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

[OlderThanMe]

I have been reading for quite a while about L6 heads and then about how more modern engines fight detonation.

 

I believe that with slight work to L6 heads (other than opening and closing the chambers to the desired CC's), that detonation can be quite subdued.

 

So many engines that I have seen in life and on the web just use a flat-top piston and create the entre combustion chamber inside the cylinder head.

I believe that the ideal chamber would be MUCH different by far from what is currently being used and would be asking much more in the shape of a piston crown than a flat-top that is just asking for trouble.

 

From my readings:

Theoretically the ideal chamber shape would be spherereical, with purely mixed air and fuel mixture that would be swiriling and evenly spread, and ignition could come from the center of the sphere.

 

This post/discussion is mainly for combustion chamber events and not for anything in the runners as of yet.

It is partially a learning expericnce that I have setup for myself as I am going for a ME degree...

So for this imaginary engine build let's just build a maximum effort 3.0 liter turbo L6 with an L28 crank and linered bores at 90mm.

It's be nice if some of the L6 builders out there that know a lot more than me about the L series motors would put in some input into my thoughts.

 

Block:

L28 crankshaft

L24 rods

pistons:90mm bore, 35.4mm pin height, 24cc dish

 

Head:

MN47 (or whatever head that floats your boat if you like to weld) head closed down to 35CC's but with lots of modifications/opening up of the chamber.

1.25mm head gasket

 

Pics(they are mine and are free to be modified/photoshopped over by others for ideas on chamber design)

 

 

Here are my thoughts (I'm throwing out "the book" on detonation somewhat):

 

If the combustion chamber(consisting of cylinder head, head gasket, and piston shape) is as closely mirrored from the head onto the piston as possible, then there should be detonation in less places than on a flat piston to closed chamber head scenario.

 

If a high compression engine was built with the afore mentioned and pictured MN-47, I would fully believe that detonation would occur on the opposite side of the cylinder head from the spark plug, where there is a gentle slope up from the squish pad to the valve seats. If you have very close piston to head clearance for good squish, then there will be a "ring" around the squish pad that will be a "detonation prone area".

 

This is my belief of why an all-or-none squish area should be used. If the squish area has a very sharp 1.5mm+ "shelf" by the squish area and moving toward the top of the cylinder head then the "low squish" area would be less fertile for creating detonation.

 

After looking at manufacturer's own high compression racing cylinder head pictures for their cars (mainly drwings of the VRH35Z in the R90CP) and actual OEM cylinder heads that were at a quite high SCR that utilize this "shelf" as what seems to be an anti-detonation implementation. I was highly influenced that the "archaic" L-gata cylinder heads could be revolutionized with modern prevailing combustion chamber designs that take cues from these high end engines.

 

 

If you look at an L28ET engine, the P90 cylinder head combustion chamber shape is quite a beautiful design. Then when you take a look at the piston crown, the realization is made that much of the squish(or quench) area on the cylinder head is just wasted and is "hanging in the breeze" not producing any(or very little) squish at all.

.

I have seen MANY posts of people with low CR L28ET engines running moderate boost experiencing detonation and melting all the pistons at once. Why is it doing this if compression ratio is the major factor in creating detonation?

 

I don't believe that a compression ratio all on it's own is what dictates when an engine experiences detonation. It is poor squish that creates detonation. I believe that higher compression ratios could be employed to make better use of boost if more anti-detonation implementations could be used.

 

 

I found this document put out by NASA which is a great read.

 

These guys know their junk...and funded quite well by the war department in 1947.

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930091978_1993091978.pdf

 

The conclusions at the bottom of the above document:

 

CONCLUSIONS

 

The following

conclusions appear justified by the data

presented:

1. The knocking reaction has been obtained indeprndcudy

of either observed pin-point autoignition or homogenous

autoignition.

2. Pin-point autoignition may occur independently of the

knocking reaction.

3. The knocking reaction may occur at various stages in

the development of homogeneous autoignition,

4. High inlet-air temperature and preasurc are conducive

to the occurrence of both pin-point autoignition and homogeneous

autoignition.

