L6 squish discussion... (the battle against detonation)

[JMortensen]

Your neighbor is senile. You don't want anything but the edge of the fire ring exposed to the combustion process. If the gasket is hanging out past the cylinders it will burn up and fail very quickly.

[Daeron]

I have been reading this post and have a few questions??????.

I am in the process of porting my p90. While performing the task, I was

confronted by my 72 year old neighbor who claims to have modified engines in the 1954. He overlooked my p90, and advised me to have a

Custom head gasket made to fully have the shape of the peanut shape

chamber due to the fact that, the original gasket only covers the round

area of the chamber and leaves a gap in the peanut shape area.

Therefore this causes gases to remain in the peanut shape area, and this results in burned and unburned gases in the gap area. According to Him it

also causes detonation, and loss of power due to the shock waves of burned and unburned gases in the gap area as used with the original gasket. PLEASE ADVISE ON THIS ONE I AM LOST!!!!!!!!!!!!!!!!!!!!!!!!!!!!!.

I think your neighbor is failing to realize that our pistons are built to all but impact the cylinder head in that section where the combustion chamber is flat and level with the deck of the head. the PISTON keeps gases out of that area (effectively enough) and prevents the "badness" he is seeing.

 

That is based on two IFs: IF I understand what you are saying your neighbor is saying;

 

and IF I am not HORRIBLY misconceived on piston to head clearances. I *am* still a n00b

[lammbn]

Ok now, I don't mean to butt in here but I think that some people aren't quite understanding the concept of squish (aka quench). I was reading a thread a few days ago where someone was having the same problem and I think Braap put it nicely and easy to understand. I think it also very nicely explains why that custom peanut shaped head gasket will not work and is not needed. Here is a link to the thread, it goes very nicely into a head's quench/squish and that relationship to detonation, once you get past some back and forth bickering that is. http://forums.hybridz.org/showthread.php?t=105653&highlight=squish&page=2 Again, sorry for butting in, it seemed appropriate in order to further the useful theories being brought up here.

 

Gollum,

Lets see.. You understand that the quench is an important aspect of detonation resistance, but don’t understand why lowering the comp ratio by .5 by only using a thicker head gasket wouldn’t help make the engine less prone to detonation?

I’ll try and explain this in a different way. (It sure would be much easier if I could draw this out. I’m much better at conveying ideas visually. Oh well, here goes any how…)

 

Quench is only achieved within a given piston to head clearance. By adding a thicker gasket you are increasing this piston to head clearance therefore the quench then no longer exists.

 

If you can lower the comp ratio on an engine with a nice high squish/quench head, then yes, the engine will be less sensitive to fuel grade and less prone to detonation, however, it is “HOW” you lower the compression ratio that makes the difference.

I’ll cover 2 examples;

1) If you lower the comp ratio by .5 just by using a taller head gasket, now the piston is further away from the head deck surface and now you have lost your quench/squish as the chamber is now an open chamber and extends over the entire cylinder, not confined to the smaller open region that surrounds the valve in the head, i.e. the combustion chamber. This would be similar to the open N-42 and Z N-47 heads.

2) If you manufacture a piston with a dish large enough to drop the compression ratio by .5 and this dish is directly under the open portion of the chamber itself, then you still retain this quench area and when the piston comes up to TDC, there is essentially NO space between the piston and the head deck surface, so the combustion chamber is now confined to only the small region that is in the head around the valves and in that little dish pocket in the piston as well.

In short, if you use a taller gasket, the quench NO longer exists.

 

Not to muddy the waters any more than they already are, by using the OE L-28 dished pistons with a P-79 and P-90 heads, you loose that quench because the OE L-28 dish covers a very large portion of the top of the piston itself. An ideal dished piston for those heads would be one with the dish under ONLY the open portion of the chamber itself.

 

Any how, hope this made sense…

 

I got a little board and decided to also illustrate this and to bring up a point I think is important here, flame speed. Flame speed is one of the main things that makes quench/squish work. Let’s just assume for the sake of the discussion that both the quench and no quench images have the same effective compression ratio and volume. The first image shows a head with no quench, the second with the quench effect.

As you can see the charge also fills the portion underneath the flat part of the head. What this does is make the flame front have to travel from one side of the whole cylinder bore to the other to consume the fuel/air charge.

