Piston to deck height?

[dvallis]

Closing this one out, Ordered custom pistons from Steve at Top End. He really knows pistons. Went with 2618 alloy, 9.5:1 compression ratio so the tuner has some room left for advance, valve pockets with 0.080 clearance on 286/272 cam, 135mm Molnar rods (since we can), gapless top ring, skirt coating, anti-detonation and accumulator grooves.

[ForcedFirebird]

Was that compression number calculated with the 84mm stroke, or 75mm?

Personally, if it were that close, would have just notched the pistons a little deeper. The crowns are very thick.

You can definitely tell those pistons are near zero deck in the pic. The sharp edge where the piston crown meets the side is difficult to measure accurately, but is what I use as a guide when building. On a stock m20, the corner of the piston is proud of the block.

Many of the engines built at work had less than 2mm intake clearance. The intake stroke has the piston following the valve, so retarding the cam a little will increase intake (and decrease exhaust) clearance(s). 4° retarded seems to be a magic number for the Schrick cams (if your block was shaved, the cam would have been naturally retarded almost a degree).

[dvallis]

9.5:1 compression is for an 84mm stroke, 135mm rods, 206.2 deck

I remember Steve saying they'll handle a bigger cam than 286/272 so there;s actually more than 0.080 valve clearance. Not sure how much. We'll clay it in any case.

Don't think I'll need to retard timing since we'll have adequate valve pockets. About that .... All I've read says retard lowers performance. And during dyno tune, we were gradually dialing in more advance to make power on the top end, then backing off a bit. What's your take on that?

[ForcedFirebird]

One of the clients saw 4whp increase with retarding a 288 to -4°.

That's quite a blanket statement to say retarding the cam lowers performance when you have no data to make that judgement. Moving the cam timing moves the power band more than it changes the peak power. Peak power is just an arbitrary number that everyone likes to look at. You know you are doing something when your engine makes the same peak power as your competitor, but you can consistently pull away from them. ;)

[digger]

9.5:1 compression is for an 84mm stroke, 135mm rods, 206.2 deck

I remember Steve saying they'll handle a bigger cam than 286/272 so there;s actually more than 0.080 valve clearance. Not sure how much. We'll clay it in any case.

Don't think I'll need to retard timing since we'll have adequate valve pockets. About that .... All I've read says retard lowers performance. And during dyno tune, we were gradually dialing in more advance to make power on the top end, then backing off a bit. What's your take on that?

When we talk about retarding we are talking about the cam timing on the adjustable cam gear. I've seen two (2) separate dynos with this exact cam a 2.8L and 3.1L and both wanted retarded from the "stock" position. It will depend on where it starts from so it depends. As the dowel pin cant be assumed to be the same unfortunately though ive never had a problem.

If you mean dialling more ignition timing advance on the dyno that's different.

ultimately its simple, you test cam position move it a bit and test again the engine will tell you what it wants. Do this before you spend alot time on the fuel and ignition timing

[ForcedFirebird]

And during dyno tune, we were gradually dialing in more advance to make power on the top end, then backing off a bit. What's your take on that?

Missed this sentence until Digger quoted you. That is EXACTLY how it should go. Reiterating above, the engine will tell you what it wants. The same thing happens when tuning, too. People often ask me questions like "what's the max spark advance for xxx build", but that is a loaded question. Albeit a couple of WHP, but two identical engines might find MBT with different timing tables.

[dvallis]

Ahhhhhhhh. I see. We actually never messed with physical retard last build. Just used a timing light to verify 0 degrees and gave that to the tuner.

Sounds like we should

1 Start with a loose spark advance on the tune
2 Do a run at 0 degrees on the dyno
3 Retard 1 degree with the adjustable cam
4.Another dyno run

Rinse and repeat until we see what the engine likes, THEN start adding advance.

And of course clay the hell out of this during build to establish the no fly retard zone

I like it!

[ForcedFirebird]

Ignition timing is independent of the cam timing. The actual ignition event happens when the ECU grounds the coil, but the rotor has a large area as to keep the cam timing independent of ignition - so, if you adjust the cam gear, the ignition timing remains static (unlike an older points type mechanical event, that would be directly related to cam position and RPM/vacuum).

[digger]

Ahhhhhhhh. I see. We actually never messed with physical retard last build. Just used a timing light to verify 0 degrees and gave that to the tuner.

