Calculating Distance Below Deck

[bajajoaquin]

I'm looking at various engine combinations, and I'm confused about how to calculate the "Distance below deck" of the piston.

It looks like a stock mid-60s 440 has a 3.375" stroke, 10.725" deck height, 2.067" compression height, and 6.76" rods. The most common measurement I see for "distance below deck" is 0.018".

However, if I take half the stroke (1.6875") plus the compression height and rod length, I only come up with 10.5145", or .2105" in the hole.

Where's that extra .1925" coming from?

 

[Big_John]

440 has a 3.75 stroke.

 

[70bigblockdodge]

1.875 - 1.6875= .1875

 

[moper]

I think you are confusing calculating dimensions vs calculating measurements. The only way to actually get the installed piston height (which is what you are referring to) is to measure the engine you have. The factory blueprint spec for the block deck height is 10.725. But that's the BLUEPRINT spec. Not the reality of factory machining. I have never seen a factory machined block that was at blueprint. They are all "tall" by some amount. Sometimes a lot (especially the 383s). You have tolerance stack between the block dimensions and parts dimensions, so the reality is these engines were never what the engineers designed. Some were closer than others but all were "less". Additionally you have at least two different pistons in the passenger car and light truck 440s depending on the year (66-69). IIRC they have different compression heights.
The deal is, its best to pull a head and measure if you really want to know for sure. Otherwise, assume it's around 9:1 static compression and choose components from that.

 

[70bigblockdodge]

As mentioned above, measured is only way to know what you have. I believe he is paper building to see what combos work out to be. None of the factory specs will be right all will be worse, or away from performance.
The 400 I slapped together for my Challenger measured out at 8.43:1 cr it was a 73 engine pistons were .130 down in the hole, feel pro gasket .040 and 516 casting heads that were milled down to 73cc (measured) combustion chamber. It will run on 89 octane with a lot of timing in it to help build cylinder pressure, I have too much overlap, but it was a cam I already owned. It runs great above 3000 rpm
Needs a cam change for drivability, it's on the list after my cage is done in my A body.
I should go back and cc the 346 heads that came off that 400 just to see how bad the ratio was dead stock.

 

[bajajoaquin]

440 has a 3.75 stroke.

Yes, you’re right. I got it right in my

440 has a 3.75 stroke.

Holy (poop)! That's exactly the problem! Doh!

(As to why I'm doing this, the paper engine is right. I'll reply more later.)

 

[70bigblockdodge]

(As to why I'm doing this, the paper engine is right. I'll reply more later

It was a test?
It's to bait us for your real question?
We gotta know. I'm sitting here bored waiting on this damn backhaul.

 

[bajajoaquin]

test? No. Just wrangling the kids for school and I want to discuss in greater detail. Essentially I want to figure out to have a taller piston with a stoker motor.

 

[Polara_500]

test? No. Just wrangling the kids for school and I want to discuss in greater detail. Essentially I want to figure out to have a taller piston with a stoker motor.

???

Huh?? A stroker is built with a shorter piston and a longer rod throw, increasing the swept volume. A taller piston defeats the very idea.

 

[bajajoaquin]

Does it? That is what is done, but is that the end goal?

 

[70bigblockdodge]

test? No. Just wrangling the kids for school and I want to discuss in greater detail. Essentially I want to figure out to have a taller piston with a stoker motor.

I calculated that out in my 400 and the stock piston sitting .130 down in the hole ×2 giving me .260 stroke available to end up at zero deck with a 3.75 440 crank will stick up .060 above a .040 fel-pro gasket. Change that to a Cometic gasket of .055 and some creative belt sander profiling of the piston top add a 91cc open chamber 452 casting with some polish work. Yields a 11-12:1 cast piston low deck 443" sleeper engine. This combo has tormented my daydreams for a while. If I had access to a lathe this would be more torturous. Lucky I'm too cheap to spend $$$ on this silly idea, still does not get it out of my head.

 

[Polara_500]

Okay, you guys are reaching well beyond the little I know. I'm gonna go sit in my corner and be quiet.

 

[70bigblockdodge]

A taller piston defeats the very idea

Taller is quiet, but heavy. Heavy is not what you want to add to make high performance. A short piston gives you less mass and a longer arm/cubic inches, win-win. Everything from pin to oiler ring is dead weight/does nothing.
Problem arises when you do this pistons only come from big $$$ nice stuff, so who wants to spend $900 on pistons, plus associated parts to go to Dairy Queen once a week.

 

[bajajoaquin]

Okay, you guys are reaching well beyond the little I know. I'm gonna go sit in my corner and be quiet.

Please don't. This is beyond my knowledge, too. The issue is that there's lots of info out there on how to make an engine for a 3000-lb car with low gears and a high stall converter. Not a lot of "performance" builds for 5500lb cars not revving over 5k.

