Are Subaru 2.5 SOHC Cog Idlers the Same for Both Timing Belt Designs?

[Artbuc]

Have not been here for many years. I am a retired chemical engineer who loves to work on cars but I know my limitations!

Subaru uses two timing belt designs on their 2.5 SOHC non turbo engines. The Forester/Impreza platform uses a modified curvilinear belt and the Legacy/Outback platform uses a curvilinear belt. Both belts are exactly the same except for tooth profile. The two platforms use different camshaft sprockets as you would expect to match the different timing belt tooth profiles. Except for the camshaft sprockets, all other components contacting the belts are the same - hydraulic tensioners, smooth idlers, water pumps and cog idlers. My question relates to the cog idler. I would think the cog idlers would have different groove profiles to mate with the different tooth profiles. I am guessing that because the idler has relatively low load and does not require precise registration, Subaru can use the same cog idler for both belts. What do you think? Thanks.

 

[Dr.D]

As you said, the idler dos not require registration, so slip or non-slip is not an issue. Hence the tooth engagement should not matter.

 

[Artbuc]

Thanks. Do you think timing belt life is affected due to increased stress caused by using a common idler for both curvilinear and modified curvilinear belts? IOW, would the belt last longer if the cog idler groove profiles were matched to the belt?

 

[Ranger Mike]

Here is the skinny on timing belts. Been a fan of these since the nascar boys switched to them years ago. I even did analysis on belt vs gear vs timing chain on this forum a few years back.Because synchronous belts have positive engagement between belt teeth and sprocket grooves, there is relatively little motion between the belt and sprocket and no slip. Synchronous belts are up to 98% efficient in transmitting power.Today the trapezoidal tooth is the most common profile. Think square teeth. This is the most precise and high speed type teeth used. Draw back to this is the pulley grooves are not deep and require more teeth groove compared to other belt drives and thus are physically larger pulleys. This presents a problem today in the insane requirement for lower hood profiles, better aero and thus better MPG.To reduce the pulley size, the curvilinear or rounded-tooth profile, was developed in the early 1970s. Called high torque drive (HTD), these synchronous belts have teeth that are fully rounded, deeper, and more closely spaced than trapezoidal timing belts.The teeth on a synchronous belt are spaced so the root lies at the pitch line. The tooth shear strength is greater than the strength of the tensile member when at least six belt teeth are in mesh with sprocket grooves. This curvilinear tooth design improves stress distribution, and overall loading on many belt drives can be higher in comparison to trapezoidal tooth profiles. Since the teeth are much longer , the sprocket / pulley diameter can be reduced. But, keep in mind, making sprockets smaller, usually leads to shorter belt service life. Sharper bend radius.A variation of the above is the Modified Curvilinear belt. These belts have high tooth flank angles and shallow depth teeth that provide enhanced performance characteristics over the HTD profile.Noise – Though not a problem in a 7000 RPM race engine, the typical grocery getter must be super quitter so as not to interrupt the drivers texting. Pitch, number of grooves, rotational speed, horsepower, and belt width affect noise levels. Two of the variables that reduce drive noise levels are higher capacity rating with small sprockets and narrower belt width. So we end up with the curvilinear belt as narrow as possible to reduce noise on a smaller pulley/sprocket to recue overall dimensions but long enough to last 50,000 to 65,000 miles. Then we get to take the vehicle into the dealership for $100 per hour belt replacements.Idler sprockets can affect the life of the belt if it is not an OEM quality product. Because there is no resistance acting on the teeth to grove of the idler pulley ( as opposed to a water pump) very little wear will occur even if the teeth to groove is not perfect. Shear forces causes wear when the tooth meets the groove and the groove does not want to move. The main cause of reduced belt life as much as the flex interaction of the sprocket / pulley diameter. The flat Idler contacting the non-tooth side of the belt has little impact on the belt. Usually the idler must be replaced as it is a non-lubricated bearing. Dealerships love to include this during belt replacement.
These PDFs are great supporting data. I copied most of the above from them.
http://ww2.gates.com/IF/facts/documents/Gf000282.pdf
http://www.sdp-si.com/D265/PDF/D265T003.pdf

 

[Artbuc]

Thanks Ranger. I have read both of the linked papers. Very helpful. This is what puzzles me about the Subaru application. On the turbo DOHC 2.5 which uses a modified curvilinear timing belt, Subie added a second bearing on the cog idler. But, they use the same groove profile used for the curvilinear belt. Obviously Subaru considers the turbo application more demanding or they would not incur the cost of a second bearing. I would think the more demanding service would also dictate a specific groove profile. Go figure.

 

[Ranger Mike]

probably a cost saving matter...my guess

 

[OCR]

This presents a problem today in the insane requirement for lower hood profiles, better aero and thus better MPG.

...

...the typical grocery getter must be super quitter so as not to interrupt the drivers texting.
Then we get to take the vehicle into the dealership for $100 per hour belt replacements.

... .

A LMAO for this, though... " so as not to interrupt the drivers texting. "

 

[HowlerMonkey]

I'm curious about the "insane requirement for lower hood profiles" since the new camaro has a much "taller" hood than the previous generation.

 

[CWatters]

How do you know the profile on the idle pulley is the same on both engines? Is that from looking at a parts list or by inspecting an engine? Could it just be a mistake? Perhaps you are the first to notice?