Valve-to-Piston Clearance, Compression and HP

#1

Elsewhere there is a 17 page discussion on Scott Gress’ engine rebuild. Lost in the pages is the topic of cutting this and that from the cylinder head and block to reuse the parts. The underlying goal, it seems, is three part:
Stay within the rules
Raise the compression ratio by making the clearance between block and head to be a minimum
Build an engine that lasts

Anyone have data to show how all of this mumbo jumbo interworks?
Right now I’ve got a fresh rebuild that is down on power. It is on the large side of clearance. How much? Don’t know, I was lazy with the rebuild.It has a .020 deck plate as well as an oversize headgasket. I was too lazy to “clay” the engine. I’ll not be too lazy to redo the work right. How much power? Don’t know, haven’t been to the dyno yet.

Where is Ray Korman when you need him?
Any thoughts from the gallery?

Regards, Robert Patton

#2

The dyno sheet I posted before was an optimized motor…165hp. There is no piston valve clearance issue since the head is not below the service limit…that is why there is a service limit.

And to address another issue raised on the l o n g e r thread, if the head is milled to the service limit, timing is retarded 2 degrees, and that does make a difference. CB (not Ray Korman)

#3

Chuck or Brendan are probably going to be the ones to speak to this with authority. At the risk of just confusing everything, my theory is that the slight change in compression ratio is insignificant as compared to other things. On a rebuild attention to piston clearance, ring gap, bearing clearance, valve spring uniformity, cam profile, valves & seats, deck heights, and balance are where you gain or loose power.

#4

cwbaader wrote:

So then using a head that hasn’t been milled near the service limit and a thick head gasket would advance the timing? Possibilities of finding a sweet spot of lower compression but making up that loss with advanced timing?

#5

What does it mean to “clay” the engine?

#6

That is the use of modeling clay to determine the learance between the valves and the tops of the pistons. Each piston gets a coating clay, the head and time belt/chain is assembled, and the engine rotated. When the head is removed the valves will have made imprints in the clay.

#7

cwbaader wrote:

Speaking of a timing retard, here I am again. :laugh: I am fascinated by internal combustion engine (and lots of other cool parts on our E30s). I have learned a ton from the experts on this site.

But I am stumped on the timing change by decking the head. As I mentioned in the other thread, here are the numbers I came up with based on a wild-a$$ guess about the size of the cam pulley:
3" dia. cam pulley
2pir = 9.4" / 360* = 0.026" per degree

All the slack created by decking the head goes to the tensioner side of the pulley, right? If you deck the head by 0.012", that is roughly 1/2 degree. I am struggling to understand the 2 degree timing change. Can anyone shed light on this for me?

#8

Steve, glad you’re here.
What does Tom the-OPM-guy say about compression and valve timing.
He has rebuilt a few engines,no?

Thanks in advance, RP

#9

Robert - I haven’t asked Tom. He has rebuilt a bunch of Mazda and Honda engines, but everything we’ve had done on the BMW was done by Bob Thornton/Race Engineering in North Carolina.

I thought it would be quicker to get an M20 answer here, but if nobody weighs in I’ll have to call Bob and spread my ignorance around a bit. I should have plenty to go around. :laugh:

#10

The 2 degrees was a SWAG with a plastic protractor. However, when I machined my pulley, I rotated it either 178 or 182 degrees in my rotary table (can’t remember which) and the timing marks lined up perfectly after rotating the crank an additional 180 degrees.

Root circle of the cam pulley is approx. 4.65". Re figure, please.

There is no low compression sweet spot…compression makes torque, and torque makes horsepower. The lower the compression, the milder the motor. CB

#11

cwbaader wrote:

2.325" x pi x 2 = 14.6" circumference, or 0.041" per degree

0.012" decked / 0.041" per degree = 0.3 degree timing change.

I think this shows that using a thicker than spec’d head gasket will cause the engine to drop power through lower compression quicker than gaining power through advanced timing, right?

#12

I don’t know. That’s why I’m asking you’se guys.

Ask Tom.

In the meantime you’re welcome to watch the adventures of clay-man as I try to get the new engine in the #8 car to make some power.

RP

#13

OK - I’ll jump into Professor Steve’s math at the risk of making a Big Fool of myself.

First: When you deck the head, BOTH sides of the timing belt are going to see a 0.012 reduction in distance from the crank pulley. So the total reduction in timing belt length should be 0.012 x 2 = 0.024

0.024/.041 per degree = 0.585 degree

But that’s cam degrees, right? And most people talk CRANK degrees (which are cam degrees x 2) - so 0.585 x 2 = 1.171 crank degrees.

So decking the head to the max buys you about 1 degree of crank timing change. Now I need a smart person to tell if that change is advancing or retarding the cam timing. I believe you want to retard the cam timing as much as possible for high RPM power (move the torque peak higher in RPM - and HP = RPM x Torque) Correct?

Also need a smart person to tell me the impact of this change on ignition timing - since the M20 distributor is driven off the crank nose - I assume there’s no way to decouple retarding the cam timing (good) from retarding the ignition timing (bad - assuming you don’t need to retard because of knock). I assume these engines aren’t knock limited on 93 octane. Can you play games with the crank sensor to change ignition timing independent of cam timing?

I mostly think I’m retarded… Who started this dang thread anyway?

#14

At the risk of getting into Ranger territory… :wink:

Hypothesis: Stacking the head by using a thicker head gasket advances timing, but causes a net loss in power due to lower compression ratio.

Earlier, I “showed” that 0.012" change in head decking causes a 0.3 degree timing change, which I believe would result in very little performance effect.

Based on Sean’s question about compression change by stacking the head, here’s what happens if you DON’T deck the head, but use a thicker head gasket as allowed by the rules.

Double check my math, of course. But it looks like you lose over 1 point in compression ratio to gain 1 degree in timing advance. Big net loss.

#15

L8Apex wrote:

[quote]OK - I’ll jump into Professor Steve’s math at the risk of making a Big Fool of myself.
[/quote]
Following me on or off track is foolish.

[quote]First: When you deck the head, BOTH sides of the timing belt are going to see a 0.012 reduction in distance from the crank pulley. So the total reduction in timing belt length should be 0.012 x 2 = 0.024
[/quote]
Good catch. I believe you’re right. Unfortunately, I posted my futher compounding of the problem simultaneously with your post. I’ll have to re-cipher…

#16

Steve, please do not underestimate my ability to screw up on or off the track. Let’s just put it this way… when you have a section in the SCCA or NASA GCR written just to cover just one of your crashes, we can talk…

Please note 9.1.1.10 below - and oh yes, I have the x-rays to prove it… Think T-bone crash - and the other car has a battering ram and his battery mounted in the nose of his Formula Vee… Ouch.

Jim

EDITING THIS SINCE I DIDN’T MEAN TO KILL THIS THREAD…

#17

Getting back on track to Robert’s question, if I had a rebuild that was down on power, the first thing I would do is make sure the bottom end is healthy (good leak down numbers).

Then I would go for max compression - without screwing up my valve or ignition timing.

Then I would spend all my money on the head (it’s all about airflow and area under the valve lift curve)

Max cam lift/duration profile
correct & equal valve spring loads
clean up the ports and valve seats as much as allowed - 3 angle grind job on the seats if allowed.
max ratio on the rockers
port match the intake and exhaust manifolds as allowed

Jim