 

[Polara_500]

My understanding of a stroker is that you want the pin as high as possible in the cylinder to maximize the rod throw and increase available cubic inch displacement. That has to be limited by deck height as you gotta have room for rings above said pin. I can kinda sorta see some upside with "pop up" pistons and such, but now you're getting into some real special shtuff like funky one off pistons or head configurations and I personally can't see any reasonable benefit - unless of course you're trying to make your living by beating the car next to you by a millisecond. Which in my mind isn't practical in a classic car. Emphasis on practical - 'cause I'm a cheapskate of Scottish heritage.

 

[bajajoaquin]

So here's more:

The goal is to have a low-revving, torquey engine in my Imperial. I am going to be rebuilding the engine anyway, so in my calculations, that's a sunk cost, and I'm trying to focus on where my marginal dollars are best spent. Depending on a lot of things, I'm not sure how much I have budgeted, but it's certainly $1000 more than a good, servicable stock rebuild, and might be as much as $2500. I was trying to avoid putting it in this thread, because that's kind of a bigger, separate discussion, and people are probably tired of it already without me adding it to every thread!

Anyway, when you stroke an engine, you change rod length and piston size so that you keep the "distance below deck" roughly the same. You have limits on rod length due to rod ratio and available piston compression heights. You want shorter pistons because they're lighter and rev faster and make more power.

However, you make those tradeoffs in the pursuit of a certain goal. That goal is maximizing performance. What if your goal is to have a quiet, smooth engine that makes butt-loads of power but feels like a refined stock engine other than being faster? In that case, short pistons aren't an advantage. They're usually forged, which expands more than cast or hypereutectic pistons. So they make more noise when cold, and have more bore wear. They also rock more in the bore than "stock"-ish pistons, causing more wear, making more noise, etc.

Rods have similar tradeoffs. Longer rods are better for some applications because you want a high stroke to bore ratio. A stock 440 has a great one at 1.8:1. A stock 454 has a crappy one at like 1.4 or 1.5:1. But lots of 454s work just fine in low-revving engines for hundreds of thousands of miles (ok, 100,000+). There's something about "dwell" at TDC and BDC that affects performance, but I really don't know how it works. I know that it is a thing, I don't know what that thing is.

So that's the premise. I've got to get back to work, and I'll talk about my thoughts in terms of what's available as a kit, and what other "standard" parts are available.

 

[bajajoaquin]

TA short piston gives you less mass and a longer arm/cubic inches, win-win. .

Half true. The short piston doesn't give you a longer arm. Only the crank throw does that. A shorter piston may enable you to run a longer crank throw, but doesn't give you a longer throw in and of itself.

 

[Mike66Chryslers]

My understanding of a stroker is that you want the pin as high as possible in the cylinder to maximize the rod throw and increase available cubic inch displacement. That has to be limited by deck height as you gotta have room for rings above said pin.

Shortening the piston compression height and increasing the rod length, by themselves, do not change the engine displacement.

To increase the stroke you switch to a crankshaft with a longer stroke, which will increase the piston travel. With everything else unchanged, the piston would then exit the top of the bore at TDC and hit the head, so you must either use shorter connecting rods and/or pistons with shorter "compression height", which is the distance between the wrist pin and the top of the piston, to make the complete rotating assembly fit inside the engine.

One downside to shorter rods is that you wind-up with a larger rod angle partway through the stroke, so there is more sideways force against the cylinder walls leading to frictional losses and faster wear. Shortening the piston, as you said, can lead to the piston rocking more in the bore, which can lead to poor ring sealing and accelerated wear.

The typical formula for a 451 stroker is to use a 440 rotating assembly (crank and rods) in a 400 block, which has a shorter deck than a 440. Relative to a stock 400, the crank is stroked AND the rods are longer (6.768" vs 6.358" rods); the new piston must be short enough versus a stock 400 piston to accommodate both of these changes.

If you are starting with a 440 block and aftermarket stroker crank, I'm sure that you would not want to use rods which are any shorter than stock 440 rods, and compensate for the longer crank stroke entirely with a shorter piston compression height.

 

[70bigblockdodge]

Throwing another wrench in the works is the latest quest for tight quench. This is because the manufacturers have figured out that creating turbulence in the combustion chamber helps to suppress detonation. Dead zone off to the side chamber and way off center spark plugs are all negatives for emissions and power. Big blocks have these issues in spades.
So you would get more bang for the buck by fixing these problems and blueprinting your engine instead of trying to stroke it cheaply.
That is my opinion. You see where my mind wanders off to on paper so have fun.

 

[Mike66Chryslers]

Throwing another wrench in the works is the latest quest for tight quench. This is because the manufacturers have figured out that creating turbulence in the combustion chamber helps to suppress detonation. Dead zone off to the side chamber and way off center spark plugs are all negatives for emissions and power. Big blocks have these issues in spades.

Yep. On my 451, I used Edelbrock closed-chamber heads and Diamond dished pistons. The dishes are milled off to one side on these, so that the flat part of the pistons lines-up very well with the flat part on the heads for quench. The block was milled to zero-deck the pistons at TDC so the quench height is the headgasket thickness, which IIRC was 0.039" for the Felpro O-ringed gaskets. IMO the combination works very well.