Sounds like we should

1 Start with a loose spark advance on the tune
2 Do a run at 0 degrees on the dyno
3 Retard 1 degree with the adjustable cam
4.Another dyno run

Rinse and repeat until we see what the engine likes, THEN start adding advance.

And of course clay the hell out of this during build to establish the no fly retard zone

I like it!

run cam straight up and get the tune at WOT fairly close then move the cam like 2 degree advance and check the power and adjust AFR if needed and from that you will see does it want another two degrees advance or does it want to go back the opposite way in which case go back 4 degrees to 2 degrees retarded. probably wont need to adjust ignition timing to run the different cam positions what's best will be what's best. Once you've got cam timing done then you do proper tune.

[dvallis]

Our Nuke adjustable cam gear finally has a purpose :-)

[digger]

i vaguely recall measuring a 284/272 cam and noting that when installed with stock cam that it was already advanced a large amount so retarding 4 degrees is probably still advanced strictly speaking. So when the other guys did their testing and found like 4 or 5 degrees retard as the optimum it may have been still advanced in a absolute sense

[dvallis]

This merits a LOT more investigation on the reassembly. So you're saying the big cam probably has built in advance, even though timing marks will say TDC. What's the best way to measure that?

[digger]

three ways:

• The correct way is to measure the inlet open and close angles at 1mm lift with degree wheel and indicator. The middle between those angles is the inlet centre line which should be 110* ATDC iirc. You can do this with head alone for simplicity with respect to cam gear with a mini degree wheel of sorts or with head on block and full degree wheel on crank;
• Check lift inlet and exhaust at TDC and compare to spec (can be done with head by itself);
• Not bother and just move it until engine makes best power or torque or whatever your preference. You won’t know what the position is you’ll just know relative to originally installed;

[McGyver]

I'm in the process of building a 2.7i m20 and have everything at the machine shop. This is the 81mm crank, 130mm rods, b25 pistons, 885 head.

I got a call from the shop that he's calculating 9.5:1CR without touching the block, which doesn't seem right to me. I've attached a calculation for a stock m20b25 that gets to 8.75:CR, which is close enough to stock 8.8. Changing to my numbers and decking the block, I get 9.46:1CR, which is close enough to the expected 9.5:1CR.

Based on my math, the current setup will have the piston 1.5mm (0.059in) in the hole, and the m20b25 has the piston 0.5mm (0.020in) proud of the hole.
Crank (81/2=40.5mm) + Rod (130mm) + Piston (34.2mm) = 204.7mm, Deck Height = 206.2mm, Deck - Stack = -1.5mm, Piston to Head = 1.5mm + HG Thickness (1.75mm) = 3.25mm, Target Clearance is 1.24mm (0.049in), Amount to deck the block = 3.25mm - 1.24mm = 2.01mm

I know from ForcedFirebird's dyno thread that I should deck the block by about 2.01mm (0.079in) to get 0.049in (1.24mm) of piston to head clearance, so this adds up.

Can one of you give me a sanity check that the math is correct and I'm not going to end up with some crazy 11:1CR engine?

My Engine Specs
	mm	in
Bore	84.46	3.325
Stroke	81.03	3.190
Rod Length	130.00	5.118
HG Bore	84.84	3.340
HG Thickness	1.75	0.069
Assumed Crushed HG Thickness	1.52	0.060
Center of Main to Deck	206.20	8.118
Top Ring Land (Down from deck))	6.35	0.250
Piston Dish (assuming domed) cc	-1.400
Head Volume cc	41.500

	Height Buildup
	m20b25	m20b27	2.7i
crank mm	37.5	40.5	40.5
rod mm	135	130	130
piston mm	34.2	35.7	34.2
total mm	206.7	206.2	204.7
Deck mm	206.2	206.2	206.2
Piston Out mm	0.5	0	-1.5
Head gasket mm	1.75	1.75	1.75
Clearance mm	1.25	1.75	3.25
Clearance in	0.049	0.069	0.128
Target Clearance in	0.049	0.049	0.049
Target Clearance mm	1.24	1.24	1.24
Amount to Deck Block in	0.000	-0.020	-0.079
Amount to Deck Block mm	-0.01	-0.51	-2.01

[digger]

the number seem reasonable.

you dont get 9.5 with the pistons massively down the hole.
:
if you replicate the stock b25 piston to deck with 2.7 then the increase is proportional to stroke increase so 8.8:1 * 81mm /75mm = 9.5:1