20* Pistons

[WhipLash96]

What are the advantages and disadvantages of using this type of piston? I know that it will require a completely different chambered head.

I just have never considered this before.

[wfolarry]

If you need high compression [12:1 & up] the dome shape is better than if you tried to get the same compression with a pop up.

[WhipLash96]

If you need high compression [12:1 & up] the dome shape is better than if you tried to get the same compression with a pop up.

Is it better burn to alleviate detonation? Head chambers would have to be redesigned?

[kd]

It creates a full 360 degree quench ring which is much more than any OEM TC head. More quench usually relates to better fire turbulence and less detonation. In fact it can be said it turns the TC into a hemi head.  As an example, both Ron Dickey at Atell and Dan Baisley are big on it.

[WhipLash96]

So Larry mentioned 12:1 and up compression.  Is doing this even worth the expense for a goal of around 10:1 corrected on a small 107? Static is 11.25:1 using a 6cc pop up and an 88cc chamber.

[aswracing]

What are the advantages and disadvantages of using this type of piston? I know that it will require a completely different chambered head.

I just have never considered this before.

When you have a dome protruding into a bathtub chamber with horizontal squish bands, the fuel charge that gets squeezed out of the squish bands runs into the dome, which forces it to turn a corner. That causes some degree of separation between the air and the fuel, which in turn aggravates detonation and reduces the efficiency of the burn.

Angled domes eliminate this issue. The fuel charge comes out of the squish bands has a direct shot at the flame front.

Angled squish bands also serve to unshroud the valves, reducing the shelf that the air has to otherwise go around as it enters and exits the cylinder. So it has advantages for breathing as well. This advantage becomes increasingly significant as the valve sizes are increased. It's also the reason that 30 degree domes are so popular in race motors. The 30 degree dome angle approximates the 31 & 27 degree stock valve angles.

The disadvantage to angling the squish band is you lose squish surface area unless the dome is made taller to match. Taller domes result in higher compression. This is what Larry was getting at. For this reason, angled dome pistons and cylinders have largely been the domain of race motors. Shallower dome angles, like the 20 degrees you mentioned, are often used in hot street applications.

However, some angled dome pistons are designed with a dish in the middle of them, so that a tall dome can be used while still maintaining a pump-gas friendly compression ratio. Dishing an angled dome piston does however increase the piston surface area that's exposed to the burn, thus allowing the piston to absorb more heat. This can be largely mitigated via thermal barrier coatings however.

And yet another disadvantage is that when applied to a forging, without any internal machining of the inside of the dome, an angled dome piston tends to be heavier than a conventional rectangular dome piston of equal dome volume. Some piston vendors offer a service ("maximum internal lightening") where they'll machine the inside of the dome, for an extra cost of course.

So it's got pro's and cons, like most things. One of the factors in deciding is how bad the original chamber is to begin with. Certain chambers reap more of the benefits of an angled squish band, for example the Evo Big Twin. Another consideration is how far you're pushing the valve sizes.

[kd]

That's a great explanation ASW.  It's probably the real reason I decided to stay with the flat tops and spend more on the heads & valve work with Mr Baisley.  I was building a hot rod 120 with a 150 square target that I use for touring too.  I am often a little immature in my riding style    and having to easily source parts while on the road made it a risk management decision.

[Don D]

Is doing this even worth the expense for a goal of around 10:1 corrected on a small 107? Static is 11.25:1 using a 6cc pop up and an 88cc chamber.

Not in my opinion

[WhipLash96]

Is doing this even worth the expense for a goal of around 10:1 corrected on a small 107? Static is 11.25:1 using a 6cc pop up and an 88cc chamber.

Not in my opinion

I didn't think so either. I was given that option but didn't know what the advantages or disadvantages would be. 

[WhipLash96]

So the question has to be asked. So would it be a bad start to use a set of 110 heads that use fairly big valves and have them re-done to use a 20° piston? Think in terms of a 124 using either a R&R 615 or Cycle Rama 630i??

[Don D]

The shrouded dome goes away with an open chamber such as the CVO head. This is a good starting point for the 124", not so much the 107. A flat top CVO headed 124 comes in at 10.7:1 with head milling in the safe range. A small dome is better yet. The 124" build I prefer uses Mahle pistons with a 6.6cc dome (can't be milled) or a flat top compression adjusted for cam. I have also used KB and CP pistons. A bathtub headed 124 is natively in the 11:1 plus range so no dome there.

[Deye76]

I had Axtell do the 20° on a set of HTCC CNC heads, 4.060" bore/113", came out at 11.0:1, ran very well. Those heads were a different animal. Check the dyno section, some good 124's running worked 110 heads without a 20° squish band. Good tune, no worries about detonation.

[turboprop]

For the sake of the discussion, I will mention that the TC 124 in my red fxr has 12:1 compression (cranks at 215 Lbs.) and is based on a set of 20 degree pistons with a reverse dome and 2.17" intakes. No detonation, ever. This is in a light bike. Would probably have different characteristics in a heavy barge. This is probably the most reliable and maintenance free high-compression big block I have ever owned. I cannot imagine not using angle top pistons with matching heads ever again.

[WhipLash96]

The shrouded dome goes away with an open chamber such as the CVO head. This is a good starting point for the 124", not so much the 107. A flat top CVO headed 124 comes in at 10.7:1 with head milling in the safe range. A small dome is better yet. The 124" build I prefer uses Mahle pistons with a 6.6cc dome (can't be milled) or a flat top compression adjusted for cam. I have also used KB and CP pistons. A bathtub headed 124 is natively in the 11:1 plus range so no dome there.

Yeah I probably won't do a 20* in the 110 heads that I have but I am interested in getting them re-done to get them to flow the same and to knock down the chambers a bit and a valve job as they weren't touched at all when I had them done for the 124. Or, I will have the set of untouched 06 and up heads that I have sittig and give them a good treatment.

[Don D]

I prefer the S&S B2 heads and matching pistons or customs which are an angled dome design.
Top tier parts that don't need large mods to get them to work well. These are high budget builds.

[WhipLash96]

I prefer the S&S B2 heads and matching pistons or customs which are an angled dome design.
Top tier parts that don't need large mods to get them to work well. These are high budget builds.

When I built this engine I did consider the B2. I didn't go that route mainly because I didn't want to use an exhaust adapter.   I didn't even bother to investigate that head much.

[WhipLash96]

So after almost a year of putting this project on the shelf, and after re-reading this short thread, it seems to me that a 20° piston would benefit me more.

So, the question is: Are Tmans 20° pistons dished?

[rigidthumper]

Here is the spec sheet for the 20° 124 upgrade. It states 12cc, but you'd have to contact them about chamber size after cutting the 20° shelf.

[turboprop]

20 degree domes have been around for a day or two. Back in the day, little EVO engines with the oem stud pattern were running 20 degree domes with STD heads with a bathtub chamber. The improved efficiency of the chamber/piston combo allowed for higher compression and less advance. For many riders the results were worth it. They required a bit of work to set up and a lot of shops that had boring bars and hones did not have the expertise to do do 20 degree chambers but flat tops were easy.

Anyways. The TC124 in my red bike has dished pistons with a 20 dome that were set up by Zippers. Cranks at 210 lbs. 158hp SAE. No issue with pump gas.