Regarding the schrick 284/272

[essi1553]

I just learned, that in order to install this cam properly you need to have an adjustable cam gear, unless you like some serious valve tapping. Lucky I spotted this before anything was installed:

Notice open valves in both cylinders 2 and 3:



and cylinder 6 is open much more than i'd like to see, definite valve contact.



After setting the cam gear properly:



That looks much better:




I didn't notice much information in regards to this cam so i'll try and keep updating this post with more things to expect. Wish me luck!

[digger]

how much valve lift did you have at TDC on number 6 @ start of induction cycle with it not adjusted and what did you end up with? did you adv or retard?

[Vin-barrett]

did you adv or retard?

gear picture above is retarded timing. idle will be lumpy

[dwight]

Are you using a msl gasket, how thick is it? Is this a stock m20b25 or stroker concept?
Thanks Dwight

[redlightpete]

[The 284/272 typically needs additional intake valve clearance with stock m20b25 pistons. Mine showed about .02" clearance with a stock motor and a full thickness head.]

Update 5/21/2017 - In case anyone is looking at this in the future. I took this measurement with the stock cam sprocket. When I set the cam to the TDC position recommended by Schrick (as well as I could) it was retarded 4.5 degrees from the stock cam sprocket position. This would provide more intake valve clearance. Sorry if this has lead anyone astray.

[essi1553]

Not sure about valve lift at TDC (thats the first shitty pic of cly 6) but I didn't take measurements, by visual inspection this needed to be done to even consider clearance.

I'm using a stock gasket, as I plan on saving money to get the bottom end redone, and do this job all over again, within 60k miles.

I asked about using a thicker gasket, but the machinist said i'd be fine. In retrospect they kind of sound like famous last words...

I'm hoping for a little lumpy, and no valve tapping, so any advice is welcomed.

[digger]

Nominally with zero valve clearance the valve lift at TDC should be 2.20 mm inlet and 1.70 mm exh with 284/272. Compared to stock which is ~1.1 mm / 1.1 mm.
What this means you lose about 1.1mm clearance at TDC on inlet and slightly more just after due to a faster ramp. In effect you lose about 1.3mm clearance on the inlet at most likely point of impact if installed as per schrick specs.
Retarding the cam 4* will gain you about 0.5mm additional clearance for the inlet as it delays when the intake valve starts to move relative to the crank.
There can be a quite dramatic performance difference with advancing and retarding a cam and a lot depends on the start point that cam is installed on and each engine is different so might require different timing to be optimum. In effect adjusting cam timing to get around clearance issues might conflict with the whole purpose for changing the cam to start with.

rule of thumb an overcammed engine will benefit most if there is some advance and an undercammed engine perhaps the opposite

[SkiFree]

Basic engine rebuilding elephant in the room. Look's like new intake valves? What work was done to them? Any work done on the valve seats?

[Vin-barrett]

In effect adjusting cam timing to get around clearance issues might conflict with the whole purpose for changing the cam to start with.

This is sound advice and from what I see in these pictures is what's happening here.

Additionally, redlightpete above mentioned using that cam required the pistons to be relieved. I expect yours do as well. Or of course aftermarket pistons.

My opinion is the car will idle horribly if it can hold idle at all retarding the timing that far. And if it does run it will pop and spit until smoothing to a high rpm you barely drive at. You really want to be advancing the cam timing

See my post 37 & 38 here. Deepening valve reliefs is not difficult

http://www.r3vlimited.com/board/showthread.php?p=3819417#post3819417

[digger]

This is sound advice and from what I see in these pictures is what's happening here.

Additionally, redlightpete above mentioned using that cam required the pistons to be relieved. I expect yours do as well. Or of course aftermarket pistons.

My opinion is the car will idle horribly if it can hold idle at all retarding the timing that far. And if it does run it will pop and spit until smoothing to a high rpm you barely drive at. You really want to be advancing the cam timing

See my post 37 & 38 here. Deepening valve reliefs is not difficult
http://www.r3vlimited.com/board/show...17#post3819417

i didn't notice anything major with idle quality advanced vs retard etc. i found that tight clearances hurt the idle more so.

when you think about advance and retard it does not alter the overlap, it alters the crank position where overlap occurs. where as tightening the clearance adds effective duration and overlap area goes up accordingly

[Vin-barrett]

not commenting on specific camshaft effects or valve adjustment- idle will suffer due to loss of cylinder filling pressure that results due to retarding the camshaft timing.

[digger]

not commenting on specific camshaft effects or valve adjustment- idle will suffer due to loss of cylinder filling pressure that results due to retarding the camshaft timing.

in my experience it doesn't work that way i ran a cam with inlet lobe retarded 15* from the norm and it idled excellent for the power it made. advancing it 8 degrees made no difference to idle or WOT dyno power however part throttle was crisper and more responsive so i left it at 8* adv. on a different cam i advanced it 4 degrees then 6 degrees and idle was no different but made more power across the board.

in general wider lobe centre gives better idle (assuming duration is unchanged) and wider is in effect retarded. this does lower the cranking compression and bottom end to a point but idle will not be affected in my experience and i would not worry about it at all.

i don't like retarding cam timing because it hurts bottom end torque and midrange quite often but this depends on the cam and engine spec. the thing that will hurt idle is overlap which is late exh close and early inl open. when you adv or retard it changes all events on the m20 simultaneously. if you are not changing overlap then you wont get any meaningful change in idle quality one way or another especially if the engine is properly tuned and functioning to begin with.

[essi1553]

Meh, thanks for the input guys. I picked up another head, with a stock rebuild, threw that on the car. I'm saving this ported head for when I have a bottom end built for it. Trying to figure out how high I want to raise the compression now.

[vertvertvert]

Let's keep this thread alive because I also have a schrick 284/272 cam I'm installing when I get my head back from being rebuilt.
I already have an adjustable cam gear, and upgraded valve springs.

I'm just kinda nervous about properly setting the correct degrees of the cam...
For gaping the eccentrics I just use the standard gaping .010" ...?

[downforce22]

Nominally with zero valve clearance the valve lift at TDC should be 2.20 mm inlet and 1.70 mm exh with 284/272. Compared to stock which is ~1.1 mm / 1.1 mm.
What this means you lose about 1.1mm clearance at TDC on inlet and slightly more just after due to a faster ramp. In effect you lose about 1.3mm clearance on the inlet at most likely point of impact if installed as per schrick specs.
Retarding the cam 4* will gain you about 0.5mm additional clearance for the inlet as it delays when the intake valve starts to move relative to the crank.

Old thread bump. I'm looking at swapping cams. I've got custom pistons and have the specs for compression height and valve reliefs as well as specs on stock m20 pistons. How do I check the longer duration and higher lift won't cause problems with my pistons?
Looking at going from stock 264 m20b25 cam to PPF Z45 turbo cam with 280 degrees, lift = 11 mm. 112 degrees peak angle.