DOHC vs SOHC PnP w/reground cams

[Splinter]

My goal is a very torquey 300hp

The major power mods are 9.5:1 compression, 2.75" turboback, FMIC, and a fast-spooling turbo, coupled with Autronic SM4 tuning and some big injectors and new fuel rails


Since it is going to be put under some pretty tough conditions, the less number of moving parts, the better. Id like to stick with SOHC non-interference if possible

So, with a good PnP, strong valve springs and reground cams, think I can make this goal?

[douglas vincent]

While it may happen, I doubt the compression ration of 9.5-1 is going to let you get anywhere near 300 hp, bhp or whp.

My goal is to get to 200 whp on my NA engine, but I bet money (and I am sure others will too) that I will break it in the process.

I am pushing 9 psi right now, with plenty of fuel and pulled timing, and still getting the occasional knock.

Right now I am at 155 hp and 175 torque at 9 psi. To get to 200 I am going to have to go to at least 12 psi on high compression. To get to 300? Unobtainable in my eyes.

Thats why I have my closedeck block project motor!!!! 23 psi here I come!

[Splinter]

300 BHP

And I do have a closed deck block

Ill prolly be replacing the crank and rods


Along with the entire powertrain

[555BCTurbo]

Do you have an Autronic?

That is my dream standalone...but since it's 2 grand, I will have to wait.

[free5ty1e]

You're going to run a turbocharged engine at 9.5:1 CR? I mean, I did that on my old Saturn, but that's only because that's the stock CR and I hadn't had a chance to change it or turn up the boost yet (4psi) before she was totalled.

I'd lower that CR a bit, 8.5:1 at least, so you can make better use of your turbo's capabilities. Which turbo, by the way?

[Splinter]

I havent really decided yet.

Im going to see how Gary likes his VF24

I want something that comes to life reeeal fast.

And no, I dont have the Autronic yet, I dont have any of this stuff yet, it's all coming this summer.

[BAC5.2]

You could probably make 300bhp on 9.5:1 compression. But it'd turn your SOHC non-interference engine into an interference engine. It's all about what you really want out of the car.

SOHC with cams might be OK, but you would definately be pushing it.

You could get crazy bottom end from a turbo with a crazy small exhaust side (fast spool) and a larger compressor side (Like a VF39). But the problem is that it runs out of steam up top. So you have to trade off. Some lag, for up top power.

You also gotta take into account the fact that with SOHC you won't be able to spin the motor as fast. Since all 4 valves are controlled by one cam spinning at engine RPM, you have that bit of increased rotational mass. With DOHC you can spin it a bit higher, but you also have interference due to valve angles as well as piston to valve contact. You have less rotational inertia, as well as the ability to get whatever cam grind suits your needs.

But with DOHC, standard maintainence is REQUIRED. You need to be sure the timing belt and tensioners and pulleys are all in good condition. You MUST do this regularly, or you'll run into problems.

Fat cams and beefy springs put a hurtin on the valve train, and can wreck up a timing belt.

Food for thought.

[sammydafish]

You're going to run a turbocharged engine at 9.5:1 CR? I mean, I did that on my old Saturn, but that's only because that's the stock CR and I hadn't had a chance to change it or turn up the boost yet (4psi) before she was totalled.

I'd lower that CR a bit, 8.5:1 at least, so you can make better use of your turbo's capabilities. Which turbo, by the way?

no way, high compression turbo motors own on the street. You just need a chamber design that's resistant to detonation. This might mean some custom piston and a reworked chamber. Pollished valves and different plugs pare pretty much a given. With a good intercooler and good engine management this is a formula that can really work well. Lag becomes a non issue and power under the curve is awesome. You might not win a dyno contest, but you'll have WAY more fun driving it.

All that beeing said, I'm not sure how far down that path the EJ22 can go. Like Dough, I'm running the EJ22e on boost and we're both putting out nearly identical horsepower. I'm almost finnished a new turbo swap so we'll see how high I can turn up the boost now and I'll get some tunning done. That limit will be met soon. Even if Dough and I bearly make it to the 200whp range our cars will still most likely be faster than lower compression subarus making 20-30hp more.

