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Old 12-20-2013, 03:28 PM   #1
WcW
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Default Hot Camming- Is this an option?

I know that inter-generational cam swaps such as swapping back to a pre-AVCS cam have been done. I'm curious about putting exhaust cams onto the intake side on DOHC heads from the ej25. Is there a fundamental incompatibility? I know that the sprockets are an issue. Is the difference in lift and increased duration not worth the work? I would walk down to the shop and look but it's 11 miles away. If my memory serves the two cams have the same journals and the only differences are in the gear end and lobes but I might be confusing the L-R intake cams. I did try to search before posting.
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Old 12-20-2013, 04:49 PM   #2
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Apart from your intake valves opening at completely the wrong time, they would open concurrently to the exhuasts and smash the whole lot to pieces.
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Old 12-20-2013, 05:19 PM   #3
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Clearly the gears would have to be modified such that that this didn't happen. You are aware that people do this in other engines? The camshafts are, necessarily, identical in respect to rotation and timing between cylinders. It is possible to reverse their function (make the engine pump air backwards) by simply shifting their relationship to the timing belt. The extra lift and duration of a "hot swap" might cause contact, but in this case it does not exceed that of custom ground intake cams, so I do indeed believe that it will not do so.
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Old 12-20-2013, 05:46 PM   #4
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Wouldn't it be easier to have the locating dowell on the camshaft repositioned rather than fiddling with the sprockets?
Why do you want to do this anyway? What happened to your intake cams?
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Old 12-20-2013, 06:34 PM   #5
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The exhaust cams have more lift and duration. Essentially you end up running four exhaust cams. There must be some problem with it because I know a lot of people with larger piles of parts than I have.
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Old 12-20-2013, 09:51 PM   #6
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if you're going through the trouble of playing with cams, why would you want to reuse stock cams (and remachine and rework them at that) anyway? It would seem a lot more cost effective to just get new/other cams to put in than trying to swap the exhaust cams as intake cams.
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Old 12-20-2013, 11:58 PM   #7
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I remember doing this with VW 16vs back in the day. The exhaust cam mod had more duration and greater lift than the intake, even moreso than the vaunted Euro intake cam but required some fiddling to get it to work.

When oriented as an intake with no modifications, it was about 3.5* retarded in relation to a regular intake cam. To bring the centers back to a "normal" state, you had to run an adjustable sprocket on it to dial it back. The problem was, the exhaust cam was driven by a belt and at the opposite end, the intake was chain driven by the exhaust inside of the valve cover.

The other caveat is that you had to machine the cam gear nub off of the cam to be placed on the intake side as well as round down the distributor drive. The cam physically wouldn't fit otherwise. I think this is still in the 16v forum of the Vortex somewhere.

What I've found in my limited research with Subarus is the opposite: the intake cam is more aggressive than the exhaust (WRX) with the exception of the STi set which is bigger all around, especially in the exhaust.

Obviously you'll have to fiddle with gear orientation and use a set of adjustable sprockets to get it within spec. The other problem lies in the fact that there's four cams (4x the cost) to fiddle with and they're not all that easy to get to. That and setting lash with buckets, VW's have hydraulic lifters...

I would say the more cost effective option is to go with a cam for your setup that already exists unless you have unlimited time and funds to play around with a maybe.
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Old 12-21-2013, 01:12 AM   #8
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The specs for the N/A cams in an ej25 from 1996 show that the exhaust cam is substantially beefier that the intake, with about .75 mm more lift at the lobe (2.5mm through the rocker?) and 8 degrees more duration. I wouldn't be at all surprised if the situation was reversed on turbo cams as they have different breathing needs and overlap can be counter productive. (then there is the whole pluse/flow turbine thing)

It looks to me like the intake cam has a nose that rides out into the back of the engine for no good purpose and the exhaust does not. Ugh, now I just have to go try it. I do agree that the sprockets will need new holes drilled in them, but otherwise the should be workable, or some combination of offset/swapping therein.

I know that this seems like a trivial matter, but a lot of us have piles and piles of head parts lying around and if there is a way to mix and match (and even drill a little) to make a few more HP then we should be all over it. In the mazda BP engines, hot siding usually garnered between 7-11 hp with no other mods.
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Old 12-21-2013, 04:38 AM   #9
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Interesting idea. Remember that there is no rocker on the DOHC heads to multiply lift. The cam lobes operate the valves directly through the lash adjusting buckets.

I dunno why the intake cams have those nubs on the rear end. Maybe some sort of reluctor idea that never materialized.

Jay
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Old 12-21-2013, 06:54 AM   #10
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subscribed because i find this idea interesting.
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Old 12-21-2013, 07:29 AM   #11
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Like I said, my knowledge is limited in Subaru and I've forgotten more than that in VW! I figured the exhaust cam would be less aggressive in FI applications versus NA.

