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Hemi piston ID #3082349 10/01/22 08:58 PM 10/01/22 08:58 PM
Joined: Mar 2010 Posts: 213 Canada C Cape OP enthusiast
Cape OP enthusiast C Joined: Mar 2010 Posts: 213 Canada	Hey every! I’m trying to figure out what pistons I have in my Hemi. Sounds stupid as you’d think I would know after having the engine built. So they were suppose to be CP 12.5:1 pistons but I think the builder put the wrong ones in. I pulled the oil pan off to see if I could see a part number and all I found was the CP logo and 426H-C, I can’t find any info on these pistons. The motors a 4.28 bore and 4.15 stroke with a 6.86 rod. The motor made 827@8000 and didn’t really perform like it should have that’s why I pulled the pan, and I have a buddy with the same short block but with Ross 13.5:1 piston which is completely different. Any help would be appreciated

Re: Hemi piston ID [Re: Cape] #3082371 10/01/22 09:56 PM 10/01/22 09:56 PM
Joined: Mar 2011 Posts: 3,068 Michigan A A727Tflite master
A727Tflite master A Joined: Mar 2011 Posts: 3,068 Michigan	Put the pan back on it and be happy.

Re: Hemi piston ID [Re: A727Tflite] #3082383 10/01/22 11:03 PM 10/01/22 11:03 PM
Joined: Jan 2012 Posts: 2,630 Stuttgart, Arkansas rickseeman master
rickseeman master Joined: Jan 2012 Posts: 2,630 Stuttgart, Arkansas	You will probably have to cc the dome yourself and figure out what you have. 2011 Drag Pak Challenger

Re: Hemi piston ID [Re: Cape] #3082392 10/01/22 11:58 PM 10/01/22 11:58 PM
Joined: Aug 2003 Posts: 43,099 Bend,OR USA C Cab_Burge I Win
Cab_Burge I Win C Joined: Aug 2003 Posts: 43,099 Bend,OR USA	Originally Posted by Cape Hey every! I’m trying to figure out what pistons I have in my Hemi. Sounds stupid as you’d think I would know after having the engine built. So they were suppose to be CP 12.5:1 pistons but I think the builder put the wrong ones in. I pulled the oil pan off to see if I could see a part number and all I found was the CP logo and 426H-C, I can’t find any info on these pistons. The motors a 4.28 bore and 4.15 stroke with a 6.86 rod. The motor made 827@8000 and didn’t really perform like it should have that’s why I pulled the pan, and I have a buddy with the same short block but with Ross 13.5:1 piston which is completely different. Any help would be appreciated 426 street hemiroid motors are hard to figure the correct compression ratio without knowing the exact deck height on the pistons above the deck, the dome volume, combustion chamber volume, head gasket volume and the bore and stroke. I would check the actual deck height at TDC first with one head off and then move the piston down to either zero deck height or maybe .500 below the deck surface and then seal the ring lands up to the cylinder walls with chassis grease and then find out what that actual volume is and then do the math on a flat top piston that far down and then due the math to get your dome volume and use that information to get the true compression ratio Don't be surprised if it is not exactly what the piston maker said they would be Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)

Re: Hemi piston ID [Re: A727Tflite] #3082456 10/02/22 12:01 PM 10/02/22 12:01 PM
Joined: Mar 2010 Posts: 213 Canada C Cape OP enthusiast
Cape OP enthusiast C Joined: Mar 2010 Posts: 213 Canada	Thanks for the suggestion. I don’t have the motor out yet, in a couple weeks it’ll be out so I’ll so I’ll get a better chance to look at it. Does anybody know if the number 426H-C on the underside of the piston mean anything? I can’t get to the top of the piston until I get the heads off. Last edited by Cape; 10/02/22 12:05 PM.

Re: Hemi piston ID [Re: Cape] #3082458 10/02/22 12:06 PM 10/02/22 12:06 PM
Joined: Mar 2011 Posts: 3,068 Michigan A A727Tflite master
A727Tflite master A Joined: Mar 2011 Posts: 3,068 Michigan	Originally Posted by Cape Thanks for the suggestion. I don’t have the motor out yet, in a couple weeks it’ll be out so I’ll so I’ll get a better chance to look at it. Personally I am still confused. You said it’s not performing like it should. 800+ horsepower and it’s not performing like it should?

