"I bought a sweet Mazda Miata a few weeks ago and now have a nice combination car."

Rupunzell

Rupunzell

Bernice Loui
Miata Story...

We purchased a new Miata in 1997. Quite reliable and OK as a daily driver for years and years unit it was put into track day duty. The perception was the Miata should be fine stock as a track day car due to the number of Spec Miatas raced each year and the number of other Miata owners who drive their daily driver Miata to track days and back..

Until this happened:
[ame="http://www.youtube.com/watch?v=57YbtyNYGiQ&feature=plcp"]When Miya Broke 071112 - YouTube[/ame]


Turns out there was far more wrong than just a broken spark plug electrode. The Crank Angle Sensor which drives the twin coil wastes spark ignition system died (very common problem in 1990 to 1998, NA Miata) causing the ignition to be 30 degrees retarded. Replace it but that did not solve the problem completely. The engine still rattles.

This caused me to dig into the inner workings of the Miata in ways never intended. Turns out the more I learned, the more I really dis-like this car.

The temperature gauge has been designed to deceive the owner/driver. It has a response curve shaper circuit designed into the temperature gauge to give the owner/driver the impression you Miata is doing fine even when the engine is in serious heat trouble. When the discovery of this circuit of the temp gauge was discovered, my eyes rolled..
http://www.miataforumz.com/how-40/how-linear-water-temp-gauge-245/

The smoking gun that I believe that drove the Miata engineers to do this was the poorly adapted cooling system for the engine that was originally designed for FWD. The cooling system was originally designed to flow coolant from cylinder one in the block and exit from cylinder four at the top of the cylinder head... similar to the Fiat. In the case of the Miata since it was a front engine/rear drive car, the Maita engineers decided to flow coolant from the front of the block at cylinder one and exit at cylinder one on the cylinder head and use the heater core to help cool cylinder three & four. This does not work so well. It might be OK for daily driven cars that are not really pushed, if pushed, the engine is very prone to over heating.

The Miata DIY folks discovered this and re-routed the cooling system to improve it significantly. Most telling is Dave Coleman who is an engineer at Mazda USA did this to their Eye Sore Racing turbo Miata that is raced at LeMons to help with the designed in cooling problem.
http://www.solomiata.com/CoolantReroute.html

This design problem appears in Miata built from 1990 to 2005 !

Then we have the issue of stuck hydraulic lifters. Very common problem in Miata. Eventually, Mazda gave up on the hydralic lifters and went to a solid lifter set-up with shim adjustment on the top... very similar to Fiat. Except their design can result in the shim popping out in during extended high RPM running as Spec Miata racers discovered.

We purchased a spare engine locally for the spare cylinder head. Removal of the cylinder head revealed more discoveries. The cast iron cams tends to wear on one edge and by the looks of the wear pattern and cracks, these cams do wear out in time. The cylinder head is typical of modern 4 valve design using a multi layer steel head gasket. The bore & stroke appears to be lifted from the Fiat Twin Cam, but 1.8L engine has a pathetic stroke to connecting rod ratio of 1.56:1 (Con rod length 133mm, stroke 85mm). No wonder why this engine does not like to rev and tends to be gravelly and not very sporty. At some point, the bottom end will be taken apart and more will be discovered.

The stock suspension corners on calibrated bump stops. Or the spring/damper combo is set up soft and when the chassis is cornered hard, the bump stops limit body roll.. Wow...

Topping that off is a chassis that is not as stiff and rigid as it should be.

Then we have differentials that tend to fail. This problem was addressed by Mazda in later years.

I'll post more about what actually failed in our 1997 Miata when the cylinder head is removed next week.

IMO, these are not as good as they are believed to be and when pressed into hard driving, more serious design problems appear. It is clear the Mazda folks designed this car for a specific road duty and not much beyond that.


Bernice
 
Why do you think....

they chose such a short ratio on the stroke/con rod length? My recollection of this general issue is that the short ratio puts more side load on the cylinder walls. Can you talk about that ratio in general terms? I'm very interested:)
 
The Miata twin cam turns out to be a rather ordinary design in many ways, It was originally an econ car engine and in time turbocharged in the Mazda GTX. This could be when Mazda added the oil jets for piston cooling.

Longer connecting rods reduces the side loads one the cylinder walls indeed. This also reduced friction. It also affects the piston velocities and various loads on the rotating parts. But the most significant item is the increased dwell or dead time when the piston is at TDC. This allows for a longer burn time extracting more energy from the compressed mixture.

For comparison, the Fiat 1500cc SOHC used in the x1/9 as stroke to connecting rod ratio of 2 to 1.

