Okay, I've been doing this every once in a while recently and I am starting to wonder the negative sides of it. I've taken off and been up at around 60ish doing about 4500 RPMs with wide open throttle. Then I come up on a car and jump off the gas but don't touch the gas. The car dies instantly in RPMs and it seems like this would be bad for the engine. Is there any bad side to this for either the engine or the tranny, or anything else for that matter. I'm just curious on this so I know what i'm acutally doing to my car.

 

doesn't anyone have any insight on this?

 

Yes, it doesn't do anything.

 

It might put a little more stress on things but I've never heard of it being a major (or even minor) cause of any kind of damage.

 

Spinning down suddenly from high RPMs will stress the rod bolts more than anything else. It's probably the #1 cause of ventilated 305 crankcases. It also isn't particularly kind to transmission cases and motor mounts.

Whether 4500 RPMs is "high" - that's a different question...

 

Really, can you please explain this, i'm not seeing it.

 

I really don't know, I am just going to take a guess at this. The rod bolts are pre-stressed in tension. When turning high rpm and suddently removing power, you are removing the compressive forces of the combustion process. The sudden downward accleration of the piston and rod masses from the rod cap without forces from the piston face contribute to additional tensile rod bolt loads.

 

Yeah, basically. Sudden spin-down makes the piston/rod mass slow down the crank. This pulls on the rod caps, and therefore the rod bolts. Snap, crackle, pop, etc.

Happens every intake stroke, but the crank isn't slowing down in cruise/power situations.

If you drag race, it's a good idea to let off the throttle slowly after the finish line.

 

So you are telling me that because there is less air+fuel mixture to be compressed and burned, there is less force pushing the piston down, creating greater tensile stress in the bolts. I will partially agree but...

Doesn't the engine slow down due to compression braking and not by mass of the piston. I will agree that a little is due to mass acceleration or deceleration in this case, but not the majority.

I'd have to say that the bolt breakage is due to an instantaneous shock loading and actual delflection and deformation of the rod, cap and bolt. This will cause major fatigue and cyclical loading of the rod bolts at a rate of 200 times per second @6000rpm.

 

Well, that and the fact that its now the pumping losses from having nothing but high vacuum present in the manifold resisting the rotation of the crank. The line of force (generalizing here) will be tension along the rod, which equates to tension in the bolts.

 

Yes, the crank will slow from compression braking and friction.

This does not change the fact that the rod/piston assembly will have to undergo the direction change without the compressive forces due to combusion, which are intuitively much larger than forces associated with compressed air.

Shock loading and delflection of the rod, cap and bolts are direct consequences of the aforementioned forces. The rod bolts experience alternating tension loads and cyclic bending. I have heard of stock rod bolts failing in fatigue from cracks eminating from the applied knurl (and hence the developement of ARP's forged "Wave-loc"). I am not sure if overload or cyclic fatigue is the more common failure mode under high rpm service.

These are all my thoughts, take it with a grain of salt.

 

Ok maybe we are getting there, i'm just kinda tired right now and can't really think this deeply. But i have come to the conclusion that an istantaneous shock loading is at fault. More precisely a tensile load due to the change in damping force during the compression stroke. Say your running at 100% VE on a 350cid engine turning 6000rpm, with a compression ratio of 10:1. The cylinder volume is about 44 cid. So say there is a cranking compression of 200 psi and a piston SA of 12.6, there is only 16 lbs pushing down and damping the piston. This is of course assuming that the cranking compression at speed is constant.

16lbs doesn't seem like much, but....

I don't know, time for sleepy!

 

I do not belive that the rod bolts will see purely tension when the rod/piston is being drawn downward. The rod end around the crank will ovalize as the crank pulls on the rod, hence bending loads along with the tension.

Too late for numbers...I agree...goodnight

 

When you do a cranking compression check, the throttle is wide open.

When you let off the throttle at high RPMs, the throttle is - well, closed. Therefore, low amount of air to fill the cylinders, low pressure during the compression stroke - the "vacuum" dimented24x7 stated.

