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Jul 27, 2013 21:52:56 GMT
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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 didn't 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..... |
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Jul 27, 2013 22:24:57 GMT
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Not doubting your ability for a minute but is the engine and box set up in correct alignment? Its the only reason I can think of for the box to be noisy with one engine and not the other - could there be a bit of load slightly off centre on the spigot bush thats causing problems at low revs? I'm not up on Mazda engine /box /bell housing combinations so this may be a complete red herring.
I wouldn't think it was something failing inside the box or it wouldn't have started with the first drive.
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sowen
Club Retro Rides Member
Posts: 2,245
Club RR Member Number: 24
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Jul 27, 2013 23:58:56 GMT
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As Dodgerover says, is the engine aligned dead on to the gearbox? The advance does look a bit high to me at low revs. This is one of the ignition maps I've used on my 4 cylinder turbo lump for comparison:  Pinking was noticeable at low rpm's and high load, generally 4th and 5th gears, sounded like a bag of nuts jangling upfront! |
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Yeah the alignment is pretty spot on- it took me bloody ages to get it right as I was paranoid about that. Another thing I have forgotten about but may well be the cause is driveshaft angles! 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! 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. |
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without referring to your build post to find out,presume it hasnt got a dual mass flywheel? these can cause the same sort of symptoms you describe,also cant remember,have your engine and box mountings got suffiecient give in them to allow some movement of box and engine?
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Jul 28, 2013 19:52:13 GMT
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Is the RX box the same as the MX one? Maybe ask on 'nutz, miataturbo.net etc?
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Koos
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Jul 28, 2013 20:21:17 GMT
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If its running at a different angle of dangle could it be a splash feed bearing thats not getting sufficient lube splashed at it till the revs rise?
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MrSpeedy
East Midlands
www.vintagediesels.co.uk
Posts: 4,786 
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Jul 28, 2013 23:16:35 GMT
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I would suggest that the piston engine is simply highlighting a worn box that the rotary engine didn't show.
It's something we come across regularly in marinised automotive engines.
At lower speeds the rotational osscillation (small variations in actual crank rotaion speed) can fluctuate quicker than the rotating mass of the gearbox/propshaft. To combat it in the marine application, we fit a progressively dampened driveplate coupling to absorb the variations.
I've never looked into the availability of varying clutch friction plates, but if you can, then I'd suggest a softer dmper rated friction plate in your cluth.
The addition of an inertia ring/heavier flywheek would also help, but obviously reduce engine responsiveness
That said, it could just be a worn plate you already have?
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goldnrust
West Midlands
Minimalist
Posts: 1,872
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Jul 28, 2013 23:30:39 GMT
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I gotta say that looks like you might be overly advanced there at low RPMs. If the mazda v6 responds to advance anything like the mx5, then idling at that ~ 15° BTDC is quite possible, but it's much less stable than at 5-10°. I found there were many peoples, especially americans, in the megasquirt community who seemed to be just going for the max advance they could without knock, but thats not always actually for the best. You could always just knock 10 degrees off the whole table, see if it feels any different, if it doesn't then you know you're barking up the wrong tree. The beauty of megasquirt is how easy those kind of tests are  |
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I would suggest that the piston engine is simply highlighting a worn box that the rotary engine didn't show. It's something we come across regularly in marinised automotive engines. At lower speeds the rotational osscillation (small variations in actual crank rotaion speed) can fluctuate quicker than the rotating mass of the gearbox/propshaft. To combat it in the marine application, we fit a progressively dampened driveplate coupling to absorb the variations. I've never looked into the availability of varying clutch friction plates, but if you can, then I'd suggest a softer dmper rated friction plate in your cluth. The addition of an inertia ring/heavier flywheek would also help, but obviously reduce engine responsiveness That said, it could just be a worn plate you already have? U what? Bit bloody clever sounding that was, Mr Speedy! :-P |
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Koos
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Jul 30, 2013 11:40:32 GMT
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Some good ideas here. cheers.
The engine mounts are quite stiff. they are poly items- mk2 cortina I think but in poly.
The flywheel is a very light cro-mo item.
