Why do engines tend to let go on lift off at the end of a long straight?

[Al Weyman]

Apart from the obvious that you are revving the nuts off them and they probably scream enough is enough but I personally have lost three engines at the end of the Rivett straight and know of many others who have done the same and just wondered why this particular straight seems to take its toll as surely the engine is caned as much elsewhere and is there any precaution like prehaps momentarily lifting off at say two thirds distance down the long straight that may help. I remember a seemingly knowledgable guy once telling me that he used to do something similiar when driving motorways at high speed.

[tristancliffe]

I think it's because the piston and rods are suddenly unbalanced by the lack of combustion at high revs (although of course on the exhaust stroke they are never balanced by combustion, and even on the over-run there are still compression effects).

However, I've also heard that the torsional vibrations change, and on 6 cylinder engines especially (with long, flexible cranks) the lifting off can cause crank failures...

I don't think lifting earlier on the straight will do anything other than make you slower.

[juliann]

Prompted by Robs 327i video Al ?
http://uk.youtube.com/watch?v=y15C-q...eature=related

A shame as he'd got such a fantastic start

[phoenix]

I am guessing that the failures are from rods breaking towards the little end. If so, I am offering an educated guess here..... or something to think about anyway.

With a wide open throttle, the greatest tension put into the rod is during deceleration of the piston approaching the momentry stop at TDC on the exhaust stroke. On the compression stroke, decelerating towards TDC, there is less tension in the rod as the piston has a load acting in the opposite direction from compressing the mixture.

So, with wide open throttle, the rod is exposed to maximum stress from tension once in every two revolutions.

On a closed throttle the loads on the rod are the same as above on the exhaust stroke, however there is little or no resistance to the piston going up the bore on the compression stroke either, even though the exhaust valve is not open. Therefore the rod is subjected to the similar tension loads from maximum deceleration on both the exhaust and compression strokes.

If this is the cause, maybe changing down later when decelerating, to reduce the maximum engine speed by 1000 rpm or so compared to when accelerating, might give the rods a greater chance of survival. Either that, or, if possible, lighter pistons and/or stronger rods may be a cure?

[AU N EGL]

Al know a few things about chevy engines here.

a few questions first:

1.how long were you at Redline? Holding a sustainged redline is a lot of stress on the engine, which you know. May need to back off a little below redline or new gearing may be in order.

2. what is your gearing?

3. did the engine "let go" as you hit the brakes?
If this is the case then a better dry sump oiling system maybe needed. Have seen many chevy sb, LS6s, LS7s and the new LS3s let go as there is too much oil UP in the heads, under hard braking, and the crank goes almost dry.

Better dry sump oil pumps and better scavaging pumps in the pan


4. How many hours between rebuilds?

[AU N EGL]

Originally Posted by juliann

Prompted by Robs 327i video Al ?
http://uk.youtube.com/watch?v=y15C-q...eature=related

A shame as he'd got such a fantastic start

WOW this one sounds like he ran out of gear and was on OLD tires.

[Al Weyman]

Originally Posted by juliann

Prompted by Robs 327i video Al ?
http://uk.youtube.com/watch?v=y15C-q...eature=related

A shame as he'd got such a fantastic start

That describes it beautifully Juliann. My engines let go at the end of the straight but on all they had the same problem the journal for 6 and 5 cylinders was definitely dry causing the rod to sieze on the crank and snap like matchwood then flay around on a path of destruction. Both times they neatly left the piston parked at the top of the bore saving the heads, one had standard chevy rods the other H-Beam Manleys. I have come to conclusion of oil upstairs and now I have rebuilt one I am running it 3 litres over the upper oil fill mark on the dipstick (about an inch in fact). Unfortunately we cannot use dry sumps which I think could be false economy as I used to use them on cheapo 1/4 mile oval racing as it was standard fare but there seems to be a reluctance to use them in Historic stuff overhere for some reason. I did just wonder about actually driving precautions at the end of a long straight maybe worth trying leaving it in the higher gear while revs are slowing under braking and changing down later as I think Phoenix has suggested.

BTW AU I only do 15 to 20 minute races so the engines should last a good couple of seasons, I had one last about 5 before I lent it to some fool who threw a belt and just carried on driving, and where was this? You guessed it Snetterton so infact I have lost 4 engines there but they do run better with a bit of water in them! :-(

[AU N EGL]

Originally Posted by Al Weyman

T
BTW AU I only do 15 to 20 minute races so the engines should last a good couple of seasons, I had one last about 5 before I lent it to some fool who threw a belt and just carried on driving, and where was this? You guessed it Snetterton so infact I have lost 4 engines there but they do run better with a bit of water in them! :-(

Dang Al

You cant run an accu-sump either? if you cant then yes add more oil and YES keep that water in there too.

