Question about engineering and modification

[medic327]

I am fully aware that a lotus esprit is an exotic car, hand made and can not be compared to a Toyota or anything of the like. But aside from this reason why exactly does the timing belt need to be changed at 12,000 miles. I am interested in a technical answer. Please reserve the hand made, special, exotic explanation, as these things are not explanations unto themselves. The engine of this or any car is a machine, and like any machine, engineering principles apply. What is it about this exotic and hand built engine that requires such frequent timing belt changes. Is it some repeated stress, vibration, poor engineering, poor choice of belt materials, etc. Finally, what theoretical modifications be done to prolong the belt life? Is it possible to fit the engine with a timing chain set up? I'm sure the answers to these questions are elementary to many on this board, but I don't have any experience with exotic car engines. I am truly interested in your answers from an academic and engineering standpoint. Thank you to all who take time to answer my questions. Stay safe.

>>> Edited by medic327 on Sunday 17th October 05:18

[mikelr]

I have heard that since the belt is exposed and not hidden behind covers like on many other common motors, that it is exposed to UV light which breaks down the material over time.
With a zero clearance motor changing a $25 belt every 12k miles is cheap insurance.

Maybe someone can add another reason...

[rlearp]

Actually, yes, you can compare the Lotus to a Toyota or a GM car since it shares many of the same parts! My personal opinion is that it does not need to be changed every 12k miles, although some might cry foul.

The belt that was just put on my car just before I purchased it was made by a large belt company which I cannot remember. At the time (over 1.5 years ago) I remember taking the company name and scouring the internet for what I could find on longitivity regarding timing belts. I found no instances of 12k intervals, in fact, I think the nearest I found was 50k.

I suppose one could argue that temps are hotter with a mid engined car and the belt sees a lot of heat stress. But, I'm not sure I buy that because I've checked "under hood" temps on my Esprit numerous times with a digital pyrometer and have not found them to be statistically higher than a front engined car provided that both are cooling correctly.

I think belt composistion has changed over the last 30+ years since the Lotus motor was introduced. I imagine the change interval is a precautionary measure and holdover from a previous era. Personally, since it is cheap (purchased from the right source) and relatively easy to change it might be a good idea to change it when needed and determine need by careful inspection when changing your oil.

I'm sure you'll find those that think the Esprit engine is special and needs special treatment, but, like you, I feel the engine is an engine like in any other car I own. About the only thing special about it is some of the poor engineering idiosyncrocies that pop up every now and again that make you scratch your head and go "WTF!?!"

It was fairly sophisticated mill back in the day. But, now with DOHC motors in many common autos, and few of them turbocharged beyond Esprit performance, it has been surpassed. Maintain it well, watch things like belts with careful inspections, and the motor will last a long time though. At least I hope mine does.

Ron

>> Edited by rlearp on Sunday 17th October 11:16

[GKP]

I think also the type of car has something to do with the way in which they are driven. A small "shopping" car probably won't be subjected to it's rev limiter and hard downchanges too many times in it's life.
An Esprit, however....

[Mike.Griese]

The reason the belt needs to be changed or at least checked every 12k miles on early cars is that it stretches with use. As it stretches, it loosens, which increases the chance the belt will jump a tooth on the intake cam at startup. A jumped tooth means a BIG timing change, jump enough and you start bashing pistons into valves.

I change mine about every 3 years. It forces me to look after other issues on a regular basis, like valve clearance, timing, carb balance, etc. I do my own maintenance, so the cost is essentially parts.

I should also point out that cars with trapezoidal belts (the belt tooth profile as seen on edge) are more prone to jumped cams because of the shape of the tooth. Cars built after 1986 have a different cam belt tooth profile, and as a result, a longer belt life.

Mike
'83 Esprit Turbo (The Fabulous Trashwagon)
'05 Elise

>> Edited by Mike.Griese on Sunday 17th October 13:13

[rlearp]

Proper belt tensioning should take care of any stretching - it should not be working loose over time. Running with loose belts is indeed very bad, as you point out! I think checking the tension and belt condition would be some of the best things one could do to make sure the motor is happy. But, unless I saw some damage to the belt or wear I don't think I'd blindly change it. Especially for those on the board that don't do their own work - it could be quite expensive over time.

