leaded or unleaded fuel

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dartlove said:
Those are absolutely correct numbers. My father inlaw came up with 14+ as well.
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You have to remember to enter a positive deck height as a NEGATIVE on Wallace's calculator. If you enter .023 deck, his calculator assumes that's. 023 in the hole.
A dome piston with a chamber that small, and a stroke that big, you can end up with lots of compression quick!
 
dartlove said:
Dome piston +6.5 comes out of the hole at the flat .023" . .060" mls gasket. 4.310 bore 4.250 stroke 70cc heads
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You have no idea what the dome volume of the piston is unless you do a downfill and measure it. Otherwise it’s a guess, and probably a poor one.

Can’t count how many engines came through the shop that the owner said has XX.XX compression ratio and 99% of them were a point or more off.
 
yellow rose said:
You have no idea what the dome volume of the piston is unless you do a downfill and measure it. Otherwise it’s a guess, and probably a poor one.

Can’t count how many engines came through the shop that the owner said has XX.XX compression ratio and 99% of them were a point or more off.
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Yes, but what if the piston manufacturer says it has a certain dome volumn? Do you not believe them?
I absolutelyagree that lots of times people don't have the cr they think they do, because they simply don't measure. I have a trw "11 1/2 to 1" engine that is really in the low 10s, determined by actual deck height and chamber size measurements . I have another with arias "12 1/2" pistons that are more than .050 in the hole ( damned if I'm gonna deck a real max block .050 !), and with an 84cc max head is barely 11 to 1.
 
yellow rose said:
You have no idea what the dome volume of the piston is unless you do a downfill and measure it. Otherwise it’s a guess, and probably a poor one.

Can’t count how many engines came through the shop that the owner said has XX.XX compression ratio and 99% of them were a point or more off.
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What is a downfill? I don't know exactly how to do the math, but the inlaw, loves to do all the figuring.
 
To do a downfill, you set the piston down in the hole a certain amount ,usually an inch, seal up the ring gap with some grease, cover over the cylinder with a head cc plate, and then cc the space. Then you calculate what the actual cc of that space should be, and compare to what you measured. The difference will be your dome,dish or valve relief volume
 
33IMP said:
To do a downfill, you set the piston down in the hole a certain amount ,usually an inch, seal up the ring gap with some grease, cover over the cylinder with a head cc plate, and then cc the space. Then you calculate what the actual cc of that space should be, and compare to what you measured. The difference will be your dome,dish or valve relief volume
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I use a half inch unless it’s a hemi with a big dome.
 
I agree, a half inch works better. Uses less fluid, less chance for measurement error. It all depends on how tall the dome is, lol. (Ever seen a 12 1/2 302 chevy piston?)
 
33IMP said:
Yes, but what if the piston manufacturer says it has a certain dome volumn? Do you not believe them?
I absolutelyagree that lots of times people don't have the cr they think they do, because they simply don't measure. I have a trw "11 1/2 to 1" engine that is really in the low 10s, determined by actual deck height and chamber size measurements . I have another with arias "12 1/2" pistons that are more than .050 in the hole ( damned if I'm gonna deck a real max block .050 !), and with an 84cc max head is barely 11 to 1.
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I believe no one. In 1994 I took on another engine builders engine because his stuff was slow.

It was a 9:1 class and it was enforced. We were reusing the pistons and I had all the paper work for the pistons and everything else.

I no longer remember the exact number of the dish, but what the paperwork said and what they were was not the same. Not by 7cc’s. They were 7cc’s too SMALL.

So when the owner went from a back marker to running up front, he got checks and FAILED. And that was on ME.

The vast majority of dome/dish volumes are not what is claimed. If you have a compression rule, you damn well better know because if you don’t, you’ll get burned.


Never trust anything. Measure everything. Then you KNOW.
 