After having a good look through all the various ignition tables on the probe forum MS thread my table is very advanced. As this is the easiest thing to change I will try this first. Stock timing at idle is 10degrees @ 700-800 rpm. mine is about 16 and ramps up very quickly from there. Hopefully just this is enough to stop pre-ignition which might be causing resonance which is getting amplified. Will check prop angles too while car is up on ramp in next day or so.
Checked propshaft runout a while back and there is almost a good 1mm!! this is certainly the cause of the vibration @ 40mph
I'll ask about the box on the miata forum too Mark because yeah- its basically a mx5 box with a different bell housing.
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This is exactly what I had on the amazon when I put the diesel in! I replaced bushes, mountings etc on the engine, box and suspension with little improvement. It turned out that the adaptor plate between the engine and box was too thin, this meant that the friction plate was at the rear of the splines and although the clutch worked fine it transmitted Huge amounts of vibration through the drive train.
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Thats interesting!
Wouldnt be too hard to slacken off all the bell housing bolts and fit a couple of washers twixt 'housing and engine, just to see what difference it made, would it?
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Koos
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I would suggest that the piston engine is simply highlighting a worn box that the rotary engine didn't show. It's something we come across regularly in marinised automotive engines. At lower speeds the rotational oscillation (small variations in actual crank rotation speed) can fluctuate quicker than the rotating mass of the gearbox/propshaft. To combat it in the marine application, we fit a progressively dampened drive-plate coupling to absorb the variations. I've never looked into the availability of varying clutch friction plates, but if you can, then I'd suggest a softer damper rated friction plate in your clutch. The addition of an inertia ring/heavier flywheel would also help, but obviously reduce engine responsiveness That said, it could just be a worn plate you already have? I'd say this is a combination of the light flywheel and the wrong clutch plate resulting in alot of torsional vibrations from the engines firing pulses being transmitted to the gearbox and making the gears rattle. I have a similar though less severe issue with my Vitesse (with Supra gearbox) caused by using a friction plate from a 1.6 Hiace which has just 4 rather pathetic little damper springs instead of the 6 beefier ones in the original Triumph plate. These have been getting fully compressed by the engine torque beyond about 3/4 throttle resulting in rattles from the gearbox. Having just dismantled it I find evidence of this in that the springs are now permanently crushed and the torque stop pins being chewed half way through. I think you need to source a friction plate with beefier springs which don't bottom out. Taking timing out and reducing the torque might reduce/stop the noise but also spoils performance........ Nick |
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1967 Triumph Vitesse convertible (old friend)
1996 Audi A6 2.5 TDI Avant (still durability testing)
1972 GT6 Mk3 (Restored after loong rest & getting the hang of being a car again)
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v8ian
Posted a lot
Posts: 3,763
Member is Online
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I can see a really light flywheel could cause a lack of damping and allow shunting, broken clutch damping springs might be an issue, another one that springs to mind, I would guess the V6 has a lot more torque over the Rotary, IF the gearbox is suspect that will be more noticeable with the application of extra torque,
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Atmo V8 Power . No slicks , No gas + No bits missing . Doing it in style.
Austin A35van, very different------- but still doing it in style, going to be a funmoble
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I agree with your comments earlier on in the post. My first thought was that if the gearbox output flange and diff pinion flange were not in the same plane, the propshaft uj's will be constantly trying to work against each other, my understanding is that the face angles have to match. The symptoms given would indicate ( to me) that the extra torque is trying to twist the prop into a knot, is may also be pronounced if the car squats under extra load or cornering.
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Last Edit: Aug 6, 2013 12:24:00 GMT by zoompod
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MrSpeedy
East Midlands
www.vintagediesels.co.uk
Posts: 4,786
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I agree with your comments earlier on in the post. My first thought was that if the gearbox output flange and diff pinion flange were not in the same plane, the propshaft uj's will be constantly trying to work against each other, my understanding is that the face angles have to match. The symptoms given would indicate ( to me) that the extra torque is trying to twist the prop into a knot, is may also be pronounced if the car squats under extra load or cornering. Propshaft issues (balance, alignment etc) are generally more pronounced the faster you go and is independent of engine speed |
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