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Limits of cast pistons on boost?
https://slantsix.org/forum/viewtopic.php?f=30&t=64431
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Author:  MadScientistMatt [ Tue Jan 21, 2020 5:33 am ]
Post subject:  Limits of cast pistons on boost?

The last dyno session ended with some pretty serious rod knock and had the motor spraying a lot of oil out the valve cover, so I'm going to pull the motor and give it a full teardown. I'll see at that point if I want to overhaul this motor or find a replacement core. Either way, it's likely I will end up doing a rebuild. The idea would be to do a mild cam upgrade, copper head gasket, maybe some head porting, and run the motor with around 15 psi boost and a 5,500 RPM redline.

It's likely I will need new pistons - there's a noticeable ridge at the top of the bores. Since this would be a stock compression ratio and not a ton of RPM, would it make sense to use a set of stock replacement Sealed Power or Silv-O-Lites? Any experience with these in a boosted motor? Or would these be the weak link and would it make sense to shell out extra for a set of forgings?

I also haven't ruled out trying to use some sort of stock replacement piston from an import turbo motor, if I can get the compression right.

Author:  Dart270 [ Tue Jan 21, 2020 5:42 am ]
Post subject:  Re: Limits of cast pistons on boost?

The cast 0.030" over Silvolites in my 180k mile 68 Dart motor have stood 12-14 psi and high 300s crank HP (high 12s in a 3300 lb racewt car with lazy 2.0 sec 60 ft times). 5600 RPM redline. I have been conservative with the timing and AFR tuneup and have driven it to-from races, including coast to coast. Water/MeOH system and no intercooler. 25-30k since turbo install.

How much HP do you want to make? Is this a real street car, or a "drive it once in a while and hit it hard and maybe break it" car? Essentially everyone I know has the latter, and lives with replacing engine parts. The only casualty with my car was one head gasket, and I still drove it home 130 miles on 4cyl... That was in 2012, before water/MeOH injection system.

Feel free to ask more questions and enjoy the building/tuning!

Lou

Author:  MadScientistMatt [ Tue Jan 21, 2020 6:08 am ]
Post subject:  Re: Limits of cast pistons on boost?

Thank for sharing! This is intended to be more of a real street car but not a daily driver, and it looks like you've got roughly the same horsepower / boost targets I have.

Author:  Dart270 [ Tue Jan 21, 2020 6:30 am ]
Post subject:  Re: Limits of cast pistons on boost?

Well, if you know what you are doing with tuning (wink, wink, nudge, nudge) then you should be fine with cast pistons and stock rods, crank, etc.

Another suggestion: Make/get yourself a good flowing head with bigger valves.

I am pretty sure I can get well above 400 HP with stock-type internals, but I like to take my time and not blow things up. I am planning a stroker motor with good pistons and such, which I hope will allow 450+ HP without too much boost, along with good reliability.

Lou

Author:  Charrlie_S [ Tue Jan 21, 2020 6:33 am ]
Post subject:  Re: Limits of cast pistons on boost?

Back in the day. I put a Corvair turbo on my 65 Valiant 170 engine. Stock pistons and head gasket. Running about 10 lbs boost. Was blowing head gaskets. Had the block "O" ringed, and copper head gasket. Cured the head gasket problem, but started hurting pistons. Went to TRW forged pistons, still hurting pistons. Problem was detonation. The OEM head gaskets were acting like a "FUSE", and blowing before hurting the pistons/rings. Once I realized the problem, and stayed out of detonation, no more problem. Back then there was no such thing as a "boost timing master", or wide band O2 sensors, so I left a lot of power "on the table", as I wasn't able to get a "fine" tune up. I would have no problem using stock pistons, today, at 10-12 lbs boost, with the proper tune up. I have pushed stock pistons harder then that with the amount of nitrous, I have used. Detonation is your enemy.

Author:  MadScientistMatt [ Tue Jan 21, 2020 10:13 am ]
Post subject:  Re: Limits of cast pistons on boost?

Thanks for the feedback, Charlie! How much power have you been making on stock pistons and nitrous?

Author:  Charrlie_S [ Tue Jan 21, 2020 12:47 pm ]
Post subject:  Re: Limits of cast pistons on boost?

MadScientistMatt wrote:
Thanks for the feedback, Charlie! How much power have you been making on stock pistons and nitrous?


Can't say about power, but my 66 Cuda with a 170 at 3250 weight, has gone 12.90 at 102 mph. But I wouldn't put too many runs on stock pistons at that level.
Many runs at 13.2's to 13.4's.
Had the car on a chassis dyno, back in 2006 at Pittsburg raceway. One pull only, and lifted early as I didn't like the sound or the feel.
Made 358 lb/ft torque at 2750 rpm, and 204 hp at 4500 rpm. The hp curve was flat within 10 hp from 2850 to 5700 rpm where I lifted. I think I was dialed 13.4x's that day.

Author:  AndyZ [ Sun Sep 06, 2020 6:48 pm ]
Post subject:  Re: Limits of cast pistons on boost?

The thing about cast pistons is they are typically in engines that were not intended to be boosted and the rings and specifically the ring end gap is for NA and not boosted conditions. The stock pistons, at least back in the day of the heavy cast rebuild pistons, were thick. They can take a lot by themselves. It’s usually the rings potato chipping and breaking a ring land and then it over. The other part of that is if you disassemble the engine to file the ring gaps larger, does it make sense to upgrade the pistons.

Author:  MadScientistMatt [ Tue Sep 08, 2020 8:33 am ]
Post subject:  Re: Limits of cast pistons on boost?

AndyZ wrote:
The thing about cast pistons is they are typically in engines that were not intended to be boosted and the rings and specifically the ring end gap is for NA and not boosted conditions. The stock pistons, at least back in the day of the heavy cast rebuild pistons, were thick. They can take a lot by themselves. It’s usually the rings potato chipping and breaking a ring land and then it over. The other part of that is if you disassemble the engine to file the ring gaps larger, does it make sense to upgrade the pistons.


Already had to disassemble the engine to deal with a clogged oil pickup and trashed rod bearing, so I'll definitely be running a larger ring gap.

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