Hey all, hopefully I won't be ridiculed or called an idiot for asking this, but I really want to know. I recently got an 05 WRX STI, I read that the DOHC EJ25 motors are interference engines. I know if you change your timing belts regularly, the odds of catastrophic failure due to pistons hitting valves is low. However, pulleys can seize and belts can fail prematurely. Also if you run without covers so you can regularly check them, that adds the possibility of something getting caught and breaking the belt.

So my question is, how hard is it to make the engine non-interference? Will installing thicker head gaskets get the clearance needed to make it non-interference? If you increase how much boost the turbo puts out, will it successfully compensate for the compression loss you would get by raising the heads? Just how much overlap is there between the valves and the piston?

 

Thanks in advance.

Edited January 22, 20224 yr by SiriusBlack

SOHC will be the only guarantee of noninterference.  With DOHC, if only one cam slips timing, it could cause the valves to collide.  Thicker headgaskets and more boost is asking for headgasket failure.

Only pre96 sohc heads were noninterference.

Change the timing belt and idlers at the service intervals.  It's usually the cogged idler that fails and takes out the belt.  Very rarely does the belt fail for no reason.

This is pretty hard to achieve and keep a drivable engine at the same time. 

Just keep replacing timing belts and idlers at the right times with quality parts. Genuine would be the best bet. 

NVU is correct about the twin cam heads interfering with each other if the belt breaks, valves will collide together and then the piston. 

Cheers 

Bennie

what they all said up there. Do you KNOW the car got a proper timing belt system service?

You could always accelerate the timing belt system's service schedule. Do it at 75K miles or 75 months for instance. Make darn sure you are using a good kit. Don't run without covers. Use good oil, make sure the turbo and avls oil screens are clean.

buying a used turbo is a gutsy move.

If you want to worry about something, read about the Killer B oil pick-up .

 

Interference is  a non issue. Put this out of your mind. 

You need forged pistons (due to ring lang failure), and a Killer-B pickup tube, and a silicone turbo inlet.

Also redirecting the turbo oil feed from the head to the engine block is an excellent idea. 

Beyond that there are many other applications so be carful..

GD

Be sure to use the full kit (belt, idlers, and water pump), not just the belt.  I thought I'd get away with just the belt on my '02 at 60k and do the full kit at 120k - WRONG!  It failed 400 miles from home and put me at the mercy of a mechanic the locals said was good - WRONG!  It was expensive, frustrating, and left a whole series of Easter eggs for me to find and repair.  Really, who breaks the head off a bolt and just epoxies it in place?  There's no harm in shortening the maintenance interval if you are concerned - much easier than totally re-engineering the engine and controls.

On 1/22/2022 at 2:17 PM, nvu said:

SOHC will be the only guarantee of noninterference

need to clarify that pre-1996 is the only guarantee of non-interference.

There are no shortcuts to good maintenance. Do it right, every time, and dont cheap out on parts. Cheap parts will do you no favors.