Most of our races are 20 to 45 min as well. I run a 3 qt accu-sump which has helped keep the oil pressure up in high speed corners and those Hard decel braking zones.

Best of Luck Al

Tom

[Al Weyman]

Yes we can and I do run an accusump and have acquired one for my older car as well which I will be running.

[zac510]

Originally Posted by Al Weyman

That describes it beautifully Juliann. My engines let go at the end of the straight but on all they had the same problem the journal for 6 and 5 cylinders was definitely dry causing the rod to sieze on the crank and snap like matchwood then flay around on a path of destruction. Both times they neatly left the piston parked at the top of the bore saving the heads, one had standard chevy rods the other H-Beam Manleys. I have come to conclusion of oil upstairs and now I have rebuilt one I am running it 3 litres over the upper oil fill mark on the dipstick (about an inch in fact).
I did just wonder about actually driving precautions at the end of a long straight maybe worth trying leaving it in the higher gear while revs are slowing under braking and changing down later as I think Phoenix has suggested.

So you think the oil is surging forwards when you decelerate or the oil pressure is dropping when the revs drop?

[VIVA GT]

I have heard of this kind of thing before, I know of a number of road going Subaru engines have failed in this way. I can't remember the technical explanation completely, but is was similar to what phoenix has written above, plus it's the sudden closing of the throttle. It was suggested that rolling off the throttle instead of allowing it to simply snap shut should also help.

[1200Datto27]

The sudden vacuum that forms in the inlet track when you back off suddenly can also place stretching loads onto the rods that can cause problems.

[VIVA GT]

Originally Posted by 1200Datto27

The sudden vacuum that forms in the inlet track when you back off suddenly can also place stretching loads onto the rods that can cause problems.

Thanks, that explains that bit!

[Al Weyman]

No I am not talking about oil surge on this occassion, more the theory put forward about the extra strain on the rods when getting out of the throttle.

[AU N EGL]

Originally Posted by Al Weyman

No I am not talking about oil surge on this occassion, more the theory put forward about the extra strain on the rods when getting out of the throttle.

Al You have to remember in most cases it is FULL throttle or FULL brake. Not much in between.

[Al Weyman]

Yes I know what you mean, i was just wondering if levelling off for a scant few seconds before getting out the throttle would help.

[Zico]

In a high end ECU, couldnt there be a 3D map overlaid which could produce a linear and slower than instant rev drop between certain high RPM's in proportion/relation to Mph or high rev duration ?

Just a thought...

[ian_w]

As Phoenix explained, when at full throttle the highest tension load on the cond rod is on the exhaust stroke. When you shut the throttle the tension load in the compression stroke will be a similar level to that on the exhaust stroke. I think the magnitude of this load will be very similar in both cases so you shouldn't get a sudden overload condition leading to catastrophic failure.

If I remember correct, cylinders 5 & 6 and an SBC are the 3rd cylinders back on each bank. I also remember that the SBC has an unusual oiling arrangement whereby it feeds the cam bearings before the crank, although I can't remember the exact details. If you are consistently failing the same cylinders it does suggest to me that it is a lubrication problem. Lack of oil to the centre section of the crank? Or possibly the crank is flexing in the centre which causes excessive wear and leakage on the centre main bearing which has the knock-on effect of starving the neighbouring big-ends? Do you have any photos of the broken bits - crank, rods, bolts, shells etc?

Are the valves OK? I have seen it where a heavy valve to piston contact causes the piston to break and part company with the rod. The flailying rod then rips itself and the bottom of the engine to pieces.

One thought - NASCAR engines don't seem to have the problem when closing the throttle at 9500-10000 rpm. The rods I've seen from these type of engines are nothing out of the ordinary ( just your 'normal' billet rod ), I think they typically run 7/16" rod bolts.

[jjordan]

Ran my small block several years ago with clear valve covers, anything over 5000 rpm and the amount of oil in the top of the heads was frightening. make sure your oil drawns are maximized, if you can limit the amount of oil to the very top end I would look at that. The amount of wrap up in the oil pump drive mechanism (which includes the cam, timing chain, etc.) is also a bunch.
This combined with the reversal of loads in the rods at the point of decel is part of the problem, although your failures sound like oiling (or lack of) issues.

[Thundersports]

I'm no engineer but a very old school mechanic once told me 20 odd years ago when thrashing my Mini over a long period of time to back off the throttle a little when wide open to allow the oil pump to catch up with doing its job. I thrashed the hell out of various machinery over the years and the only road car engine I ever seized happend to be a dry sumped 3.2 Porsche 911 which was very expensive but thats another story. (Oil starvation)