Buccaneer said it did not matter and the tighter it is the more susceptible to detonation you are.....I think he typo'd that though given the dudes attention to detail and knowledge.
If quench did not matter......then why does a 500ci Pro Stock engine have 50cc chambers on a 4.700 bore and flat top pistons at 17 to 1 compression!

Jebby

 

Not that I give a damn about some of the comments from the know it alls, what I said, was "I did not worry about the quench " (being right at .040 was omitted and understood)................and yes if the option is hit .040 quench perfectly with hypers OR get close and go with forged pistons, yes I went with the latter.

Buccaneer was pretty specific about his thoughts on quench and further clarified in his following comments..no typo from what I can see..........

 

Before I get flamed for bragging or being a know it all, stating facts is not bragging and sharing your knowledge and experience is not being a know it all!

Quench or squish, whichever you choose to use, is very important and does have large effects on the engine. I have built race winning and record setting engines from small 2 cycle engines, Pro Stock, road race, land speed and Top Fuel, naturally aspirated, turbo charged, supercharged, gasoline, methanol, and nitromethane, as well as decades of street engines and years of dyno testing.

The statement "The closer you get to zero quench, the more it is prone to detonation." couldn't be more wrong! Tight squish absolutely makes the engine less sensitive to detonation. Engines with tight quench will run more efficient, burn cleaner, sound crisper, need less timing advance and will make more power.

I do not consider .040" on a SBC to be a tight squish. Beyond .040" is where quench begins to have little positive effect. I like to run as tight as possible without the piston colliding with the head. I run .034" on my engine in my signature. I have run as tight as .025" in engines before. In race engines I used to keep tightening the quench until the piston and head would just start to shine each other a little where they touched and then back off about .002". How tight you can run depends on a lot of things like rod and piston weight and material, stroke, block material and rpm the engine will be spun to.

There are other things that come into the equation like quench to bore ratio. A low rpm engine may run as much as 55% quench to bore ratio whereas running that much on a high rpm engine would cause a pumping loss and may even put the flame out. High rpm engines do better with between 35% to 40% quench to bore ratio. At extremely high racing engines rpm, quench becomes less of a concern because of such short combustions times and the speed of the intake charge. Of course this is not of concern to the OP or most street engine builders. Run between .035" and .040" quench and you will be fine. You will get the benefits of a tight quench.

Smokey Yunick professed the merits of a tight quench all the way back in the early 60's, so this is nothing new. He liked to set engines up so the piston and heads were just starting to shine the surface of each other. There is no wonder where I got that idea!

Mike

 

I don't see any reason to not optimize it. I fail to see why guys build engines and don't take just the little bit of extra effort or care to insure the best chance of optimum performance.
I'm not an advocate of spending racing $$$'s for a street build, but things like this cost little to nothing and will add a layer of insurance against poor or unsatisfactory performance.

Not doing it and then ending up with an engine in which you need to pull timing or enriching the fuel mixture just get it to run without deto will lead to wishing you had done it when you had the chance.

OP, Super important, make real sure you follow the manufactures recommendations for end gap on the top ring when you install them. Failure to do so and you will rip the top ring land right off a piston. I have those same pistons and done right (no deto and correct top ring gap) they run great.

 

The number one reason for failure of hypereutectic pistons is the ring gap is too tight!

Mike

 

with it all and
the bold "shine" part is EXACTLY what I was taught & try to achieve with circletrack motors. Thanks Mike

 

I have not built any engine to a racing standard (chevy SBC or BBC). So mine currently is at .038. Tighter would be ok with me. On a 350 with original bore, hypers and 5.7 rods with a 6500 RPM limit, I could probably run .030" or maybe even .025 (eek).
But for this engine .038 is just fine and I cannot get it to detonate regardless of the timing or mixture.

 

If anyone can post the ProComp 190cc SBC head airflow chart, I can load this up into the Dyno Sim program, but I did a number of searches on the interwebs and can't find the airflow charts for this.
I'm sure the numbers posted on here are good ballpark estimates.

St Ovo, You're looking to run an equivalent of a US 91 Octane fuel (95 RON), getting that quench distance right can help it run without detonation on lower octane fuels than is otherwise possible. (As a few folks have quoted getting the piston to stop closer to the head, especially with your pistons causes a high-speed mixing of the air / fuel mixture which eliminates hot-spots and helps keep reignition / detonation at bay.)

-You should use a Dynamic CR calculator to estimate your DCR, which is a good rough indicator of what octane fuel you'll require. (Especially with your octane limitations and your small cam and large squish, you should probably calculate it...)

You can also do things like make sure you have a cold air intake to try and avoid detonation if you're close to the edge with your build. -You're in Northern Italy so at least you don't have the scorching heat of the south, which will help.
You can also change the jetting of the carb to richen it up in the summer to help with detonation or keep a couple containers of octane booster with you -or retard your ignition timing a couple of degrees in summer.