5. The occurrence of knock, pin-point autoignition, or

homogeneous autoignition under given engine conditions

varies greatly with fuel type.

 

Since our beloved L6 engines aren't producing very high static compression ratios and are run with quite thick air, detonation should be less prevalent than it is.

 

 

So in order these thigs are most important for building serious power in an turbo L-series engine

 

1: Airflow. The ports pretty much stink when compared to modern engines. Even when ported they take sharp bends. There are some things I have heard of things done to L6 heads that would be almost considered disturbing but were quite helpful but haven't seen done recently.

 

2: Detonation. This is the real killer. Turns a grand of pistons into scrap.

 

3: Lack of displacement. Plenty of things can be done here to "up the ante".

L6 displacements range from 2 liters all the way up to 3.4 liters but some displacements depend on your pocket book a little more than others. 3.1-3.2 liters is the "reasonable" limit for most of us. That extra .3 liters to 3.4 will probably cost 4 grand or more over a 3.1 liter.

 

Any way I've been typing and deleting for a long time now. I'd like to hear the input of people more knowledgable than me of the L series engines. (I'm a sophomore in the school of the L6... there are many PhD's out there...)

 

[1 fast z]

Ideal NA motor, for our heads is a DIRECT flat top, no dish, for perfect burn travel, with .022" piston to head clearence with ofcourse a P or M series head.

 

For Turbo motors, you NEED to ONLY have the dish as a MIRROR of the combustion chambers. I build the motors, bore, etc. then I stick EACH piston up to a P series head that has prussion blue on it, and bump each piston to the head while assembled with NO head gasket. This then gives the exact outline of the chamber, and I then cut INSIDE that area, to make sure there are no trapped areas (such as a stock dish piston on a P90 head). I ordered my last set of pistons to be 9/16" thick domes, with a direct flat top so I can machine the correct dish to get my 8.25-1 compresson ratio, and reveresed imaged the chamber on EACH piston independently and cut accordingly. I then go on the inside and lighten up the uncut areas to .200" thick. NO chamber is the same, this is a casting, thats why each piston needs to be fitted to each chamber.

[JMortensen]

This subject has been beaten to death. You're taking a slightly new track on it, but if it goes down the same old road, I'll close it out.

 

Now to approach the issues you bring up:

 

Your first sentence says modern engines "fight squish". They actually use it to fight detonation. Not sure if that was a typo or what.

 

Yes, the sharp lip around the quench pad is prone to detonation. Sand the sharp lip to make it a bit duller, and then it is no longer as detonation prone. People do the same to the valve heads and the piston as well, and it does make a difference.

 

Look at the best 2 valve high performance engines out there like the LS engines or some of the CAD designed Harley engines, and none of them (that I've seen) have spherical combustion chambers. I think the reason why is because you can't have a chamber that large (assuming 2 valves) and then dish the piston to match and still maintain a reasonable compression ratio. Take a look at pics of high compression L series race heads and they usually have the closed chamber like the P heads or the MN head, and then the top of the piston is machined to fit up inside that tiny remaining space to get even more compression. A sphere just won't work on our application, and I think it would be hard to achieve on any application where the valves were large enough for it to be a "performance engine".

 

I'm also not convinced that the sphere would be the "perfect" shape. Seen any newer diesel pistons? They basically use a flat head and put the chamber in the piston, and they've been coming up with some WEIRD shapes to take full advantage of the combustion. I watched a show on the BMW diesel factory the other night and they were showing the piston shape, seen similar shapes in magazines and such. While diesel isn't directly analogous to gas, I think the idea of capturing and using the explosion of the fuel to the drive the piston is sufficiently similar that we might draw the conclusion that spherical is not perfect. I'm open to other ideas here, I just think it would have been done by now if it was.

 

I agree with you on the L28ET part, seems like a flat top would be in a much better position to take advantage of the chamber shape. If you still needed to reduce compression it would be better to use the "1/2 dish" pistons like the V8's do. Essentially its a piston with a dish in a shape that corresponds to the chamber shape. Again, sanding the corners down is important, and when the pistons come out of the box they're sharp right at that corner.

