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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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Re: Piston Design Questions

Post by skinny z »

rebelrouser wrote: Fri Jan 08, 2021 11:54 am
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 can appreciate that.
The only thing not in hand at this time is the piston. That said, I'll spec the deck once the mock up can be measured. Then it's cut to fit.
I should be able to nail the CR then.
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Re: Piston Design Questions

Post by David Redszus »

skinny z wrote: Fri Jan 08, 2021 2:04 pm
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.
The clearance from piston to head is only one part of the squish velocity equation.

What is of even more importance is the squish area ratio of the piston/head interface. which has
a strong influence on squish velocity.

And the shape of the squish area which determines the direction of the squish jet.
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Re: Piston Design Questions

Post by skinny z »

David Redszus wrote: Fri Jan 08, 2021 2:11 pm
The clearance from piston to head is only one part of the squish velocity equation.

What is of even more importance is the squish area ratio of the piston/head interface. which has
a strong influence on squish velocity.

And the shape of the squish area which determines the direction of the squish jet.
Is this to say the shape of the dish is of significance?
The 19 cc version is a full dish with a rim that would provide the only squish area. I'm more inclined to go with an example from Ross pistons which has a D shaped dish so as to coincide with the head deck. The RHS heads have that Vortec shaped chamber which provides plenty of area for this.
Is that a more reasonable path if the order of the day is to maximize the effect of that tighter clearance?
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Re: Piston Design Questions

Post by frnkeore »

I haven't seen any exact dimensions given for your parts and in the end, the thing that will matter is the measured deck clearance, on the assembled engine and the accuracy of the chamber volume.

Part of that chamber volume, is the volume, above the first compression ring, depending on the diameter of that area and the distance from the ring to the top of the piston, it will be +/- 1cc, in your case. Not much, but, it all adds up.

Since you have such a high concern for your CR, have the heads CCed, measure the deck clearance get the top ring info (pist diameter and dist down). You then need the head gasket info. Select a HG with no more than .045 PTH clearance and find out the cc capacity of it. that will give you your SCR

After calculating all that and then select cam timing with a dynamic CR of between 7.8 & 8.0/1 and you should be good to go.

We can't tell what your actual CR is w/o the above info but, it will probably be a little lower than your thinking, could be higher too ;) but, I think it will be lower.
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Re: Piston Design Questions

Post by skinny z »

I can't say I need any assistance with calculating the CR but I can appreciate what you're saying.
I've done the measuring and what isn't assembled will be and the balance of the spec can be completed. Specifically decking the block.
FTR:
Chambers measured at 65.4 cc.
Head gasket is VR 5746: .026" x 4.100".
Spec for the deck is .014" down the hole.
4.03" x 3.75".
Piston choices (as an example) are 6 cc or 19 cc.
Admittedly I haven't calculated the crevice volume or included it in my calculations.
FWIW, I've never bothered to use that value and in this way, while the number may not be 100% on the money, it allows me to compare what's worked for me in the past vs those examples where the success hasn't been so great.
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Re: Piston Design Questions

Post by PackardV8 »

skinny z wrote: Fri Jan 08, 2021 2:17 pm
David Redszus wrote: Fri Jan 08, 2021 2:11 pm
The clearance from piston to head is only one part of the squish velocity equation.

What is of even more importance is the squish area ratio of the piston/head interface. which has
a strong influence on squish velocity.

And the shape of the squish area which determines the direction of the squish jet.
Is this to say the shape of the dish is of significance?
The 19 cc version is a full dish with a rim that would provide the only squish area. I'm more inclined to go with an example from Ross pistons which has a D shaped dish so as to coincide with the head deck. The RHS heads have that Vortec shaped chamber which provides plenty of area for this.
Is that a more reasonable path if the order of the day is to maximize the effect of that tighter clearance?
FWIW, back in the day, we spent large money and sent casts of the combustion chamber to have a piston dish matched to it.
Image

Then, a piston manufacturer said, "I shouldn't be telling you this, but some NASCAR testing proved a simple concave dish actually produced better than a matched dish."

I've never seen the published data, but now I order dished pistons which look like what Studebaker V8s came with back in 1955.