Adam

 

This subject about quench has been beat up so many times that I really can't remember the number and of course everyone has their opinion on the subject which is fine. Whatever you decide with, go for it any see how it runs. If it runs poorly, change the gaskets to a different thickness. I'm not saying that quench doesn't matter at all, just that the "magic" number of .040" is not a "in stone kind of thing" to make the motor run. If you were racing the car for money, than I would want to get every single advantage I could get out of the motor and then it may be a very important player. I currently have 10.6 CR now and if I went to .040 it would go to 11 or more which I don't want for a street car running on 91 octane pump gas which is as high as it gets here in AZ unless I go buy race gas by the track...no thanks at the current prices. I can't see building a motor that requires higher octane than I can buy at the pumps just to drive it and have fun or go for a cruise. Again, this is only my opinion and others will differ. I wish you the best with your 383 build and keep the rubber side down.

 

Just had a text conversation with my race builder who built my short block and he confirmed that he built my street engine to .030 quench with my AFR heads and builds his race engines to .015-.020. I guess that explains why I have zero detonation running 89 octane fuel with a 425+ Gross HP 355 with a static compression of 10.2:1........

 

so.....YOU didn’t have to worry about quench because YOU didn’t do it .

 

Detonation / preignition at it's most simple is simply about exceeding the TEMPERATURE at which a fuel of your octane begins to self ignite like a diesel motor.
There are MANY factors, that can influence it up or down, but Static Compression is actually quite far removed from the problem.
Cylinder temperature and pressure are HIGHLY correlated in an n/a engine so you can think of them together as one, but things like air temp, thermal coatings, oil temps, and coolant temps can change temperature without changing pressure -which helps with detonation still; beyond that they're still VERY highly coorelated.The same subject in terms of forced induction is useful: you've got way more cylinder pressure from compressing the air, but you usually see air cooling ala intercoolers or chemical intercoolers (water / meth injection) to keep the combustion TEMPERATURE down so you don't have detonation (again because it's the temperature of combustion that actually matters/ makes detonation happen).

Preiginition/ detonation is also concerned with LOCALIZED temperatures, and not necessarily just average combustion temps; this is also where a tight squish helps as it gets the air fuel mixture spinning and tumbling at a high speed to keep temperatures even and help avoid hot spots (even after you get carbon build-up)- I think it's actually primarily HOW a good quench helps increase the pre-ignition / detonation resistence (evening the temps throughout the combustion chamber to avoid hot spots).

"Dynamic Compression Ratio", which isn't actually dynamic, is a MUCH better indicator of cylinder pressure/temperature than static CR. But it's still ONLY guestimating the actual compression / actual amount of trapped air and cylinder pressure/temp based upon the static CR and the intake valve close event.
-IMO, PipeMax comes the closest to estimating actual cylinder pressures as it takes into account intake and exhaust systems and wave tuning and cam event timing to estimate quite accurately how much air will get into an engine (even if needs a dyno graph as an input to be at it's most accurate) - and even PipeMax doesn't look at things like intake air temperature, coolant temp, oil temp, flow volumes, etc... (PipeMax even goes so far as to convert it's cranking pressure and running cylinder pressure (at torque peak where its' at it's max) into a recommendation of fuel octane to safely prevent pre-ignition /detonation. -Although again air, oil, and coolant temps and coatings can change that number.)




It's is completely uncontroversial and as close as we can get to established fact that a tighter quench helps prevent detonation faster than the increase in compression that you get from running a tighter quench. David Vizard has graphed the differences from the change in quench along on enough dyno pulls that he now has an estimate of how much power increase and detonation resistence you get from an increasingly tight squish (I've seen it posted on SecTalk a few times by attendees of his seminars in the past year, but I couldn't find it with my mediocre Google Kung-fu).



-The items that you're posting (HP, Quench, Static Compression ratio) aren't very well coorelated to detonation: garbage in, garbage out problem. It proves very little.

Engines are more detonation resistant at a given DCR and temp with a tighter squish and good quench motion and make tiny bits mroe power with a tighter squish and good quench motion. It's not really a point that any experienced professionals argue.
Anecdotes of one guy's uncles, brother's builder who had a high compression motor that ran on cat **** and wouldn't detonate, don't prove a thing and are counter-productive for helping people make informed risk-reward decisions.