 

Strangely though you will occasionally see some very high hp turbo cars that cut out the quench pads (RB's in particular I've seen doing this). I don't know the purpose, but I would guess that at some point lessening the compression ratio by opening up the chamber has more of an effect in reducing detonation than using the quench to its full effect.

[Daeron]

Could we Little Folk please get some pictures of pistons that have been machined accordingly? I, for one, have never (knowingly?) seen a photo of a piston that had been machined to match the combustion chamber shape of an L series head.

 

I think what OTM is reaching for is further visualization of the hard parts, to latch on to all the theoretical knowledge in his head. At least, that was what I thought as I was reading his post.. "This is pretty much the question I am asking myself in my progress in understanding all this. I wonder if anyone has any pictures of pistons cut like that?" Photos of the pistons assembled into a shortblock, with one or more at TDC, would be ESPECIALLY helpful.

[Guest TeamNissan]

There are some pics of said pistons in How to mod your datsun/nissan ohc engine. If no one can find some online I'll scan.

[HS30-H]

Could we Little Folk please get some pictures of pistons that have been machined accordingly? I, for one, have never (knowingly?) seen a photo of a piston that had been machined to match the combustion chamber shape of an L series head.

 

Photos of the pistons assembled into a shortblock, with one or more at TDC, would be ESPECIALLY helpful.

 

 

Here's a very mild ( 10.5:1 compression ) example.

 

1971 Nissan 'works' rally car engine strip. Semi-domed piston crown with machined valve cut-outs. Not quite at TDC, but at least gives an impression of the shape.....

[76280z]

I'm currently building up a mn-47 head. The head info on the forums here has been invaluable, however I think the mn-47 is seriously underrated. I figured I'd try an mn47 because of favorable forum reviews. However, here at the shop I work at, we ordered some GM performance "fast burn" cylinder heads for a customers car. The gm fast burn heads are "high quench" heads, after seeing them I went to the junkyards I got a couple more mn47's, just as extras (since I know they are prone to cracking). Here's a link to a pic of the GM performance fast burn, high quench heads.

 

http://www.superchevy.com/tech/0310sc_increasing_horsepower/photo_03.html

 

The web site I got this pic from also said this: The name "Fast Burn" refers to its combustion chamber's ability to quickly and completely burn the air/fuel mixture. The chamber shape is designed to work best with flattop pistons and GM does not recommend that you try to re-shape the chamber for more power.

[OlderThanMe]

Ideal NA motor, for our heads is a DIRECT flat top, no dish, for perfect burn travel, with .022" piston to head clearence with ofcourse a P or M series head.

 

For Turbo motors, you NEED to ONLY have the dish as a MIRROR of the combustion chambers. I build the motors, bore, etc. then I stick EACH piston up to a P series head that has prussion blue on it, and bump each piston to the head while assembled with NO head gasket. This then gives the exact outline of the chamber, and I then cut INSIDE that area, to make sure there are no trapped areas (such as a stock dish piston on a P90 head). I ordered my last set of pistons to be 9/16" thick domes, with a direct flat top so I can machine the correct dish to get my 8.25-1 compresson ratio, and reveresed imaged the chamber on EACH piston independently and cut accordingly. I then go on the inside and lighten up the uncut areas to .200" thick. NO chamber is the same, this is a casting, thats why each piston needs to be fitted to each chamber.

 

I would have to disagree with you on the idea that a flat-top is the ideal chamber for an engine (+ or - for turbo compression and valves).

 

So far I have only talked about DETONATION resistance.

I have other ideas on how a piston can enhance other aspects of an engine's operation. A flat-top piston does work great since it has good detonation resistance, and is cheap, but not ideal for a max effort engine.

I think a piston with a "valve pad" that comes very close to the intake valve can create a great vacuum behind the valve and "suck in" slightly more air. Same goes with an exhaust valve but in reverse. Spent gasses are forced away from right behind the valve and into a more favorable exit path.

