18 replies to this topic

[STEVE HUNT]

Greetings fellow endurance racers. I've searched high and low for a forum that specializes in endurance racing the Miata but haven't had any luck finding one. That being the case I thought this seemed like a really good place to start a forum thread pertaining the the specific technical challenges likely to be encountered when enduance racing a Miata. I've been racing my Miata for 11 years now, starting out in SCCA sprint racing then moving on to Lemons, Champ Car, SCCA and AER endurance racing. The setup of my NA Miata is based fairly close to the west coast Super Miata with NB suspension, NB2 VVT motor w/coolant reroute, MegaSquirt ECU, 4.77 rear gear, diff cooler, 5 speed trans, Sport Brakes, and fiberglass doors and top.

 

The most significant challenge we've faced when racing 9 hours a day has to be heat. We've experienced and seen many, many failures through the years and I'm convinced that I can attribute the large majority of those failures to the build up of heat taking its toll on the equipment. Here's what I've done to mitigate the heat issues:

• Header - I sent our Racing Beat header to JetHot and had it coated. I've tried header wrap but found it to be to fragile plus I do believe it makes the header susceptible to cracking. JetHot says their coating knocks down 60% of the heat and I believe them.
• Transmission - The header is pretty close to the trans so I fabricated a heat shied that fastens with a couple aluminum straps.
• Diff - I did wrap the exhaust pipe where it comes closest to the diff. I'm not as concerned about it causing cracks in a fairly cheap piece of exhaust pipe and I also painted it with the product recommended for this purpose.
• Diff Cooler - With only one quart of gear lube, heat build up is a huge problem when racing for 9 or even 24 hours. After seeing multiple diff failures, all heat related, coming late in the race day I decided that a diff cooler was really the only option. I've only had it for one race weekend and that was in February but I've fairly confident that it will prevent future failures.
• Foot well - insulating the foot well is a must, I actually had a driving shoe fail because the foot well was getting so hot.

That's it for now, time to head the shop to start installing the Penske shocks I just purchased for the Miata.

[Ron Alan]

Are you running a stock Torsen with your different gear ratio? What failures have you had? Granted, my experience comes at a time of year when outside temps are cool but I have run 15 Miatas in the last 5 years at the NASA 25 HR(300+ hrs)and never had a failure. In that same time I had 4 motor failures and 2 transmissions. I'm wondering if your diff failure is not related to how it was set up with the different ring and pinion...internal heat vs external heat? The rest of the things you mentioned certainly help...especially the foot well...driver comfort

[STEVE HUNT]

Thanks for posting. Yes, it was a stock Torsen. The last failure went as follows; with 45 minutes remaining the left CV shaft seal failed which dumped gear lube on the exhaust system creating quite a smoke display. I disassembled the diff and found that the fluid was badly burned and the ring and pinion were badly scared. It seemed pretty obvious that the gear lube had overheated and failed. The CV shaft seal basically melted from the heat build up. This was in the summer with an outside air temperature around 90. The exhaust was wrapped so I'm not attributing the heat build up to it so it had to be internal heat buildup but the diff was several years old and had been used for two previous endurance events and numerous sprint events. What gear lube are you using? Also, what gear ratio are you running and what's your maximum stint duration?

[Ron Alan]

For no reason other than its readily available to me and I have never had issues...always use Redline for diff and tranny. 

[Michael Novak]

Why the 4.88 Gear? I assume fuel and stops are not of concern with the series that you run in, but Champcar it would never allow you to get to the 2 hour mark?

 

We had 110 hours on our Miata and even after 2-24 hour events the fluid would come out looking pretty good. Redline Shockproof lite.

[Ron Alan]

Why the 4.88 Gear? I assume fuel and stops are not of concern with the series that you run in, but Champcar it would never allow you to get to the 2 hour mark?

 

We had 110 hours on our Miata and even after 2-24 hour events the fluid would come out looking pretty good. Redline Shockproof lite.

Agreed on the ratio...if anything I go the other way when I can!

[EMatoy]

Public service announcement—Big heat problem I have seen on most of the Miata’s (and nearly all the BMW’s) running is exhaust and diff heating the fuel tank. Result is gas spewing from the gas cap due to the pressure in right hand corners. My car did it without my knowledge- a teammate driving behind me told me on the radio. My reply was I was just going to tell you the same thing! If you stop in the pitts and there is pressure when you take the cap off - you have a problem. Some cars spew so much gas I have turned on my wipers. Insulation and ventilation of the tank are your friend.

[gerglmuff2]

Public service announcement—Big heat problem I have seen on most of the Miata’s (and nearly all the BMW’s) running is exhaust and diff heating the fuel tank. Result is gas spewing from the gas cap due to the pressure in right hand corners. My car did it without my knowledge- a teammate driving behind me told me on the radio. My reply was I was just going to tell you the same thing! If you stop in the pitts and there is pressure when you take the cap off - you have a problem. Some cars spew so much gas I have turned on my wipers. Insulation and ventilation of the tank are your friend.

 

interesting. when i used my autocross car at a track day it was doing this, probably because the exhaust blows directly on the gas tank .... i dotn use that car very often and never had any other issues with it doing other things, so i never thought much of it. but now that makes sense. 

[Steve Scheifler]

interesting. when i used my autocross car at a track day it was doing this, probably because the exhaust blows directly on the gas tank .... i dotn use that car very often and never had any other issues with it doing other things, so i never thought much of it. but now that makes sense.

