Hypereutectic pistons?

[JKarow08]

Ahoy,

Has anyone on here used the AA hypereutectic pistons with success, in particular their 10.7:1 86mm version? I know most people use forged, but for a street motor hypereutectic should be more than sufficient, and may have better longevity and less blow by due to their low expansion (even lower than 4032). Henry mentioned in a previous post that their hypereutectic pistons were high quality, but he questioned if they were sufficient at this compression level. I'm curious if anyone has put some miles on a set to confirm or refute that. Thanks!

https://aapistons.com/products/porsche-911-86mm-piston-cylinder-kit-2-2-2-4

[JKarow08]

Nobody?

[PeteKz]

No experience with them, but one thing I would check out with AA: The cylinders in Porsche engines since 1974 are aluminum, so the expansion coefficients are much more similar than aluminum pistons running in iron blocks. I'm just speculating, but it might be the case that hyper eutectic pistons won't expand as much as the cylinders. Thus, your running clearances would increase as the engine reaches operating temps.

I stick to Mahle piston and cylinder sets, because I know Mahle has extensive engineering experience in making sure they work together.

[Henry Schmidt]

The challenge with Hypereutectic pistons is molecular structure is less resistant to high cylinder pressures. The weak spot seems to be the top ring land.
We see these kinds of failures in a lot of the 9.3:1 Kolbenschmit pistons in 911 SC. The pistons used in Alusil cylinders are cast rather than forged.



That said, the description of the piston and cylinder set in the AA description is inconsistent.
They are suggesting that you can use one set for 66mm stroke and 70.4 mm stroke.
In order to make 9.5:1 with a generic chamber size of 69cc the piston dome would be around 30cc.
The problem is, if it makes 9.5:1 with a 66mm stroke the same piston will only make 10:1 on a 70.4 crank.
As for expansion rates: The AA cylinders are Biral in design. That means it has a cast iron liner in aluminum fins. These cylinders have a very low expansion rate that should match a cast iron cylinder even better than a forged piston.
Everything has to match so even the ring pack is different from the Goetze/Mahle rings.
When we use the AA cylinders, we send them out to have them Nikasil coated. We do this to guarantee the bore consistency and to reduce friction. The resulting cylinder can use Goetze ring packs and custom made pistons.
AA piston and cylinder sets have a nice price point but offer a challenge in functionality.

[JKarow08]

Originally Posted by PeteKz

No experience with them, but one thing I would check out with AA: The cylinders in Porsche engines since 1974 are aluminum, so the expansion coefficients are much more similar than aluminum pistons running in iron blocks. I'm just speculating, but it might be the case that hyper eutectic pistons won't expand as much as the cylinders. Thus, your running clearances would increase as the engine reaches operating temps.

I stick to Mahle piston and cylinder sets, because I know Mahle has extensive engineering experience in making sure they work together.

Both the Porsche OE/Mahle cylinders and the AA are aluminum with a iron sleeve so the expansion coefficient differences between the two should be negligible. The Mahle pistons from pre-74 S motors were forged (unknown alloy assumed to be close to 4032 from what I've read), so while they are the lowest level of expansion for a forged piston, they still have greater expansion than a hypereutectic piston. They are definitely stronger than an hypereutectic piston, but if that extra strength isn't necessary (non race/boosted motors), they will provide no real gain yet have more blow-by, piston rock, and increased cylinder "coke bottling" as a result. I definitely don't mean this as a slight to the OE Mahle setups, they are obviously a proven commodity, I just question if they are necessary (or even better) than a $1200 AA set. My biggest concern is if the AA 10.7:1 setup pushes the limits of hypereutectic strength, necessitating utilizing forged at that level.

[JKarow08]

Originally Posted by Henry Schmidt

The challenge with Hypereutectic pistons is molecular structure is less resistant to high cylinder pressures. The weak spot seems to be the top ring land.
We see these kinds of failures in a lot of the 9.3:1 Kolbenschmit pistons in 911 SC. The pistons used in Alusil cylinders are cast rather than forged.



That said, the description of the piston and cylinder set in the AA description is inconsistent.
They are suggesting that you can use one set for 66mm stroke and 70.4 mm stroke.
In order to make 9.5:1 with a generic chamber size of 69cc the piston dome would be around 30cc.
The problem is, if it makes 9.5:1 with a 66mm stroke the same piston will only make 10:1 on a 70.4 crank.
As for expansion rates: The AA cylinders are Biral in design. That means it has a cast iron liner in aluminum fins. These cylinders have a very low expansion rate that should match a cast iron cylinder even better than a forged piston.
Everything has to match so even the ring pack is different from the Goetze/Mahle rings.
When we use the AA cylinders, we send them out to have them Nikasil coated. We do this to guarantee the bore consistency and to reduce friction. The resulting cylinder can use Goetze ring packs and custom made pistons.
AA piston and cylinder sets have a nice price point but offer a challenge in functionality.