 

I'm not knocking your work, as your engines have made good power and work. Thing is that engineering doesn't go by the tried and true old ways of engine building when other advancements in engines are taking place but our old heads, for the most part, still look the same.

 

This subject has been beaten to death. You're taking a slightly new track on it, but if it goes down the same old road, I'll close it out.

 

Now to approach the issues you bring up:

 

Your first sentence says modern engines "fight squish". They actually use it to fight detonation. Not sure if that was a typo or what.

 

Yes, the sharp lip around the quench pad is prone to detonation. Sand the sharp lip to make it a bit duller, and then it is no longer as detonation prone. People do the same to the valve heads and the piston as well, and it does make a difference.

 

Look at the best 2 valve high performance engines out there like the LS engines or some of the CAD designed Harley engines, and none of them (that I've seen) have spherical combustion chambers. I think the reason why is because you can't have a chamber that large (assuming 2 valves) and then dish the piston to match and still maintain a reasonable compression ratio. Take a look at pics of high compression L series race heads and they usually have the closed chamber like the P heads or the MN head, and then the top of the piston is machined to fit up inside that tiny remaining space to get even more compression. A sphere just won't work on our application, and I think it would be hard to achieve on any application where the valves were large enough for it to be a "performance engine".

 

I'm also not convinced that the sphere would be the "perfect" shape. Seen any newer diesel pistons? They basically use a flat head and put the chamber in the piston, and they've been coming up with some WEIRD shapes to take full advantage of the combustion. I watched a show on the BMW diesel factory the other night and they were showing the piston shape, seen similar shapes in magazines and such. While diesel isn't directly analogous to gas, I think the idea of capturing and using the explosion of the fuel to the drive the piston is sufficiently similar that we might draw the conclusion that spherical is not perfect. I'm open to other ideas here, I just think it would have been done by now if it was.

 

I agree with you on the L28ET part, seems like a flat top would be in a much better position to take advantage of the chamber shape. If you still needed to reduce compression it would be better to use the "1/2 dish" pistons like the V8's do. Essentially its a piston with a dish in a shape that corresponds to the chamber shape. Again, sanding the corners down is important, and when the pistons come out of the box they're sharp right at that corner.

 

Strangely though you will occasionally see some very high hp turbo cars that cut out the quench pads (RB's in particular I've seen doing this). I don't know the purpose, but I would guess that at some point lessening the compression ratio by opening up the chamber has more of an effect in reducing detonation than using the quench to its full effect.

 

Yes that was a typo and I fixed it. Thanks.

 

About the sharp lip, I tend not to believe that the sharp lip itself causes detonation. I think it is the place where the piston to head clearance is .035"-.045" which is most often right next to the sharp edge.

The sharp lip does provide a place for super high heat and a place where the detonation shockwave could effect.

 

Looking at my 1993 VH45 cylinder head(for a 10.2:1 SCR), the squish pads have a .055" little "shelf" that goes vertically from the squish pad before it starts to blend into the combustion chamber. Since the VH45 was Nissan's baby I believe that this was not for no reason at all. This seems to be part of the "all-or-none" squish idea that I am advocating.

 

Obviously we cant have a perfectly sphereical chamber. The sphereical chamber part seems to me to be ideal for the combustion process, not really for anti-detonation in sense. Swirl needs to be induced for more even combustion which can be done by other methods.

 

 

Could we Little Folk please get some pictures of pistons that have been machined accordingly? I, for one, have never (knowingly?) seen a photo of a piston that had been machined to match the combustion chamber shape of an L series head.

 

I think what OTM is reaching for is further visualization of the hard parts, to latch on to all the theoretical knowledge in his head. At least, that was what I thought as I was reading his post.. "This is pretty much the question I am asking myself in my progress in understanding all this. I wonder if anyone has any pictures of pistons cut like that?" Photos of the pistons assembled into a shortblock, with one or more at TDC, would be ESPECIALLY helpful.

 

This is a Honda, and it does produce 300hp.

The roof-top pistons are matched to the pent-roof chamber that the Honda engines have.

 

For our closed wedge chamber heads I could see something like this but with one bigger wedge rather than two opposidte ones...