It can be a real problem on the dyno as well, radiant heat off exhaust & muffler and little airflow. Recently had an old Camaro belch large quantities of fuel out the filler just sitting there.

[STEVE HUNT]

Excellent, that was my next topical question. I'm definitely leaning towards swapping out the 4.77 diff and going back to the 4.30. What ratio are you guys running and what's your max stint duration?

[Michael Novak]

Excellent, that was my next topical question. I'm definitely leaning towards swapping out the 4.77 diff and going back to the 4.30. What ratio are you guys running and what's your max stint duration?

4:30 or 4:10   I like the 4:30 best as, you have a little extra power when you need it--Short shift or back out of it earlier to save fuel. 2 hours if you take it easy 1:45-1:50 running hard depending on track.

[Jeff Wasilko]

2nd on the 4.30. Feels like the perfect diff ratio.

[EMatoy]

2nd on the 4.30. Feels like the perfect diff ratio.

3rd on that. It also allows common spares with my other car that is a SM

[Ron Alan]

Excellent, that was my next topical question. I'm definitely leaning towards swapping out the 4.77 diff and going back to the 4.30. What ratio are you guys running and what's your max stint duration?

Considering you have a vvt swap in your car...which is controlled by a megasquirt ecu...your lower rpm torque and power should look really good compared to a stock ecu. Keeping your car running in a lower rpm window/range without losing any real grunt/acceleration is the goal. What you run your AFR at will obviously be the biggest factor in your equation if fuel mileage is being considered...which also affects how happy your car is running...balancing act!

 

4.3 IMO is far better than what you are running but my choice would be 4.1...at the same speed you have effectively lowered the rpms 200-300rpm over the 1600 rpm working range. I think if you look at your dyno curve and compare power/torque 2-300rpms apart its tiny. The benefits of running lower rpms in endurance racing far out weighs the ideal sprint set up(which 4.3 is for sure!)

 

Finding a 4.1 Torsen(94-97)would be your easiest solution. Finding a 4.1 open ring and pinion pumpkin and installing your torsen would be next. Another fun trick is to find an aluminum housing/carrier from a RX7 diff to use! You have to change the bushing mounts in it to fit Miata chassis but it is stronger and WAY less likely to break in a minor confrontation!! And it actually takes just a smidge more oil to fill...but if you are running a cooler this wont matter! Replace 4.3 in this paragraph if thats the way you want to go

 

This is a fun page to mess around with all the variables when it comes to transmissions, gear ratios, rpms and tire size!

 

https://www.flyinmia...ech/gearing.php

[Alberto]

The higher rear gear really taxes the oil temps in the diff.  

 

For footwell and cockpit insulation in general, consider DEI floor and tunnel shield under the transmission tunnel and on the firewall (engine side).  It helps a lot.

[Steve Scheifler]

The higher rear gear really taxes the oil temps in the diff.

For footwell and cockpit insulation in general, consider DEI floor and tunnel shield under the transmission tunnel and on the firewall (engine side). It helps a lot.

By higher you mean lower/shorter? Or do you reslly mean the 4.1 causes more heat than the 4.3? People tend to mix up the terms. Higher number effectively lowers all the other gears hence shorter, a lower number makes the others taller or higher.

Regardless, I’m surprised the small difference in ratio would be at all noticeable on temps.

[Alberto]

By higher you mean lower/shorter? Or do you reslly mean the 4.1 causes more heat than the 4.3? People tend to mix up the terms. Higher number effectively lowers all the other gears hence shorter, a lower number makes the others taller or higher.

Regardless, I’m surprised the small difference in ratio would be at all noticeable on temps.

 

I suppose I do.  I always get confused with the terminology.

I never tried on the Miata but on the RX8, the 4.7 and especially the 5.1 generate more heat than the 4.4 

[Michael Novak]

I suppose I do.  I always get confused with the terminology.

I never tried on the Miata but on the RX8, the 4.7 and especially the 5.1 generate more heat than the 4.4 

Anyone understand why---I would guess that the larger ring and pinion would cool even better with the less contact??  Or is the exhaust more of the factor?

[STEVE HUNT]

Considering you have a vvt swap in your car...which is controlled by a megasquirt ecu...your lower rpm torque and power should look really good compared to a stock ecu. Keeping your car running in a lower rpm window/range without losing any real grunt/acceleration is the goal. What you run your AFR at will obviously be the biggest factor in your equation if fuel mileage is being considered...which also affects how happy your car is running...balancing act!

 

4.3 IMO is far better than what you are running but my choice would be 4.1...at the same speed you have effectively lowered the rpms 200-300rpm over the 1600 rpm working range. I think if you look at your dyno curve and compare power/torque 2-300rpms apart its tiny. The benefits of running lower rpms in endurance racing far out weighs the ideal sprint set up(which 4.3 is for sure!)

 

Finding a 4.1 Torsen(94-97)would be your easiest solution. Finding a 4.1 open ring and pinion pumpkin and installing your torsen would be next. Another fun trick is to find an aluminum housing/carrier from a RX7 diff to use! You have to change the bushing mounts in it to fit Miata chassis but it is stronger and WAY less likely to break in a minor confrontation!! And it actually takes just a smidge more oil to fill...but if you are running a cooler this wont matter! Replace 4.3 in this paragraph if thats the way you want to go

 

This is a fun page to mess around with all the variables when it comes to transmissions, gear ratios, rpms and tire size!

 

https://www.flyinmia...ech/gearing.php

I already have a 4.30 diff as a spare, I think you've convinced me to swap out the 4.77 with it.