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Piston Design Questions

General engine tech -- Drag Racing to Circle Track

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skinny z
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Piston Design Questions

Post by skinny z »

Subject: SBC 383.
I've more or less zeroed in on a balanced rotating assembly from Scat. One of their all forged offerings with Pro Comp 6" rods and a choice of two piston styles. Flat top (6cc) or 19cc dish.
I have cylinder heads ready to go as in aftermarket Vortec (anyone remember RHS Pro Torker?) that have been worked over a little (255@.550") and chambers measured at 65 cc.
There will be an element of street driving but am looking at drag racing plus some open road events more than anything.
The flat top will put the CR too high for what my plans are. About 11:1. (iron heads)
The dished piston, I think will be unreasonably low at 9.5:1.
All of the above are with a .040" piston to head clearance.
Here's where the questions come in.
If I choose the flat top piston, what am I giving up by opening the PTH clearance to say .060"? CR would fall in closer to 10.5:1 which I believe I can find manageable. Similarly, what is the sacrifice with a 9.5:1 CR and a full dished piston. That is one with no quench pad to speak of.
The only reasoning I can come up with for the flat piston and increased clearance is that it gives me an option for different heads and larger chambers down the road. As always I appreciate all insights and comments.
Thanks in advance.
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Re: Piston Design Questions

Post by rebelrouser »

If you are wanting to optimize the compression ratio, why stop at only looking at the pistons available? A die grinder and a graduated burette, will allow you to adjust as much as you need, I would think. Plus if you grind the chamber correctly, you will most certainly pick up some flow in the cylinder heads.
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Re: Piston Design Questions

Post by skinny z »

Valid point for sure. Thing is these have had that done already. That was back when they on a 350 and the 10.4:1 and primarily street driving was giving me a lot of grief with the short cam.
I'd just a soon leave the heads as they are and work with the short block. Perhaps that's a little laziness or eager to get on with it on my part. But I will say that is an interesting thought.
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Re: Piston Design Questions

Post by lefty o »

just buy the crank and rods, not some supposed balanced kit that will need to be balanced again anyways. shop for the piston that works for your needs instead of compromising by buying some hack balanced kit.
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Re: Piston Design Questions

Post by David Redszus »

An increase in dynamic compression ratio of one point is worth about 3% power increase.

But squish velocity (and direction of turbulence) could be worth much more and will
avoid detonation as well.
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Re: Piston Design Questions

Post by PackardV8 »

skinny z wrote: Fri Jan 08, 2021 10:31 amHere's where the questions come in.
If I choose the flat top piston, what am I giving up by opening the PTH clearance to say .060"?
Thanks in advance.
Agree with David. In this case, more is less.

When the PTH is more than .050", most feel more is lost than gained.
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skinny z
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Re: Piston Design Questions

Post by skinny z »

lefty o wrote: Fri Jan 08, 2021 11:10 am just buy the crank and rods, not some supposed balanced kit that will need to be balanced again anyways. shop for the piston that works for your needs instead of compromising by buying some hack balanced kit.
This is something I'm considering. In discussions with my machinist, he mentioned chucking up the crank and checking Scat's work anyway. Thinking this will be something I'll pursue further after your comment regarding the balance being more than the first time I've heard that. There are a couple of piston choices that have a better fit than what the kit offers.
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Re: Piston Design Questions

Post by skinny z »

PackardV8 wrote: Fri Jan 08, 2021 11:15 am
skinny z wrote: Fri Jan 08, 2021 10:31 amHere's where the questions come in.
If I choose the flat top piston, what am I giving up by opening the PTH clearance to say .060"?
Thanks in advance.
Agree with David. In this case, more is less.

When the PTH is more than .050", most feel more is lost than gained.
Such is my understanding.
While not a maximum effort by the standards of our local residents, I'd like to take advantage of whatever gains that are there for the taking.
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Re: Piston Design Questions

Post by Stan Weiss »

11:1 with Iron heads should be able to be run on pump gas with the right cam. Why don't you talk to Mike Jones?

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Re: Piston Design Questions

Post by rebelrouser »

lefty o wrote: Fri Jan 08, 2021 11:10 am just buy the crank and rods, not some supposed balanced kit that will need to be balanced again anyways. shop for the piston that works for your needs instead of compromising by buying some hack balanced kit.
One other thing from my experience is that while they say you will have 11 to 1 compression with the flat top, I seldom see as much compression ratio as advertised when I actually CC the chamber, measure the deck height etc. and calculate it. How far that shelf piston is down in the bore will make a big difference. And again I seldom get a shelf piston that is at zero deck.
I remember many years ago when I was young and dumb. I had a 340 mopar, and it ran pretty good, so I of course wanted to bump up the compression to make it run better. So I ordered the replacement pistons to go from 10 to 1 up to 11.5 to 1 and they were identical in every respect, and different companies I might add, but if you read the fine print they started saying @ different combustion chamber volumes. I learned that lesson very well.
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Re: Piston Design Questions

Post by piston guy »

Detonation is FAR more common with iron heads , we all agree. "I" don't like to increase piston to head clearance beyond .040 with iron heads because of that. Custom pistons from some piston manufacturers "can" be as little as $100 extra and worth every penny.
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Re: Piston Design Questions

Post by skinny z »

Stan Weiss wrote: Fri Jan 08, 2021 11:42 am 11:1 with Iron heads should be able to be run on pump gas with the right cam. Why don't you talk to Mike Jones?

Stan
I've worked with Mike (via his website) and he's offered a couple of specs although the combination was slightly different. That difference being the CR. IIRC, those recommendations were based around 10-10.2:1.
What I'm coming to learn though is that the higher CR, the limited flow of the heads and the CID add up to mismatched package.
I will however give him another shout and see what he has to say.
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Re: Piston Design Questions

Post by skinny z »

David Redszus wrote: Fri Jan 08, 2021 11:13 am An increase in dynamic compression ratio of one point is worth about 3% power increase.

But squish velocity (and direction of turbulence) could be worth much more and will
avoid detonation as well.
That's as I've come to learn it.
I used to suffer the tenths in a CR spec and in my particular applications it wasn't worth it. Or more accurately stated, the issues with being near the edge didn't outweigh the benefits of being able to develop a proper timing curve and all the rest that goes along with staying out of detonation. Which is another reason that the PTH clearance is something I put value in.
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Re: Piston Design Questions

Post by Mark O'Neal »

Run the flat top and a .080 gasket.

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Re: Piston Design Questions

Post by skinny z »

Mark O'Neal wrote: Fri Jan 08, 2021 12:46 pm Run the flat top and a .080 gasket.
I take it then that you've little concern for an excessive gap between the piston and the head? Seems the conventional thinking is that it deserves attention.
This is part of the reason for my wanting to stay away from a full dish as opposed to something with a D shaped dish. That would be to take advantage of that tight squish area.
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