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Old 05-17-2018, 10:14 AM   #1
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M42 Cam Adjustment for Turbo

I am working on a turbo install on my m42 after gathering some parts and getting an upgraded an rebuilt T25 GT25R turbo and an old school mosselman bottom mount turbo manifold. I have successfully built a plug and play megasquirt unit to run the engine and am working on installation and soon tuning. I will be able to adjust spark, timing and fuel for any camshaft adjustment I make.

Garrett T25 turbo (turbo has just been rebuild with heavy duty internals, 360 thrust bearing and step gap piston ring) Upgraded to GTX style billet compressor wheel, flows enough for over 300HP. Compressor side .48 (but the wheel has been upgraded and there is no way to calculate the exact A/R now) turbine .49. Very responsive turbo for this application, should spool almost instantly. This turbo can be upgraded even further, if you decide that you need more at a later time.Exhaust housing and bearing housing have been fully ceramic coated, the compressor housing has been powder coated "chrome".

It is essentially a Garrett GTX2554 turbo at this point. It was upgraded with the billet GTX2554R 1.658" (42.12mm) inducer and 2.1410" (54.39mm) exducer. Originally it was a much smaller 38mm inducer, that is what Mosselman had spec'd out back then. The bearings are upgraded to 360 and it has a stepped gap piston ring on the exhaust side, again all of the best parts that you can get for this turbo. The bearing housing, turbine housing, adapter and manifold was all ceramic coated and the compressor housing was powder coated chrome.
Long story short I am looking to optimize the camshaft timing for the turbo installation.

Why: I am worried the turbo will be too responsive and will choke on the top end. The m42 is a good engine but falls off after 5000 rpm. My goal is to shift the powerband higher in the rev range and utilize the quick spooling turbo to pick up the slack on the low end. All this should do is shift the powerband up or down, not change top end power.

The 6 degrees you get of adjustment on the m42 is pretty useful. Use is to change how you build boost. Advance the exhaust cam a little to give the engine a tad more bottom and build boost a little faster. Adjusting the intake cam is going to give you a tad better up top. With this you are just shifting the power curve ever so slightly. Change is more noticeable on a F/I engine.
After doing some research it looks like the enemy to the turbo cam is overlap and the exact timing of the cam. I learned about this subject at this link. With the help of this link I was able to find the maximum speed of the piston is at 75* ATDC which is when you want the intake valve open the most. If I retard the intake cam a few degrees (counter clockwise) it will open the intake valve later and increase overlap. If I advance the intake cam (clockwise) it will reduce overlap, reduce top end but increase midrange performance. Here is a quote from a bimmerforums thread.

I spoke to Rennie at Redline BMW about this. He's really knowlegable on the M42/M44. He used to race a 318ti sponsored by Grassroots Motorsports magazine. In the race car he said they picked up 12 whp by advancing the intake cam and retarding the ust cam. He didn't recall how much adjustment was made, but it sounded moderate. This adjustment increases overlap, which, in general, can be good for naturally aspirated cars. If I had an N/A M44 I would I would start with +2 intake and -2 exhaust and work from there. For cars with forced induction increasing overlap can cause a decrease in power. I would love to hear some results from someone with a DASC trying different cam timing settings. In a previous car of mine (not a BMW) I picked up over 15 hp in the midrange of my power band doing the same thing. The car felt slower after the adjustment because the power band be came more gradual and less peaky, but I actually picked up .4 seconds in the quarter mile.
So far I have tried advancing the inlet, retarding the inlet, advancing the exhaust and retarding the exhaust. The best gains have been from advancing the inlet about 6/7 degs. It gave good gains from 2000 - 4000, (up to 6lbs.ft and 3bhp) This is great for the road but not very helpful for a race car. I have all the dyno charts for this but they have my details on them, so until I edit them I won't put them up as I don't want displayed yet. Another word of warning...be very carefully when retarding the exhaust cam, as I think I had some valve / piston contact with a modest decrease of exhaust valve timing (-4/5 degs). Also, a 2 deg shift in either exhaust or inlet timing can cost you 10bhp, so make sure all your work is dyno tested as these engines are sensitive to pulse tuning when adjusting cam timing.
For those who are interested in cam timing... (So far I have tried advancing the inlet, retarding the inlet, advancing the exhaust and retarding the exhaust. The best gains have been from advancing the inlet about 6/7 degs. It gave good gains from 2000 - 4000, (up to 6lbs.ft and 3bhp), those are my findings based solely on adjusting the cam timing not a rwhp figure based on an engine thats had several modifications including cam timing.
In short, I am going to have to compromise. If I advance the cam, then it shifts my power lower. If I retard the cam, it increases overlap which causes the compressed air to blow out the exhaust. What would be the best bet for timing? Or do I work backwards, find the duration of the stock cam and try to figure the intake cam location that places the maximum lift at 75* and adjust the exhaust cam to match that one?
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Old 05-17-2018, 08:22 PM   #2
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Overlap in a turbo engine is not the devil like its made out to be in old turbo books. Way too much overlap can be a negative but if you retard the inlet slowly you should be able to fond where the power does not increase up top any more and the midrange is suffering.

With log manifolds and turbines that cause high backpressure I would start by retarding the exhaust cam and then adjusting the inlet.

Some info:

M42 on VEMS
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