I am thinking of changing the cam gears on my Jensen to the later style gears and belts that are more resistant to jumping teeth. The Jensen has the early square tooth design and I'm not sure I like that so much.

>> Edited by rlearp on Sunday 17th October 13:35

[toyroom]

I would like to know why (according to the service manual) The timing belts on a V8 only need changing every 102,000 miles in the USA but every 24,000 in the UK ? Perhaps this recommendation has changed recently but surely the climate alone can't account for this difference. Maybe the ratio of startups to total mileage is different in America, butfour times different seems a bit much !

[medic327]

First off I want to thank everyone for their replies. So it seems that with maybe some extra cooling measures, careful selection from a good belt manufacturer and maybe something to shield the belt one theoretically could extend the belt change interval. Do you guys think that the belt replacement interval (with or without modifications) could be extended to 36000 miles or even 60000 miles safely? I agree that changing the belt early may save the engine from expensive damage but I don't really consider spending $1000+ every 12000 miles cheap. Say for instance you changed the belt at 60,000 miles (as is the standard for most cars) instead of 12000 miles, during this time you would have saved yourself $4000. So the question becomes this: is the cost/benefit ratio of spending $4000 VS a theoretical failure in favor of absorbing the cost? If you only needed to change the belt 2 or 3 times during the life of the engine, you could theoretically buy a new engine with the savings. The question I'd really like answered is if it is possible to fit a timing chain instead of a belt to the engine, I'm wonder if this would be well worth the one time cost. Thanks to all who took and take the time to answer these questions. Stay Safe. -medic

[lotusguy]

medic327 said:

So it seems that with maybe some extra cooling measures, careful selection from a good belt manufacturer and maybe something to shield the belt one theoretically could extend the belt change interval. Do you guys think that the belt replacement interval (with or without modifications) could be extended to 36000 miles or even 60000 miles safely? I agree that changing the belt early may save the engine from expensive damage but I don't really consider spending $1000+ every 12000 miles cheap. Say for instance you changed the belt at 60,000 miles (as is the standard for most cars) instead of 12000 miles, during this time you would have saved yourself $4000. So the question becomes this: is the cost/benefit ratio of spending $4000 VS a theoretical failure in favor of absorbing the cost? If you only needed to change the belt 2 or 3 times during the life of the engine, you could theoretically buy a new engine with the savings. The question I'd really like answered is if it is possible to fit a timing chain instead of a belt to the engine, I'm wonder if this would be well worth the one time cost. Thanks to all who took and take the time to answer these questions. Stay Safe. -medic

HI,

Zigging when everyone else is Zagging is on rare occasion innovative. Most often, it's a recipe for disaster!

Lotus didn't just pick the service interval out of the air. I suppose they did add some fudge factor to be well inside the safe zone, but generally the interval chosen is sound. I have seen, and almost experienced, the result of a broken belt. $4k to redo the potential damage is a low side estimate, and that's if you do your own work.

The reasoning that the money saved could equate to the cost of repair if you're wrong is faulty logic. And it doesn't take into account the value of your aggrevation, loss of driving time or depressed resale value if the failed engine is revealed in the provenance.

Your same logic could be used to rule against changing the oil. If you could go long enough without changing it, it would be possible save a sum equal to a new engine, but again, I doubt you'd get many disciples.

If the cost of maintaining the car exceeds your will or abilities, better you search for a car that better suites your needs and taste.

So far as replacing the belt with a chain, in the realm that anything is possible, this could be done. But, it would bring conversion and maintenance costs (Yes you'll have to maintain this system too) that exceed those of changing the belt at regular intervals.
Happy Motoring! ...Jim'85TE

>> Edited by lotusguy on Sunday 17th October 21:49

[medic327]

