DavidNJ

There is a trend to machine 10-20cc of the combustion chamber into the top of the piston. Then milling the head accordingly for the correct compression. As I understand it, this eliminates the need for a valve clearance notch.

My questions are: why is done and and when is it appropriate?

I've been told it is use with a small 'peanut' combustion chamber. However, aren't the ET heads and AFR heads getting close?


I don't know why it is used. My guess is it unshrouds the valve by placing in further in the cylinder. Then the combustion chamber would be shaped to control combustion around TDC, which is around a millisecond after ignition at say 6000 rpm.

Thanks,

David

airflowdevelop

I'm not sure what you are even asking....But am concerned if you asked the appropriate parties if you can use there images openly (especially the third photo).


What do these photo's have to do with increasing the area on the piston?

Is the question more directed to why people are placing quench pads on both the piston and cylinder head, and gaining area to capture the cylinder pressure?

Dennis

DavidNJ

First, this is a non-commercial equivalent of showing someone a picture in a brochure. These are links to the original site, not copies. When a site protects its pictures, it prevents links not originating from its pages. For example, try doing this on wikipedia.

Second, your purpose in announcing you are not familar with reverse domes was? Read some of the articles on the Engine Master's Challenge for examples.

This is a picture of a piston with a reverse dome and its matching cylinder head. Note the matching shapes. In this case the combustion chamber was cut down to 27cc, and the balance about 20cc was in the piston.



People have written all sorts of stuff about quench this and quench that being the reason. Stuff that didn't make a lot of sense to me. Does the mixture really care if the piston/head seam is in the middle of the combustion area rather than at the bottom? In this picture, the intake valves are cleary at the lower edge of the head, supporting the unshrouding argument, IMHO.

This engine builder, who has finished 1,1,4 in the past three years, spend a lot of effort to do this. The spark plug as moved, the valves relocated, and it appears the peanut shape enhanced. He must have had a good reason to do it.

SStrokerAce

iTrader: (2)

David,

Dennis is well aware of what a reverse dome is used for.

So much for you getting any good info out of him now.

How about you do some reading, this is from 1985!

Start here.... http://www.theoldone.com/articles/Th...5FHead%5F1999/

http://www.theoldone.com/articles/Ci...Commentary.jpg

http://www.theoldone.com/articles/Ho...oft_Head_1.jpg
http://www.theoldone.com/articles/Ho...oft_Head_2.jpg
http://www.theoldone.com/articles/Ho...oft_Head_3.jpg

Good reading, it's F1 Qualifing time.

Bret

DavidNJ

i don't know who Denis. There isn't a web site link, signature, or any info attached to his public profile. I make a consious effort to keep my posts informational and non-hostile. Where possible, I use pictures to illustrate a point. With the wide range of expertise on these forums, I believe they are very helpful.

If I unfairly attacked Dennis unfairly, I hope he accepts my apology. Sometimes my posts in the early morning (was that 2am or so) are not as well thought out.

Back on topic, if I read them correctly, those articles deal with swirl, which maintains an even air/fuel mixture and promotes fast, even combustion. It mainly discussed the port. The role of the piston was described as maintaining the swirl on the compression stroke. Piston pictures showed a dome.

Where I am confused is from several inputs. First, the piston seems to have a depression with the shape of the combustion chamber but a flat floor. How does that maintain swirl? When the flow is at its highest, the piston is pretty far from the combustion chamber. Why is matching the combustion chamber an advantage?

Second, although all many of these engines in the contest had custom built pistons, none of the head builders seem to do that with their general market products. With their super thick deck, ET, AFR, Dart, World, and other heads could probably be brought down to 35-40ccs. But none of the manufacturer's are offering a head/piston set. And the piston manufacturer's aren't offering it either.

Third, in a conversation about making a piston like this, where my focus was mainly on cost, the piston manufacturer's representative volunteered that the head I was looking at didn't have enough of a peanut shape to justify it. Ok, what does a peanut shape (say vs. a heart shape) have to do with it.

Fourth, matching the combustion chamber shape would for combustion would seem to of benefit only near TDC at ignition. However, why would that want a flat bottom? Shouldn't the bottom be shaped to provide a uniform distance from the ignition source?

