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401 cylinder pressure numbers |
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BassBoat 
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Joined: Aug/29/2008 Status: Offline Points: 1719 |
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 Posted: May/05/2011 at 9:42am |
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Per Mr. Sanchez's post. "Cranking compression is dependent on compression ratio, ring seal, valve seal, camshaft. Some what on intake and exhaust system(restrictions will change comp)"
Cranking compression is not a good indicator of static compression ratio. Most likely the reason you see more cranking compression is that you have a mild cam with wide lobe sep. and early intake closing.
bb Edited by BassBoat - May/05/2011 at 9:44am |
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Johna
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Joined: Feb/02/2010 Location: NS Canada Status: Offline Points: 35 |
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Posted: May/05/2011 at 9:59am |
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I called Maddog and got the phone# of the manufacturer of the pistons and there going to fax me a copy of the piston drawing so I can get my volumes and clearance numbers. Tech rep said that they are 9.5:1 pistons (4 pocket) for standard 72-75 heads.
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whizkidder
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Joined: Mar/03/2008 Location: North Georgia Status: Offline Points: 2972 |
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Posted: May/05/2011 at 12:29pm |
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I'm running a Comp 280H -- about .490 lift, and about 230 duration at .050.
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Ron Frost
marne1ancient @ gmail.com 910 nine two two 0563 "There is no limit to what a man can do, so long as he does not care a straw who gets credit for it. Charles Montague |
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amx39068
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Joined: Feb/21/2008 Location: Arizona Status: Offline Points: 11576 |
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Posted: May/05/2011 at 1:42pm |
Don't take the reps word for it . Make sure you get the actual compression height and cc volume for the 4 valve recesses so you can figure it out for yourself. "Standard" valve reliefs are 28cc for high compression 401 pistons but 41cc for 72 and above 401 engines. Both early and late engines should have a 1.508 compression height but on the later pistons it is only on the outer ring land area at the outer edge of the soup bowl dish.
If you have 28 cc pistons and 1.508 compression height then indeed your pistons are the same as stock pistons and a standard bore will yield around 9.5:1 with 58cc heads on a block that has not been decked and with heads that have not been milled. If the heads have been milled it will shrink the chamber volume depending on how much was milled.. Also, over bore raises compession so that needs to factored in as well.
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Dan Curtis-Owner and CEO AZ AMC Restorations; Facebook: https://www.facebook.com/amcmusclecars/ & Curtis Real Estate Development
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amx39068
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Posted: May/05/2011 at 1:48pm |
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Almost forgot, the reason they are saying their cast pistons are 10:1 at .030 over is that they are as suggested by BB to be most likely .030 shorter than stock compression height.
The reason they make cast pistons shorter is to allow overbore, block decking and head planing to create roughly the same compression ratio as the stock taller pistons with in the original unmolested block and heads.
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Dan Curtis-Owner and CEO AZ AMC Restorations; Facebook: https://www.facebook.com/amcmusclecars/ & Curtis Real Estate Development
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Johna
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Joined: Feb/02/2010 Location: NS Canada Status: Offline Points: 35 |
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Posted: May/05/2011 at 5:04pm |
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whizkidder, that's starting to make sense. You cam has a wider duration and it's allowing more time to bleed down the cylinder pressures. When I was selecting the cam for this rebuild I called Crane and spent some time with the rep. He was concerned about me wanting to use a torquer cam with these pistons. He was concerned about bleed down time. I stated that I was not using this engine for racing but only cruising and it would not see rpms above 3500. He still didn't want me to use the torqer cam and he reluctantly agreed on the next one up the H-260-2. It's worked out well. I'm happy with the performance of the engine but I can now see with all these discussions that at 193psi I am at the limit of cylinder pressures with pump gas.
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17tamx
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Posted: May/05/2011 at 6:53pm |
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190 to 195 psi is what I shot for on my 70 390 I built for my AMX. I run the 91 octane pump gas with out any pinging or detonation problems. I am running an older TRW (close to group 19 spec) cam with 222 222 @ .050 .478 .478 lift 108 LSA installed 4 degrees advance. I also wanted to make the most torque possible with readily available pump gas and am very happy with the results.
I want to be able to make some road trips to some of the future upcoming events and shows. If I tow the car than I have to take the kids, not as much fun (did I type that) or if I take the kids then I have to tow the car, not as much fun (what I should have typed).
I think you should be very happy to be making 192-197 psi in your 401 without having and pinging and detonation problems.
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Kirk P. Fletcher
70 AMX BBG w/Shadow 390 4sp 71 SC360 Wild Plum Ram Air 4sp 67 Rogue Convert 343 4sp 66 Rogue Hardtop 290 Auto 66 440 Convert 232 Auto |
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addic
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Joined: May/01/2008 Location: Kansas City Status: Offline Points: 370 |
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Posted: May/05/2011 at 8:58pm |
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I have 220 psi on my 401 , somebody goofed on my piston order and I never checked when I put it together,couldn't get the pinging out, but guys on the forum helped me figure it out,flat tops with 54cc heads, still dont know what the exact comp. .... I just run e 85 now and runs great
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Mean71
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Posted: Aug/21/2011 at 7:22pm |
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Hey, saw this topic while doing a search...what's normal readings for a stock 401 cylinder pressure?
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Johna wrote: