Quench dimension 2

[sierra4000]

Hello,

What is minimum (ideal) Quench dimension for flat-top pistons?
Piston to head distance for racing engine.

Thanks
Radek

 

[mfgenggear]

More info on definition

 

[sierra4000]

V6 engine bore 94, stroke 68 mm, redline 7000 rpm,
original steel connecting rods, forged pistons, 100 octane fuel,
we want to get some compression ratio with block milling but keep safe piston to head distance.

 

[BrianPetersen]

Squish clearance? (Piston-to-head clearance)

This depends on about a million things that haven't been specified.
- How much rod stretch at rev-limit revs between the exhaust and intake stroke (no compression force opposing it.
- And that depends on the cross-section and length of the rods (which usually isn't uniform along their length) and the mass of the pistons and pins.
- How much the wrist-pin deflects under those same conditions.
- How much the piston deflects under those same conditions.
- How much the crankshaft deflects under those same conditions. It isn't a rigid body under those conditions.
- How much the block deflects under those same conditions. It also cannot be assumed to be perfectly rigid.
- An allowance for piston rock due to necessary clearance between piston diameter and cylinder diameter - which varies depending upon the temperature of everything involved. And how much it's worn - which will increase with distance driven.
- Allowance for differing thermal expansion of many parts made of different materials that operate at different and variable temperatures.

General guidelines are available for engines of different types.
The more specific guidelines usually arise from the school of hard knocks.
Been there, and blew something up because of it.

 

[sierra4000]

I'm aware of all the mechanical reasons,
but from a combustion efficiency perspective?

 

[BrianPetersen]

For DOHC-4valve chambers, which are usually short on squish-band area due to having plenty of valve area, all I've ever heard has been that the tighter you can get it without piston-to-head collisions at high revs, the better off you are. It's better to have air-fuel mixture pushed into the chamber where it has a chance of burning and is closer to the spark plug and flame front, than to have it out in the far reaches where it can contribute to knock. It's better to have plenty of small-scale charge turbulence near TDC to encourage faster controlled combustion near TDC or shortly afterward. Any air-fuel mixture that you can controlled-burn, can't knock.

Only experience I have with this has been with a high performance motorcycle engine (DOHC 4-valve) which had squish clearance set tighter than stock according to the Kawasaki race-kit manual (via select-fitting of head gasket thickness), which I was not about to second-guess. Combustion chambers are otherwise stock, pistons are not (Wossner 13:1 via a very slight dome, inboard of all the squish-bands). This engine ended up needing less ignition timing advance than stock by about (if I remember right - this is getting to be quite a while ago) 3 degrees - indicative of faster combustion.

Old skool 2 valve per cylinder "wedge" chambers can have much, much bigger squish areas, and I've heard rumours and speculation that going too tight can initiate knock because you're almost trapping charge in there without giving it enough time to get out, but I've never messed with those.

 

[mfgenggear]

I am possibly out of touch. and only have limited experience. but in my Dat it was described as a high performance and high compression.. and can be accomplished many ways. an engine with a low compression engine but with a turbo can be a high performance engine. the fuel has a immense play in this factor. such a fuel formulated as not to knock at high compression. there was a nice documentary. explaining fuel formulas.
the high performance pistons as well as the rods and crankshaft are forge to with stand higher moment and stress.
the piston cylinders are designed to clear the valves.
name of the game these days is 3 d cad.
to adjust and verify clearances.

 

[Lou Scannon]

There is a continuum and it depends where a particular combustion system falls within it. As alluded above, you want the controlled combustion to proceed as far as possible before knock, and have the quench occur to prevent knock when controlled combustion can no longer be assured.

 

[SwinnyGG]

Old skool 2 valve per cylinder "wedge" chambers can have much, much bigger squish areas, and I've heard rumours and speculation that going too tight can initiate knock because you're almost trapping charge in there without giving it enough time to get out, but I've never messed with those.

This is correct. The controlling characteristic isn't the squish area, but the shortest path from any point to the flame front. So you can have very large squish area that won't cause a problem, if it's a constant diameter 'ring' around the perimeter which has a consistent, and short, path to the flame front.

If that shortest path gets too long, the inertia of the charge becomes a factor; essentially it has too much inertia to be quickly pumped out of the squish area, so it stays there and basically undergoes compression ignition locally. And you wind up with ventilated pistons or crankcases.

Off topic but an interesting phenomenon from the 2 valve wedge head days.

 

[mfgenggear]

High-performance engine heads differ from regular engine heads by having larger intake and exhaust ports, larger valves, better valve spring design, optimized combustion chamber shapes, and often lighter materials like aluminum, allowing for increased airflow and significantly higher power output, while regular heads prioritize efficiency and drivability in everyday driving situations, usually with smaller ports and valves and heavier materials like cast iron.

