Anytime I had the opportunity to remove a head on a "Nail" that had the proper quench/squish area it had a PERFECT as can be for the design burn pattern along the top of the pistons going to the side. This is with NO LESS than .025" & NO MORE than .050". Again, just my observations. On a decent running stock-er type "Nail" with a greater quench/squish area was NEVER as good. One reason that one engine from the factory for some unknown reason seemed to run better than the next. The "Pent-Roof" combustion chamber design used on the "Nail" is basically a "Semi-Hemi" design & many newer cars today have the same type combustion chamber style because of the benefits. So as far as we've come in engine design the "Nail" was WAY ahead if it's time, nothing new for Buick. If we go back to the original design of the heads back in '53 the combustion chambers were out to the ends of the cylinder wall & they had major detonation problems even with only about 8.0-1 compression. The fix back in the day at the dealers was to install double head gaskets to lower the compression even more to help the detonation problems in the early days of the "Nail". I believe this was fixed by middle/late '54. A sign of a good combustion chamber design is not as much advance is needed for a total on the timing. In stock form the "Nail" has approx. 4000RPM's of swirl. When you start porting the swirl becomes less, but since the heads flow better it doesn't appear to be detrimental. As an example if the intake part of the head was cut at a 45* angle with a ported head the swirl goes down to approx. 3200RPM's. Compare this to a more modern aftermarket Chebby head they flow approx. 2300RPM's. Because of the design of the original pistons if the heads were cut too much more than .035" the possibility of the side of the piston by the combustion chamber in the head many times the piston will hit the head. There have been a few that have had that problem as recently as less than a year ago. Started out the thought was lifters. After a head was pulled they noticed the piston hitting. That's why the "Forged" pistons I designed have that area rounded. Just more thoughts on my part. Tom T.
thanks Tom, good stuff. with custom pistons moving the piston to the top of the bore, you can now make the dome a little shorter. wonder why the factory always puts the piston so far in the hole ? must not knew about quench back then.
yup, that was me moral: never let your customer make you cut corners on measuring clearances, as he'll blame you, not himself
They've known that stuff loong before you think they have. What the engineers know, the accountants say, and procedures used to keep the assembly line moving profitably are all completely different things. It's smarter business to drop the pistons in the hole. Believe me...these things are far from maximized and they know it. It isn't their job to give you their best. If you read differently, that's just marketing. Thanks Telriv for the comments on more quench area. I may have mis-stated area for distance and apologize for any confusion. That .025-.050" "sweet spot" holds true to universal engine design principles. Outside that requires different tactics, all well known. I commented in a general way that it's known about exceeding the point of diminishing returns and causing a detriment somewhere else. The temperature quenching characteristics don't show much if any better even to the point in which the piston contacts the head. Now something non-quench related is happening. This isn't voodoo, it's well understood and the formulas are proven and predictable. Rpm naturally replaces the artificial influences of swirl, tumble and squish, but the speed range in which it negates those techniques is often higher than the powerband used. They have known this stuff since before WWI, and refined the practices during WWII. The basic formulas are still sound today, and taught to engineering students. With Direct Injection gasoline or Diesel engines, note the "swirly" shape of many piston tops. It's there for a reason. Dynamic compression magic is not because of special cams. There are innumerous factors that allow much higher compression than what hobbyists think is safe.
So I'm trying to digest all this. If I read correctly, what is required is the more narrow squish band the better --- which will increase compression ratio ? Then a higher octane fuel is also required to avoid detonation. So by decking the block and using thinner head gaskets, what effect does this have on pushrod travel.
the thinner head gasket .017 thousands was stock. so how much you deck the block is how more the pushrod will go farther into the lifter. you can get your narrow squish and put your compression ratio any where you want by making your piston dome bigger or smaller with your custom piston. only need to deck the block just to square the block, then move your piston up the engine bore with the pin height on your made to order pistons.
From personal experience, I would pay close attention to what Tom has to say. I brought him my relatively freshly overhauled 425, and with his forged pistons and absolute attention to detail, It probably gained 100 hp (or more - it was really pretty doggy) and it runs beautifully.
The engine manufacturers needed to make a one number FITS ALL pistons to be used in EVERY engine. I have taken apart MANY "Nails". I ALWAYS check clearances BEFORE just ripping them apart. At least I KNOW where I'm starting from!!! In my experience the deck heights vary by a considerable amount. For instance a 401 I recently checked had a deck height of 10.0135" in the front & 10.0075" in the rear. Was the opposite on the other deck. OK, so cut .006" from the block to square it up & we would be OK. NOT SO, After cutting .006" the bottom, outer deck wasn't even touched. We had to cut an additional .003" to get satisfactory results. Before we cut the last .003" we double/triple checked to MAKE SURE we had the block properly set-up & ALL checked OK. This particular engine was said, "it ran really good compared to others". Now our deck height is 10.004". The deck started out shorter than the more common deck height of approx. 10.020" & there are variations between these numbers. I've seen as much as 10.022" to as low as 10.004". That's quite a range from the factory, .018". The reason one runs better than the next. Another reason off the shelf pistons are a CRAP SHOOT. NOT ALL engines check the same. In most instances the deck heights don't normally vary by more than .001" side-side. Some higher decks, some lower decks. On a 425 I took apart, the recently rebuilt engine with the new off the shelf pistons in the "Hole", if I remember correctly, was at about .065" in the hole. Reason he ordered a set of my 'Custom Made forged pistons'. NOW he is HAPPY with the way his vehicle runs. It's ALL ABOUT ATTENTIONS TO DETAILS GUYS!!!!! Of course this attention takes time & costs extra $$$$, but makes for a better than normal running engine. Now, with the proper "Custom" made pistons you don't have to opt for, the more expensive & becoming much harder to find as time marches on, the "Steel" head gaskets. You can now use the customary "Composite" gaskets readily available in gasket kits because you can tailor where the piston sits in the hole. As an example if using steel gaskets the piston will sit approx. .020" in the hole. If using composite it will now be closer to zero deck. NOT THE MORE NARROW THE SQUISH/QUENCH AREA THE BETTER. GO BACK & READ WHAT I SAID!!!!!! Really not trying to be offensive. If you read the former posts correctly & have an understanding of the proper quench/squish area, even though it may in many instances raise compression, will normally NOT require higher octane fuel. Just the opposite. It mostly ALL relates to "Volumetric Efficiency". Pushrod lengths have a HUGE role to play for proper lash, lifter travel, etc. I've done a couple now that have had the "Rocker Assemblies" rebuilt. In some, not ALL rockers the tips were replaced with "NEW" repro tips. Those tips were about 3 times the thickness of the originals. In this instance the rocker ratio comes into play. Could have gone with diff. pushrods for those needed, but opted to replace tips. The lifter was all the way to the bottom of it's travel. Would have made for a poorer running engine when warm. Replaced ONLY those tips & everything ended up where it SHOULD be. AGAIN, DETAILS!!!!! Sorry guys, just more of my thoughts. Tom T.
