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fly-cutting pistons
I have been soliciting advice from the good folks here about what to do to increase the performance on my car. I have a new 351W build with new forged flat top pistons and new forged rods. I am getting 450hp but I want more. I am running too high a compression (11:1) to bolt on a hair dryer. I would like to swap out the crank and increase the stroke but don't want to toss brand new pistons for which I just paid $600 bones. I heard about flycutting and want to know what exactly this will allow me to achive. I know it will bring my CR down. Can the pistons be worked to allow me to install a bigger crankshaft while keeping the CR to 11:1 or lower? Is this something fly-cutting is used for?
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It is a combination of things actually. How much engine do you know?
Are you the builder? When you did the piston to valve clearance test, what kind of numbers did you come up with? How did you determine you have 11:1 right now? Flycutting the tops is for valve clearance and, if done too much, will weaken the top of the piston allowing a hole to be punched in it with any number of anomolies. New pistons, the openning of combustion chambers and special head gaskets can be used to reduce the compression ratio. The amount of material on top of the piston is probably not enough to allow a you to "just drop in a crank". Then there is the deck. How high is the deck? The list goes on. Hope this helps. :3DSMILE: |
Fly cutting is usually done to increase piston to valve clearance. When installing a more agressive camshaft with increased lift and duration there is more chance that a valve will come in contact with the crown of the piston with disasterous results.
Flat top pistons often don't have a valve relieve in them so a fly cutter in a milling machine is used to cut a half moon into the top of the piston under each valve make clearance for the edge of the valve as it opens. It's probably not a good idea to cut too much material out of the piston crown to lower the compression as this will weaken the piston. Dished blower pistons have a lot more material in their tops for the dish to be machined into. To drop your compression I would suggest a set of thicker head gaskets or finding a set of heads with bigger chambers. Cheers |
Ain't that always the way. You type out an answer and someone beats you to the post button. **)
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Thanks for the advice.
I guess I missed my chance during the build. The shop that put my car together suggested stroking it but I was already over budget and still had more stuff to do at the time. On top of the motor, I spread my money around on a new racing suspension, new disk brakes, and new wheels/tires. I am not displeased with the car; it hauls more ass than a rickshaw driver pulling Jennifer Lopez up a hill but I wouldn't have minded a little more. I guess I will just take the money I would have spent on the extra motor tweak and add it to a better paint job than I originally planned.
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How about dyno tuning it!I have done a few cars[customers] and have found an average of about 50 HP by dyno'ing[chassis].Including a 43 hp gain by removing the stelling air cleaner on my car!If you know how, the dyno people will let you do the work on the car and just rent the dyno for time instead of pulls.Stan
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To be honest, I'd be happy with the 450. Anytime you can get better than 1 to 1, cubic inch for HP, you are doing fine.
Find a 427, 428 or 429 and have a little fun that way. Take your time. Dyno it to what you want. |
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I don't know if 11:1 compression ratio will cause any problems with N2O injection as I'm not an absolute expert on this topic, but I guess a 351W bottom end built to handle 450 natural aspirated hp should withstand an extra 150 nitrous hp. A simple carb plate system should be able to deliver about 150hp, if you want more you'll need one of them port fogger systems. Simon |
11 to 1 is pretty high, even if you could cut the pistons (and I doubt that is possible) you couldn't cut enough to drop much compression. Going with a FAT head gasket would help, but again, not "that much". I mean 11 to 1 is WAY up there!
It's likely you'll need new pistons with a stroker anyway. Assuming your piston to deck clearance is typical at or near "zero" a stroked crank will push the piston above the surface of the block. You need to change the piston pin height (compression height). Not to mention valve to piston clearance issues! I was bummed I had to buy new $800 CUSTOM pistons, but that was the only way to get my compression from 12.5 to 1 down to 9.8 to 1. My "old" Ross pistons are in excellent condition,,,,, hmmmm,,,, maybe e-bay those bad boys! |
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Flycutting pistons is used for adding piston to valve clearance when necessary. It's effect on CR is minimal.
If you're pulling 450 ponies out of your 351W, that's about as good as it's going to get with a street driven car, and keep it streetable. In a Cobra, you'd have little competition. A '69 Mustang, however, is not a Cobra. It is a full size and heavy car. If I recall, these things come in at 3,600 - 3,800 pounds. The hotter ones in '69 sported 429's, and even then had trouble keeping up with the Chevies, and Mopars. For a heavy car you need torque, and to get torque, you need displacement, supercharging, or both. Dollars wise, the easiest thing to do is add the stroker crank, and pistons. If you want to go hunting big blocks, dial your CR in at about 8.5:1. Otherwise, go about 9.5 - 10.0:1 for pump gas. |
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