BTW, you multiply the pressure times surface area to get force, not divide.

pounds/inch^2 x inch^2 = pounds

 

Wow... this is some of the most advanced tech I have seen on here in a long time...






I LIKE IT!!! KEEP IT GOING BOYS!!! I need to learn something new today!!!

 

The compression braking and pressure calculations ME Leigh is talking about occurs during the during the compression stroke tip into power stroke. At this time, the valves are not open. Manifold pressure is not an issue during this stage, other than the VE effect.

 

That's why I said, "BTW..."

 

This thread has some of the most stupid posts I've seen in a while. The whole thing about decelerating and "stress" on rod bolts is retarded. 5-7, you are talking about how because of the absence of combustion pressure above the piston, there is a stretching force applied to the rod bolts at TDC, right?

Folks, these are and always have been 4 stroke engines! That means that whether we're "on the gas" or "off the gas", there will always be at lease one out of two TDC's per power stroke that has no combustion pressure above the piston (between the exhaust and intake stroke) You'll notice that engines don't spew rods every time they pass between the exhaust and compression stroke, so why would they under no throttle?

Yes, there is a strain on the rod bolts at TDC under no load, but no more so than at TDC between exh and intake, under WOT. Point? The rod bolts were built to thake that level of stress, and they will take it just fine, for about, um... forever.

Question: Why don't race cars break rod bolts when they decel from 8500+ RPM into a turn at no throttle?
Answer: Because the rod bolts, rods/ etc. were designed to handle the load at TDC under all conditions, TDC on exh/int and decel being one of those situations.

 

Yes, "race" engines are built to take it.

These momentum forces are velocity squared, though, so the difference between 2000 RPMs and 8500 RPMs is substantial.

And, it does make a difference what all the crank shaft is slowing down. If it's just the engine, probably not a big deal. If it's the crankshaft and vehicle with the transmission in a lower gear (or geared lower), it's a bigger deal, especially with stock rods & bolts.

 

I agree with this

...but not this. How in the world does the ROD BOLT, know if the flywhee is engaged to a load or not?? The stress on the rods, bolts and crank and pistons will be the same whether it's decel-ing in neutral or in gear. Just in gear, it'll more than likely be at the higer ROM for a longer period of time.

Still, you offer no explaination as to why engines don't desintigrate at WOT, 5,6,7000 RPM(whateve max is for a given engine), between the Exh and Intake stokes.

 

I said it a long time ago - the piston/rod mass becomes the slowing-down force (along with piston/ring friction, which I failed to mention) of the crank. Under power, the crank is pulling the rod/piston down on the intake stroke, but the force is higher on the rod when it is attempting to slow the crank down as well (and the chamber vacuum is higher).

Maybe we could start a poll - "What were you doing when the rod came out of the crankcase of your 305?" Some will say over-reving under power, but I'd bet a lot more had just downshifted when the rod cut loose. Or, if they're honest, they had done so and hurt it, and it finally cut loose later under less stressful conditions.

 

I highly doubt that the vacuum above a piston on decel is going to be the factor that breaks the rod bolts! lol. It can handle the forces generated by 6000 (or whatever) RPM, but throw in 14.7 PSI of atmospheric pressure on the bottom side of the piston and BAM! the rod bolts let go? I doubt that highly.

I ususally agree w/ most everything you post, but I guess we squarly disagree on this one. I think we can agree, however, that the original poster, decel-ing from 4500 RPM is never going to hurt anything as a result.

 

Okay.

Reality can have a funny way of changing our assumptions.

 

Yeah sorry i knew that, thats what you get when your working on 5 hours of sleep and its 12:00.

Now back to the original question. Now that i think about it i know for sure that sudden deceleration put stress and strain on the rear end. When you are applying power to the rear end, force is applied to one side of the gears. When you are decelerating the forces are opposite because the drivetrain is actually slowing down the car or mass. The ring and pinion gears in the rear are not designed to take this type of loading, even though it is quite small, it is still significant. So.....

I think the same thing happens to the engine. An instanteous shock loading in the opposite direction of where the load was and instant before.