Interference Engines

[dj_yen]

We on this forum should all be very well versed in the dangers of interference engines and the importance of regular timing belt changes but my question is, why an interference engine in the first place?

What is the advantage of an interference engine over a non-interference engine? Are they cheaper to make?

[scheistermeiste]

ive wondered that my self...

[jm951]

valve lift & duration v compression ratio

[Britwrench]

True, but it also has lot to do with cylinder head design, valve location and angle and other fun stuff.

[gearhead290]

Maybe somebody at Porsche wanted to make sure that the mechanics keep busy and that Porsche makes some money off of selling the belts?

[Brando]

Not a worry for me! haha!

also, early 928 guys have 2x my engine... teehee, non interference!

[nize]

from what i've read about this in the past, interference design allows greater piston movement which in turn allows for greater compression. and as we all know, more compression = more power.

[Makenzie71]

^correct

[gearhead290]

Here's another good question, why did Porsche use a timing belt instead of a timing chain? This has kept me awake with headaches many of nights

[Makenzie71]

Belts...though they wear faster...are cheaper and easier to deal with. Also, almost every aluminum head/engine uses a belt as opposed to a chain because of stress factors. A belt is elastic and will go with the stress on the mass of the head or block...a chain will not. Belts are also easier to change and maintain and inspect. The only obvious sign of wear a chain will show is rust, and loosening a tensioner is far easier than removing a cam or sprocket to change a chain. Belts are also quieter and smoother to operate which creates less wear on the engine's hard-parts (if you're a bike fan, this will make sense to you).

The biggest reason, however, is probably that pullies and belts are cheaper than sprockets and chains.

[nize]

i actually read about that one a while ago also. although a chain would make belt changes unnecessary, the added weight/resistance would make the motor less nimble and rob power. i suppose with modern technology they could design it better (i believe the honda s2000 has a chain timing belt) but we're talking about 20 years ago.

the real question is: why doesn't someone design a better belt, like out of graphite or steel-belted or some other higher-durability/longer-lasting material/design? i'm sure there's a market for an improved +60k mile lasting belt.

makenzie, you reading this? i know you're looking to fabricate some of your own parts to sell to other porsche owners, here's something you can make and sell that hasn't been done before (to my knowledge).

and if someone knows of such a belt, please let me know!

[Taz's Master]

The belt question is more significant in my mind. How many non-interference perfromance engines are there?

[Makenzie71]

Originally posted by nize
sure there's a market for an improved +60k mile lasting belt.

It's actually suggested that chains be changed as often on interference engines. Non-interfenrence engines usually make no reference to timing belt maintanance.

A radial timing belt would be the same as using a chain. The majoraty of th reasoning behind using a belt is it's elasticity.

[Makenzie71]

Originally posted by Taz's Master
The belt question is more significant in my mind. How many non-interference perfromance engines are there?

4mE
5mE
6m-GE
6m-GTE
7m-GE/GTE
1jz-GE/GTE
1g-GE/GTE
2jz-GE/GTE
1zuf or whatever the V8 is
4A-GZE
4A-GE
4E-FTE
3S-GTE

12A
13B
13B-T
13B-REW
13B-REE
20B
20B-RE
26A

4G63T

RB20DET
RB25DET
SR20DET
CA18DET (I think)

(those are the only ones that pop into mind right now)

[nize]

Originally posted by Makenzie71
It's actually suggested that chains be changed as often on interference engines.

the honda s2000 is a chain-driven interference engine.

recommended timing belt replacement on the s2000 is 90k miles.

[nize]

Originally posted by Makenzie71
A radial timing belt would be the same as using a chain. The majoraty of th reasoning behind using a belt is it's elasticity.

considering how tight the 944 timing belt needs to be, and how it needs to be tightened again after being driven a while, i doubt elasticity is the reason they used a belt.

if it wasn't as elastic, i'm guessing it probably wouldn't even need to be re-tightened after initial installation.

[Makenzie71]

My area of expertise kinna starts fading around 1997. But, since it's an intereference engine, a routing changing of the timing assembly is still important and noted by the manufacturer.

MR2's, apparently DO have interference engines...not sure which ones yet.

[Makenzie71]

Originally posted by nize
considering how tight the 944 timing belt needs to be, and how it needs to be tightened again after being driven a while, i doubt elasticity is the reason they used a belt.

if it wasn't as elastic, i'm guessing it probably wouldn't even need to be re-tightened after initial installation.

When you consider the change in the block from heat being less than 1mm, there doesn't need to be much elasticity. That much change, however, could bind a chain easily.

[nize]

Originally posted by Makenzie71
That much change, however, could bind a chain easily.

uh, we're not talking about chains, we're talking about a longer lasting +60k mile belt.

[Makenzie71]

ok...I'll try it a different way...

The belt has give to it. A chain does not...because a chain is made out of metal. Adding steel will give the belt all of the qualities of a "chain".

Ever hear the phrase "only as strong as it's weakest link"?