Discussion in 'Small Block Tech' started by FLAME ORANGE GS, Jan 12, 2018 at 7:32 PM.
whats the optimum quench for a 350?
At the current time there is no real quench area since the head chamber is the same size as the block bore (open chamber). I don’t think it really matters if the pistons sit way in the hole with the iron heads, other people disagree. I had good luck running flat top 3.0 v6 pistons in a 350 and they were way in the hole.
The TA heads will be closed chamber style and I am planning for 40 thou in quench distance.
If you are wondering how quench works in general, there's plenty of science-y info to be found on the net or in votech books.
You can get as advanced as whatever latest MIT or SAE info is available for your perusal.
Information, working theories or actual data isn't exclusive to engine types as much as how you understand, utilize and incorporate it to your overall build strategy.
There are other engines besides SBB's that have open combustion chambers that builders have found ways to utilize quench, such as step-head pistons (protrude the piston past TDC) or hemi-headed engines with tall matching domes.
Generally speaking, any closer than appx .032" doesn't show much more than trivial improvements, and might show diminishing returns.
There are cases in which a builder runs much less piston clearance while looking for a few more hp and even finds light piston to head contact. This is well beyond street driven hot rods, lol.
I shouldn't have to point out what happens when you take up all of your vertical rod clearance by hitting the head with an elongated rod and piston from heat and stretch at full rpm
Also consider that many combustion enhancing effects are both subject to a performance curve as well as hang in the balance with other effects. One detrimental side effect of misused effects are how fuel can be pulled from rapidly swirling charge as it overspeeds the spin.
For anyone to begin suggesting specs on a particular engine's range of specs...one must have a certain amount of information to work with.
What is it you are trying to do?
What are (many of the) engine's measured or planned specs?
What conditions are you running the engine in and what are it's expectations?
Also if the pistons are let’s say 60 this in the hole like many factory engines and then you have a 40 thou thick gasket then there will be no effective quench if a person used a closed chamber is used. The funny thing is that there is a “danger zone” where detonation can result. But once you go beyond this zone there is no issue with detonation although the benefits of quench are gone at that point.
Each engine design is different but the “danger zone” is commonly accepted that over 0.100 quench distance there is little to no chance of detonation caused by this distance measurement.
There are even lots of pro race teams (boosted applications) running huge quench distances on purpose using flat top pistons way in the hole. They claim it reduces stress on head gaskets due to the flame front being further in the hole. Boosted engines are far less reliant on optimum quench vs NA engines.
Not only are you talking .060" in the hole and a .040" gasket...the distance from the head's deck to any flat areas within the chamber that would otherwise act as quench areas if they were close enough to the piston.
So far the generality of the question is like asking, "how close can Helen Keller get her hand to a wood stove before she detects a problem? What about if that stove is red hot? "
Calculate the rate of safe escape based on reaction time, areas of callouses, rate of heat transfer through dead skin, hand to elbow length and angle of lean-in, etc.
Thats right and I have posted a pic on here before of the distance of the head chamber flat part vs the deck of the head and it is deep.
A lot of Buick replacement pistons sit .090 down hole. Along with the 3.0 flattops
I'm assuming one asking about quench on a sbb is prepared to deviate far from the norm or is considering aftermarket heads when released, making the entire dialog relevant.
Agreed... Yes. Unfortunately people read general built info and think that “zero deck” on pistons is needed but the open chamber heads do not care.
A fellow ran 12.50 in the quarter with flat top 3.0 pistons way in the hole giving 10.5:1 so there is something to this.
Plus when people talk about aftermarket pistons that sit way in the hole plus the dish cc it really kills the cylinder pressure.
I know a guy who used low compression 8:1 big dish pistons, with zero deck and heavily milled heads and it was 10:1 static comprsssion. More than one way to skin a cat lol.
No one has done a side by side comparison on the Buick 350 to prove or debunk quench theory's or their irreverence .
There's nothing to prove or debunk. Not theory.
To suggest that would be arguing with 60+ years of OEM and MIT level engineering.
How you 'choose' to incorporate any combustion sciences is totally up to what results you need to see.
Can you exceed the need for any particular kung fu by other means?
The point being 'what is it you are trying to accomplish?'
'How do you intend on incorporating this aspect into the goals?'
What are the goals here?
Lol there is no quench to be had with iron heads so it doesn’t matter lol. No need to prove it disprove anything.
I should clarify that the end results of 'the engineers' intentions are all that matters.
If one wants to rethink things or change drastically, they are on their own.
There are many times in which various strategies aren't needed.
ok, put a different way, what deck height or piston down in the hole seems to work in a iron head 350? i have not measured my molnar rods or the 2343 trw forged pistons for height yet, but my originals were .055 in the hole. just wondering if a bigger than clean up cut does any good on the deck. do i want them as close to zero as posible? seems like .050 deck is alot to cut the deck.
Are you going to use iron heads or the alum heads? If iron then just setup the compression ratio to match the cam. As we are saying there is no quench area on the iron heads so it does not matter. You already have the pistons and rods so CC your heads to see the chamber volume and then you may or may not want to mill the block. Zero deck may be too high of compression for your combo, or maybe not. The deck height and head volume are the last two variables in the compression ratio so do some more measurements before you do anything else.
What year engine is this? If it is an early engine and you are using 20 thou thick steel shim gaskets then you could leave the pistons 20 thou in the hole and still have optimum quench distance. If it is a later engine and you are using a 40 thou thick gasket then I would try for zero deck. It is common to cut 50 thou off the blocks, not a big deal when building a performance Buick 350.
72 motor, using a .040 gasket. zero deck give 10.9 comp. is it safe to cut that much from the deck?
Yes it is safe to cut that much off the deck. Just be sure to look at the intake fitment as it may need to be milled a bit.
And depending on the cam you are using you may or may not have too much dynamic compression.
You should be sure to degree the cam and THEN check the valve to piston clearance as you may need to add valve reliefs to the pistons.
Might be easier to temporarily disregard blueprint 'zero' and call it zero deck according to your new parts' stack height.
On a tangent...at 10.9 you must be considering a really serious cam or alternate fuel?
...or is this largely behind the quench Q's?
Also on a tangent, if this is performance intended... simply raising the port opening to match offers more benefits than the cash savings from not cutting the intake.
pistons are already fly cut. just trying to figure out if cutting that much is worth it. even at .025 deck its still 10.3. going to let tim at TA recommend cam, something roller.