[sammydafish]

You also gotta take into account the fact that with SOHC you won't be able to spin the motor as fast. Since all 4 valves are controlled by one cam spinning at engine RPM, you have that bit of increased rotational mass. With DOHC you can spin it a bit higher, but you also have interference due to valve angles as well as piston to valve contact. You have less rotational inertia, as well as the ability to get whatever cam grind suits your needs.

the SOHC is still a 4 stroke, so the cam still spins at half the engine speed, it just has double the lobes on it. The DOHC will still have more mass (4 cams -vs- 2) but the SOHC has rockers which require mildly stiffer springs (they need to return the weight of both the valve and the rocker to the cam lobe) You could spin either setup faster than you could make power if setup properly, though the DOHC setup might be better stock -vs- stock. The big bennifit you get with DOHC is the ability to control the cams individualy. This lets you use cam gears to adjust cam timing which can drasticaly change the characteristics of the engine. Due to the design, you may also be able to run a cam with a much larger amount of lift on the DOHC setup. The other advantage might be a change in valve angle (though I'm not sure if Subaue changed it) which would also change chamber design and may be more efficient or detonation resistant. I'd assume the DOHC head is a more performance oriented and detonation resistant chamber design as Subaru runs it on all thier performance models.

[Matt Monson]

Save your pennies. You can use either one of those heads in STOCK form to hit the power you want. That kind of power is easy and lies in your choice of turbo, not your choice of heads. With respect to the heads, use what you got...

I personally favour the SOHC heads, as do a number of tuners currently playing with them. They can spin to 8000rpms in stock form and the only change you would need to make for high psi boosting is upgraded springs. Porting won't matter. Cams won't matter. Read the sticky at the top about Xephyr's old set up. He used DOHC heads, but it makes no difference. He now uses SOHC heads and is making 480+whp. 300whp with an EJ22T bottom end and either kind of Ej25 heads is EASY!

Like I said, the biggest single thing is turbo choice. A Vf24 won't get you there. There's not a single IHI turbo that will get you there. You will need something like a 20G or one of the Garrett options to hit that. You will also want injectors in excess of 700cc's and good EM, which you have already committed to with the Autronic...

[Splinter]

A bigger turbo might achieve the goals of 300BHP, but only up around 5000-6000 RPM

I want to be making 300BHP by 5000rpm, and I dont care if it drops off after that. The VF39 is looking pretty good for this.


I really want to avoid going DOHC if possible, the car is probably going to be taking quite a beating. Are there any better SOHC heads that will bolt on? Id like to avoid screwing around with plumbing if possible.

[BAC5.2]

Matt - Are you saying that heads and cams won't make any difference? That stock EJ22T heads flow just as well as, say Ver. 7 Spec-C heads?

And are you saying that more lift and longer duration cams don't do anything for power production?

I'm not quite sure I follow you.

[Splinter]

I think he's saying that the stock heads can hit 300BHP with a big enough turbo

Im not sure I even believe that, but regardless, thats not my goal.

[IronMonkeyL255]

I think he was saying you could hit 300bhp with either the SOHC or DOHC RS heads, not the stock heads.

[Matt Monson]

http://forums.nasioc.com/forums/showthread.php?t=686942

9 months ago...

http://www.awdpirates.net/phpBB2/viewtopic.php?t=7796
A different car last month with VF-39. He now has a Momentum turbo on there and while he hasn't posted the dyno plot, he is now up over 300whp, with stock SOHC heads.

And I could go on and on. I know of at least a dozen cars with EJ22T or Ej257 bottom ends with unported, stock cammed EJ25 heads up over 300whp.

I reiterate, read Mark's personal experience in the sticky up top. Yes, porting will get you a higher overall HP number. But as he has found on both SOHC and DOHC heads, when they are ported the bottom end suffers and you lose torque. The power under the curve and manners of the car are far better with the unported, uncammed heads.