If you have piles of spare cams, a head and some cam gears, by all means experiment away! Its what I did when I delved into the dual exhaust cam mod for VWs. Before all of this though, make sure the bearing locations etc are the same and that the cam will fit in it repurposed location.

When I did the mod, the cams were solid cast steel so I was able to cut and score them with a cutting disc on an angle grinder, smack the scored end off with a hammer and then finish off the end on a grinder. Like I said, I don't know how Subarus are but I'd imagine the construction is similar.

I would recommend at least eyeballing the cams first with the correct gear for the side (intake cam gear on the exhaust) just to see if you can get the center lines and lobes close or even if the lobes match. You can probably tell by just looking at both from the cam gear end and comparing the location of the cam gear pin to the closest lobe.

Once you've done that, you can determine if you can dial out any advanced or retard with a cam sprocket, which you'll want anyway for fine tuning.

Happy experimenting!
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Old 12-21-2013, 07:47 AM   #12
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Just to add: what a terribly pornographic thread title!
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Old 12-21-2013, 08:07 AM   #13
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keep in mind, the cam sensor takes its info off of the intake cam gear, so you can't just move the gear's location relative to the lobes willy nilly

other than that, sounds like a neat trick
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Old 12-21-2013, 12:48 PM   #14
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Personally, I would relocate the indexing pin in the camshaft to preserve reluctor location and belt timing mark orientation. Determining the correct position is something that would take some research.

Jay
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Originally Posted by rocketperson7 View Post
keep in mind, the cam sensor takes its info off of the intake cam gear, so you can't just move the gear's location relative to the lobes willy nilly

other than that, sounds like a neat trick
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Old 12-22-2013, 11:26 PM   #15
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Going to get out into the shop tomorrow morning and see what I can see.
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Old 12-24-2013, 05:23 PM   #16
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Default Observation Report

Ok, spent some time on this today and the results are pretty interesting. For starter, the cams could be swapped with only minimal work. On the right side there is a nice match up between the +165* mark, where the exhaust cam needs to be shifted to, and the spacing of the cogs on the sprocket. In that case all you need to do is move the timing marks, and voila, you have just installed an exhaust camshaft on the intake side. On the left side, you need to drift out the pin and drill a second hole or put a second slot in the drive end of the camshaft. I did not actually attempt this but It looked like putting a new slot in the cam would be the easier DIY approach for those with a vise and a large file. BUT WAIT. it gets interesting. I feel like an idiot for not knowing this:

!!The cam lobes on the exhaust camshaft are not the same!!

The valves are the same, the ports give no clue, but one of the valves on the exhaust side has a much hotter lobe than the other. One has about a .4mm more lift, and a lot more area under the curve and looks pretty hot. and the other has about .5mm less lift and looks like an economy cam. The first lobe has the overlap, the second lobe has almost none. They share identical closing profiles.

This explains why I though though that the exhaust cam had a hot profile. the lobes that I was measuring do! But when you put them side by side I'm not sure if it makes sense to do the swap, especially since you will be putting the two high lifting, high overlap, valves right next to each other. I don't think that they would hit each other, it just looks like you would start dumping mixture into the exhaust for no good purpose.

So, from here: Is this eccentric exhaust camshaft profile universal to the DOHC? Did they use other profiles in the DOHC heads that we might still do this swap with? Is the hot swap here actually to run two intake camshafts?

Time to do a little more research.
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Old 12-24-2013, 11:56 PM   #17
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that's very interesting!
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Old 12-25-2013, 09:01 AM   #18
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Once again, I had experienced something similar on VWs.

The cam lobes do not match exactly, perfectly. Some are more advanced in relation to the normal cam, some are retarded. This is where it gets tricky and you have to split hairs as far as timing the cam. Only experimenting will be your guide here.

On an aside: I had tried this on my motorcycle as this is a common mod on the SV650. The same issues crop up but are a little easier to sort out by mixing and matching drive gears. We had tried this on a Bandit 400 but there were large, very detrimental differences:

-Cam profile was asymmetric: there was more profile ground into the lobes that were specific to the application, not just a normal lobe.

-Greater lift, shorter valve: even if you were able to time them and get them running, the higher lift in the intake position was enough to cause the retainer to contact the valve stem seal. This was exacerbated by high revs: smoke show on decel and cramped quarters to replace them.

-Valve clearances: thankfully the Bandit 400 uses adjustable rockers. We set lash to the exhaust cam spec on the intake side.

The reason these mods even happened: these are a cheat in any production class vehicle. Since the vehicle came with this cam standard, it is generally "allowed" by the rules of whatever organization you are with. If you get broken down in tech, the ex cam has all the factory stampings and obviously no sign of regrind/reweld.

Now that you know the certain caveats, now you know why people just go with a tried and true camshaft selection. I wasted a lot of time on the VW experiment (which was successful) and on the Bandit experiment (which was terribly unsuccessful, costing me a motor at one point).
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