Re: Hemi piston ID [Re: Cape] #3082477 10/02/22 01:37 PM 10/02/22 01:37 PM
Joined: Aug 2003 Posts: 43,099 Bend,OR USA C Cab_Burge I Win
Cab_Burge I Win C Joined: Aug 2003 Posts: 43,099 Bend,OR USA	Originally Posted by Cape Thanks for the suggestion. I don’t have the motor out yet, in a couple weeks it’ll be out so I’ll so I’ll get a better chance to look at it. Does anybody know if the number 426H-C on the underside of the piston mean anything? I can’t get to the top of the piston until I get the heads off. I'm thinking that 426HC means 426 Hemi Chrysler so they can grab the correct tool (CRS on the proper name of that tool ) to use when they stamp, forge, the aluminum blanks into a piston There are several other brands of race hemi motors like Brad Andersen, Allen Johson performance and as several others I can't think of now, which do not use the same combustion chambers on their race heads hence the need for different piston forging to match the heads The original TRW, Speed Pro factory 10.5 to 1 compression ratio street Hemi pistons had the piston dome height above the deck right at .550, the race pistons with 12.5 to 1 ratio dome height was right at .750, the 13.5 to1 was near .800 with a lot taller and narrower dome Last edited by Cab_Burge; 10/02/22 01:41 PM. Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)

Re: Hemi piston ID [Re: Cab_Burge] #3082493 10/02/22 02:24 PM 10/02/22 02:24 PM
Joined: Mar 2010 Posts: 213 Canada C Cape OP enthusiast
Cape OP enthusiast C Joined: Mar 2010 Posts: 213 Canada	Thanks Cab. Combo 69 Dodge Charger 38-3900lbs 478 Hemi 4.28 bore 4.15 stroke G101A 4 speed Advanced clutches twin 8 inch 4.88 gears 10.74@128 @ 3850DA 827hp. Hope the confusion is cleared up. There should be more in it is all I’m saying. I had the heads off a couple years ago to get them CNC but never looked to see if there’s a part number on the dome. I was told they were 12.5:1 but they don’t look that much different then the 9.5:1 mopar ones that came out of it. It started life as a mopar crate 426. The first outing this year the car went 11.37@123 with exhaust. When I went again a couple weeks later I took the exhaust off and did a couple jet changes. That’s where the 10.74@128 came from. Then the yoke split in half. If I could post the pictures of my 9.5:1 pistons and the ones in it now you’d see the difference and if I could post my buddy’s piston which is 13.5:1 you’d really see the difference. His is the exact same combo just the pistons are different.

Re: Hemi piston ID [Re: Cape] #3082506 10/02/22 03:24 PM 10/02/22 03:24 PM
Joined: Mar 2011 Posts: 3,068 Michigan A A727Tflite master
A727Tflite master A Joined: Mar 2011 Posts: 3,068 Michigan	Originally Posted by Cape Thanks Cab. Combo 69 Dodge Charger 38-3900lbs 478 Hemi 4.28 bore 4.15 stroke G101A 4 speed Advanced clutches twin 8 inch 4.88 gears 10.74@128 @ 3850DA 827hp. Hope the confusion is cleared up. There should be more in it is all I’m saying. I had the heads off a couple years ago to get them CNC but never looked to see if there’s a part number on the dome. I was told they were 12.5:1 but they don’t look that much different then the 9.5:1 mopar ones that came out of it. It started life as a mopar crate 426. The first outing this year the car went 11.37@123 with exhaust. When I went again a couple weeks later I took the exhaust off and did a couple jet changes. That’s where the 10.74@128 came from. Then the yoke split in half. If I could post the pictures of my 9.5:1 pistons and the ones in it now you’d see the difference and if I could post my buddy’s piston which is 13.5:1 you’d really see the difference. His is the exact same combo just the pistons are different. A bit clearer now- the car doesn’t perform to your satisfaction, and the 827 hp came from a dyno test? There could be a million things wrong here - but if it dynoed at 827 I doubt the pistons are your issue. According to the calculators you are off around 5 tenths using that altitude. Going from 11 to 1 to 13 to 1 back in the day was worth about 2 mph and a tenth and a half. May be with climate change it’s more now. Last edited by Transman; 10/02/22 03:51 PM.