Image

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Re: Piston Design Questions

Post by skinny z »

Interesting.
An engineering thesis (that I'm familiar with) was based on combustion chamber design. Way back then it was determined that a sphere was the most efficient design. Of course this meant a centrally located source of ignition.
That said, I'd often wondered if a full dish (as would be the 19 cc piston) would have better combustion properties than a flat topped piston.
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Re: Piston Design Questions

Post by frnkeore »

skinny z wrote: Fri Jan 08, 2021 2:42 pm I can't say I need any assistance with calculating the CR but I can appreciate what you're saying.
I've done the measuring and what isn't assembled will be and the balance of the spec can be completed. Specifically decking the block.
FTR:
Chambers measured at 65.4 cc.
Head gasket is VR 5746: .026" x 4.100".
Spec for the deck is .014" down the hole.
4.03" x 3.75".
Piston choices (as an example) are 6 cc or 19 cc.
Admittedly I haven't calculated the crevice volume or included it in my calculations.
FWIW, I've never bothered to use that value and in this way, while the number may not be 100% on the money, it allows me to compare what's worked for me in the past vs those examples where the success hasn't been so great.
Based on the above info, your SCR is very close to 10.78, with -6cc pistons, with the first ring .3 down.

Based on that, you should be ok, with a cam, that has the intake closing at about 74*.
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Re: Piston Design Questions

Post by skinny z »

I don't have my notes handy but IIRC that's within a tenth of my calculation. I think ~10.8:1 is where I ended up.
I have probably 4 or 5 cam specs from various sources but none with that high an SCR.
Thing is, at least from what I'm understanding is that these heads are somewhat flow limited for a 383. Some simulations suggest peak HP RPM in or around 5600. Cams have been spec'd as such. But I'm well aware of the workarounds for situations like that.
That said, that kind of RPM and with the torque that those other specs offer (with a 10:1 CR) falls into the rest of drivetrain package if I were to consider drag racing as the premier application.
To tell you the truth, if I were designing a 383 from the ground up, I'd skip over these heads and get something more suitable. However, such is not the case.
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Re: Piston Design Questions

Post by bobmc »

JE-SRP has a 4 valve relief piston with 10cc volume, comes with trick rings pn 324863
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Re: Piston Design Questions

Post by rebelyell »

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^^^^
and get a crank that'll easily internal balance ... that usually means at least 6" long rods coupled w/ appropriately short pistons
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Re: Piston Design Questions

Post by David Redszus »

PackardV8 wrote: Fri Jan 08, 2021 2:51 pm
skinny z wrote: Fri Jan 08, 2021 2:17 pm
David Redszus wrote: Fri Jan 08, 2021 2:11 pm
The clearance from piston to head is only one part of the squish velocity equation.

What is of even more importance is the squish area ratio of the piston/head interface. which has
a strong influence on squish velocity.

And the shape of the squish area which determines the direction of the squish jet.
Is this to say the shape of the dish is of significance?
The 19 cc version is a full dish with a rim that would provide the only squish area. I'm more inclined to go with an example from Ross pistons which has a D shaped dish so as to coincide with the head deck. The RHS heads have that Vortec shaped chamber which provides plenty of area for this.
Is that a more reasonable path if the order of the day is to maximize the effect of that tighter clearance?
FWIW, back in the day, we spent large money and sent casts of the combustion chamber to have a piston dish matched to it.
Image

Then, a piston manufacturer said, "I shouldn't be telling you this, but some NASCAR testing proved a simple concave dish actually produced better than a matched dish."

I've never seen the published data, but now I order dished pistons which look like what Studebaker V8s came with back in 1955.

Image

jack vines
Jack,
Can you calculate the area ratio of those pistons, and the clearance that you run?
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Re: Piston Design Questions

Post by PackardV8 »

David Redszus wrote: Sat Jan 09, 2021 4:14 pm Jack,
Can you calculate the area ratio of those pistons, and the clearance that you run?
Hi, David,
It's been several years since that Packard V8 build went into my Studebaker pickup. The only spec I remember today is .005" deck height and a .025" steel shim head gasket. Seems tight, but it's a relatively low RPM truck engine. Doesn't ping with good fuel other than a strong uphill pull on a very hot day. Runs like a freight train, which is a obsolete term of satisfaction.

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Re: Piston Design Questions

Post by skinny z »

Well, it would seem it's a tale of two points of view.
One camp firmly endorses the importance of an ideal quench and the benefits it entails.
The other is suggesting that the PTH is inconsequential and that the importance is in the CR target. (Unless I missed the point there).
In a perfect world, I'd have both. Flat top piston that's 40 thou from the head. Like the 355.
That isn't going to happen given the heads in hand and the 383 architecture.

As suggested, it looks more like a mix and match deal rather some pre-packaged "kit".
Something I'll have to discuss with my machinist.
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Re: Piston Design Questions

Post by lefty o »

call some of the piston companies. there's been enough 383's built over the years, they have likely encountered it before, and have something sitting on the shelf.
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