Hi guys,

In trying to learn as much as I can before ordering parts. I have a "ponderment". How much quench is too much for a street-driven, pump gas motor. I'm always reading about trying to achieve around 0.040".

I have a stock deck ht. 350 block that's still in the machine shop. I will find out for sure later during assy., but I persume that the pistons will be rougly 0.025" in the hole.

1. With a certain Felpro gasket (0.041" compressed) this would give me a quench distance of around 0.066"...right?

2. How and why is it bad to have a large quench distance? Or is it bad?

Any guidance is helpful
Thanks,
Dan

"quench" effect causes turbulence in the cumbustion chamber as the piston comes up to TDC. "Good" quench reduces an engines tendency to ping. Also, the space is so small in cannot support cumbustion, this also reduces the tendency to ping.

Roughly over .060"-.080", the quench effect stops, that area is just another part of the combustion chamber.

How big a deal it is for a particular motor depends on how close to detonation that motor is in the first place. If it's not close, quench doesn't matter that much. If you're on the raged edge, it's important.

MANY factory engines have what we'd consider excessive quench, and run fine.

To close it up, you could use a thinner gasket, or deck the block.

A lot of people with pretty mild motors are extremely concerned about it. My 355 has a "bad" quench of .056" and it ran fine on 91 octane. And that is with 11:1 compression with 215-230 psi cold cylinder pressure.

Other factors than just quench influence a motor's tendency to detonate on pump gas. If you haven't purchased the parts or had the machine work done it wouldn't hurt to try to get the quench to .040". If you already have the parts don't worry about it.

Many use Quench as part of the equation when building a optimal Hi performance engine, and use .040 as a good number to have.

For me, the importance of quench came into Play when I ran pump gas in a high CR engine with optimal timing and a short duration Cam, The Power was way down low. Detonation wasnt so apt to come into play in this engine with a good quench number. But that said, The best solution is to use a cam with longer Duration @.050 moving the power up a bit and out of the range for possible detonation.The quench isnt so important with the right matched Cam.

On my stock 350 block my machinist had to deck my block .030 due to the eagle rot kit that I ordered had a piston head volume of +10.cc

The pistons were .045 down in the chamber and that was going to give me somewhere around 8.5.1 ratio when the kit was for 9.3.1 using 64cc heads. In the end I should have 9.5.1 and that should be a good compression ratio for the street with todays fuel.

OK, thanks for the responses so far.

I'm building a mild performance 355 for my street '70 Camaro.

I really don't want to spend the extra money to have it decked if it really doesn't matter. So I'm wondering why quench plays an important role in performance motors. See where I'm coming from?

For pump gas street machines:

So what happens when I have roughly the 0.066" clearance as mentioned before? Would the motor be more apt to pinging?

What happens when the quench distance is too tight?

How much is too tight?

I want to the science down. Then I'll include what parts I plan on running for this 355 to see if I'll be ok for pump gas on the street.

Thanks,
Dan

Piston's kiss the head!!

pdq67

"So what happens when I have roughly the 0.066" clearance as mentioned before? Would the motor be more apt to pinging?" - YES

"What happens when the quench distance is too tight?" - Pistons kiss the heads

"How much is too tight?" - depends on parts, .035 is good to shoot for on an SBC. Go with the 1094 Felpro steel shim that has a compressed .015 thickness. With your .025 piston in the hole, and that .015 gasket, you are looking at a good .040 quench. Perfect.....

Agree. you don't want it any tighter than that tho. Be sure to measure, not assume.

"as for the science"... The flame front starts at the plug and proceed outward, an expanding ball of fire. That expanding ball of fire "supercompresses" the remaining unburned mixture. If the remaining unburned mixture reaches it's flashpoint and ignites before the flame front reaches it, you have ping.

In a tight quench area, the mixture trapped there is not burnable, it won't ignite, simply not enough space.

"turbulence" is also important. As the piston reaches TDC and closes down the quench area, the mixture at the edges of the cylinder is squirted back towards the center, kinda like stepping on a toothpaste tube. This helps mix the mixture in the combustion chamber and make it more consistent for a smoother burn.

"Piston kissing head" of course is a really bad thing that needs to be avoided.

If your deck ht checks out at .020 or more, use the 1094 gaskets. I've used them many times. They seal well. They won't blow out. And they're thin. The decks do need to be good and flat though. Maybe a light pass just to be sure.

when i first screwed together the vortec headed, HOT cammed 355 in my Nova, i used a cheap gasket set that had some really thick head gaskets. i'm not sure how thick- but i think .040 is the "standard" SBC replacement head gasket thickness.
anyways, with that gasket, i was at around 9.5:1 compression ratio with my 2 vavle relief flat top pistons. it would rattle at 36 degrees total on 93 octane gas and the best mpg i ever saw was 15.
one winter, i decided to put some .015 steel shim Victor Reinz head gaskets that i had laying around in it. with just that change- which pushed the compression up to around 10:1 or so- i was able to "get away" with running 87 octane at 36 degrees total if i didn't beat on it, and 89 never rattled at all. power and mpg noticeably fell off with any more timing than that.
with only the thin head gaskets, my mileage went up to an average of 17 no matter how i drove it.
so, in some cases, i'd say quench is mighty important.

You want to know about quench, here is an excellent article. http://www.popularhotrodding.com/eng...eze/index.html

since this forum is about learning and all....What is quench?

Quench

From Wikipedia, the free encyclopedia


A quench refers to a rapid cooling.

As JimM explained in his post, Quench is the process of cooling the compressed charge in the combustion chamber to prevent pre-ignition by turbulence. Too large of clearance, not enough turbulence, too little, too fast, no cooling effect.

Look at any cylinder head, you will see a void housing the valves, called the bowl. Beside it is the flat deck of the head. As a note, (not required for quiz on Friday) the definition of a combustion chamber is the volume created by the piston head, cylinder bore, head gasket thickness, and the bowl area at TDC.

Imagine turning the cylinder head over and placing atop the cylinder and have the piston rise in the cylinder compressing the charge. As distance becomes shorter between the piston head and head, the compressed charge gets hot. At the same time, the charge is squeezed from the volume created by flat deck and piston head and gets squeezed over into the bowl area. This squeezing action during the compression stroke, sets up a turbulence and cools the charge. Remember using a room fan? It's cooler with moving air than stagnant air.

Questions?

Ok, Thanks for everbody's input. It has help me tremendously.

So all-in-all, a distance somewhere between before the piston kisses the head and say 0.040" for a sbc is ideal.

Can we ponder on this for a while:

I always wondered why the engines had pistons down "in the hole" a fair amount and didn't detonate.

Since the factory engines usually have their pistons "in the hole" a fair amount leads me to believe that I can leave it like that on my rebuild and be fine.

But...this thread has lead me to believe that a greater distance from the head to the piston at TDC will be more apt to pinging.

Is this correct?...In general conversation of this thread.

Thanks,
Dan

The quench effect is due to the close proximity of cool metal surfaces to the combustible mixture during pressures high enough to cause detonation. The mixture can't self-ignite because it can't get hot enough.