 

 

 

There are some pics of said pistons in How to mod your datsun/nissan ohc engine. If no one can find some online I'll scan.

 

Here's a very mild ( 10.5:1 compression ) example.

 

1971 Nissan 'works' rally car engine strip. Semi-domed piston crown with machined valve cut-outs. Not quite at TDC, but at least gives an impression of the shape.....

 

That is exatly what I was thinking...Thanks for the pic Alan. Would you happen to have a picture of the combustion chamber of the same engine?

 

 

I'm currently building up a mn-47 head. The head info on the forums here has been invaluable, however I think the mn-47 is seriously underrated. I figured I'd try an mn47 because of favorable forum reviews. However, here at the shop I work at, we ordered some GM performance "fast burn" cylinder heads for a customers car. The gm fast burn heads are "high quench" heads, after seeing them I went to the junkyards I got a couple more mn47's, just as extras (since I know they are prone to cracking). Here's a link to a pic of the GM performance fast burn, high quench heads.

 

http://www.superchevy.com/tech/0310sc_increasing_horsepower/photo_03.html

 

The web site I got this pic from also said this: The name "Fast Burn" refers to its combustion chamber's ability to quickly and completely burn the air/fuel mixture. The chamber shape is designed to work best with flattop pistons and GM does not recommend that you try to re-shape the chamber for more power.

 

Remember that the Maxima L24 also had dished pistons, not making use of most of the quench pad.

I believe that with the exhaust liners cut out, the MN-47 can be an incredible head. Even with the liners it would be a great choice for any non-race engine.

 

 

 

 

I believe that if detonation can be dealt with, then progress can be made to raise compression ratios and still be able to use pump gas. As-is, without further engneering of the combustion process in the L series engines, detonation is going to be the weakness of the L6.

 

Also notice that one of the most powerful L6 powered cars on this site, owned by TimZ, uses NO SQUISH at all? Open chamber N42 head.

 

So it seems to me that all parts of a cylinder head should either have full squish or have enough clearance to be out of the detonation zone.

[JMortensen]

About the sharp lip, I tend not to believe that the sharp lip itself causes detonation. I think it is the place where the piston to head clearance is .035"-.045" which is most often right next to the sharp edge.

The place where the head is .035" - .045" is the quench area. That part is important in reducing detonation.

The sharp lip does provide a place for super high heat and a place where the detonation shockwave could effect.

The "super high heat" is the cause of detonation. Sand the sharp corner and it won't get as hot, less heat, less detonation.

 

Looking at my 1993 VH45 cylinder head(for a 10.2:1 SCR), the squish pads have a .055" little "shelf" that goes vertically from the squish pad before it starts to blend into the combustion chamber. Since the VH45 was Nissan's baby I believe that this was not for no reason at all. This seems to be part of the "all-or-none" squish idea that I am advocating.

Pictures would help here. In general I would agree that for an NA engine utilize the quench as much as possible. This is why the E31 is not as good as the P or MN heads.

 

For our closed wedge chamber heads I could see something like this but with one bigger wedge rather than two opposidte ones...

Dig around and see if you can find pictures of John Coffey's Sunbelt head. It is basically as you describe.

 

Also notice that one of the most powerful L6 powered cars on this site, owned by TimZ, uses NO SQUISH at all? Open chamber N42 head.

The chambers and pistons are coated (not sure about the valves). I can't remember what Tim said he did to the chambers, but I'm pretty sure at those power levels they aren't stock. Comparing his head to an off the shelf head is not the best way to get a good result. I think a major difference is as I said before, turbocharging. This is a pure SWAG, but there is something about the turbo that seems to make the quench less effective. I'm guessing that the fill of the chamber is so much more dense that the quench doesn't have as much effect.

[X64v]

Here's some detonation evidence that we don't see as often. We've all seen pictures of head gasket elongation on dished piston/p90 head engines, but here's bad detonation on a flattop/open chamber head. L24 and late E88 head (8.4:1 SCR). Note the odd and usually uneven distortion.