Now in this picture you can see that the flat top of the piston closes off the flat area of the head (the piston isn’t actually touching the head obviously, but it is close enough that it might as well be because it forces almost all the charge to be pushed into the dome of the chamber.) Now with this design the flame only has to travel from one side of the dome to the other, a much shorter distance. This helps detonation because the charge in both designs (remember same volume and CR) burns at the same speed (in meters per second.) So if it has to travel a shorter distance it will reach it much sooner than if it had to go the longer distance. What this does is create an ideal situation for the combustion to complete sooner than later which allows us to run an ideal amount of advance at higher RPMs and get complete combustion and maximum pressure at the ideal time (~15* BTDC), which gives us better power than running less advance and only getting partial combustion and pressure at that point. This also helps because at higher RPMs we have less time to complete the combustion for the same number of degrees of crankshaft rotation as lower RPMs, which is why we hear more ping at higher RPMs.

 

I think I got all in there and at least mostly right. I’m feeling a little loopy right now so I may have it way backwards, so correct me if I have anything wrong.

[Daeron]

GREAT drawings, and thanks for the plain word confirmation that the piston comes asymptotically close to the cylinder head on that "squish pad" (dunno if I am using that term right, but at least you know which spot I mean)

 

If you'll forgive me, I took that "quench" jpg and cut out the combustion chamber, the first image is a straight horizontal inversion of it, and the second is both horizontally AND vertically inverted:

 

inverted X axis:

 

inverted X and Y axis:

 

now, the plain and simple version of it ALL is....

 

On a closed-chamber, "Quenching" or "Squishing" head, you would want a Flat top piston with a combustion chamber carved into it corresponding most closely to the second picture; that is, flipped on two axes.

 

 

Right?

[76280z]

I guess everyone just ignored my first post on this thread. It doesn't really matter what kind of piston we 'think would work well'. Engineers have said the flat top pistons are supposed to be used with the peanut shaped combustion chamber head. Given, the head I used as an example was a fast burn SBC head, but come'on, they've only had 50 years of R&D with that engine. And the chamber shape is identical to a MN or a P series head.

[lammbn]

I think you missed the point of carving out an identical shape of the chamber onto the piston. its to lower the compression to a more managable and sane CR range and still keep the high quenching affect of using flat top pistons with these heads, not to magically make it more powerful or efficent. with the high quench effect it is a so called "fast burn head" because the flame still has a shorter distance to travel than all the way across the cylinder from one side to the other..

[OlderThanMe]

I guess everyone just ignored my first post on this thread. It doesn't really matter what kind of piston we 'think would work well'. Engineers have said the flat top pistons are supposed to be used with the peanut shaped combustion chamber head. Given, the head I used as an example was a fast burn SBC head, but come'on, they've only had 50 years of R&D with that engine. And the chamber shape is identical to a MN or a P series head.

 

I don't think compari ng an SBC to an L6 is that great of a comparison but...

 

There is a reason why our L6 engines are so detonation prone. I am trying to figure out what the reason for the detonation is. My theory is that the narrow band of .050"+ piston to head clearance is the cause of detonation.

The one thing about the sharp edges is that Nissan and other OEM's had shar edges in their high comression(10:1 +) engines. They had radiuses elsewhere but not on the quench to non-quench areas.

 

10.2:1 VH45 qench pad lip measures .055" from quench pad to the pent-roof combustion chamber:

 

I have seen this lip on other modern import cylinder heads laying around at the junkyard...

 

*edit*

With a piston that has a smaller combustion chamber in both the head and the piston, the flame travels less and travels the same amount of time before reaching the different areas of the chamber to push the piston down. This is why I said that I believe a sphere is the best for COMBUSTION. Flame makes impact on all combustion chamber surfaces at about the same time. With that said we can't have sphere for a combustion chamber. We have a peanut(in the L series) or a wedge (or something in-between like the N42).

What I am thinking is having a piston that is domed on the intake side (possibly creating more vacuum directly under the intake valve when it opens...increasing VE) and have a big dish on the exhaust side for a more spherical combustion chamber.

 

Basically what Daeron's picture but also squished down to just on the exhust side.

[TimZ]

Basically what Daeron's picture but also squished down to just on the exhust side.

 

Squished - get it? heh, heh.

[OlderThanMe]

Squished - get it? heh, heh.

 

For some reason I like squish...Your setup works very well though... LOL

 

I think that I'm trying to figure out how to increase compression on a turbo motor so that boost has more of an effect.