NO, I am not saying they are better than Spec C heads. Furthermore, I did misunderstand the opening post and was thinking the decision was between SOHC EJ25 and DOHC EJ25 heads. SOHC Ej22T heads will absolutely need a port and polish to his 300whp. I don't know that I would change the cams much, other than maybe going to the NA cams or a Delta regrind. I know of one or two people who broke the 300whp barrier on a complete Ej22T w/the factory SOHC heads back in the day, but you have to work so hard to do it, why bother. The Ej25 heads are so cheap and plentiful. And if you go with the DOHC ones, all the higher performance fuel injector options are a direct drop in. Since you are going standalone, managing any of that won't be a problem. If you were going for 250whp, I would say you could do it on the cheap with the Ej22T SOHC heads, but 300 is a bit further out there and no worth the effort. That is unless you are like me and ," they said it couldn't be done, so I did it!!!"

[BAC5.2]

We have tons of cars making well over 350whp using stock cams in stock EJ20 heads on EJ257 bottom ends.

I mean TONS of them. The last car we had made 360whp and 380lb-ft at the wheels.

But toss cams into them, and they jump up CRAZY power. A 2.5 with ported heads and 272 duration cams made 455whp at 16psi.

That's a lot of whp at just a little psi. Hard to argue with. With lesser heads, I'd be sure the setup would make less power. Wouldn't you think so?

[Splinter]

So the 2.5L SOHC heads?

What the plumbing like on that swap?

[555BCTurbo]

Matt makes a very valid point...

Todd (-k-) and I both believe that if the SOHC heads don't flow, you make them flow with more forced air, which is basically what Matt has said. The SOHC heads provide the "under curve" torque that the EJ22T is famous for...When I build my 2.35 Stroker, I was actually starting to think about using RS Legacy DOHC heads (as I have an endless source for them), but I think I will use Phase II EJ25 heads instead. I want to put down a reliable ~250whp, which I believe can be done with the stroker and 2.5 SOHC heads, and some engine management of course...

[free5ty1e]

Interesting stuff.

So just by swapping the EJ22t stock heads out for some EJ25 SOHC heads, this goal becomes more reasonably-attainable?

Any EJ25 SOHC heads? Or how would one identify which?

This would, of course, allow the engine to remain non-interference, correct?

Also; the EJ25 SOHC heads would only need stiffer springs for an overall redline of 8000 RPM...? Which could be replaced on the way in...

Are EJ25 heads pretty much bolt-on? Do the exhaust ports change, require different headers...? I've got STi spec C twin-scroll headers here waiting for me to install my VF36 (and to get a notched oil pan and matching pickup)

Let's go into what would be required to raise the CR to say 9.0:1 safely for boost on the EJ22t with the above mentioned setups - I'd like to hear more on this.

[BAC5.2]

9:1 would need custom pistons fo sho.

I think I misunderstood Matt. I think he is saying that the power you want is attainable by running a lot of boost.

[free5ty1e]

I'd figured custom pistons to change the CR any more than a few points... but what shape would be best for the EJ22t running ~20 PSI of boost? Dome? Dish? Flat?

9.0:1? 9.5:1?

No changes would need to happen to the EJ25 SOHC heads, using a stock gasket and stock-shaped headers, still maintaining a non-interference engine status... right?

And -- any year EJ25 SOHC heads?

I'm liking this idea so far.

[Binford]

I'm with you, Free5style. I'm very interested as well.

[free5ty1e]

By the by, I did do some searching; I found lots of conflicting information, such as the EJ22t stock heads would flow better than the EJ25 heads with some work -- and that the EJ25 heads are already better than the EJ22t's heads could ever hope to be worked.

Anyone know what models / years had a SOHC EJ25 to pull some good bolt-on heads from? So when I get out to the great rust-free junkyards out West, I know what to search for...

[0perose]

By the by, I did do some searching; I found lots of conflicting information, such as the EJ22t stock heads would flow better than the EJ25 heads with some work -- and that the EJ25 heads are already better than the EJ22t's heads could ever hope to be worked.

Anyone know what models / years had a SOHC EJ25 to pull some good bolt-on heads from? So when I get out to the great rust-free junkyards out West, I know what to search for...

rusty junkyards are what you want for engine parts.. cars rot away with 100k hooray

[Splinter]

Yea its not like Aluminum rusts...

Im still confused about this whole thing.

So the RS SOHC heads flow really well, but they're a bitch and a half to plumb? Is that where we're at?