 

Cylinder 1

 

Cylinder 2

 

Cylinder 5

 

Cylinder 6

 

The chamber shape.

 

My point is that the distortion is happening in a very different way from the heads with quench pads.

[MONZTER]

Maybe some have you have read this http://www.theoldone.com/articles/The_Soft_Head_1999/

 

Good overview

[OlderThanMe]

Here is a picture of a VH45 combustion chamber.

 

 

I am getting some better pictures with my own head but this all that I had online.

There is a .055" step from the squish pad (between the valves)to the penthouse part of the chamber.

 

MONZTER,

Yes I have read all of the articles on that site. Great stuff!

[OlderThanMe]

The place where the head is .035" - .045" is the quench area. That part is important in reducing detonation.

Oops my bad... another typo.

 

 

Right outside of the quench area is what I meant where clearances are close but not close enough.

[MONZTER]

OTM,

Have you ever seen the heads I modified?

 

Here is what I currently run a N-42 welded for max quench with 36cc chambers. On a 1mm over 240 lower end with flat tops .015 out the deck with a comp gasket. Runs great on 91 octane all day

 

 

 

Here is a picture of my next head (P-90), not finished yet. I took it to the next level with welding up the chambers, ports, and plug holes. Made a CAD model for the CNC to cut them out all the same. High quench, lots of swirl. I will be doing as 1 fast Z states, in the fact the piston will mirror the head. I will be using this on my 280 turbo project.

[TimZ]

OTM,

Have you ever seen the heads I modified?

Damn that's nice work, Jeff. Love the modified plug angles.

 

Regarding my N42, yes the head has had some work done to it, but mostly to the ports. The chamber has been smoothed a bit and ceramic coated, but not that much has been done to it. I'll get a pic of it and post in the next couple of days. I think that a great deal of the detonation resistance that I have is due to the fairly deep dish in the pistons, and the 7.5:1 CR that results from it...

 

For whatever reason, I do make good power with this head, and I've repeatedly been able to run 26psi on 94 sunoco pump gas. I feared that maybe I just wasn't hearing it, but at the last teardown there was absolutely no evidence of detonation.

[Guest TeamNissan]

That gorgeous work Jeff, head work has always been a art to me.

 

A question though, what are your thoughts behind the offset and angled plug holes? Doesn't a centered, strait plug provide better combustion?

[MONZTER]

That gorgeous work Jeff, head work has always been a art to me.

 

A question though, what are your thoughts behind the offset and angled plug holes? Doesn't a centered, strait plug provide better combustion?

 

Yes, the angled plugs are all part of the master plan. First a few more details. Like I said above the head of the pistons will be a mirror image of the head. What I did not say was that the shape of the piston under the intake valve will be domed, and the shape under the exhaust valve will be dished. The idea is to use the domed portion under the intake to make some squish which will force the charge to the exhaust side, right where it needs to be to flow out. The angled plug will now be closer the true center of the mixture, just like you said. You just have to imagine the chambers are no longer traditionally shaped, they will now be offset towards the exhaust valve. So, a few things that will make this work. First all of the chambers are designed in 3-d on Pro-Engineer, so it will be easy to design the piston tops to be exactly a match to what I am trying to do. Second, since the chambers are all CNC machined at one time, they will be perfect in shape, size, and volume to perfectly match the CNC’d piston heads. I am hoping the small tight chambers from the above idea will make for a very fast burning set-up, which means less detonation, and less timing. All good things.

 

Check out the link I posted above, It goes into much more detail

 

BTW The head above (P90) is just roughed out. Once the head is surfaced, and all of the valve guides and seat installed, it will be put back in the CNC machine and cut to its final depth with a much finer step-over

[TimZ]

Well, it's not as sexy as Monzter's but here are a couple of pics of my combustion chambers...

 

....no squish in the head, but a pretty good match to the pistons above.

[JMortensen]

That's a pretty well cleaned up chamber Tim. A big improvement over stock I'd say.

[HENRY MACIAS]

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!!!!!!!!!!!!!!!!!!!!!!!!!!!!!.