Getting the quench clearance to the .025-.050" range WILL reduce the need for a higher octane fuel, EVEN IF it raises the compression. The compression # is not the rule. The temps in the chamber prior to firing is. Quench clearance and squish is kind of like cooling off parts of the combustion chamber by misting a garden hose. It pulls heat away from the hot spots. The excess temps in the far away corners of the chamber, along with residual gasses from the last firing cycle leads to abnormal combustion. This is NOT load related detonation, but does play along with that. To re-iterate...going from a piston .080" down the hole to .040" will REDUCE the need for higher octane, even if compression goes up, and make the engine possible to be tuned to run better.
at .025 quench your piston might be hitting the head at high rpms why risk that.. I was always told .035 minimum.. ok , now you tell me or tom for that matter how much compression and timing is good before the motor starts pinging on my nailhead. pistons are 1-2 thousands in the hole with 33-34 thousands compress head gasket, so 35-37 thousand total qeunch, can't get better quench then that can you. on 91 octane gas. in a lighter skylark and not a heavy Rivera which is more inclined to detonation or pinging as some would call it. you are going to have everybody building high compression engines with detonation, just because they think with qeunch they can get away with it. remember there is a limit even with a nailhead and good qeunch, nothing worse then a pinging motor. oh yea now you get the cam factor how much duration and what LSA. let me help you, 10.37 compression with 192-195 cranking compression. that was pushing the limit, because on a warm day even with a fresh air scoop it will ping with over 35* timing, lucky it only needs 32-34*to run best. so next time no more then 10.2 compression and 182-185 cranking compression.
so next time no more then 10.2 compression and 182-185 cranking compression. this is all at 700 feet above sea level, which matters too.
Joe, did you figure out what the DCR is on your setup? When does the intake valve close? A water injection setup might help you out. Had one on my old '79 Turbo Regal that worked real well. And I think the pinging with the extra timing is a different issue..referred to as 'knocking' .... the mixture is burning too fast, not pre-igniting due to high compression or hot spots. I'll bet the perfect quench/hi-compression ratio combo isn't worth a damn without proper fuel and timing control. Is that even possible with a carb and distributor? There's been some good reading in this thread. Thanks for the lessons!
I never knew about the difference in pinging and detonation, my intake closes at 42 at .050 duration. my DCR is 9.33 and says that my cranking compression should be 193.77, so that makes sense, like I said when I checked it was 192-195.
DCR at 9.3 and cranking compression at 190+ lbs is more than pushing it for that build, I can see why there would be problems. Different engines definitely have varying ranges of tolerance. Suggesting a tolerance range for quench distance is ONLY to the effect of quench itself. I was referring to combustion technology exclusively and not at all to the g-force of pistons or rod stretch. I can say that a 3.3 stroke crank swinging a 450 gram piston can go 7800-8000rpm's @ .028" from the head and never have an issue with a buttery-soft stock rod stretching enough to hit the head. I can build 12:1 carb'ed pump gas engines all day long for me, but I NEVER do that for other people. Any of these builds assume good tuning ability and a compatible vehicle. There are many, many factors beyond the engine's specs related to what an engine will tolerate. Static compression is literally only one factor in 50+. Vehicle factors can have way more influence on what the engine will tolerate than the engine itself. I've tried to make this abundantly clear. Temp of intake air is a biggie. A 2800lb car with a 5000 converter and deep gears with an engine's powerband being 6000-7500 rpms would be much easier to start with. (obviously foolish to intentionally use pump gas, just an example)
I forget when I tested the compression the intake valve closed at 40, then latter I retarded the cam 2*. so that takes the compression down to 10.3, not 10.37. still the same dcr 9.33
I know but when Walt asked DCR, I ran it with 42 intake closing, I hade forgot my exact static compression, but was positive of my cranking compression. plugging in numbers to double check. 10.37 made me think because when I was building the motor I was shooting for 10.1- 10.25 max. always counted the head gasket at .037 compressed but after measuring the old compressed gaskets there at .033 believe it or not, and I have measured another set that was .033 too. point is that's why I over shot my 10.1-10.25 and came out at 10.3 in the first place.
9.33 sounds too high. I think you're closer to 8.5 - 9.0 DCR, but I'd have to redo the calculations. I did a chart once upon a time. Stock was around 8.5 DCR with intake close at 36* (0.050") and static CR around 9.75. Nowadays they recommend around 8.0 DCR for pump gas. Anybody feel like doing a DCR spreadsheet?