Thanks for chimming in here Henry, I know you likely have the most experience with these AA P/Cs.

I was also curious about the claimed compression #s. Based on your knowledge, what would you estimate the compression to be with these 86mm 10.7:1 pistons to be with a 70.4mm stroke and 2.4s heads?

I've definitely seen plenty of ringland issues with hypereutectic pistons in the Subaru world, primarily in greater than stock boost situations. With that said, I had a "Porsche" motor in my Subaru (ez30r h6) that had 10.7:1 hypereutectic pistons that saw 8200rpm almost daily with no issues. If I was building a race motor or throwing boost at it forged would be the only way I'd go, but on a 200-250hp motor that won't see over 7k I'm just not sure if it is necessary and question if forged will result in less longevity, hence why almost all OE manufacturers use hypereutectic pistons these days.

[JKarow08]

https://youtu.be/bXYaePMKvcI?si=YdS5U2XkUvwZx5Ad

My "Porsche" motor running 10:7:1 hypereutectic pistons screaming to 8200rpms...

[PeteKz]

Okay, then the pre-question is: What are you building, and what do you plan to use it for?

[JKarow08]

Originally Posted by PeteKz

Okay, then the pre-question is: What are you building, and what do you plan to use it for?

I'm waffling a little bit on the final specs, part of why I posted this thread and a few others. If these AA 86mm P/Cs sound like a safe option, I'm likely going to build a 2.5l "long stroke" motor with a 70.4mm crank and 86mm bores with 10.7:1 pistons, 2.2/2.4s spec twin plug heads, dc30 cams, and an ITB/twin COP/Megasquirt setup with full sequential and knock control. If it sounds like 10.7:1 is too high for hypereutectic pistons, I'll probably drop down to their 84mm 9.7:1 set with single plugs, everything else the same. The goal is to turn my '73 into a spicy street build that is reliable and easy to start and can be used as a daily, capable of putting on a lot of "spirited" road miles with an occasional autox/track day.

With modern efi/ignition, knock control, and twin plugs I'm hoping I can get away with these 10.7:1 pistons...should result in a very "spunky" motor that would last a long time on a reasonable budget.

[JKarow08]

This is my "barn find" donor chassis I'm putting it into. It's the matching #s case so I don't want to bore the case spigots to take it to a 2.7l.

[Henry Schmidt]

Originally Posted by JKarow08

I'm waffling a little bit on the final specs, part of why I posted this thread and a few others. If these AA 86mm P/Cs sound like a safe option, I'm likely going to build a 2.5l "long stroke" motor with a 70.4mm crank and 86mm bores with 10.7:1 pistons, 2.2/2.4s spec twin plug heads, dc30 cams, and an ITB/twin COP/Megasquirt setup with full sequential and knock control. If it sounds like 10.7:1 is too high for hypereutectic pistons, I'll probably drop down to their 84mm 9.7:1 set with single plugs, everything else the same. The goal is to turn my '73 into a spicy street build that is reliable and easy to start and can be used as a daily, capable of putting on a lot of "spirited" road miles with an occasional autox/track day.

With modern efi/ignition, knock control, and twin plugs I'm hoping I can get away with these 10.7:1 pistons...should result in a very "spunky" motor that would last a long time on a reasonable budget.

The AA pistons are not 10.7:1. They are probably close to 10:1.
The idea of running 10.7:1 on street gas is a mistake. I have covered this in the past but simply put, dynamic compression on a streetable cam is far too high.
Lower compression is always the safer choice when dealing with questionable fuel All street gas is questionable especially if you cross the line where boutique furls are mandated. IE: Cal.

[Henry Schmidt]

I ran some quick numbers on your proposed build, using a DC62 camshaft. A pretty aggressive cam for the street. Generally speaking a milder cam will yield a higher DCR .

DC 62

Static compression ratio of 10.7:1.
Effective stroke is 2.27 inches.
Your dynamic compression ratio is 8.95:1 .
Your dynamic cranking pressure is 183.78 PSI.

Using a DC40 (Mod S)

Static compression ratio of 10.7:1.
Effective stroke is 2.31 inches.
Your dynamic compression ratio is 9.09:1 .
Your dynamic cranking pressure is 190.34 PSI.


Most engine builders would suggest that 8.95:1 dynamic compression is too high for pump gas.
There are many other factors to consider but given that the Hemi-head two valve Porsche engines are prone to detonation, it's always better to be safe than sorry.

[JKarow08]

Originally Posted by Henry Schmidt

I ran some quick numbers on your proposed build, using a DC62 camshaft. A pretty aggressive cam for the street. Generally speaking a milder cam will yield a higher DCR .

DC 62

Static compression ratio of 10.7:1.
Effective stroke is 2.27 inches.
Your dynamic compression ratio is 8.95:1 .
Your dynamic cranking pressure is 183.78 PSI.

Using a DC40 (Mod S)

Static compression ratio of 10.7:1.
Effective stroke is 2.31 inches.
Your dynamic compression ratio is 9.09:1 .
Your dynamic cranking pressure is 190.34 PSI.


