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Check My Math - Compression Ratio

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Ken_Parkman View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ken_Parkman Quote  Post ReplyReply Direct Link To This Post Posted: Sep/19/2018 at 10:01pm
I get 10.2 as well with my spreadsheet. Wiseco lists that piston in the catalog at 10.5 for the 58 cc head, probably computed with a stock deck; why you have to measure and check your particular combo.

BTW not that you asked, and I probably should shut up cause I go against the general opinion here. But I consider 10.2 too much for pump gas and a stock head if you drive it normally and will get in a traffic jam on a hot day. Seen way too many broken pistons in the past from those pushing the limits. For real use with an iron head I suggest stay to about 9.5 max. You can go higher with the better chamber in the Ed head and you have your deck clearance right, and pay attention to things like inlet air temp and operating conditions. Race use you can go higher than street use cause it is easier to control the conditions.

You will get different opinions.

But it is a massive mistake to compare a modern centrally located plug chamber on a small bore where the flame travel is a fraction of the distance to an offset plug chamber on a big bore. Different animals altogether.
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PHAT69AMX View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PHAT69AMX Quote  Post ReplyReply Direct Link To This Post Posted: Sep/19/2018 at 10:05pm
No there ya go confusing the issue with facts, good sense, and real world hands-on experience... Wink
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AMXrated Quote  Post ReplyReply Direct Link To This Post Posted: Sep/19/2018 at 10:12pm
Thanks.  I plan to measure everything out once I have all the parts and pieces. I was using Wiseco's CR as a reference only and couldn't figure out why I couldn't come up with anything close to their number.

I figured I was going to be pushing the envelope with CR/pump gas.  The car is not going to be a daily driver, but your point is well taken.

--Jack 
You can have my Hurst shifter when you pry it from my cold dead hand.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PHAT69AMX Quote  Post ReplyReply Direct Link To This Post Posted: Sep/19/2018 at 10:32pm
There's more than one way to get a woman undressed... uh... I mean Skin a Cat...
One approach to take, which means nothing in particular, is...
1st Selected an honest working target RPM Range...
That "dictates" the Camshaft and once chosen the Intake Valve Closing Angle is known.
With that known it "effects" the Dynamic Effective CR and Static CR "relationship"...
Head material and "style" and Fuel Octane "dictate" the "tolerable" Dynamic Effective CR.
Then measure all real finished machined parts.
Then "select" Pistons that yield a Static CR that results in the "tolerable" Dynamic Effective CR.
The Pump Gas "tolerable" Dynamic Effective CR is the "crux" of the biscuit...
and "the crux of the Biscuit, is the apostrophe..."



Edited by PHAT69AMX - Sep/19/2018 at 10:41pm
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AMXrated View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AMXrated Quote  Post ReplyReply Direct Link To This Post Posted: Sep/19/2018 at 10:57pm
"It isn't necessary to imagine the world ending in fire or ice. There are two other possibilities: one is paperwork, and the other is nostalgia."
--Frank Zappa

Man was a genius...
You can have my Hurst shifter when you pry it from my cold dead hand.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 1982AMCConcord Quote  Post ReplyReply Direct Link To This Post Posted: Sep/20/2018 at 6:49am
Originally posted by AMXrated AMXrated wrote:

  
Have a set of Wiseco Pro Tru Flat tops (p/n PTS539A3) with a stated CR of 11.22:1.

Also, it appears the extra .010" on the Scat rods do not have an extreme effect on CR.


I have been told that the Scat Rods won't work with the Wiseco pistons without added work.  

Does anyone know if they will work together or if they need work? 

This was the combo I am interested in.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 304-dude Quote  Post ReplyReply Direct Link To This Post Posted: Sep/20/2018 at 6:58am
Originally posted by AMXrated AMXrated wrote:

<span style="color: rgb51, 51, 51; font-family: "helvetica neue", Helvetica, Arial, sans-serif; font-size: 14px;">"It isn't necessary to imagine the world ending in fire or ice. There are two other possibilities: one is paperwork, and the other is nostalgia."</span>
<span style="font-size: 0.75em; white-space: pre;">                                                  </span>--Frank Zappa

Man was a genius...


A lot of artists are geniuses... The ones doing their work for big money usually are not the genius peeps think they are. Most every genius thst I have noticed never got paid what they were worth.

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79 F150 rear & 8.8 axles
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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: Sep/20/2018 at 7:42am
I'm in agreement with Ken Parkman on compression. 9.5:1 (maybe 10:1) on pump premium is about tops for a street engine. That is with old style chambers like all AMCs. Typically you can go a full point higher (10.5:1) with aluminum heads because they reject heat (cool) better.

Modern engines that run higher compression like the Hondas not only have better head/chamber designs, but they also benefit from precise computer control for fuel mixture and timing. Take the computer off and try running one on a carb and distributor and it will be a lot harder to get it to run right.
Frank Swygert
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AMXrated Quote  Post ReplyReply Direct Link To This Post Posted: Sep/20/2018 at 7:53am
Originally posted by 1982AMCConcord 1982AMCConcord wrote:

Originally posted by AMXrated AMXrated wrote:

  
Have a set of Wiseco Pro Tru Flat tops (p/n PTS539A3) with a stated CR of 11.22:1.

Also, it appears the extra .010" on the Scat rods do not have an extreme effect on CR.


I have been told that the Scat Rods won't work with the Wiseco pistons without added work.  

Does anyone know if they will work together or if they need work? 

This was the combo I am interested in.

I can't find any mention of this online anywhere but that probably doesn't mean anything.  I ordered a set of the Scat rods last night so I hope not...I emailed Scat to ask the question and will let you know what they say.
You can have my Hurst shifter when you pry it from my cold dead hand.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 304-dude Quote  Post ReplyReply Direct Link To This Post Posted: Sep/20/2018 at 8:14am
Originally posted by farna farna wrote:

I'm in agreement with Ken Parkman on compression. 9.5:1 (maybe 10:1) on pump premium is about tops for a street engine. That is with old style chambers like all AMCs. Typically you can go a full point higher (10.5:1) with aluminum heads because they reject heat (cool) better.

Modern engines that run higher compression like the Hondas not only have better head/chamber designs, but they also benefit from precise computer control for fuel mixture and timing. Take the computer off and try running one on a carb and distributor and it will be a lot harder to get it to run right.


Correct, but I did mention rich AFR, was part of my expected setup. Honda runs their engines a bit rich. Many have added 50 to 100 hp NOS kits without a tune, and never found any issues as a result.

It will help running rich, but aluminum heads are key to getting high compression to work with pump gas. Another is water or methanol injection. Some have been used since the early 70s. It can help a lot with iron heads. My 65 Riviera had an aftermarket water injection kit. The 401 engine had 11:1 compression and ran very well on pump gas, as long as I maintained the water at every fill up.
71 Javelin SST body
390 69 crank, 70 block & heads
NASCAR SB2 rods & pistons
78 Jeep TH400 w/ 2.76 Low
50/50 Ford-AMC Suspension
79 F150 rear & 8.8 axles
Ford Racing 3.25 gears & 9" /w Detroit locker
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