Hi Jim,
First off, I'd like to say that I do not advocate skipping belt changes, I am just asking critical questions to educate myself. Secondly, I dont think that skipping relatively cheap oil changes will ever be worth the value of a new engine, the cost/benefit/risk ratio is not favorable, thus I would never even suggest such a thing. You suggest that the service interval of 12000 miles to be mandatory through your experience. This brings me back to my original question about 12000 miles being a very short interval and asking why this was the case. The vast majority of automobiles do not have such a short belt change interval, and I was wondering in essence what they were doing right that Lotus was doing wrong. Hopefully I might gain some insight into the problem and apply a solution; thus searching for a solution instead of just griping about an issue.
In so far as searching for a car that I could better be able to afford is concerned, I can tell you that I consider critical questions such as this before I make any major purchase. It has less to do with financial means and much more to do with financial sense. This is sort of like reading a contract and considering what it means before overzealously applying pen to paper. But this is all I'll say about financial matters, since like discussing religion and politics you'll never know who you may offend.
I am still looking for the right car and taking the time to make an educated and well considered purchase. Thank You for taking the time to answer my post, and thank you for all of you previous posts from which I've learned much. Stay Safe. -medic.









>> Edited by medic327 on Monday 18th October 03:11

[country]

im a little confused from reading the posts,i have a 03 1/2 esprit i was told the timing belt should be replaced at 30,000 miles,and to ck the tension every 10,000 miles.is this correct.
thanks
adam

[Esprit2]

country said:

im a little confused from reading the posts,i have a 03 1/2 esprit i was told the timing belt should be replaced at 30,000 miles,and to ck the tension every 10,000 miles.is this correct. thanks adam

For a V8, that's correct. In the USA, LCU will check and re-tension the V*'s timing belts for free of charge every 10k miles (12K ??... check with your dealer) through the last scheduled interval prior to the odometer clicking over 100k. Do it.

Early 4-cylinder Esprits had belts with trapezoidal shaped teeth. Those belts were supposed to be changed every 25,000 miles by the book. But in light of the severe damage that is done if the belt breaks, and an early history of problems with that happening, the practical self defense is to change that belt well before the 25k spec.

One US independent Lotus parts supplier recommended I use 12K / 2 years, which ever comes first and that's what I use. A skeptic might counter that the vendor was just trying to sell more belts, but I doubt that. The engines did have issues in that regard and I trust this guy.

In 1986 (Fed HCI) a round tooth HTD timing belt was introduced and the change interval was increased to 31K (50K in California). In 1995 an HTD HSN (Highly Saturated Nitrile) belt was introduced with a 100k mile interval for California. The 50k & 100K intervals were mandated by California and do not reflect design advances in proportion. If the car is off warranty and you pay the repair bills, do not go that long. Use the HTD HSN, but don't go past 50k. That's just my opinion, and if it were my car, I'd go 36k/ 3 years. It's cheap insurance against a big, expensive repair bill.

I recently posted the following on the turboesprit list on YahooGroups...

Belt replacement interval spec - miles...
A907D0191Z Trapezoidal
12,000 miles/ 2 years, whichever comes first - "what I use".

25,000 all 907s
5,000 check & re-tension

25,000 carb turbo 910, except California
31,000 carb turbo, Calif (mandated, I wouldn't go there)
12,500 tension

A912E6697F HTD (round), standard
31,000 G-Turbo HCI, Calif
31,000 1988-92 manual
12,500 check & re-tension

36,000/ 36 months 1993 onward manual
12,500/ 12 months check & re-tension

50,000 (California std's req', I wouldn't go there).
12,500/ 12 months check & re-tension

B912E6697F HTD HSN (introduced 1995, California)
100,000 (1995 Calif std's required it -- be still my heart).
12,500/ 12 months check & re-tension

I would really like to know what the service warranty claims were like with the 100k belts... I don't know how well they performed. but I wouldn't be comfortable with that long interval if my checkbook were on the hook.

Later,
Tim Engel
Lotus Owners Oftha North

[Esprit2]

medic327 said:

But aside from this reason why exactly does the timing belt need to be changed at 12,000 miles. I am interested in a technical answer. Please reserve the hand made, special, exotic explanation, as these things are not explanations unto themselves.

The original 9XX 4-cylinder engine was penned in the late 1960's. Timing belts were relatively new then and materials and design technology weren't as well developed as they are now. The belts are not very durable and that was a concern in the market place when belt-timed engines were introduced.

The early belt design and layout is almost a 40 years old and it doesn't hold up to a 21st century critique. They were learning. That's as complicated as the answer gets. It may not be as scientific as you want, but that's as good as it gets.

Lotus was (is) a small company and can't afford to re-design something as significant as an engine very often. Once the design was set, it did't receive many large changes. The belt tensioner was revised several times in response to warranty/ service issues, but the belt itself wasn't upgraded until 1986, then again in 1995. Re-designing, and more importantly, re-certifying an engine for the US market was (and still is) just too expensive.

medic327 said:

What is it about this exotic and hand built engine that requires such frequent timing belt changes.

I know this seems flip, but...
Because the belts break if you don't change them often enough. And the expensive valve train can get wiped out when the belt does break.

medic327 said:

Is it some repeated stress, vibration, poor engineering, poor choice of belt materials, etc.

It's fatigue from constantly flexing around pulleys and then straightening out. It's the materials available and used. It's leaving compliant materials under constant stress. It's rubber of the era being exposed to ozone, oil, solvents and heat, it's about a less than perfect choice of tooth shape... etc.

The original belt technology was early generation. The trapezoidal tooth design was sub-optimal in that a tooth shape made up of straight lines and angles was asked to do it's best work wrapped around circular pulleys. The round-tooth HTD tooth is much more appropriate and a real improvement.

I think the biggest flaw in the pulley/ belt system design was a lack of compliance in the belt tensioning system. The engine is aluminum and grows considerably as it warms up. With the non-compliant eccentric tensioner, the belt simply has to stretch and work under greater tension.

A properly tensioned belt at 92-95 Burroughs will go to about 125 Burroughs as the engine heats up. That's why the manual specifies setting the belt tension with the engine at room temperature and before being started for the first time that day.

One might expect the belt tension to drop as the rubber & fabric belt heats up, expands and becomes more soft, but that's not the dominant case. The engine's expansion stretches the belt and increases tension significantly.

The simple eccentric tensioner is non-compliant and does not adjust for that increase. The belt must be tight enough to function without skipping when the engine is started from cold (even winter cold), and then just live with the stretch imposed upon it by an expanding engine.

Lotus did try to use a spring loaded, semi-automatic tensioner. But the spring tensioner design they came up with had it's own teething problems and went through a series of re-designs. Ultimately, Lotus abandoned it and went back to the simple but non-compiant eccentric tensioner. I don't know, but I think they just got fed up with messing with the semi-auto tensioner and said, "Let 'em change belts".

medic327 said:

Finally, what theoretical modifications be done to prolong the belt life? Is it possible to fit the engine with a timing chain set up?

The only basic "layout" flaw is the minimal amount of pulley wrap on the intake cam pulley and auxiliary pulley. It's only about 90° and should have been more. Altering that would be tough.

Personally, I like the semi-auto tensioner and think it has the potential to extend belt life more than any basic layout change. It can be reliable "IF" you keep it in a good state of repair, but it doesn't last forever. It needs attention and that's something many/ most American's don't lavish on their cars. A compliant tensioner is one area where a back-date would be an improvement... IMHO.

I think the best solution would be a new tensioner (don'hold your breath) with a more robust design with a stronger hinge and a ratcheting hydraulic self-adjusting pre-loader for the spring. Power the hydraulics with engine oil pressure which is consistently pretty high on start-up. Then a ratcheting piston would hold the force as the oil pressure dropped as it heated up. You would reset the tensioner's spring pre-load everytime you started the engine.

You could convert to chain drive... anything is possible if you have the time and/or money. But chains don't last forever either and you would just be trading one problem for another. Just ask Lambo and Ferrari owners about the ease and economy or servicing or replacing timing chains.

No one offers a chain conversion kit. Doing so yourself would be more work and cost than servicing the current belt design (IMHO) and would be a matter of jumping out of the frying pan and into the fire.

Regards,
Tim Engel
Lotus Owners Oftha North

[judson]

The problem isn't the belt, it's the valve shims.

The valves need to be shimmed every 12k miles, without it, they burn. The belt is a $20 after-thought, but most assuredly cheap insurance.

I've had numerous 907s in my shop with 30k-50k on the belt, but they never considered the valves. Now they come to me needing new valves and a valve job.

[Lotusacbc]

judson said:

The problem isn't the belt, it's the valve shims. I've had numerous 907s in my shop with 30k-50k on the belt, but they never considered the valves. Now they come to me needing new valves and a valve job.

Whats that run ya?