High lift cams,but how high?

[jc4orr]

I just picked up an 87 325is as my weekend toy/project. Its completely stock now, and needs a little work, but its all what Im replacing. Im getting the head stage 2 ported and polished, rebuilding it and installing high lift cams. The only problem is I havent decided on how high yet.
Since its going to be my weekend toy and Im doing stage 2 port and polish, I was thinking 288s. I would like to do a stroker kit or install high compression pistons when I get some more money. And I dont want to have to replace the cams again when I do that.

[325ix]

if you are going to do a stroker and cams, you want something in the 11:1 - 12:1 compression ratio with a 288. Check out the stroker faq on e30tech.

[nando]

above 11mm of lift and you have to start being concerned with piston/valve clearance at TDC. basically you'd need to cut the valve pockets a bit deeper to maintain the minimum 1.5mm clearance between the piston and valve.

also above 11mm of lift you have to be very particular with port work - the port flows fine but it gets noisy. Supposedly, using a 30/40/70 3-angle port works best. Don't make the inlets larger..

[Cinnabar325is]

above 11mm of lift and you have to start being concerned with piston/valve clearance at TDC. basically you'd need to cut the valve pockets a bit deeper to maintain the minimum 1.5mm clearance between the piston and valve.

also above 11mm of lift you have to be very particular with port work - the port flows fine but it gets noisy. Supposedly, using a 30/40/70 3-angle port works best. Don't make the inlets larger..

Just curious to hear what you think about this: when I bought my car recently, the motor was internally stock except for a pretty huge Cat Cams 298* cam. Max lift is 11.95mm. There wasn't any interference between the valves/pistons. I don't doubt that above 11mm, interference can become an issue. Why was I not having that issue with such a large cam? Valve pockets and headgasket were OEM.

[Sagaris]

Perhaps the cam timing was just right for it to work, assuming the 11mm lift is truly the limit.

[Cinnabar325is]

The car also has the stock cam gear, so no real adjustability in crank to cam timing.

[nando]

Just because it was internally stock doesnt mean the valve pockets were not clearanced. Either that or the lift at tdc isnt very much and the 298* advertised duration is overstated. did the engine come with any documentation? Was it a regrind?

Even a schrick 284/272 with 11.4mm of lift is dangerous to use without at least measuring intake valve clearance at tdc. The 288 opens even further at tdc, even though the peak lift is identical. It would be extremely foolish to just stick it in the motor without considering the consequences - bent/broken valves, spun bearings, bent or even thrown rods.

[Sagaris]

there would be a little cam timing difference if the head was shaved, but yeah, I am not trying to argue or anything, just throwing out ideas.

[nando]

I want to say the 284/272 lift at tdc is 1.6mm, the 288 is 2.2mm. The 284/272 did not make min clearance on my stock motor - it grenaded james crivellone's motor in like 100 miles. Add another .6mm at tdc, when it didnt clear at 1.6mm, and you can see my concern.

[Cinnabar325is]

Just because it was internally stock doesnt mean the valve pockets were not clearanced. Either that or the lift at tdc isnt very much and the 298* advertised duration is overstated. did the engine come with any documentation? Was it a regrind?

Even a schrick 284/272 with 11.4mm of lift is dangerous to use without at least measuring intake valve clearance at tdc. The 288 opens even further at tdc, even though the peak lift is identical. It would be extremely foolish to just stick it in the motor without considering the consequences - bent/broken valves, spun bearings, bent or even thrown rods.

Cat Cams are good units and definitely not regrinds. I removed the cam from the head to be certain it is what I was told and the part number matches up. You can look at the basic specs here: http://www.catcams.co.uk/acatalog/BMW_M20.html

Lift at TDC is 2.80mm. I don't know how that compares to the Schrick 284* but that would be interesting to find out! (NVM, I see you posted that info).

I can almost guarantee the pockets weren't altered from stock, the bottom end in the car when I bought it was straight out of a junkyard with 180k miles. That bottom end was quickly tossed into the car b/c the last bottom end threw a rod bearing.

[Cinnabar325is]

there would be a little cam timing difference if the head was shaved, but yeah, I am not trying to argue or anything, just throwing out ideas.

Sure, I'm welcoming any ideas. I was told that the head was just lightly resurfaced, so no significant material was removed. And that looks to be accurate judging by the circular impressions in the deck surface of the head.

One other factor: I just built a new bottom end for the car using the M52 crankshaft w/ 130mm eta rods. So I'm gaining 9mm of stroke, split that in half and the pistons would sit 4.5mm higher at TDC than stock. However, the rods are 5mm shorter. So all said and done, the pistons will be sitting 0.5mm lower than before at TDC. My thinking was that if the cam worked before in the stock motor, I should have even a bit more room to breath with this stroker 2.8.

[jc4orr]

Thanks for the help everyone. So from what I gather, it would be a lot easier to go with something a little smaller then 288?

Could I use a thicker head gasket to add some space between the pistons and valves?

[nando]

You would probably lose more power from the thicker gasket than youd gain from the slight change in lift. Either cut the pockets deeper or dont do it.

[matt0300]

Get the new 366 cam