Adam

 

I think the simplest explanation of the benefits of a tight squish that provides a good quench is: A tight squish (piston to head clearance) acts as a free "octane booster", pushing pre-ignition/detonation out to higher cylinder pressures/ temperatures. Increasingly tight squishes provide higher levels of preignition/ detonation resistance, but increase the risk that at higher RPM as you get more rod stretch the pistons will meet the head potentially resulting in catastrophic damage. (Tight squish/quench also provides small increases in power.) (-On SpeedTalk you see builders thinking about peak RPM and rod materials when making recommendations on squish / piston to head clearances.)


It's maybe worth noting that certain piston designs help provide a better quenching action at the same squish (piston to head clearance) -the OP's pistons with the quench pad and quench ring all the way around the piston are a really great design to benefit from a nice tight quench as it pushes the mixture from the outside of the cylinder towards a smaller area in the center of the chamber. Also making a really nice 3 dimensional chamber shape that will burn quickly and probably need less timing / making VEERY slightly more peak torque because of the rapid combustion.

-If your engine will already run on 87 octane is there a power benefit to running premium? (not a significant one -it's kinda the same with squish / quench a little bit) If you already have a low DCR engine that will run on cat **** a tight squish isn't going to gain you much, but with a tight squish you can push the envelope on DCR and make more power with the same fuel.



If you were offered the chance to be given a free octane booster with each fuel fill up throughout the life of your engine by your engine builder, would you take it? That's what building an engine with a tight squish / good quench is offering.

Adam

 

Squish definition: In engine building, refers to the piston-to-head clearance. Primary driver of "quench".

Quench definition: In engine building and theory, a high-speed mixing of combustion fuel / air that eliminates hot spots and helps prevent ignition / detonation.
Quench practical definition: An engine building physical (rather than chemical) "octane booster" that works via high-speed mixing of combustion fuel / air immediately prior to spark ignition.



Adam

 

Dont hear too many talk about squish...important also guess it all is. We used to get that so tight the pistons would just about touch the head back in the Saugus Speedway days
H beams woulda helped but definitely would be an immediate DQ.
Your car will run plenty strong, you have learned a bunch over the last yr
At leats you splurged with Mike Jones that is never a wrong move. Some of the grinds he comes up with seem way the hell off on paper but they sure do work.

 

Ok...I see what you are saying and I get it..........I once put a top end and cam on a 69' truck 350 that had the pistons .050 in the hole! I went with 64cc 180 Dart heads, a 292H Magnum (one of my favorite cams) and a Tunnel Ram with two Holley 450's on it.......it worked out to 9 to 1 and ran like a scalded *** ape in a 37' Ford business coupe.....the engine had never been rebuilt from 1969 and was perfect internally so I took a chance and went with it......it worked out because of the low compression.....but the piston to head was .090!!! This was about 9 years ago.....
fast forward to 2016 when I was putting together my 406 and wanted 10.5 to 1 come hell or high water......the pistons were set zero deck and a .039 gasket run.....pretty standard.....and no marbles in a can much less any detonation....so as always, it depends on what you are doing and the circumstances of it.......

Jebby

 

Not to ruffle feathers but one reason why guys who spend SO much on restoring a car with their #s engine turn around and run the cheapest gas possible not realizing that piston can be pretty far down there;detonation starts long before you can hear it.
One reason blueprinting/mocking up an engine a few times is so crucial...noone wants to pay for it so they skip that part. A blueprinted engine will usually come with a sheet that gives every last detail, you wont be getting that with any mail order/crate motor. Why 2 different motors with the same parts can run noticeably different.

in the 70s and 80s the popular answer was to lower compression, not really needed wih todays parts in fact you can run more & increase its efficiency. Opinions may vary

 

Not exactly ......He did the machine work on the bottom end and all bottom end parts (crank, piston, rods, pan oil pump etc). I did the roller cam and AFR head installation (top end) and engine/clutch reinstall.......unbolted heads sitting on the short block in my garage

 

Good point Jebby and I do bet it ran just fine...Nice! My tuner in Havasu has a 66/67 box Nova with a SBC, single turbo, EFI motor and it runs 7.50s. His is at .120". Not a great example, but... His new motor is a billet Dart block and will have twin hair dryers, should be a beast for sure.

 

Well put Adam. I'm somewhat amazed that there is still a debate regarding squish.

I would beg to differ a little on the octane and power produced however. Granted there is not a large difference. But for a given engine using a lower octane can produce slightly more torque.
Lower octane fuel is more volatile. So it burns faster, lights off easier therefore burns more completely extracting more power from the same volume of a higher octane fuel.

That being said if ethanol is in the fuel then I wouldn't run it in any case. It's typically easier to get premium without ethanol than other fuel grades.

In my area I can get 85 or 91 without ethanol. So I have no issue with putting in 50/50 mix if I want to. That works out to 88 octane.

The original engine with it's lousy cooling, fuel metering, timing, cast iron heads and dish pistons ( no squish to speak of) detonated even on 91 on a hot day.