M20 maching specs

[Ether-D]

ok. That was a great explanation. Thanks L.

[aventari]

Retarded cam timing doesn't "lose horsepower" either. It just shifts the powerband up the rev range. Advancing the cam timing just shifts the powerband lower. We're talking advancing or retarding a reasonable amount here of course, not 30 degrees.


Depending on your motor setup, it could benefit from either advanced or retarded timing, that's why adjustable cam gears and dynos exist.

In my opinion the bump in compression from milling the head should fill out the low end and the retarded timing should help the top end.

[Ether-D]

Oh I really like that one^^

[digger]

Cam timing is measured in crank degrees. Since the cam is running half speed compared to the crank, .75 degrees at the cam is 1.5 degrees at the crank.


I'll go double what digger says and say 2 degrees doesn't matter on a street engine.

The more important question is where did the timing start at before it gets retarded ? I have timed enough cams to know they are hardly ever where they are supposed to be exactly.

If you start off 3 degrees retarded, mill the head and lose another couple for 5 degrees retarded then you are giving away horsepower. Every performance engine should have it scam timing checked/ optimized in my opinion.

thinking about this boggles my mind sometimes....lol
on a race engine high performance build 2 degrees on the crank can make a difference but the effort to do this is not warranted on a normal build unless you just are curious and like playing around seeing what things do

[digger]

Retarded cam timing doesn't "lose horsepower" either. It just shifts the powerband up the rev range. Advancing the cam timing just shifts the powerband lower. We're talking advancing or retarding a reasonable amount here of course, not 30 degrees.
Depending on your motor setup, it could benefit from either advanced or retarded timing, that's why adjustable cam gears and dynos exist.
In my opinion the bump in compression from milling the head should fill out the low end and the retarded timing should help the top end.

Years ago i advanced my MM rally cam 4* and 8* degrees and the power curve was identical (on the dyno). Part throttle response was a lot better.

Later on when i did 4* and then 6* on my catcam with same setup i gained power everywhere including the peak but the power fell off the cliff right at the topend a bit sooner.

I did not even do the fuel and spark maps to suit and there were substantial gains, at low rpm low throttle the engine hesitated because it ran so much leaner from a better trapping efficiency (less reversion).

An engine seems to respond to advance at light load and low rpm more than anywhere else. This is logical since the primary reason for advancing providing any benefit would be the earlier inlet valve closing point. This builds higher cylinder pressure due to less mixture reversion. Or in other words is more efficient at trapping the mixture rather than pushing it back into the inlet port/runner. At low rpm and light throttle the air speed is lowest and hence most prone to reversion so this would be where the biggest improvement % would be.

Slightly milling head is barely going to make any gains IMO. If you are in a competitive racing category it would be worth it but its not going to do anything dramatic.

[Ether-D]

I'm in a "get the kids 25 miles to school twice a day" category. But other than that, I like driving and making something work better than it did yesterday.

[Vin-barrett]

I looked and couldnt find solid reliable answer.

How much can i take off an 885 head before,

1) there is valve/piston clearance issues using an oem thickness gasket?
2) i have use an adjustable timing gear?
3) before i have to use a fatter gasket? similiar to 1 but a bit different. Not concerned w interference as much as, what the most you can take odd and still use oem... maybe it is the same question after all.

this is on a 88 supereta. so im not 100% on if the pistons have the same height in relation to the block as a regular "i."

please only reply w first hand experience, not "i heard of a guy who..." If you did it and it worked lemme know.

Thanks!!

and the head is off my car now, so fast replies would be nice.

I have done this.
The clearance between the piston & valve are very close on these engines.
Below is the result of a super-eta bottom end with the 885 decked to the bottom of the dimples. This was 50 thou off the head plus whatever was cut off the head in it's previous life. 1.75 headgasket.
Less than 1mm

[Vin-barrett]

You can easily deepen the piston pockets with a sacrificial head and drill.
This might look scary but works fine. Done this for years with many cars, never a problem.

[Ether-D]

^^Vin-B I love it! What a cool way to make that happen. Machine shop-schmachine shop!

I just clayed the intake clearance on my decked head (past the dimples) & spare block. It was exactly 1/8" or .125" or 3.175mm. So I think I'll be well in the clear as far as piston/valve clearance goes. My mind is eased.

Thanks everyone for the info.

[digger]

great photos...you are best targeting about 1mm but no more 1.25mm on the squish piston to chamber clearnace which looks ok in those photos. then deepen the pockets as needed which is what Vin-barrett did. as opposed to running a thicker gasket to increase piston to valve

[Leo1234]

I was goin to start on a 885 head swap but need more he's adjusting the valve clearences ect.


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[ADEN]

Bringing this thread from the grave lol.
My head was milled beyond the limits now i am hearing a piston to valve at 2k and gets noiser when going higher rpms if i play with my cam timing (a tooth advance) is that going to stop the p/v contact without taking the head off and correct it?

What are the impacts of doing so?

[digger]

what cam, pistons, engine specs?

how do you know that is the noise? if you are hearing the valves impact they may be a whisker bent.

To answer your question if you're going to move the cam timing you will want to get an adjustable cam gear to do a smaller change than a whole tooth. Retarding the cam will improve PtoV on the inlet side but make it worse on exhaust. Normally the inlet is the problem.

[ADEN]

what cam, pistons, engine specs?

how do you know that is the noise? if you are hearing the valves impact they may be a whisker bent.

To answer your question if you're going to move the cam timing you will want to get an adjustable cam gear to do a smaller change than a whole tooth. Retarding the cam will improve PtoV on the inlet side but make it worse on exhaust. Normally the inlet is the problem.

Thanks digger engine is stock but as i said i milled the head too much that now i have a knocking noise when warm and starts at 2k rpm your answer stat that playing with the cam timing is dangerous whether to advance or retard. I am opening my head to check and maybe going for a thicker gasket.

I enjoyed the thread to the point that i tought i can get away with it by playing with cam gear without poping the head lol.