Mr. Luos

iTrader: (9)

I was reading through a couple others threads and I was touchy on some things.

Mahle -12cc pistons. They are a full dish piston. Probably not the best piston to run N/A with I would assume. They come .008" out of the hole and I am running .045" Cometics. Quench should be good for my application I would think.
Compression to be right around 11:1.

How will these pistons affect power output?? Or will they?
I went with these pistons planning on boosting later on, but some things have changed recently and that might be WAY down the road.

Discuss please. The motor isn't in the car yet, so if it is going to be a mistake to run the Mahle's N/A for a while I might change them.

Thanks!

Mr. Luos

iTrader: (9)

Seems some of you have an opinion on this until I ask.

Just trying to figure out if the full dish affects the quench and how much it will affect the performance of a N/A motor.

calongo_SS

iTrader: (7)

How are you getting 11:1 compression with a -12 cc dish?

Steve Bryant

iTrader: (1)

What is the bore and stroke of the engine?
What is the bore diameter of the .045" head gaskets?
What is the cc volume of the combustion chambers in the heads?

Answer these questions and we can verify the 11.1:1 CR question.
A piston is dished for two reasons:
1. to reduce the compression ratio
2. to allow additional piston to valve clearance which can be a problem with high lift, long duration cams.

Typically, a dish that is over 10 cc's will yield a CR in the range of 9.0 to 10.0 depending on the stroke and combustion chamber volume. What CR do you want to run?

Regarding quench, you want to have a quench of about .040" to minimize detonation and the resulting knock retard and power loss it causes. Your numbers indicate that your quench would be about .037", so that should be fine regarding KR. I think that your biggest problem may be lower than expected CR.

All my best,

Steve

Mr. Luos

iTrader: (9)

62cc heads
4.0 stroke
4.0 bore
-12cc pistons
Gasket is a .045 thickness 4.030 bore.

Probably closer to 10.8 or so.
Iron blocked 402ci.

Steve Bryant

iTrader: (1)

I ran the calculation for you on DynoSim Pro and the result is 10.68:1. I think that that is a pretty good CR for a daily driver, occasional drag strip car. How do you plan to use the car?

Steve

Mr. Luos

iTrader: (9)

Couple trips down the track a season. Might visit the track 3-4 times over the spring/summer.

Mostly just for fun.

Beast96Z

iTrader: (21)

Steve, one thing about that dyno sim is that you need to enter -.008 for your deck clearence because the piston is "out of the hole", not in. Notice on the calculated deck volume that the amount of volume increased to 13.65 when in fact it should have been just a little over 12 or so. When I entered it in my calculator, I got 11.09 scr, which is just about where he said he would be.

strokedls1

iTrader: (1)

I have a -19cc dish and 56cc heads at 11.17 static ratio and it work great. I run 91 octane with 28.5 degrees advance with no knock. I even sprayed a 100 shot on top of that without pulling timing and still had no knock. Most of the engine building competitions use a small chamber head with a dished piston. The dish also gives you some extra room for a bigger cam.

Beast96Z

iTrader: (21)

Most piston companies will put some flat area on the piston to make a "quench pad". If you find a company with a good desighn, it's deffinatlly better to run the dish and a smaller chamber. If the piston is just somewhat of a "bowl" desighn, I wouldn't be looking to run it in my N/A car. There are diffrences between N/A and F/I pistons. One is designed to draw air in and force it out while with the other, all the air is forced in and out.

Mr. Luos

iTrader: (9)

Okay...

So are there any adverse affects of running the dish type pistons N/A for now??
There is no quench pad on these Mahle pistons.

Beast96Z

iTrader: (21)

You'll probablly sacrifice a few HP, but I don't think I would change pistons if your gunna go F/I later.

blu00rdstr

One question I haven't seen answered, is why some think that higher horsepower F/I motors would not need a quench area. The higher dynamic compression a motor has, the higher the odds of detonation are. You can richen and retard an F/I motor to reduce cylinder pressure and power, but you begin undermining the whole reason to go F/I. Which is power.

strokedls1

iTrader: (1)

It is usually because you rely more on the chamber design of the head rather than the piston. The boost will in effect help remove the exhaust gasses. I think some of the boost guys have the most advanced setups out there for ls1's. I think keeping quench tight with a thinner gasket and dishing the pistons is a better idea than trying to use thick ones to bring the static ratio down.

blu00rdstr

Yes, boost will help push exhaust out.

Quench surface creates a cool area furthest from the spark plug, which is the most likely heat spot that can cause detonation. The quench area also creates a tight squish between piston and head to clap mixture towards the plug which also reduces detonation.

Both quench and squish are important to reduce detonation, and F/I motors are prone to detonation due to higher cylinder pressure. So why would someone say quench not required for F/I, but is required for N/A?

strokedls1

iTrader: (1)

I have no idea. Maybe experiece with a thicker gasket. I personally would run the same quench on either application. We build nitrous motors, but other than static compression it is very similar to how I would do a boosted engine. The only difference is I would just swap to a bigger cc head to reduce compression ratio. My je pistons on my current motor are a full dish with a flat bottom and have worked out great. The tune is just as important as the mechanical setup. If the tune is off (ie not enough fuel, too much spark) then problems will occur.

Mr. Luos

iTrader: (9)

That is what I was planning on doing....If I ever get to that.

Thanks again for the help.

QuickSilver2002

I think most people here are confused about what the original poster is asking.

I'll use some different terminology in an attempt to clarify things. There are two types of piston designs to lower compression (basically anything that has more than just valve reliefs).

1. Full dish (most of the bore area is dished out) and there is very little quench pad left intact. Picture a piston where the only flat part is a small edge around the entire bore.

2. Reverse Dome pistons (the chamber area is the only part of the piston that is dished out. This keeps all the quench pad intact. Picture a piston where the only part that is not flat is the part that matches up with the chamber on the head.

Up until recently I had only heard of reverse dome used in ls1 applications (Diamond and JE are all I've ever seen ). And just assumed they were all done that way.

Based on few things I’ve recently read, it seems that some people are getting away with a full dished piston, but if you ask anyone who has been doing ls1 fi stuff for a while, they will say that they would never personally use anything but a reverse dome and that reverse dome is better.

Now for a NA pump gas car, I think you would be crazy running a fully dished piston. This has nothing to do with compression ratio. It is just the fact that you then have no quench. and are going to be way more prone to detonation.

I'm not a professional engine builder or engineer, so get other opinions before making a decision.

Mr. Luos

iTrader: (9)

That is exactly what I am worried about.

I have done the research, and now I am starting to think these pistons aren't going to work too well with this set-up.
Which sucks, because I have a decent amount of money and work into it so far.

Any other opinions??

The only thing I am thinking is this....
-12cc full dish or a -12cc reversed dish will have the same amount of quench area total anyways right?? Or is it just the quench pad that matters??

Why would shops sell the full dish if they basicly sucked? Even further than that...why would a company continue to manufacture pistons that don't work well??

Brandon Boomhauer

iTrader: (10)

edit.
check this post by jrod out