View Full Version : Turbo pistons and why gearheads are poor in addition to being sick.

08-15-2005, 10:41 PM
So here is how this story goes. To start with, I used to own a Buick Grand National and that is how all of this craziness started. I've seen the stock pistons in these cars...dished symetrical, no valve reliefs. My main consultants for all things automotive are a body man and ford Master Mechanic, but in their heart of hearts, they are Buick Turbo guys and they have learned a lot, so I trust them, sort of. :_paranoid

Well, They have been down the road of flat top pistons in turbo motors and have had, well to say the least, less than ideal results.

So in one our many bench racing sessions, The buick boys told me to spend the bucks and get the right pistons for a turbo motor and the thing will live so much longer it will be well worth the dough.

That wasn't enough for me to spend $1100 with JE. So, time goes on and it bugged me so much, I give Slow4dr a call and he doesn't encourage or discourage either way but hints that dished pistons are the hot set-up, he talks to me about tidal waves of air. That still wasn't enough.

You must understand I have perfectly fine set of Lunati Flat-Tops.

I'm thinking about the diesels, the ultimate turbo motors, I know what those pistons look like and how long those motors live...continuing to ponder my existence as the days go on.

The August issue of Hot Rod arrives...
The plot thickens even more.

I go to Marlan Davis' Pit Stop Department and the question from a dude in Ventura, California:

Dished Vs. Reverse-Deflector Pistons

Recently I've noticed that some serious performance engines are using dished pistons(very similar to the ones in my turbo Buick)...I'm all Ears...Instead of a D-Shaped cup or a reverse-deflector configuration. It would seem to me that the larger squish pad on the the D-shaped cup or reverse-deflector design would induce significantly more turbulence, resulting in a better air/fuel mixture, assuming the head has a corresponding pad. Perhaps at elevated RPM this not an issue because the incoming mixture is at a high enough velocity that it mixes itself. The rest is blah blah blah...

So Marlan writes a dissertation about the issue.

The other article, Go Fast on Pump gas talks a lot about tumble and swirl, heat across the piston deck. The more turbulent the chamber, the more cylinder pressure you can have before detonation.

Duttweiler does experiment with reverse-dome Vs. dish and concludes it's all in the tune.

Mixture Motion, Combustion Chamber Shape, Quench, Tumble, Swirl, Money, What should I do? My head is swimming.

The Hot Rod issue seems like it was written directly for me and Long Story short, I have dished pistons only it's a spherical dish.

Now I had enough, so I call up JE and tell them what I'm doing. He recommends a spherical piston top, Not a cone, Not round, not 3d, tells me in not so many words, why are you running such crappy heads for such a cool motor.

I answer.

So here they are on my soon to be famous couch.

Sphoerical Pistons Vs. Flat Tops. I'm out of my league. I've bitten off more than I can chew. But I am playing and I'm gonna swallow it. It will just take a little time.





I guess that the lesson is, if your building a turbo motor, do your homework on the pistons...I guess if your building any motor do your homework on everything.

Anybody want to buy a set of Lunati Flat tops for a .040" over 400 block with a 327 crank and 6.125 rods?

In Case your wondering Why?


Oh well, I hope they help.

Rick =)

08-16-2005, 01:12 PM
I agree with Duttweiler

"it's all in the tune."

All flow/burn issues aside, a flat top typically has the top ring land very close to the chamber and will most likely fail sooner with the higher temps.

10-23-2005, 11:35 AM
Duttweiler does experiment with reverse-dome Vs. dish and concludes it's all in the tune. agreed................it's all in the tune.

10-23-2005, 05:12 PM
I am just running stock replacement cast dished pistons with a .050" deck height :yes: