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70 510 - Endurance Racing: 2 Different Transmissions w/ Synchro Key Issues


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Hi All -

 

relative noob, breaking things and figuring it out as we go (or vice versus)..  I have owned 2 dimes in my time and currently have a stripped down 1970 510 endurance racer.  Here are the basics of what I am working with:

 

L18 with a .490 cam with new rockers, springs, lash pads, etc.

MSD ignition / datsun comp distributor

4 barrel mikuni 40mm

stock 4 speed

adjustable suspension (camber plates, adjustable shocks, rear slotted cross member)

etc...

 

We spent the entire winter rebuilding her, getting ready for the NJMP 24 hrs of Lemons race this past weekend.  The car was much faster and handled on rails....  An absolute blast to drive, we were class B and based upon our lap times, could have been a top 10 contender if we were able to keep her on the course.

 

We lost 3rd and 4th gear on the 7th lap.  We had a spare, so did our first transmission swap in under 4 hours and got her back out.  

 

We took apart the first transmission, now out of the car, and discovered two things:  

 

1) the roll pin on the linkage to 3 and 4 gear fork was sheared.  Easy fix.

2) All of the synchro keys had popped out!  

 

My guess is that with the keys out, we sheared the pin trying to get it back in gear

 

On to the 2nd transmission, still in the car and and now stuck in 4th gear.

 

1) We drained the gear oil, which was high temp 90W.  It looked cooked, as brand new that AM and dark brown in the late PM.

2) Took the pan off and quickly noticed that the that synchro key was jamming the synchro.  

3) We popped it back in, filled the gear oil up to the drain plug (likely 4+ pints vs. the 3 pints the haynes manual called for)

4) We raced all day, from about 9am - 3pm.  In the last 20 min, she stuck in 4th gear on me and had to finish the race that way.

 

So, two transmissions, the same issue leads me to conclude we were addressing the symptom and not the problem.  Are we just cooking the transmission and the keys are not hardened, so expanding and popping out?

 

Thoughts?

 

Thanks in advance..

 

Ian

 

 

 

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Are you talking about the stock square pan 4speed,,, i ruined 3 in 2 years by just adding wide tires and showing off slamming gears and it only took a few times each as i'm old granny driver most of the time... Never opened one up but they all made this Christmas bells noise from inside before taking them out..   Almost completely stock engine with L20b cam and an open rear end even...  weak very weak ...

  Can you located a longtail truck 4speed those would probably handle the smashing much better and should be as cheap as the small trans.  You would have to shorten driveline but thats only a one time cost.

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Thanks fellas.   Yes, stock 4 spd square pan

 

What about a fs5w71 from a 280zx?  I know I would have to shorten / balance the drive shaft, but given the ratios, durability, and greater availability seems like the best way to go.  Any downsides?  Too little power?  Reliability?  

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The F4W63 should stand up to an L18 as described but if you drive it like you stole it, nothing will last. In an endurance race it's all about lasting to the end, not getting out in front and staying there. Don't be aggressive with the shifts there is little to be gained from this.... consistency is what improves lap times. Not how fast you can bang shift. Races are won at the end, so save something for that. Keep in mind that the transmission is from some old 510 with likely well over a hundred thousand miles on it already. 

 

 

Make sure the transmission outer case is clean so it can radiate the internal heat. Flat black paint radiates heat better than bare aluminum and better than any other color.

Try a synthetic oil. Synthetics can be run much hotter than dinosaur oil without 'cooking' it.

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Fifth gear is actually an overdrive and not really intended for heavy throttle use like hauling a load, passing or climbing a hill.... best to use 4th for this. There are NISMO 5 speeds that have no over drive.

 

The transmission and the differential should be chosen to match the race course. To some extent the tire diameter can be tweaked to fine tune this. You should have differential gears that allow low speed corner exits with the engine well revved up and not out of it's power band and a top speed (if there are any straights, that is just running out of revs. Other than braking to enter a corner, at all other times on the course, your car should be accelerating hard in it's power making RPM band. If running a race mostly in 3rd gear likely you are under your power band on some corners and running out of breath on straight-aways. Lowering the differential gearing would allow 3rd to be revving higher in the turns and a quick shift into 4th at top speed. You may also get the same effect from a close(er) ratio transmission. Second would be closer to third and third closer to forth than the 4 speed you now have. This would allow a shift to second on some turns and a good torque-ey exit with the engine well revved up making more power than the old third gear. 

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Thanks Mike..  Agreed on all points.  I found at the last track, lots of hard lefts, rights, lightbulbs, etc. that the ratio with the 4 speed was lacking...  Downshifting to 2nd would put me at the top of the power band in most corners (e.g., 5500+ RPM), with little gain in terms of speed coming out, and then shifting to third would drop the RPMs (around 4000 RPM) and lose what little speed i gained as the RPMs picked up.  

 

Having 3, close ratio gears (e.g., 2-4), I think would help significantly on the tight, curvy tracks, with lower RPM drops and helping to stay in the power band.

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