd.

anyways, lesson learn, don't street race so you won't miss-shift and breaking.