# Engine gurus step in please



## Pontiacpurebrred (Jun 22, 2011)

As many of you know I am preparing for a 455 rebuild so I have been digging around reading everything I can so that I will be up on the latest tips, tricks and techniques before I start.

I came across this article and I have never heard of this combustion chamber prep but I guess it sort of makes sense. Here is the article from Popular Hot Rodding Mag about a 455 Pontiac rebuild and pictures below of the chamber prep.

DCI 455 Pontiac Engine - Murphy's Law - Popular Hot Rodding Magazine



















So discuss, I am interested to see if anyone else has heard of this or seen this. 

THANKS!


----------



## geeteeohguy (Feb 2, 2008)

OK. I'll bite: No way would I install hundreds of sharp cornered little hot spots in my chamber and on my pistons. The divots on the piston will hold carbon and cause hot spots, as will the divots in the head....hot spots mean detonation and pre-ignition. Hundreds of little glow plugs ready to get hot and pre-ignite the charge. I did not take time to read the article, but I take it that this is for "swirl" or "atomization". If it was an improvement, the factory would have done it, as it would be a no-cost mod. I am definitely against it. Good way to ruin an engine, IMO.


----------



## Pontiacpurebrred (Jun 22, 2011)

Here is the part of the article about the dimpling of the combustion chambers (and bringing the spark closer to the combustion)

Ross provided the forged pistons that Johnston and Milano modified in-house. "We did dimple the tops of the pistons and the combustion chambers to help atomize the fuel," he says. When asked about any empirical evidence that the dimpling did anything to actually help, Johnston referred back to a 409 Pontiac he built for a previous EMC competition. They tested that engine with stock-type chambers first. "It looked fat in areas and the combustion chamber had wet spots where it would puddle in there. I thought, 'You know what, I'm gonna try this.' Because I've seen some articles done on this but I've never seen any testing and nobody knows where to put these things, so I just kind of randomly made it look like a golf ball in the combustion chamber. We didn't do anything to the pistons on that motor." Between just the dimpling of the chamber and also changing the spark plug depth where the electrode would protrude slightly into the chamber, that engine picked up almost 40 hp and Johnston was sold. With the dimpling done to this 455, the pistons and chambers looked like fuel had never touched them. They were also able to run significantly less timing for best power.

I have no clue if it's voodoo or if it might have some good ideas in it.


----------



## facn8me (Jul 30, 2011)

If it was that easy to gain HP stop all carbon buildup all with less timing then every manufacturer would already do it. Kinda like truck tail gates. You think if driving around with a tailgate down actually increased milage evey new truck would have some type of flow thru tail gate as EPA regulations are tough enough to reach as it is. On some engine in a certain configuration it may work. Whaking a piston with a hammer and punch really does not sound like a good idea to me and if it shelled at some point I would not be suprised.


----------



## geeteeohguy (Feb 2, 2008)

40 HP gain? REALLY?? If I saw a dyno pull before the dimpling, and a dyno pull afterwards, and there actually _was_ a difference, only then would I even _think _about lending credibility to this modification. Definite backyard engineering, IMO, and not very credible at all. I don't believe everything I read...."Trust---but verify".


----------



## Pontiacpurebrred (Jun 22, 2011)

:cheers

Thanks guys ... see that's why I bring things to the forum, for other credible opinions.
But really how do you guys feel? Don't hold back? I want to know the truth.
LOL

Thanks again.


----------



## BearGFR (Aug 25, 2008)

I tend to go with GeeTee on this one. My first thought too was sharp edges = hot spots = detonation city. I read the article and it's unfortunate that they had to run it under the conditions they did (last minute, broken timing light, tune by ear, wrong carb, etc...). I'm very curious though, if I could see a controlled before/after test both with and without the dimples and had some proof from it that they did anything positiive, then I might be convinced -- but I'm not going to buy it on face value though.

Bear


----------



## Pontiacpurebrred (Jun 22, 2011)

BearGFR said:


> I tend to go with GeeTee on this one. My first thought too was sharp edges = hot spots = detonation city. I read the article and it's unfortunate that they had to run it under the conditions they did (last minute, broken timing light, tune by ear, wrong carb, etc...). I'm very curious though, if I could see a controlled before/after test both with and without the dimples and had some proof from it that they did anything positiive, then I might be convinced -- but I'm not going to buy it on face value though.
> 
> Bear












BEAR!!! I was hoping you'd chime in. I, like you, am open to being convinced either way but there was not enough info here to go with it "at face value". That's why I thought I would bring it out in here and see if anyone had heard of such a thing. 

I am old school and was always told to polish all the surfaces "smooth is good"
so I was taken aback by this as well. Though as I said before I suppose it's not all "bad science" it kinda makes sense. 

I am certainly not gonna do anything like that unless I hear something concrete.


----------



## ALKYGTO (Mar 29, 2010)

I've seen intake manifold runners dimpled like this to brake up "boundry layer" air flow but I agree I would'nt want to do it to the combustion chambers or pistons because of the risk of detonation or a place for carbon buildup. If you're looking for that "extra" 5 hp i don't think you'll find it here. :lol:

It's either a bandaid for another problem or an attempt at "trick of the week" IMO. If something like this worked I think you would see it in Nascar or some other professional racing where every little bit counts.


----------



## Eric Animal (Oct 28, 2007)

I, not being an engineer, would go on the PY forum and post in the appropriate section.....there are several engineers such as Tom Vaught, and Mark Luhn who will probably be happy to give you an opinion. My heads are already on my motor and I have never seen or heard of the technique discussed in the article......Veeeeeery Interesting !!:cheers Eric


----------



## BearGFR (Aug 25, 2008)

It made me curious so I ran some Google searches using variations of "dimpled combustion chambers" for search terms. What I found were lots of speculations, lots of comments about the Pontiac 455 article, lots of theorizing, but no hard evidence or any validated testing results. There were posts back to 2004 and earlier, so this is not a new idea. I'm coming to the conclusion that, like GeeTee said, if it was a good idea, everyone would be doing it already. I did find one writeup from John Erb (chief engineer for KB pistons) that lead me to think that perhaps the results they got with the Pontiac were more due to the bump/ramp/whatever they talked about placing near the intake valve than the dimples. Here's a link to the article:

KB Performance Pistons-Bench Racing

Bear


----------