Fifth, the proximity of the intake valve to the bottom of the head seemed to be the limiting factor. None of the discussion, unless I am misreading them, refer to this as a flow improvement technique (unshrouding the valve). Is it an important factor? And if so, why specifically shape the dish around the intake valve to promote unshrouding near TDC at the beginning of the intake stroke?

Related to those articles, their seems to be a big push to measure ports in in volume and maximum flow. However Engine Master's challenge top finishers seem to have relatively small ports (180-220cc) for engines peeking in the mid- to high-6k rpm range. The range of the hydraulic lifter setups that dominate this forum. Why are all the heads here so large? Marketing?

Thanks,

David

SStrokerAce

iTrader: (2)

That's a good idea. He actually knows something compared to some people around here.


There are some pistons out there with a concave floor in the reverse shape of the chamber. Matching the chamber to the piston maximizes the qunech area!

It's because of cost #1 and #2 the product has to be bolt on for the average user so you can sell enough units to make it profitable to sell.

This gets into the concave vs. convex shapes of combustion chambers, but either way maximizing the quench area is going to give you better burn.

Yes it should be, but that is also much harder and more complex to machine, but it has and can be done.

It might unshroud the valve more but you also can easliy screw up the valve job by cutting too much off of the deck surface and get close to the valve seat and that will hurt flow.

Or why are some LS1 heads so small? Marketing?

I assume ( and I don't want to make a *** out of you and me ) that you are talking about LS1 heads here. The problem with LS1 castings is that the effective cross sectional area of the port, or the portion of the port that is being used is actually a lot smaller than the physical cross section. You couldn't run a 180cc LS1 head and make any power because the velocity profile of the port would be so fast in areas that the port would just not work.

Bret

DavidNJ

I'm on my third reading of the 1999 article. It is taking a few minutes to sink in.

On the quote above, is a large part of the cathederal actually not used? If so, would it pay to fill it in? In Ford Cleveland heads, it is common to fill in quite a bit of the intake port.

AFR sells a lot of related parts; adding pistons shouldn't be a big deal. And ET sells heads that require high end Jesel rockers. Adding a piston option shouldn't be outrageous. Some head manufacturers have specific piston designs on file with different piston manufacturers, specifying a model number or even a contact person. World manufactures complete engines. Dart has nearly all the parts (although no pistons).

David

P.S.
From that article I get the impression we are targeting some 20 deg timing numbers and .4 BSFCs. Pretty extreme stuff.

racer7088

David,

That cathedral shape and that notch on top is soley a plce for the injector to reside and nothing more. There's nothing particularly good about that part of the LS1 heads.

Also the LS1 heads flow and use cross section fairly similar to all other heads I have seen. They are not that different really if you throw out that little injector relief.

We have had LS1 heads go 360+ cfm at 250 cc range and the port lengths and such are all in pretty normal territory. You probably know we did go 150+ mph at almost 3300 pounds so these 345-365 cfm heads do work and made over 750 RWHP on motor.

If you flow an LS1 head with a normal orifice though the air will be able to take an easier line of sight dive into the short turn and bowl area that's not normally available to it with a manifold on.

You are right though that it would be very advantageous to have a shelf number matched piston for certain apps but people love to be different even if their ideas are much worse. Although reverse domes can be very lopsided in weight and just plain heavy if you don't have a dedicated forging.

MadBill

The heavy part concerns me. Maybe that "hot tub" bowl is good for squish and creates a compact chamber, but if I was winding up an SBC or LSx engine, say to over 8,000, I think I'd want flat tops at ~ 150 or so grams lighter.

DavidNJ

I'm very new (say 5 days) to the reverse dome concepts. How heavy are they? When I look in the catalogs, 10-20cc dishes seem to be about the same weight as flat tops. Within 10gms. After the lightening milling, shouldn't it all even out?

Overall, while I read a few articles that talk about a tight quench area and confining the mixture a small uniform area at TDC. However, if the floor of the dish in the piston is flat, how is that different from having a flat top? The small remaining seam will be a the bottom instead of the middle. Otherwise, the chamber should have the same 3 dimensional shape.

Still confused.

Thanks,

David

P.S.
Erik, are many flow bench tests of LS1 style heads inaccurate for the reasons you stated?

DavidNJ

Well. I received an answer on another forum that moving the quench area to the middle of the chamber provides a beneficial effect from the force of the squish flow.

Are there any test experiences available? Assuming there are about 10cc/.100" dish depth, what is the effect a 10cc dish vs a 25cc dish? Wouldn't the first have a minimal effect on piston weight?

In the picture above, it appear that the deepest part of the dish near the exhaust valve is approximattely double the depth of the shallowest part near the intake valve. And the walls have a slight (.005"?) taper. Is that common practice?

Does this require the spark plug to point to the exhaust valve? It does appear that the ET heads (pictured at the top), the AFR heads, point the plug toward the exhaust valve. However the LS1 heads seem to point toward the intake.

Thanks,

David

airflowdevelop

Hey Dave,
Everything is fine...I have pretty thick skin! And you are correct concerning the legality...But it is always A nice idea to ask for permission before directly posting a picture (IMO). Larry may not even care, But I have personally found pictures of my own heads and other peoples heads show up in posts and websites of some of these so called "SpeedShops"...with heading like "look what I did!"

Looks like Bret has you heading in the right direction! Keep in mind, a large part of what you will read and here on this subject is more of a theory than a law. Take everything together, then prove and digest. Also, as you may have found out by now, Kaase is part human...and mostly alien. He is capable of disecting and improving areas that normal humans have a tough time even comprehending, truly a guy above the rest.

Bret,
This is absolutely correct! Plus, the cathederal port heads are pretty hard to keep stuck at high velocity without major modification...but as tall as the port is, and how shallow the valve angle, being a little slower never really seems to bother them.

Dennis

Oh, and I updated my profile.

Dart331Stroker

Companies like Diamond Pistons for example have a piston "map" which is basically a cnc traced copy of the chamber this will allow them to make a piston to fit the Dart chamber exactly, taking into consideration the head gasket thickness, piston to deck, and valve to deck numbers, can give someone an exact piston to exact compression ratio desired.

Spark plug location is something alot of people dont realize, but we at Dart spend alot of time wet flow testing our head designs, specifically to how the chamber and plug placement relates to the port shape, and how fuel enters and exits the chambers.

Wet flow testing allows us to test the same way a head will work on an engine. ET heads in comparison have many different valve angles and even a canted valve head, so when comparing the plug locations it is important to know what ones are being compared against which head.

thanks
john

briannutter

Theoldone is a guy by the name of Larry Widmer. Very famous or infamous depending on who you talk to, but he's done a lot of research on the matter and is a really nice guy that's presently messing around with hondas.

I'll throw out a couple things for you. A cup pistons is spherical dish or reverse dome (just slightly) even when they "Could" build it as a flat top. I wouldn't worry about it too much though because a NHRA Pro Stock piston still has a small dome on top even when they "could" get it down to a flat top. Combustion is a compromise with head flow.

Our reverse dome pistons weight 20 to 30 grams more than a reverse dome when built for the same "horsepower". "Big" Reverse domes often weigh MUCH more than flat tops though because they're usually built for Turbo/Nitrous at the same time. It's pretty rare to have someone call for a "light duty" reverse dome as in Milling their heads to an extra low value. A disk of 2618 aluminum that's 4" in diameter and .060" thick weighs 34 grams, so imagine that a flat top needs a certain "minimum thickness" in the crown and the ".060" deep reverse dome" has to be built above that.

DavidNJ

From what I could tell, a .060 reverse dome (6-8cc?) would probably not be useful. However, what about a .100, .150, or .200 reverse dome with 10cc-20cc?

That leaves space for considerable shaping of the edges of the chamber bottom. And may allow moving more of the chamber over the exhaust valve/spark plug.

racer7088

David remember that Nextel and prostock are higher compression so you won't see that much dish ever.

The Kaase EMC dish is more like what you are talking about and could be done with the lower compression limit and large engine size of that years EMC and still make sense.