Somewhere I read that every point of compression is worth about 4psi that you don't need to boost to get the same power level. That is just hear-say though and I haven't seen any hard evidence of actual power levels.

[TimZ]

Somewhere I read that every point of compression is worth about 4psi that you don't need to boost to get the same power level. That is just hear-say though and I haven't seen any hard evidence of actual power levels.

 

As I recall it's more like .75 to 1 psi to "make up" the power loss from losing a point. In general, the amount of additional boost you can run with the lower CR more than makes up the difference.

 

EDIT:

Here is the relationship for thermal efficiency for the Otto cycle:

thermal efficiency = 1 - 1/(CR^k-1)

A typical value for an air/fuel mixture is k = 1.35.

 

...so for a CR = 8, the thermal efficiency would be ~0.517

... at CR = 9, the thermal efficiency = ~0.537

This is an approximate 4% difference in efficiency

[OlderThanMe]

As I recall it's more like .75 to 1 psi to "make up" the power loss from losing a point. In general, the amount of additional boost you can run with the lower CR more than makes up the difference.

 

EDIT:

Here is the relationship for thermal efficiency for the Otto cycle:

thermal efficiency = 1 - 1/(CR^k-1)

A typical value for an air/fuel mixture is k = 1.35.

 

...so for a CR = 8, the thermal efficiency would be ~0.517

... at CR = 9, the thermal efficiency = ~0.537

This is an approximate 4% difference in efficiency

 

It was just another of those internet myths about the 4PSI per point of CR I guess.

 

If compression ratio can be altered enough that it has a considerable gain, it may be worth it (i.e. 10:1 and turbo in an L series). I see detonation as a main obstacle of running a higher CR and thus increasing the effectiveness of each PSI. When you get into high boost on a maxium effort turbo engine many turbos can't handle the pressure that boost puts on them. (unless you have a diesel turbo like a holset that loves 35psi+ of boost)

There are guys with VH45's making double(and more) the stock power with the stock 10.2:1 CR cast and coated pistons(flat-top) in a turboed setup.

I was just looking at what they have for combustion chamber anti-detonation tools.

Obviously the engines aren't related... but there are things that I believe can be carried over from the millions spent in engineering to fight detonation and produce power.

 

I have seen one L28 with a 4 barrel Holley that was at 11.25:1 CR and run on pump 93 octane and the guy was quite hush about his head work which he admitted was severe. All I have heard on here is that 10:1 is about the max for street use on a N/A Z.

 

I will admit that my L6 has never pinged and it has a stock E88 and the L28 crank rods, and L24 pistons. 9.2:1 and run on 87 octane. I keep hearing of the L24 guys getting pinging on a stock L24 but mine has basically the same setup with L28 stroke and have never had any problems even on watery 87 grade. Why? I don't know. I'm trying to get my head around the whole series of chamber events and it is quite complex...

The flat-top and squish is a tried and true method for cheaply building a high compression motor. I believe that with a re-engineered design with a particular head and a piston that is custom designed for the particular engine could have much greater posibilities.

 

I have a quick question...

What is the angle of the valve centerline to head surface? I'm going to draw up some pistons and a combustion chamber in Solidworks to show my ideas better.

[Guest TeamNissan]

I thought quite a few people were running comp passed 10:1 just the right combo. I may be mistaken. Isn't a flat top l28 with mn47 head a "ok" combo for 91+ octane with arguable timing advance? Thats 11:0+.....

[1 fast z]

For the record I run 12-1 on 91 octane with 36 degrees full timing.

[Guest TeamNissan]

Motor specs?

[1 fast z]

Specs:

 

89mm bore

83mm stroke

HIGHLY ported P90, shaved .110"

DIRECT flat top pistons, with .022" Piston to Head clearence

Megasquirt for fuel only

Had a 270 duration with .470 lift cam, but now a .590" with 286 duration.

Pistons weigh 310 grams

12-1 compression

Custom ITB intake,

blah, blah, blah

[Guest TeamNissan]

What aspect/s do you believe is allowing you to run 12:1? Besides the tunability of ms.

[MrWOT]

I would guess the .022 piston to head clearance, that's a tight squish. Must be very confident in your rods

[OlderThanMe]

I would guess the .022 piston to head clearance, that's a tight squish. Must be very confident in your rods

 

And oil clearances??

 

How high do you rev it?

[1 fast z]

Motor sees 8k often, almost everyday.

 

Here is a vid

 

[MrWOT]

Overview of the build if you would?