Key differences:

• Port size:
  High-performance heads have larger intake and exhaust ports to allow more air to flow in and out, maximizing power at high RPMs, while regular heads have smaller ports for better efficiency at lower RPMs.
  
  
• Valve size:
  Performance heads utilize larger valves with higher lift capabilities to further enhance airflow, while regular heads have smaller valves designed for good overall performance.
  
  
• Valve spring design:
  Performance heads often have stiffer valve springs to handle high valve lift at high RPMs, whereas regular heads have less aggressive springs.
  
  
• Combustion chamber design:
  High-performance heads have meticulously designed combustion chambers to optimize air-fuel mixture and combustion efficiency at high power levels.
  
  
• Material:
  Performance heads are often made from aluminum for weight reduction and better heat dissipation, while regular heads may use heavier cast iron for durability and heat retention.
  

Important considerations:

• Application:
  Choosing the right head depends on your intended use. For a high-performance racing engine, high-performance heads are necessary, while for a daily driver, regular heads might be sufficient.
  
  
• Cost:
  High-performance heads are usually more expensive due to their complex design and superior materials.
  
  
• Drivability:
  While high-performance heads can significantly increase power, they might negatively impact low-end torque and drivability in everyday driving situations.

 

[mfgenggear]

high-performance piston, typically made through a forging process, is significantly stronger and better suited for extreme temperatures and pressures compared to a regular (usually cast) piston, making it the preferred choice for engines producing high horsepower or operating at high RPMs due to its superior durability and resistance to wear and tear; while a regular piston is lighter and cheaper, but less robust in demanding conditions.

Key differences:

• Material:
  High-performance pistons are usually forged from aluminum alloys with specific compositions for strength, while regular pistons are often cast aluminum with lower silicon content, making them less resilient.
  
  
• Strength:
  Forged pistons have higher tensile strength, allowing them to withstand greater forces and heat generated in high-performance engines.
  
  
• Design features:
  High-performance pistons may have specialized features like slipper skirts, optimized ring grooves, and specific crown shapes for improved combustion efficiency and heat management.
  
  
• Cost:
  Regular pistons are generally cheaper to produce than high-performance forged pistons.

 

[sierra4000]

So, is mainly mechanical problem to maintan head/piston in safe without mutual contact?
Yes , this area is quite short

Original dimension is 1.4mm head gasket+0.4mm piston top under block face
now block is milled -0,7mm
new area dimension 1,1mm

 

[mfgenggear]

OP
The amount of max stock removal will depend on the make and model of the manufacturer. In engine specification
For reworking warped heads ot will detail max stock removal permitted. After that depending on the engine it could have metal to metal interference. Or excessive pressure
Could cause head gasket failure.
Aquire the correct detail specifications for your engine. Do not guess at it.

 

[dtimberlake]

For simpler automotive size cylinders I think quench in the range of .04" to .07" is commonly considered "good". Much closer than .04" can invite piston>head collision at max rpm, and may even have some effect on esoteric pumping losses.

Looser than .07 starts to lose the important detonation resistance from "quenching" effect cooling the end gas. Hot end gas is where detonation initiates.

Naturally Formula 1 and others have delved into stuff like higher squish velocity causing excessive swirl and tumble, and the benefits and even necessity of tapered squish/quench clearance.

 

[mfgenggear]

this may be usefull

Definition and Description​

Quench is the distance between the top of the piston at TDC and the cylinder head surface. It is also called "Squish" or "Piston-to-Head Clearance."

How is it measured?​

Quench is a calculated dimension. It uses the following formula:

Quench = Deck Clearance + Compressed Head Gasket Thickness​

How does it affect performance?​

The right amount of quench can promote fast, complete burn of the air/fuel mixture. It creates turbulence, which forces the air/fuel mixture toward the spark plug. This reduces the possibility of Detonation. It also leads to cleaner emissions.

What should it be?​

Quench should be as tight as possible, without the piston contacting the head. Recommended quench depends on your max rpm and the type of connecting rods you use.

Max RPM​	Connecting Rod Type​	Quench Range​
6,000 or less​	Steel​	0.035 - 0.045 in.​
6,000 +​	Steel​	0.038 - 0.043 in.​
Any​	Aluminum​	0.050 - 0.065 in.​

Notes​

• Aluminum rods get longer as they get hotter.
• • Increased quench will prevent the piston from contacting the head.
• Don't run more than 0.060 in. quench trying to lower Compression Ratio.
• • This will slow the combustion process and could cause Detonation.

 

[sierra4000]

Thanks,
so new size 1,1mm with steel rods, with cast pistons , redline 7000rpm is in safe range,


Radek