Most engine builders would suggest that 8.95:1 dynamic compression is too high for pump gas.
There are many other factors to consider but given that the Hemi-head two valve Porsche engines are prone to detonation, it's always better to be safe than sorry.

Hi Henry,

Thanks so much for taking the time to run these numbers! I'm no dummy when it comes to engines but this type of math is beyond my level of intellect, but I understand the outputs.

A couple of follow up questions... is your 8.95:1 DCR limit for pump gas for twin plug motors as well or just single plug? And if you estimate these AA pistons at closer to 10:1 than 10.7:1 they should be under the acceptable pump gas limit even with dc30 cams then correct?

I'm not trying to pound a square peg into a round hole, but this "high compression" 86mm set definitely seems like a lot of bang for the buck (pun intended) when building up a 2.4l case for street work.

Thanks again for sharing your insight!

-Josh

[JKarow08]

Also curious what everyone's opinion is on how upper compression limits differ (if at all) when using full sequential COP ignition and knock control. Both should significantly reduce the incidence of detonation (if tuned correctly) which should increase the acceptable upper DCR limits. Thoughts?

[Henry Schmidt]

Originally Posted by JKarow08

Hi Henry,

Thanks so much for taking the time to run these numbers! I'm no dummy when it comes to engines but this type of math is beyond my level of intellect, but I understand the outputs.

A couple of follow up questions... is your 8.95:1 DCR limit for pump gas for twin plug motors as well or just single plug? And if you estimate these AA pistons at closer to 10:1 than 10.7:1 they should be under the acceptable pump gas limit even with dc30 cams then correct?

I'm not trying to pound a square peg into a round hole, but this "high compression" 86mm set definitely seems like a lot of bang for the buck (pun intended) when building up a 2.4l case for street work.

Thanks again for sharing your insight!

-Josh

I think you misunderstood my data. The upper limit is not 8.95. I did the calculations and that was the number I got given the perimeters. Then stated that was too high.
8.35 is closer to the acceptable number for an aluminum Hemi head.
You seem to looking for the limit when a reasonable position might be more like what is safe given that high compression if not part of the overall build really offer very little enhancement.
Porsche never ran giant compression numbers in there air-cooled engines for a reason.
Even the early twin plug racing engines [running on race fuel] ever topped the low tens.
Most claimed 10.4:1 and were actually closer to 9.9:1.

[JKarow08]

Originally Posted by Henry Schmidt

I think you misunderstood my data. The upper limit is not 8.95. I did the calculations and that was the number I got given the perimeters. Then stated that was too high.
8.35 is closer to the acceptable number for an aluminum Hemi head.
You seem to looking for the limit when a reasonable position might be more like what is safe given that high compression if not part of the overall build really offer very little enhancement.
Porsche never ran giant compression numbers in there air-cooled engines for a reason.
Even the early twin plug racing engines [running on race fuel] ever topped the low tens.
Most claimed 10.4:1 and were actually closer to 9.9:1.

You're right, sorry, I guess I read that too fast and mistook what you said in my response. Based on your 8.35:1 DCR limit on pump gas what compression piston should I target to reach that on DC30 cams? Their 9.7:1 84mm hypereutectics should be below that correct? Any opinions on those pistons at that compression ratio?

I'm not necessarily trying to find or push the safe limits with this build, just figured it would be wise to squeeze as much power as I could out of the build (safely) if the price is going to be the same either way. Again, I really appreciate you taking the time to help me out here, there is a lot to consider when building one of these motors.

[Henry Schmidt]

DC 30 cams

Static compression ratio of 9.2:1.
Effective stroke is 2.45 inches.
Your dynamic compression ratio is 8.25:1 .
Your dynamic cranking pressure is 167.57 PSI.

[JKarow08]

Originally Posted by Henry Schmidt

DC 30 cams

Static compression ratio of 9.2:1.
Effective stroke is 2.45 inches.
Your dynamic compression ratio is 8.25:1 .
Your dynamic cranking pressure is 167.57 PSI.

Thanks Henry! It looks like that is right in the wheelhouse of your suggested DCR limit. That should be fine on a single plug setup correct? Do you have any experience using these pistons? Thanks again for your help!

[Billiam 911 2.8]

Kudos to Henry for being so generous with his knowledge.
I have the luxury of not daily driving my 2.8, so I run 96 octane, non-ethanol fuel from a station that is near enough to me. When I track the car, I bump up to 100 octane race fuel (leaded).

[JKarow08]

Originally Posted by Billiam 911 2.8

Kudos to Henry for being so generous with his knowledge.
I have the luxury of not daily driving my 2.8, so I run 96 octane, non-ethanol fuel from a station that is near enough to me. When I track the car, I bump up to 100 octane race fuel (leaded).

We are all definitely in debt to Henry for sharing his knowledge, I definitely owe him a beer or twelve

What compression and pistons are you running? Twin plug?