Gearbox rattling at low revs under load- timing?

[yoeddynz]

So ever since fitting a V6 to the front of my Rx7 gearbox it has been noisy. It was always very quiet behind the rotary- not a single peep, rattle or squeak.

 

Now behind a piston engine it has decided to start being noisy. Was noticeable from the very first drive.

 

How would I describe this sound?...

 

It rattles the insides of the box as if there is lots of of backlash between the gear teeth. Massive vibration right down the drive train at its worst.  It will only do it under load from low down to about 1800~2000 rpm. Then it goes. If I try to accelerate out from a bend or corner in say 3rd gear it will do it- unless I really squeeze the pedal gently.  I either have to keep my speed up or drop into second. Any normal car would be fine at the speeds I am getting this sound at and with the rotary I never got the sound even when pulling from way down low. Or staying in 4th/5th at low speed and dropping below say 2000.

 

Otherwise the box is really quiet and smooth.

 

Its not the release bearing, clutch fork/ spigot bearing. In fact its not a high pitched rattle of loose item. Its a proper rumble from deep within.

 

If the box was previously noisy behind the rotary I would just strip it and rebuild it. But it was quiet as. 

 

I am now wondering if it might be the inherent low rev roughness of the V6 firing order creating this vibration?

 

Or could it be as simple as my ignition is too advanced under load low down and this creates pulses or vibration that is amplified by the gearset within the box?  I think I might be under estimating the noise or vibrations this can create? There isn't any noticable pinking from the engine but I'm always learning about these things so might not be detecting it..

 

I understand that V6's are not as naturally balanced as other engines- hence Mazda put much effort into engine mount design and vibration damping.

 

I will have a play with the timing hopefully today and see if the sound relates to high advance on the map. This is my current ignition table...

 

 

I have asked on Ausrotary about noisy rx7 gearboxes but have only had one reply. I didnt even mention that I have removed the rotary- I knew that their small minds would not be able to understand why. But yeah- no real help there.

 

Its bloody annoying. I can drive around the sound fine but others cant. And the engine wants to pull so strongly from low down too but I just cant. Would make town driving that bit more easy being able to ride the wave of torque.

 

Ideas?

 

Off for a drive now so will tune in later.....

[Brock-Lee]

My bmw does this exact thing. I have no advice on how to fix it but mine only makes noise if you really load the engine up in a high gear so not really an issue.

 

 

LING POST

[ta63-1uzze]

I highly doubt it is caused from ignition. I understand your theory but ignition advance/retard changes instantly with any pedal movement , things like pedal position and oxygen present in exhaust stream will drastically change ignition  depending on what you are asking the engine to do.

 

The thought pattern that I can see in my mind tells me that its more likely that you input/output shaft bearing and more likely your thrust bearing's are worn inside the gearbox.

 

I would say you are experiencing the sound now because the new engine makes more torque from very down low where its spinning at a much slower rate whilst being applied with a lot more torque then the pervious engine would supply at the same gearbox rpm. basically Appling more thrust down the shafts loading the thrust and shaft bearing more. and by what you have said it sounds like it is doing it in more than just 4th , I say just 4th because this is direct drive and cancels out all other gears. thus leading me to assume that the fault is either input or out put shaft or drive line.

 

I think the first thing to do is to jack it up on 4 stands, apply light braking , have someone run the car up to the speed that makes the fault, then get under the car with a stethoscope or a hard handled screwdriver , put the listening device on the gearbox , around the tail shaft housing , then the centre driveshaft hanger bearing , the pinion of the diff, etc. basically listen to everything you can to determine where the sound is defiantly coming from .  

 

it could be a hanger bearing, diff pinion bearing beginning to fail or internal side bearing failing inside the diff transmitting sound through the drive line etc.

 

fist thing to do is to find the source of the sound , and then make logical steps from there, best to do this sooner then later because sounds like this can be the indicator before total destruction.

[ta63-1uzze]

if you have a dti gauge and a manual for the gearbox , you can quickly whip of the tail shaft and measure the out put shaft end float. this will be a great way to check the back end of the gearbox's thrust bearing . you will need to drop the box to check the input shaft thrust bearing, but its done in the same way.

 

this is what I would do if you locate the sound to the gearbox .if they are worn , you will have excessive end float  , this makes the backlash between gears huge and provided play and movement for the shaft to bounce back and forth between load and unload ie light pedal feathering , and that is where I think you are getting your problem . but its a little early for diagnoses like that . but its an idea to go with. now just have to try and prove it he he he

[Brock-Lee]

surely with a rattle it will not be thrust bearings but shaft bearings allowing the gears to separate under load?

[ta63-1uzze]

the gears are helical gears , constant mesh. if it was not thrust bearings then only the contact patch would change, with lateral change ie the thrust bearing worn out then the backlash would change. contact pattern would make it whine, but  backlash cerates clunking or rattling

[mjrstar]

How close did you get the dimensions for the adaptor plate?

[capone]

Not driveshaft angles?

[yoeddynz]

Cool bananas fellas- thanks for the excellent replies.

 

mjrstar. I spent a long time getting the adaptor plate spot on and Im pretty happy with that.

