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Phil, you say:
"Shaving the head raises CR, but does nothing to change the clearance. We do that last after committing to a known deck height and gasket thickness."
Which is what I hear other people saying too when discussing squish/quench. If you shave the head, does that not bring the top of the head down closer to the top of the piston? How is that different from shaving the deck which does the same thing? In my mind anyway. I see it as shaving the deck makes the pistons stick up more towards the head compared to shaving the head which brings the head down to the pistons. Which is also what is supposedly good... But all in all, the head and pistons will be in the same place considering they were both shaved the same amount... Care to enlighten me? =D I wanna know!
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Kyle - '68 Volvo 142 w/71 b20b and m40...
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Kyle,
Picture that the piston is round, but the chamber is oval (roughly). Therefore there's a part of the piston that faces the flat bottom of the head, and if the piston came up too high there'd be contact (a Bad Thing).
So, say the head gasket crushes down to .050" and the pistons come up even with the top of the block (numbers for illustration purposes only). The piston-to-head clearance is .050". Shave the head as much as you like and it's still .050" and the squish/quench is still the same, although you've raised CR because the volume of the oval cutouts is smaller.
In this case, if we wanted the clearance to be .030", there's two ways to do it: 1) use a .030" gasket, or 2) deck the block so the pistons stick up .020" out the top. Now we've got the tight squish/quench, AND we've raised the CR a bit, but not a whole lot. Only now can we calculate how much more volume to take out of the oval cutouts by shaving down the head.
See? Ideally, we want zero clearance without contact at maximum revs and maximum temperature, but the pistons and rods "stretch" with heat and revs, so we need to allow for that and build in some clearance measured with the motor cold and at rest.
Two real-life examples... my MPPE motor has a 93mm bore (2175cc), so that means using the Elring big-bore gasket that only comes in .050" nominal thickness, and crushes to .047". I wanted .032" clearance, so the block is decked so the pistons stick out .015".
Now I'm putting together a 2036cc motor for a guy (only .040" over), so I can use a .030" gasket that crushes to .028". For this one we want .030" clearance (it won't rev quite as high so a bit tighter is safe), so we're cutting down the block so the pistons end up .002" below the deck.
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but still, compared to stock, shaving the head will improve squish correct? But you run into the limits of how thick the head gasket is and thus need to deck the block to make up for it... Makes sense now that I think a little more about it =D. And actually, i was just reading, and still am, about someone on TB building up a b21ft for higher compression and good squish and crap. So my understanding of it's growing. I was also thinking about my 74 engine, and it's future buildup... And got to thinking something along the lines of the engine you're describing, the 2036. I'm not sure if I should worry about going .040 over rather than .030 because what if i need to rebuild the engine again and i'd need to oversize it that much more because over a .040 boring costs more and is rarer, unless I go with b21 pistons, which is actually what I've always wanted to do, but might not end up doing. So yeah, i guess if the engine's done well, i shouldn't worry about going .040 over rather than .030.
That clearance of .030 sounds pretty darn low... How low exactly are you talking about for his rpm range? I don't really know how much stretch really comes into play, but yeah, that number's a bit lower than what i've seen elsewhere, and your .032 sounds lowish almost too, but i guess it's alright. How high have you spun your MPPE up to? What rpm did you figure was high enough? I want something that'll run up to 7k, use HIF6 carbs to their max, and be good. As in, possibly something hotter than V-performance's regular street performance kit, but still with SUs. And of course a properly modified head. Along with M rods and blah blah. Possibly a custom ground cam as well... I was actually just looking at KGTrimming's catologs and saw that they're street performance head comes with 37mm exhaust valves where as JP uses 36mm, and stock is 35... But this is going to be a big topic and one i shouldn't really get myself involved in because this won't be happening for two years at the least...
=D I love this stuff! If only school wasn't a factor, i'd be doing it this coming summer. And probably drive up to Canada to do it all with Rhys or something. Oh well.
EDIT: Anywho, also, what camshafts are you using on your MPPE and what are you planning to use on the 2036? Are your rockers modified at all?
and if you were wondering what i was/am reading:
Turbobricks
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Kyle - '68 Volvo 142 w/71 b20b and m40...
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"but still, compared to stock, shaving the head will improve squish correct?"
Nono, I'd suggest you read Phils post again. Shaving does not affect quench at all, only compression.
cheers, ben
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P131, '65, B20B+M47. P131, '69, B20E+AW71L+LSD. (www.tinustechniek.tk)
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Actually, since the head chamber has a slanted wall instead of a vertical one, the squish area is VERY SLIGHTLY increased when the head is shaved. Of course if you modify the combustion chamber to yield additional volume you will likely decrease that area even farther than it was stock!
If you happen to have dished pistons, of course, all this is blown into a cocked hat as the effective squish goes to zero in a hurry (unless they are custom pistons with squish areas incorporated...). Only use flattops myself but I thought it bore mentioning.
The only time you have to worry about piston contact vis-a-vis the bearing clearances, is on the exhaust stroke as the compression stroke is pushing down so hard on the piston crown (an "air-spring"). This is also present on the exhaust stroke but is lessened. I expect at lower RPM with a wild cam (lots of overlap) you may get a little "bounce" out of the piston. The contact is most likely when the heat of the engine is greatest (aluminum expands rather a lot) or if you are near engine failure anyway from lack of lube or poor cooling (lots of heat and suddenly expanded clearances).