33IMP said:
You have to remember to enter a positive deck height as a NEGATIVE on Wallace's calculator. If you enter .023 deck, his calculator assumes that's. 023 in the hole.
A dome piston with a chamber that small, and a stroke that big, you can end up with lots of compression quick!
Click to expand...
Agree!
will be real interesting how much crank compression it has.
 
I mix 3 gal of 93 pump gas to 2 gal 110 race gas. I do this b/c I really don't need the oct level of race gas, but with the ethanol in pump gas I had issues with pure pump gas boiling in the carb esp on hot days. I do this in 3 of my cars without an issue. They all run very consistent. The compression rations are only 10:1 to 11:1 in all 3 cars though.
 
Here is an example of why you need to cc the piston(down fill) to be accurate on your compression.
My 340 piston set .017" above the deck.
as you can see below that creates a 3.57 cc dome above the deck.
upload_2020-7-30_7-6-22.png

now drop it down the hole .500" and cc the volume.
4.04 bore .500 stroke=105.03 cc. My down fill volume was 112.04 aka like a dish piston would do. but it has a pop up??????
The reason why is that the Valve reliefs were larger then the pop up of the piston.
In other words i had a pop up piston. but the valve reliefs made it act like a dish piston.
aka a 10.5 piston figured out to be only 9.4:1 compression.
upload_2020-7-30_7-33-17.png


I may have fudged a little on my math but you get the point.
 
Cudafever said:
Here is an example of why you need to cc the piston(down fill) to be accurate on your compression.
My 340 piston set .017" above the deck.
as you can see below that creates a 3.57 cc dome above the deck.
View attachment 1715570023
now drop it down the hole .500" and cc the volume.
4.04 bore .500 stroke=105.03 cc. My down fill volume was 112.04 aka like a dish piston would do. but it has a pop up??????
The reason why is that the Valve reliefs were larger then the pop up of the piston.
In other words i had a pop up piston. but the valve reliefs made it act like a dish piston.
aka a 10.5 piston figured out to be only 9.4:1 compression.
View attachment 1715570033

I may have fudged a little on my math but you get the point.
Click to expand...


Perfect example of why you MUST down-fill and measure for compression ratio or you have no clue what the CR really is.

If you think you have 11:1 but it’s really 10:1 (or less) and you order the cam for the higher CR, the engine won’t be happy.

I can’t stress enough that you have to verify the CR. If your engine builder won’t or can’t, find a new one.
 
So then more than likely a 8.5 to 1 340 is only 7.5 as the piston is .100 thou shorter. I hate to think what a lo po 440 is with the piston down in the hole .150 thou. Kim
 
I have said this before but the ironic thing is that a 10.5 340 piston was based off a 62 cc head(shaved) and a .020" piston above deck height(block decked)
How Ever! the 11.5 domed pistons were 11.2 with out shaving the head and with out decking the block.
When i build my first 340 and figured out it was only going to be 9.4 i bought one, 11.5 piston, hoping that it would come in around 10.5 NOPE 11.2 So i settled for the flat top at 9.4:1
If i new then what i know now, i would have ran the 11.5 piston, on 91 octane fuel and been happy!
 
oldkimmer said:
So then more than likely a 8.5 to 1 340 is only 7.5 as the piston is .100 thou shorter. I hate to think what a lo po 440 is with the piston down in the hole .150 thou. Kim
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oldkimmer said:
So then more than likely a 8.5 to 1 340 is only 7.5 as the piston is .100 thou shorter. I hate to think what a lo po 440 is with the piston down in the hole .150 thou. Kim
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The .150 plus in the hole 440s are somewhere in the low seven range. 7.2 to 7.4 to one.
I've got a factory aluminum head chrysler eight, with 6 to 1.
 
Cudafever said:
Agree!
will be real interesting how much crank compression it has.
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170psi cranking pressure. Wow kinda lower than I thought it would be. I thought for sure it would be around 190- 200psi
 
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The cam has a lot to say with how much crank pressure you have.........but 170 sames kinda........well 170 isn't low but......
did you give us your cam spec's?
 
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