 

Capone- drive shaft angles. Yeah this is something I thought about ages ago and had posted it up in another thread. It may well be just that. I need to make some adjustable top links and then adjust the pinion angle. I do know that the engine/gbox centre line points down more than it did behind the rotary. I think it may well be sort of like this...

 

 

So I think the angles don't match. If this is so then at low revs under load it could be causing the output shaft to knock up and down as it rotates causing the rattle/noise? I have until now actually forgotten about this possible issue. Whoops. Thing that has me not sure about this is that under load the axle nose will try to rise and so make the angles even better?  As per my fine piece of engineering artwork- I think I need to have the nose pointing up to begin with. I have bought a decent protractor ages ago and not even removed it from its package. Must check this!

 

Ta63-1uzze. Thanks for that info. I can check the output end float next week and see if its in spec. Dropping the box out aint a major issue as far as access goes but cold weather and lack of garage at home is. I wonder now that if the prop angles are wrong and there is some slack in the bearing that supports the output shaft then it can move up and down.  I should really look at the angles first and rule them out.

[kpr]

if your drive shaft is in a straight line with the tail shaft, as in your picture. you may want to do some adjustments at that end. lift the gearbox or drop the engine, if either is possible.   will give you some rotation on the uni's.

 

drive shaft angles may not be the problem. but definitely worth doing first, to rule it out, as you say.

[Yowzer]

I think you want around the same angle on each end of the driveshaft because the angle of the UJs effects its rotation, and the other end is meant to cancel it out. If the angles are different it may not cancel out the rotational vibrations. I could also be talking out my ass, but it makes sense in my mind.

[Yowzer]

Alternatively it could simply be the firing force of the engine creating the noise. Perhaps a better dampened clutch or a heavier flywheel to even out the firing pulses.

[Unclejake]

I totally haven't read the thread, but the first couple of sentences sounded like engine pre-detonation or an input shaft to pilot bearing mis-match

Forgive me if both of those things have already been covered after the second line. I'll have a better read later - but you would want to be certain that the input shaft is snug into the crank or else you'll bugger the box pronto. It's possible that the shaft is too short and it isn't locating into the pilot bearing at all

[yoeddynz]

UJ- the it's Motion shaft to pilot bearing is all good. I took a long time to make sure I got it spot on. My boss the other day pointed out as well that if it was out it would vibrate worse as revs went up and would rattle vibrate while idling and free revving. But it's all good in those situations.

Jesse- it could well be accentuated by the lightweight flywheel I have. Which would be a shame because I love the way it revs.

But I think, hopefully, it might be the prop angles. Will check this week. Also have been comparing my ignition table to many other KLv6 ones on probe talk etc and mine is really too far advanced. Will double check timing with light at work and then adjust. Would be awesome if it was just that but I fear it's more mechanical.

[flyingbrick]

Everyone has great ideas.

Your story reminds me of a few cars Iv had that kind of shuddered and made horrible sounds at low rpm and high load- I always thought it was pre detonation.

With that said though- I once was tasked with finding vibration in a Mazda ute- it ended up being a dented and put of balance drive shaft- it behaved NOTHING like anyone expected

[kpr]

i didn't even take notice of your ignition map there, when i read this last time..     i don't know what those engines like timing wise.  but that does look like way too much.  at light load/cruise it is probably ok ish.  you will find somewhere between 20-30degrees at full load is where it needs to be.    36 is way too much for most engines.  make the map go past your max rpm also

[Spencer]

Yea neither, wow thats lots of timing on a road tune/in general.

[yoeddynz]

Yeah it is terribly advanced. It's just the default ignition table that came on the code I uploaded from MS extra. According to mr Haynes the funny bearded man, my ignition timing should be 9~11 btdc. But it's not just the idle timing that is way too advanced- it also ramps up very very quickly.

I still have much to learn about tuning and timing. My boss reckons that apart from obvious pinking at lower revs to much advance can make a car feel bogged down? And can cause surging at cruise. Which this car does have a bit of and I had sort of tuned out by adding a touch more fuel.

Never got timing light on engine today as spent time replacing fucked ball joint. Must update thread. Steering soooo much better.

[kpr]

make sure you base is set right to start with.   get the timing light on there, and make sure its reading the same as what is in the ms ignition map.   there is a setting in tuner studio somewhere to lock the timing.   set it to 10. make sure reads same on timing light.   while your at it.  may as well free rev the engine and make sure you get a solid 10degrees as revs increase. 

 

unlock it.  

 

i would pull

10 degrees out of 100 row

6-7 degrees out at 50 row

maybe a few degrees  down at light load. 

obviously want to interpolate  between ^^^

idle timing is actually ok

from here on tweak it a few degrees either way. if you feel no change. its about right.  if you feel a big change its miles off.

thats about the best i can do for ya without making the map myself.

 

over advance will feel same as not enough advance,  sluggish.   but is worse for the engine. 

 

you want the minimum timing to net best power. 

[yoeddynz]

Sweet- I'll do that. Thanks very much. Will look up how to lock the timing on TS.

 

Have a look at some of the tables on this thread if you can be bothered sometime. It seems that these engines do run fine with quite a bit of advance- although not ramping up as quickly as mine does. But I need to make sure avoid getting into copying other peoples tables as all setups are different eh.

 

http://forums.probetalk.com/showthread.php?t=1701288659&highlight=ve+tables