Mike!
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Mike!
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Actually, since the head chamber has a slanted wall instead of a vertical one, the squish area is VERY SLIGHTLY increased when the head is shaved. Of course if you modify the combustion chamber to yield additional volume you will likely decrease that area even farther than it was stock!
Yes, and there's also a balance to be struck between what flows best and what combusts best. It's not always beneficial to have as much squish area as possible, but it's always a plus to reduce the clearance to the minimum.
If you happen to have dished pistons, of course, all this is blown into a cocked hat as the effective squish goes to zero in a hurry (unless they are custom pistons with squish areas incorporated...). Only use flattops myself but I thought it bore mentioning.
Flat pistons are the way to go, IMHO.
The only time you have to worry about piston contact vis-a-vis the bearing clearances, is on the exhaust stroke as the compression stroke is pushing down so hard on the piston crown (an "air-spring"). This is also present on the exhaust stroke but is lessened. I expect at lower RPM with a wild cam (lots of overlap) you may get a little "bounce" out of the piston. The contact is most likely when the heat of the engine is greatest (aluminum expands rather a lot) or if you are near engine failure anyway from lack of lube or poor cooling (lots of heat and suddenly expanded clearances).
The biggest factor seems to be thermal expansion of the pistons themselves, and secondarily the rods (which don't actually stretch mechanically -- that's a misnomer). Bearing clearances are only a few thou and I don't think that enlarges in any way. I suspect crank flex is more of a factor... dunno, really. Whatever the causes are, we know experimentally how tight we can and cannot go.
I emphasise again: You CANNOT run such tight clearances as part of an average rebuild -- we put a lot of work into the rods and extreme accuracy throughout the motor to get away with this stuff.
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Amen to that.
I did a quickie thermo-mechanical analysis on a theroetical engine with 3" bore, 5.5" stroke, Skirt clearance of 0.005", Main bearing clearance of 0.0015" and wrist pin clearance of 0.0005".
I assumed that the DeltaT of the engine was about 300 degree F and the piston body remained "cool" and the rod correspondingly "hot." Steel rods.
I came up with a contact distance (absolute minimum clearance) of about 0.0210" When you toss in wear limits and a FOS I get closer to 0.032" (and that's if I am very careful about wear!).
I hear the Honda-boys go 0.030" routinely, USA V-8's are 0.060" factory and 0.040" "hot-rod".
Mike!
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Mike!
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The 'squish' Phil is talking about is not the air being squished up into the domed portion of the cylinder head by the piston, that's just regular compression. Instead, it involves the air being 'pinched' between the top of the piston and the flat shelves on the sides of the combustion chamber (somewhat on the spark plug side, mostly on the opposite side). These shelves are part of the shaved surface of the cylinder head, so shaving the head doesn't affect their position relative to the block (and thus the clearance to the piston when bolted on).
The way Phil is building his motor, the piston comes *very* close to making contact at higher rpms. So when running, the air is just compressed normally as the piston moves upwards, but right at TDC the pistons comes very close to touching, and the air trapped between the shelves and the piston shoots out sideways and would tend to create a swirl in the center domed portion of the chamber. The swirl helps mix the fuel better and also promotes a quicker combustion. The less pinch or squish between the piston and the cylinder head shelves, the less swirl.
Shaving the head just brings the top of the domes combustion chamber closer and reduces its volume, and would have no affect on squish. Decking the block, however, makes the pistons poke up farther and thus closer to the shelves.
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I'm JohnMc, and I approved this message.
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"These shelves are part of the shaved surface of the cylinder head, so shaving the head doesn't affect their position relative to the block (and thus the clearance to the piston when bolted on)."
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Kyle - '68 Volvo 142 w/71 b20b and m40...
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http://www.vclassics.com/mppe1.html
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Yeah, i've seen that before... I don't know if all of it is still accurate, but i shoulda checked nonetheless. Good deal.
So my motor will be less extreme than that... I guess the equivelant(sp?) to kgtrimmning's KG17 cam would be nice for my needs.
"KG17 drar bra från låga varv passar utmärkt till veterancupen med exempelvis dubbla SU-förgasare. Går givetvis ännu bättre med dubbla 45:or. Lämplig topp steg 2. 11.1 107° 285°"
Most of the stuff would probably be from John Parker, unless somewhere else seems cheaper.(kgtrimmning, unitek, or OJ Rallye, etc.)
Ooh the fun it will be...
And Phil, "Shootout at Portland International Raceway, anyone?", we're still waiting for that to happen ;) along with some dyno results. =D
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Kyle - '68 Volvo 142 w/71 b20b and m40...
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It's somewhat outdated. The MPPE now has 48DCO Webers instead of 45DCOE, and I'm using different air horns. There have been a lot of detail changes that don't affect the listed specs.
It'll spend some time on a dyno within the next three weeks -- trying to schedule that now.
I now live 200 miles from PIR instead of 6 miles... that makes those track days a bit harder to pull off.
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It'll spend some time on a dyno within the next three weeks -- trying to schedule that now.
I think a lot of people (including myself) will be looking forward to the results of that!
cheers, ben
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P131, '65, B20B+M47. P131, '69, B20E+AW71L+LSD. (www.tinustechniek.tk)
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