I posted this in the general tech area but have not gotten a whole lot of responses. I figure it might be better placed here in the Advanced Tech section. Here is my issue:

I am at a crossroad. I am building a fully forged 383 but am stumped on piston choice. The specs on the engine are below:

6 in rod
3.75 stroke
4.030 bore
zero deck height block
0.038 head gasket
stock aluminum F-body (52-53cc) heads worked over
future holds a 150 shot of NO2

The above should yield around 11.~2ish? compression with the -16cc forged pistons I am looking at.

There is no difference in cost between the SRP and Mahle. They are both forged and from what I can tell there is very little weight difference between the two. The biggest difference is that the piston tops are completly different. The Mahle is a true dish where the SRP is a D-shaped dish. Which would be better for my setup and WHY? I am planning to go a LE2 head/cam package so the heads are not currently set up one way or the other.

Mahle: http://www.karl-ellwein.org/ESE5/pistoninstall.jpg
SRP: http://www.karl-ellwein.org/AllensEn...tonfirstin.jpg

Thanks for helping,
TOM B

 

My preference would be the D shaped dish over the round dish. The reason being with the extra squish area on the piston, the squish to bore ratio is higher. This provides increased mixture movement as the piston approaches TDC during combustion. The increased squish action improves air/fuel mixing as well as stimulates the combustion process for a faster more complete burn.

 

I would prefer a preped flat top.... It's going to bump the compression to over 12:1 but it all depends on the cam.

You might want to look at the hemispherical piston dishes out there as well, they are common in Import and Mod Motor pistons....

Bret

 

DITTO, normally on the small bore engines we are trying to build more compression

 

This is going to be a street car running on pump gas (max of 92 octane as that is all we have around here). I hope to visit the strip a few times a year as long as I don't keep breaking things. I have the choice between these two pistons so am wondering which one would work better for my needs and why. What is the pros/cons of the full dish (Malhe) vs the D dish (SRP)?

Thanks all,
TOM B

 

I only do BBC thinking and the theory with a BBC is to work with a typical aftermarket BBC head which has a large combustion chamber and very big valves. The big combustion chamber unshrouds the valves for better air flow. To get the compression ratio you want, you chose the piston either dished, flat or any sized dome. My pistons have a huge dome giving me about 13:1 compression. I can't remember what a flat top piston would give me. Something like 9:1. I use JE lightweight racing pistons. www.jepistons.com

 

I think automotivebreath covered it. The D gives better squish action and causes more swirl in the combustion chamber. Flat tops would do the same and give higher compression. I have a set of -16 srp's that have been sitting in a box for two years with about 3500 miles on them. They have been polished on top to reduce sharp edges and are for a 6" rod on 4.03 boreif you are interested.

 

With my LT 396, I'm running the "flat top" SRPs w/ AFR 195 LT4 heads (58 cc chambers) and a .049 H-gasket. Therefore, my compression is at 12.2-1. It runs great with the PCMforLess tuning using 93 octane, however, it runs much better ETs with the Sunoco GT-Plus (104 unlead). I like the higher compression engines and don't mind the higher octane gas, and my car is running in the 11.20s @ 120+.

WD

 

Bret,

Is that what the Mahle (the one pictured) is? Or is the "hemispherical piston dishe" you refer to different. Do you have a link to what that design looks like?

TOM B

 

These concave dish pistons are worth about 5 HP in high RPM applications.

 

Yep, that picture is what I'm talking about.

Bret

 

So the Mahle (http://www.karl-ellwein.org/ESE5/pistoninstall.jpg) is considered to be a "hemispherical dished piston". So far I have heard from both camps and quench was mentioned but that is why I am asking. It is a package deal (Compstar Speed Pack) and I have my choice between those two pistons:

SRP : SRP 138103
Mahle : SBC125030I16

Just wondering which one is better for my application.

Thanks for the offer on your old pistons but I am buying a balanced assembly so I'd have to get the thing balanced which is what I am trying to avoid.

TOM B

 

Tom, I couldnt get the photo/link of the SRP piston to work, (on my computer).

http://www.karl-ellwein.org/AllensEn...tonfirstin.jpg

That's the photo I'm pretty sure.

Bret, what do you think about that -16 inverted dome Mahle for a natural 383 with mild nitrous? Seems like the D-dish SRP would be best for this application with that squish pad. But I have no experience.

Karl

 

Hi Tom,
I don't consider the piston shown to be "hemispherical dished" The bottom of the dish is flat not concave.

You haven’t detailed the intended RPM range. Regardless I would choose the D shaped dish over the dish piston shown to maintain squish to bore ratio. I'm assuming you will be running some amount of time at low to moderate RPM, squish action plays a huge role when piston velocity is low.

 

The car is a T56 equipped, 4.10 geared, 4400lb Impala. I don't plan to rev it to the moon so I figure I would keep it under 6500 RPM.

Does that help any more?
TOM B