Beyond this, the bore diameter of 86_ish mm is near ideal ( why 87 mm bore on the Fiat 1500cc engine is good) . Turns out the ideal bore diameters for a petro internal combustion engine is about 88 mm. This is driven by the burn time and flame propagation once the mixture is ignited and the flame wants to travel radial from the point of ignition, not axial or down the cylinder. This is why the World Race Engine proposed for everything from WRC to Formula One has a bore of 88 mm and short stroke with long connecting rods.

Look up the various great engine designs over automotive history and one will note the 88_ish mm bore and short stroke with high stroke to con rod ratio.

The other significant factor is valve area -vs- friction.

I think this say an awful lot about Lampredi's understanding of the combustion process and performance engine design and just how good that Fiat SOHC engine really is.

Anyway, this Mazda engine has me curious to see if longer connecting rods can be had and pistons with shorter compression height made to improve this design deficiency.

Otherwise, this Mazda Miata engine makes a good turbo or supercharged engine, NA it is not ideal.


Bernice

usta in Portland said:
they chose such a short ratio on the stroke/con rod length? My recollection of this general issue is that the short ratio puts more side load on the cylinder walls. Can you talk about that ratio in general terms? I'm very interested:)
Click to expand...
 
I looked into .....

one of my classic engine books and found some comments on the effect of short conrod ratios on the " instantaneous velocity"(IV) of the piston off TDC during the intake stroke. The implication is that the very high IV creates too much turbulence initially so that a full charge is not so easily acheived. This is somewhat offset by the torque advantage of the short rod getting to an advantageous lever angle quicker on the power stroke but as engine speed increases that advantage becomes less important and the turbulence gets worse-the writer even suggested that this is, in fact, why power curves in general begin to flatten on most engines. Does all that make sense?
 
In short, shorter con rods help flow at low engine RPMs due to the higher initial velocity, then becomes a problem as the RPMs goes up.

It is also why a Mazda Miata engine does not rev very well.. beyond it's long stroke relative to bore.

IMO, this engine is designed to please drivers that are accustomed to high torque at low RPM's.. aka Detroit iron.

The Fiat engine, Italian and engines in general can be RPM monsters that are happiest above 4,000 RPM. Not really sale-able to the majority of American drivers who are accustomed to shifting below 3,500 RPM.

The other factors beyond stroke to con rod ratio becomes more significant in fuel burn efficiency (energy extracted from fuel used), emissions reduction, fuel consumption and etc.


Bernice

usta in Portland said:
one of my classic engine books and found some comments on the effect of short conrod ratios on the " instantaneous velocity"(IV) of the piston off TDC during the intake stroke. The implication is that the very high IV creates too much turbulence initially so that a full charge is not so easily acheived. This is somewhat offset by the torque advantage of the short rod getting to an advantageous lever angle quicker on the power stroke but as engine speed increases that advantage becomes less important and the turbulence gets worse-the writer even suggested that this is, in fact, why power curves in general begin to flatten on most engines. Does all that make sense?
Click to expand...
 
Several years ago...

I slightly offended a friend who was baby sitting a Miata when I told him that I thought the engine sensationally unremarkable, especially against the slightly modified 1800 unit in my 124 Spider. I still think so and precisely for the reasons you give. In fact the whole car feels like a compromise taken too far in the name of broad market appeal. Still I do occasionally think they make sense.
While you are here-do you have any opinion about the Honda S2000. It is a car I have thought might become a modern classic in the next decade and I have always admired their understated good looks. The zinger engine concerns me a bit though and I'm not fond of the digital dash (maybe they changed that). Anyway I don't know much about them and magazine stories are useless for this stuff mostly.
 
Regarding the deceitful temp gauge

This is a common design on late model cars. For example late model VW and Audi temp gauges read dead center of the gauge (usually labeled "190" or "90" depending on the market) from around 170F to around 210F. There are a lot of other cars that work in similar fashion.

Twenty years ago when I worked at a Ford dealer they had a lot of complaints about oil pressure fluctuation. Many Ford back then had factory installed gauges that read actual oil pressure. Unfortunately it was difficult to convince many car owners that the fluctuations were normal. At that time new car warranties were just starting to go beyond 12mo/12K miles that had been the norm for quite a while. Since the cars were often still under warranty people tended to believe that the dealers were lying to them about the oil pressure fluctuation being normal to avoid fixing it under warranty. So, Ford came up with a "fix" that made the gauge read dead center whenever the oil pressure was in the normal range.

