4AGE and 7AGE piston information

I don't have time to edit the full article but I did want to mention that over the past several months Arias has worked tirelessly with me to dial in new shelf pistons that are a better design and use much better numbers for compression height, dome volume and everything else. 
While I am not sure if you can find these pistons elsewhere you will find them in our 16v pistons section and they will all be titled new design. 
We can also get 20v pistons made with modified specs and are now working on improving the 20v shelf designs. 




Here we will discuss piston specs and details on stock and aftermarket pistons for the 4AGE and the 7AGE hybrid.


Let's start with the OEM pistons. Across all 5 generations of 4AGE these piston specs are the same.

Bore 81mm

Compression height 30.5mm

As you can see from this picture compression height is the distance from the wristpin to the top of the piston.

Deck clearance is the height between the top of the piston and the top of the block or the deck.

The stock compression height gives the 4AGE a nominal deck clearance of zero.

I do believe that from the factory many 4AGE blocks come on the tall side of nominal meaning the piston still sits slightly below deck.

On a performance build I highly recommend measuring the actual deck clearance on your motor. If the block gets decked it will be important to measure this as well.

It is very important to pay attention to the compression height on aftermarket pistons because most of them are a good bit off.
For example Arias's designs used to be way off on some of their pistons. They used 30.226mm compression height for most of their 16v pistons so they will sit below deck but even more odd is their 8:1 piston used 30.5054 which will sit slightly above deck.

I have been working with Arias to improve their designs and now we have new shelf pistons that are all much more correct and ideal. I hate using the info below now because Arias has done so much to work with me on improving their designs however I already used them as examples and don't have the time to completely redo it. All of the new Arias designs use the stock compression height of 30.5mm. Stock dome volumes have been corrected for each head and we have also refined the design of the 16 valve pistons. 
The new pistons are available in 8.5:1, 9.5:1, 10.5:1 11.5:1 and 12.5:1. You can also adjust squish and combustion chamber volume to move each up or down slightly. 



CP shelf pistons have a compression height of 30.6832 so they sit a good bit above the deck.



Another place that most piston manufacturers mess up is with the dome volume.

For example the dome volume for the 16 valve head tends to be between 35CC and 37CC and usually very close to 36 which is generally considered nominal.
For your average build or for pre build rough calculations this number can be used. For a high performance or optimized build you will want to CC your head because they are all different.
Ideally you would want to match all the chambers. So say your biggest chamber is 36.5 you would want to remove material from the other chambers until they were at the same 36.5.
It is also best to debur and remove all the sharp edges from the combustion chamber then open them all up to the biggest one. Combustion chamber cleanup will wait for another article though.

Another thing to consider is squish gap or thickness.

This is the distance between the piston and the head in the squish/quench (squench) area.

To learn more about squench follow these links.



Ideally you want the squish gap as tight as possible but you have to leave some distance between the piston and head.
When the rod heats up it expands. An aluminum rod will expand more than steel so it will grow more vertically than the steel block and get closer to the head.
Even a steel rod may expand more than the rod if it's average temp is higher than the block.
This means at operating temp the rod will be longer and closer to the head.
Then there is tensile stress that actually stretches the rod slightly.

For these reasons you need some gap between the piston and the head. Generally speaking .75mm is about as close as you will want to get. Tighter than that and you could risk the piston hitting the head.
In racing applications they will sometimes start around here then go slightly thinner and thinner until they see light contact marks between the piston and the head. I have read of instances where race teams found this to be where they got the most power without detonation.

For most of us in the world of light to moderate builds .75mm to 1mm is a great and safe target. The 1.2mm of the stock 4AGE is what I consider to be on the wide side of acceptable. If I am building a motor I really like to get it under 1mm.

With stock pistons you can calculate your squish gap off your headgasket thickness since the piston should be flat with the deck. With aftermarket pistons you must first consider their compression height.



I will be focussing on Arias today because I have scored a great deal on their pistons and they seem more open than most to work with me to improve their pistons.
Hopefully soon many of these inaccuracies will be resolved with Arias but this information will help you calculate for any piston and check to make sure the numbers are right.
Or in the case of most shelf pistons out there now to be able to come up with your own numbers that will be right.

As you can see here Arias calculates 39CC for their 16v combustion chamber volume.


This means their numbers will be very far off.

You can use this compression ratio calculator to crunch numbers.


For this examle we will use the 16 valve head. Smallport and largeport are the same.

I will use the 9:1 compression ratio piston for this example.


Units 2 for mm

Bore 81.5mm for the Arias 9:1

stroke 77

Headgasket bore OEM is 82mm

Headgasket thickness 1.2mm

Combustion chamber volume 36

Dome -.8

Deck clearance A stock piston would be 0 but for the arias we must take 30.5 -30.226 which is the 1.190 in converted to mm. = Deck clearance .274

Now if you calculate the compression ratio is 10.01:1
Unfortunately though your squish gap is 1.2 + .274 = 1.474 which is quite horrible.
So you can change the calculation. The thinnest headgasket I know of is Cometic which go down to about .68mm. This gives you a gap of .954 which is pretty good.
So now if we enter .68 into the compression calculator we get a compression of 10.6:1.

Another option would be to shave the block down .274. This brings the piston back flush with the deck. This gives you more room to tweak compression and squish gap with the headgasket.

Now with a stock headgasket you could get back to a 1.2mm squish gap which would put your compression at 10.3:1.
Since your squish gap is now the same as the headgasket thickness I wouldn't run thinner than a .75mm headgasket unless you really know what you are doing.
You could run a .8mm headgasket which would give you a .8mm squish gap and an 10.79:1 compression ratio.

Here is a link to common compression ratios assuming a stock head.


7AGE 7A-GE Hybrid

If you are making a 7AGE hybrid you have a couple options. The stock 7A rods and most aftermarket rods put stock pistons about .6mm below the deck. When doing calculations you will have to factor for this. For example the above arias piston on a 7AGE would sit .6mm + .274mm = .874mm below the deck. This means there is no headgasket that will get you an acceptable squish gap.
Option 1. is to shave the head by a determined amount to get the piston back flush with the deck or to whatever point you decide will be best.

Option 2. would be to use our 7AGE Brian Crower rods that are .6mm longer bringing a 30.5mm piston back flush with the deck. They are also designed for 4AGE 20mm floating pistons. This is a very easy solution to a number of problems.
You can find more iniformation on the details of the 7A conversion and piston and rod information here.

Here are some numbers I have calculated for quick refrence.


courtesy of webmatter.de