Interference Engines

[Taz's Master]

Makenzie, I think there is a comparison being made here between the 944 timing design, and American iron V-8 timing design. I believe (I'm not certain, but I do know tha valve float can cause piston to valve contact, so I assume the design is interference) a Small Block Chevy is an interference design. There are concerns with the engine (the afore mentioned valve float issue for one), but timing chain failure really isn't one, nor is there any real emphasis on timing chain maintenance.

But I think that is comparing apples to oranges. An OHV engine's timing design is much simpler than an OHC design. There are maintenance and reliability issues with chains on some OHC engines, the 944 isn't unique that way.

[Makenzie71]

Yes, the majoraty of american made V8's are interference engines...the majoraty of ALL american made engines are interference engines....actually.

Outside of that, I really didn't understand what you were saying there, Taz...because I wasn't comparing the 944 engine to anything else. I'm comparing interference engines with non-interference engines. It really doesn't matter how the head is setup up...it's either one or the other.

[Porsche-O-Phile]

I've always wondered why they just didn't gear the damn things. . . So it'd weigh a couple of pounds more, require lubrication, and make a little noise. Big deal.

[Makenzie71]

Originally posted by Porsche-O-Phile
I've always wondered why they just didn't gear the damn things. . . So it'd weigh a couple of pounds more, require lubrication, and make a little noise. Big deal.

Again...stress caused by fluctuations in the block and head would cause the gears to bind and probably destroy themselves. Sure, it's possibly, but it would need such an elborate setup it'd be ridiculous.

[Moneyguy1]

You would have to explain the VR6 VW engine then...double chain drive (intermediate sprocket).

Also, "notching" the pistons has been an age old way of reducing the interference problem.

I would say that the problem of expansion is far more serious with the 944 all aluminum engine than with a cast iron design. The 928 , although aluminum, uses a variable adjuster not engineered into the 944; hence less chance of belt failure. The 928 also includes a "check engine belt" indicator which could have been but wasn't included in the '44. The 944 engine is a series of compromises that should have never been put into production. When it works, it works well. When it doesn't, it is a difficult article to work on without special tools and two elbows on each arm.

The reason for the design of the 944 is cost. Interesting for a car that cost multiples of "lesser" four bangers, the company could not come up with a better design.

And, while they are at it. why can't the belt be "pre-streched" to eliminate that PITA overnight trip BACK to mechanic for anothe exfusion from your wallet?

[bluebullet]

mackenzie I think you just listed all the interference motors..not the non-interference ones.

I for one specialize in work on the 4G63 and my friends work on SR20s and honda motors. Those are interference motors and they will bend valves or worse if the belt or chain goes. The early honda motors were belt driven, Im not too sure on the S2000 (think it is chain driven now to increase time between maintenance). Dont ask how I know about them being interference...Ive learned the hard way, even with my own motor.

Rotaries dont count because they have no valves. Altthough u can **** the rotor up really bad if you use the wrong plugs (they stick out into the chamber and as the rotor comes around it will wack the plug and either break it off and jam it or crack the rotor..my friend used to have a Turbo 2 and did that..not good)

Most performance engines are interference motors...thats how you get better power out of them than non interference motors.

I believe the 4G63 SOHC (used in some early galants) and the 4G64 are non interference. My friend has a 2002 Galant with the 63SOHC and he broke his belt without damage to the motor. Usually base model engines or tiny economy engines are non interference (although my stock 1.5L motor that was in my colt wasnt...i broke the belt and wacked ALL the valves..and broke one and jammed it in the head )

[Makenzie71]

^lol...I was wondering if anyone was going to catch the rotory reference...

I've learned that the MR2's all had interference engines...something I was sure was opposite. The Nissans I was unsure of. However, all the rest, especially ALL the Supra power-trains are non-interfernce. I have torn open almost every variety of Supra block in existance...

[leakyfaucet]

Actually, they do make super strong kick-ass timing belts from kevlar fibers. Well for DSMs, (eclipse, talon, laser) which is the world I come from.

Maybe someone can find something like this for Porsches?
http://www.machv.com/ketibe.html

[scheistermeiste]

the rotary engine is the reason i went with a rx7 as my daily driver! i drive today not fearing for my motors life because a belt might break!

as for the plugs... if you are going to replace plugs you should at least know what kind to replace with... and taking out the old you should notice something "isnt quite right" when you compair it to a non rotary plug...

[jm951]

Belt setup has lower mass, therefore less inertia when revving up, than the chain setup. Chain setup costs more to manufacture. Belt setup is cheaper and easier to service. On an aluminum engine, the chain needs to have a sophisticated tensioner to allow for expansion of the block and heads, changing the distance between the crank, waterpump, cam and whatever centers. In short, the belt is the best setup for this car. There are lots of cars using belts for nearly these exact reasons.

[Porsche-O-Phile]

I've actually seen a geared modification for high-powered SBC engines (forget the manufacturer, it's bookmarked at home if anyone cares), that's why I'm wondering. It probably would make some noise, but I'd care less about that than eating valves.

[Britwrench]

We have a F5000 Chevron (SBC, 5.0L around 650hp) with a Jessel belt drive. Seems to work ok.

[VINMAN]

Most american stock V8's are not interference engines. The only time it really comes into play is with radical cam grinds, and high dome pistons.
Valve float will be a problem even with a non interference engine. Valve float is caused when the lifter loses contact with the cam at high rpms. it is usually combated with stronger springs.

Britwrench, that Jesel belt drive is an awsome set-up, Ive put them in many motors. (way pricey though , but bulletproof)

[jm951]

Valve float is also combatted by using lighter springs (not lower strength), lighter lifters and valves. Lower interial mass allows the follower to keep up better with radical profiles at higher rpms.

[fast924S]

I think they did the belt because, One they had to deal with a full aluminum block that expands when hot and contracts when cold, 2 they wanted less weight and wanted it to be free spinning, I think the reason for the interferance engine is the CR ration and packaging, They didnt want this engine to take alot of space, Hence why its on a slant, Lower center of gravity

[nize]

i've got a timing belt change coming up soon.

anyone find a super-strong belt for porsche?

[Taz's Master]

The weight/inertia thing I don't buy. It would have been very easy to reduce the weight of the flywheel the same amount that the chain would add (over the weight of the belt). Cost, simplicity, and expansion seem like reasonable explanations though.

[jm951]

Taz- the weight/inertia thing was related to the actual reciprocating mass of the valves themselves. In order for a valve to open/close, it needs to have a change in velocity exactly 180 degrees opposed to the original motion. Kinetic energy is KE=.5m*v^2. The more rapidly the valves move, the more energy that needs to be overcome to change directions. Said velocity needs to be at the very least sufficient to insure the follower (lifter) maintains contact with the cam lobe. As the lobe gets taller (lift) or changes profile (duration) these values get radically changed. The best way to change the amount of KE needed to insure delta V, is to change the mass of the system. Hence, lighter weight springs, keepers, retainers, and valves.

In relation to the overall engine, some rotational engine mass is desirable in a street car to make it easier to drive for most people. (Conservation of Angular Momentum) Light flywheels and low mass make it easier to rev the engine, but make it much more imperative that the driver be comptetent with a clutch. Said mass also makes for slower revs when the throttle is opened.

The main reasons for not using a chain in an aluminum engine are the thermal expansion coefficient, additional parts, cost. Simply put, a belt is cheaper and less prone to problems in adusting to an aluminum block. HTH

[dj_yen]

jm951:
well, said!

[MikeCT]

Holy Physics!