I think gauges like this are not so much a cover for a design deficiency as much as they are a response to many car owners not being able to handle the truth. Especially when the car is under warranty so they are looking for problems because someone else is paying. People want to feel like they got their money's worth out of the warranty. Gauges in most late model cars are not there to tell something is wrong they are a propaganda tool to tell you everything is fine.
 
I love my 90 Miata... but no argument from me. I've broken piles of parts...

With the stock ignition timing, it is gutless down low, and it sure doesn't zing and pull up to redline like you think a little 1.6 16valve would. I remember test driving Honda's, a Suzuki Swift GTi, and a Toyota Corrolla GTs (86) and those engines were much smoother zippy suckers. My cars supercharger makes my car HIT redline, but in NA form it surely doesn't 'wind out' like many 4cylinders do. I don't think I've ever 'bounced it off the rev limiter' when it was NA...
 
In the case of the Miata, the cooling system definitely has a designed in problem. While the deceptive stock temperature gauge with a dead area was likely done to address the problem of owner/drivers expectation of how the temperature gauge behave.

The oil pressure gauge appears to have a dead area designed in for the same market expectation reasons.

This entire discussion has evolved into a marketing, buyer expectation and market inertia discussion beyond just the problems inherent in the Miata.

If one looked at the commercial and market success of the Miata, it is indeed a FIVE STAR winner by this metric. With nearly one million units sold world wide, the Miata holds the world's record in sales for a car of this type. It says that the folks at Mazda who designed the Miata did their work well and delivered a product that meets and/or exceeds owner expectations. They pegged the potential owners and market directly on target.

It also says much of what these owners expects from a, "sports car."

The Miata will never be the Lotus Elan it was copied from. Worth noting, the developers of the Miata used an x1/9 as one of the comparison cars along with two Lotus Elans and MGB.

For comparison, the Honda S2000 is in many, many ways the better sports car, superior engine (Yes, there was a time when I schemed about installing this engine into an X1/9 or Lancia. It IS that good), transmission, diff with LSD. Good chassis and overall just a better design than the Miata. Using the same marketing metric as the Miata, the S2000 was a market failure selling about 112,xxx units over it's production life time.

This comparison tells much about what the majority of car buyers want and will accept... Even if one builds a far better mouse trap, the world will never beat a path to your door based on the better mouse trap alone.

Sad but the harsh truth and reality of marketing and market expectation.

These factors are also why the exxe will continue on it's current market path.


Bernice




Matthew said:
I think gauges like this are not so much a cover for a design deficiency as much as they are a response to many car owners not being able to handle the truth.

Especially when the car is under warranty so they are looking for problems because someone else is paying. People want to feel like they got their money's worth out of the warranty.

Gauges in most late model cars are not there to tell something is wrong they are a propaganda tool to tell you everything is fine.
Click to expand...
 
Last edited:
[ame="http://www.youtube.com/watch?v=I1Lkyb6SU5U"]Supersize Me -Trailer - YouTube[/ame]

The film:
[ame="http://www.youtube.com/watch?v=H-eRXuuH9AI"]Supersize Me Movie / Documentary - Full length - YouTube[/ame]



Bernice


ianlawson said:
........adding to the "grossly expanding" US Health Dept problems!! :dead:

cheers, Ian - NZ
Click to expand...
 
This is yesteryear's fast food...

controversy. We are onto more interesting things, like chicken fillets and gay marriage. Yep, Americans are a creative lot, Ian :D:D:D
Next week-pizza and the Taliban:eek:mg:
 
Yep, yuz guyz shore move along fast...........!

usta in Portland said:
...... We are onto more interesting things, like chicken fillets and gay marriage.......
.......Next week-pizza and the Taliban:eek:mg:..
Click to expand...

......looks to me like Mitt Romney is now even like "last week's newspaper"! :bored:

cheers, Ian - NZ
 
Good mixing of topics :)

Bernice, I recently had something similar happen with my BMW. I'm still having issues with the car - now I believe it's the cats that need to be replaced but my attention to this came in most unfortunate timing.

Which one of these things doesn't belong?

3ecec765.jpg


One of these USED to be a Bosch Platinum 4...

fc6eda89.jpg


...which destroyed the ignition coil

be062552.jpg


It's a long, long story but I was fortunate enough to break down TWICE in the same location near Tony Natoli's place in Santa Clarita - once coming south, and the other going north on the 5 freeway.

The car has 217k on it and after the cats get replaced, I'm hoping to take it up to 500k. The problem with my meltdown (also in the #2 chamber) also lead to completely losing the threads in the head so the new spark plug is being held in place with an aluminum insert and Loctite....
 
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