Re: Hemi piston ID [Re: A727Tflite] #3082519 10/02/22 04:14 PM 10/02/22 04:14 PM
Joined: Mar 2005 Posts: 3,021 NY B B1duster master
B1duster master B Joined: Mar 2005 Posts: 3,021 NY	I’m pretty sure CP does not sell Hemi shelf pistons, only custom. Measure the dome height and compare with different brand hemi 4.280 shelf pistons that list dome height and CC’s. Then use online compression calculator to get a rough idea of where it’s at.The increase in stroke increases compression.

Re: Hemi piston ID [Re: B1duster] #3082540 10/02/22 06:35 PM 10/02/22 06:35 PM
Joined: Mar 2011 Posts: 3,068 Michigan A A727Tflite master
A727Tflite master A Joined: Mar 2011 Posts: 3,068 Michigan	I feel you are wondering just about the dome - buy a Harbor Freight fiber optic camera. On a Hemi, piece of cake to see the dome and maybe the part number. If intent on pulling the motor I personally would leak it and check compression too. Point of reference.

Re: Hemi piston ID [Re: A727Tflite] #3082560 10/02/22 08:01 PM 10/02/22 08:01 PM
Joined: May 2019 Posts: 710 Lake Villa Il INTMD8 super stock
INTMD8 super stock Joined: May 2019 Posts: 710 Lake Villa Il	I didn't think CP had shelf hemi pistons either. No part number on mine 69 Charger. 438ci Gen2 hemi. Flex fuel. Holley HP efi. 650rwhp @7250 510rwtq @5700

Re: Hemi piston ID [Re: A727Tflite] #3082592 10/02/22 10:40 PM 10/02/22 10:40 PM
Joined: Mar 2010 Posts: 213 Canada C Cape OP enthusiast
Cape OP enthusiast C Joined: Mar 2010 Posts: 213 Canada	I found an old picture of the top of the piston, it has carbon on it and I can’t see any numbers on it. I am concerned that the pistons are wrong, so the motor was built back in 2012 and I had the heads CNC a couple years later so the heads came off. I called CP the other day and asked if they had an order from the engine builder, they told me his last order was 2007 and I have the piston box they came in which has the 2007 date on it, the guy at CP said the pistons that were ordered were 11.1. I’ll be calling them tomorrow to see if the 426H-C means. I’m pretty sure there wrong and the heads haven’t been decked at all. Thanks for all the suggestions and help

Re: Hemi piston ID [Re: Cape] #3082669 10/03/22 10:28 AM 10/03/22 10:28 AM
Joined: May 2019 Posts: 710 Lake Villa Il INTMD8 super stock
INTMD8 super stock Joined: May 2019 Posts: 710 Lake Villa Il	I checked again and mine do have a job number on the underside. The 426 HC is the forging number. You would have to measure everything of course but you would probably need around 100cc dome for 12.5-1 69 Charger. 438ci Gen2 hemi. Flex fuel. Holley HP efi. 650rwhp @7250 510rwtq @5700

Re: Hemi piston ID [Re: INTMD8] #3083044 10/03/22 09:40 PM 10/03/22 09:40 PM
Joined: Mar 2010 Posts: 213 Canada C Cape OP enthusiast
Cape OP enthusiast C Joined: Mar 2010 Posts: 213 Canada	Thanks for posting your order sheet. Mine look nothing like yours, there’s not much more dome on them then the ones that came with it and there a 9.5:1 piston. I did find out from CP today that the part number on the box was for the 11.1. I’ll be pulling the motor and measuring them up. Thanks again for all the help.

Re: Hemi piston ID [Re: Cape] #3083169 10/04/22 10:04 AM 10/04/22 10:04 AM
Joined: May 2019 Posts: 710 Lake Villa Il INTMD8 super stock
INTMD8 super stock Joined: May 2019 Posts: 710 Lake Villa Il	Curious if you have the dyno sheet and what rpm were you shifting at? As Transman alluded to, making 827 at 8000, it should still put down numbers for that power level even if the compression is less than you thought. Might even need more gear if you're shifting it at 8400 or so 69 Charger. 438ci Gen2 hemi. Flex fuel. Holley HP efi. 650rwhp @7250 510rwtq @5700

