Upgraded Subaru 2.5 Phase II Cylinder Heads
eBay Item number: 2481104500
Auction ends tonite Sunday June 13 at 9:00 PM Pacific standard time
At least enjoy the pictures

http://cgi.ebay.com/ebaymotors/ws/eB...RK%3AMESE%3AIT

 

What are the pros and cons of 2 cams vs. 4?

I am trying to do a bit of research because heads look like the direction I need to go in, but my learning is coming pretty slow.

Less or more wear on springs? Single springs? What about wear on the valves themselves? What are the angles that you had the ports cut and the valves backcut? In your ad, it says that stock cams are included. Does that mean that these don't have more aggressive cams? Which cams come with?

Thanks

 

Hello gqpatmac,

It sounds like you are getting very deep into the subject

I will try to answer those questions I can, and defer others to those with better answers

Spring wear, to my knowledge is a function of spring material (toughness), cam design (lift/rate), and engine operations (speed/cycles), and is independent of head port and valve seat modification work. Also I usually hear concern for cam lobe wear, rather than spring wear, since excessive spring pressures used to combat valve float at higher rpm, can result in shortened cam lobe life. With todays modern materials, unless you are turning unreal RPM's or crazy high lift, I would not expect spring wear to be a factor in valve train component life.

Engine builders might use various combinations of stiffer springs, multiple springs, or even springs with different coil properties in a single spring.

A precision cut valve seat, that more closely matches the valve contour, will preserve valve life. I do not know what angles were used in the valves and ports. Those were details I left in the hands of Dan Paramore, and never thought to ask about. He might have some info for you if you contact him directly

http://www.dprracing.com/ and/or http://www.dprracing2.com/home.htm

Cams included were the factory cams that came in the engine (SOHC 2.5 Phase II), so yes that means these did not have more aggressive cams than stock, however, the stock cams from this Phase II engine are fairly aggressive (see Trey Cobb discussion below)

The heads were reworked for a turbo application so we didn't want too much overlap like in agressive normally aspirated cams or you end up blowing some of the boost right out the exhaust. A little higher lift might not be a bad idea, and a cam mod is part of the overall plan.

Pros and cons of 2 cams vs 4 is a very interesting subject, and I will defer to a discussion offered by Trey Cobb, duplicated in the following post due to it's length

Hope I provided you with some useful info....let me know

see Trey Cobb discussion in next post

 

Here's Trey Cobb discussion on SOHC vs DOHC

It's important to look at the differences in design SUBARU used when making these heads to specifically pin-point out advantages and disadvantages.

In regards to the extra power (torque) the 2.5L SOHC had over the 2.5L DOHC, I believe this is primarily due to the camshaft profile. In terms of stock flow characteristics, the DOHC is equal or marginally (2-3%) better in stock form than the SOHC head. It's not necessarily overlap where it's making the power though, it's ramp rates and lift.

The key differences in camshaft design are a result of DOHC cams using a 'flat tappet' lobe style due to the 'shim over bucket' head design and the SOHC cams using a 'roller rocker' lobe style due to the rocker arms which have a 'roller' which rolls over (follows) the cam lobe.

On the DOHC design (late model EJ25 and non-STi WRX), the way the cam pushes directly against another piece of metal (the shim sitting on top of the bucket), you are limited by the ramp rate (how aggressively you open the valve) and your total lift. This design doesn't allow for the valve to stay at high lifts for very long durations in street use (ie: short durations). Get too aggressive with these, and you'll end up with a fast wearing cam that doesn't last very long (ie: race cam) or at worse, you'll dig the camshaft lobe into the side of the shim and/or bucket. Another concern with DOHC cam design is the problem of lifting the shim off the bucket at higher RPMs. The shims are a solid chunk of metal that sits on top of the bucket and is used to take up the slack between the camshaft's lobe and the bucket. At high RPMs and/or with more aggressive camshaft profiles, you can actually pick this shim up off the bucket. When this happens, the potential is there for the shim to jam up between the camshaft and the cylinder head creating a very nasty mess. So, even though the valvetrain mass is considered "lighter" in some respects to a SOHC, you still have to worry about RPM's for potentially different reasons.

With the SOHC heads, you have a centrally located camshaft with 3 rocker arms per cylinder that have little 'rollers' which roll over the camshaft lobe and push the other side of the arm (think of a sea-saw) against the top of the valve. Since the roller doesn't have as much tendency to dig into the lobe of the camshaft, you can normally push the valves open faster with this design. This improves your cylinder fill because you can open the valve faster to get it lifted enough to take advantage of good flow rates, and keep it open at these higher lifts longer without actually increasing seat-to-seat duration. In practice, this means you could even get better cylinder fill and more power with less duration than a flat-tappet (DOHC) style cam. This is great for street cams. Another feature of the SOHC heads are the rocker arms and their 'ratios'. Unlike the DOHC design, which essentially places the cam on top of the valve for a 1:1 cam lift to valve lift ratio, the rocker arms on the SOHC allow for a higher valve lift compared to cam lift. For example, the SOHC heads used on the 2.5L North American models uses a 1.75:1 intake rocker arm ratio. This means that if our max lift on the camshaft is 0.200", then our valve is actually opening 0.350" (0.200" x 1.75). In a DOHC head, to get a 0.350" lift at the valve, we would need to run a 0.350" lift on our camshaft. These ratios allow us to more easily take advantage of the incredible flow these heads have at higher valve lifts without being limited by a shim diameter or bucket height. And for those wary of extra mass in the cylinder head, it's ideal to keep the weight of your camshafts down and if at all possible, run as much mass close to the centerline of the cam as possible. Running big lobes far off of the centerline will increase rotational weight and make your engine expend more energy turning it. Ultimately, due to this roller rocker arm design, you can run more valve lift with the Subaru SOHC than you can with a Subaru DOHC head, and with a higher ramp rate.

The DOHC heads offer much more precise valve timing and, if you have adjustable cam sprockets, you could fine tune your cam timing on the dyno. This is a particular advantage to all-out race motors.

The SOHC design doesn't offer as much precision with valve timing, and the overlap is fixed into the cam. In this case, it's more critical for the cam designer to get it right.

You have to look at the entire picture. A properly designed cam in a SOHC head could very likely outperform a DOHC head tricked out with adjustable cam sprockets. If the cam designer did his homework on the SOHC head, they should be able to take advantage of it's features and make a cam that performs extremely well. If not, then the adjustability of the DOHC head will ultimately win.


Kind Regards,
Trey
Cobb Tuning www.cobbtuning.com