Re: Hemi piston ID [Re: INTMD8] #3083376 10/04/22 08:29 PM 10/04/22 08:29 PM
Joined: Mar 2010 Posts: 213 Canada C Cape OP enthusiast
Cape OP enthusiast C Joined: Mar 2010 Posts: 213 Canada	I do have the dyno sheet, it’s 827@8000, 8000 is where the curve started to flatten out. I’m shifting it at 8000 and going through the traps around 76-7800. I don’t have any data logging equipment besides my playback tach which wasn’t working at the time, the tach was working so I saw 76-7800 through the traps. The guy that built the motor passed away earlier this year so I can’t ask him, I’m putting the motor back on the exact same dyno as it was on back in 2018 so I’ll see exactly what it made when I ran the 10.74@128.

Re: Hemi piston ID [Re: Cape] #3083499 10/05/22 09:50 AM 10/05/22 09:50 AM
Joined: Jan 2003 Posts: 13,351 Marion, South Carolina [><] an8sec70cuda I Live Here
an8sec70cuda I Live Here Joined: Jan 2003 Posts: 13,351 Marion, South Carolina [><]	How are you launching it? 60 foot? That cubic inch making that kind of power is gonna have a pretty high torque peak. My 575" hemi makes similar power and needs a 5,000 rpm launch. Yours should be a lot higher. CHIP '70 hemicuda, 575" Hemi, 727, Dana 60 '69 road runner, 440-6, 18 spline 4 speed, Dana 60 '71 Demon, 340, low gear 904, 8.75 '73 Chrysler New Yorker, 440, 727, 8.75 '90 Chevy 454SS Silverado, 476" BBC, TH400, 14 bolt '06 GMC 2500HD LBZ Duramax

Re: Hemi piston ID [Re: Cape] #3083507 10/05/22 10:12 AM 10/05/22 10:12 AM
Joined: May 2019 Posts: 710 Lake Villa Il INTMD8 super stock
INTMD8 super stock Joined: May 2019 Posts: 710 Lake Villa Il	Maybe I'm misunderstanding but it peaks at 8000? Would need to do the math on it but you would need to shift it past peak hp so you have max average hp going down the track. 69 Charger. 438ci Gen2 hemi. Flex fuel. Holley HP efi. 650rwhp @7250 510rwtq @5700

Re: Hemi piston ID [Re: Cape] #3083601 10/05/22 04:35 PM 10/05/22 04:35 PM
Joined: Aug 2003 Posts: 43,099 Bend,OR USA C Cab_Burge I Win
Cab_Burge I Win C Joined: Aug 2003 Posts: 43,099 Bend,OR USA	I use engine dyno testing to see what the changes I do on the motor that day read on the dyno sheets, I have found once I have maxed the motor out on the dyno I may, and I have had to several times, retune the motor in the car at the track to get the best time slip and spark plug readings The only thing that can't normally be made to LIE is the time slip and spark plugs when you shut the motor off at the finish line after shifting it into neutral My message is the engine on the dyno is sitting in a control environment room with no oil or gasoline being slosh around like actually happens to the motor in the car As already brought up you need to test at the track on the starting line RPM and shift RPM to find out what makes YOUR CAR go the fastest that day Last edited by Cab_Burge; 10/05/22 04:37 PM. Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)

Re: Hemi piston ID [Re: an8sec70cuda] #3083885 10/06/22 05:53 PM 10/06/22 05:53 PM
Joined: Mar 2010 Posts: 213 Canada C Cape OP enthusiast
Cape OP enthusiast C Joined: Mar 2010 Posts: 213 Canada	I’m still trying to find the sweet spot for the launch, 5200 is blowing the tires off and 4000 it just dead hooks the tire 4800 it still spun but not as bad. I’m also still adjusting the clutch and shocks as well. I just ran outta time to figure out the suspension and clutch.

Re: Hemi piston ID [Re: INTMD8] #3083887 10/06/22 05:56 PM 10/06/22 05:56 PM
Joined: Mar 2010 Posts: 213 Canada C Cape OP enthusiast
Cape OP enthusiast C Joined: Mar 2010 Posts: 213 Canada	It did make peak at 8000. So the power difference between 7500 and 8000 wasn’t much so that’s why I’m shifting it at 8000.

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