You'd think SOMEONE would make a forged piston that is good for about 8.7:1 compression in my motor.... Ithink I'm going to have to get custom machined pistons. Sigh...
Building an engine can be a pain in the ass.
- Thread starter notfred
- Start date
ScoobMaster
Platinum Member
- Jan 17, 2001
- 2,528
- 10
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You are having them fail at only that low? what kind of car? Did you try some racing component shops?
You need blower pistons, most of them are custom made. The other chioce is to buy 9.0:1 and machine a dish in the center to reduce the ratio.
eh? Your comment doesen't really make much sense, I mean... what do you mean by "piston that is good for about 8.7:1 compression"?
Although I realize domed/dished pistons will increase/decrease the compression ratio, it's mostly a function of the heads...
Use any race piston thats meant for your engine. Flat topped or dished if you need to mess with the CR.
I don't mean they're failing, I just mean that I can't find one. They're all like 10:1, or hyperreutectic.
61cc, but that page says "NOT RECOMMENDED FOR USE WITH FORCED INDUCTION SYSTEMS." on it, so I don't think I want those
Only 6PSI. I'd be willing to go up to 9.0, but I don't want to run any higher than that cause I'd have detonation problems.
Originally posted by: Tyler
61cc, but that page says "NOT RECOMMENDED FOR USE WITH FORCED INDUCTION SYSTEMS." on it, so I don't think I want those
tyler,
explain your application. is this a turbo or supercharged application? what motor, what head, you need to be a little more specific
I figure most people on here know this by now, but it's a ford 302 w/ Trick Flow twisted wedge heads and a supoercharger making 6 PSI of boost. It's also getting 1.7 roller rockers in it, but trick flow tells me there shouldn't be any valve clearance problems so that shouldn't be a problem.
Here's some that say they're suitable for Forced Induction, but you're going to be at less than 8:1 with 61cc heads.
check out JE Pistons look on the 2nd page where it shows the inverted dome pistons. looks like you could get 8.4 or less with a 64cc head.
I should also mention that I'm using stock length 5.090 rods and anything mentioning a stroke of 3.400 is for a 347 stroker motor (which is not what I'm building).
also, the KB pistons are good also. a lot of people don't like them because they are not forged, and people have been told for years the forged are the way to go. but, if you build a motor to the specs that the KB people tell you they work excellent. if you use KB pistons, be very careful and use specs the KB people give and not the people who supply the rings. the KB pistons tops get a lot hotter and you must make sure the rings, especially the top ring has the correct gap, or else you will have problems.
Demon-Xanth
Lifer
- Feb 15, 2000
- 20,551
- 2
- 81
Think that's hard? Try finding stuff that actually works for a Jeep/Dodge 4.7 SOHC V8. Lots of promises, few real results.
i know, JE are expensive, but they work well. i used to work in a machine shop and we used either JE for "race" jobs or KB for "street" engines.
Schadenfreude
Lifer
- Dec 28, 2001
- 11,391
- 3
- 0
Getting a foot stuck there can be a pain in the ass too . . ..
<---- Jehovah's 'advice' of the day
<---- Jehovah's 'advice' of the day
Tyler,
You should try to find pistons that are flat-tops that meet your HP requirements. I would not worry too much about CR. It will be around 9:1 which is optimal for your combo. Trying to find 8.7:1 pistons is going to be hard and expensive plus they will be dished which is not optimal for efficient flamefront travel during combustion.
Your problem was not your engine but either an inadequate fuel system or improper computer tune. At only 6 PSI you should have no trouble controlling detonation. One of the Mustangs I built had iron Twisted Wedge heads on the stock shortbleock (around 9:1 CR) and we put 17 PSI of boost to it reliabely with a ProCharger blower resulting in 507 rear wheel Hp with 512 LB/FT of torque, since your goals are not quiet as lofty the stock compression ratio is not your problem.
Do not down-build your engine to meet a blower package that is not working properly. You need to investigat your fuel system, it is the likely culprit. I assume you are using a stock fuel system with helper pump and an FMU. I usually don't like FMUs since they try to meet the upgraded fuel requirements by simply increasing the fuel pressure an on the stock sized fuel lines flow can actually be adversely affected if the pressure is set too high. I would also consider upgrading the fuel system if it is in your budget. We changed the lines and everything to larger capacity to feed the engine with the blower. Although you are only using 6PSI I like to have a better fuel system than 5/16" feed lines and 1/4" return lines since they are marginal on a 300 RWHP NA vehicle and you should always try to build headroom into the fuel system of a blown vehicle.
I would also look into a programmable chip for your EEC VI processor. The one we use has USB and is capable of data logging so we can be driving along and read everything on the laptop. I know the manufaturer here in Houston personally and can help you procure one if necessary. Although they run about $500 they are worth their weight in gold. We picked up over 75 HP on the dyno with the chip after we tuned it properly. You can also fatten up the fuel curve or take a little timing out as necessary to control detonation. As I said, we made over 500 RWHP with a blower and much more boost than you on iron heads with no detonation at all and I feel you should be fine if you address the real problem which is not too much CR. This chip we use also allowed a friend of mine and myself to shave over half a second off his 1/4 mile times with tuning alone on a NA '94 Cobra.
Good luck, and if I can help let me know.
You should try to find pistons that are flat-tops that meet your HP requirements. I would not worry too much about CR. It will be around 9:1 which is optimal for your combo. Trying to find 8.7:1 pistons is going to be hard and expensive plus they will be dished which is not optimal for efficient flamefront travel during combustion.
Your problem was not your engine but either an inadequate fuel system or improper computer tune. At only 6 PSI you should have no trouble controlling detonation. One of the Mustangs I built had iron Twisted Wedge heads on the stock shortbleock (around 9:1 CR) and we put 17 PSI of boost to it reliabely with a ProCharger blower resulting in 507 rear wheel Hp with 512 LB/FT of torque, since your goals are not quiet as lofty the stock compression ratio is not your problem.
Do not down-build your engine to meet a blower package that is not working properly. You need to investigat your fuel system, it is the likely culprit. I assume you are using a stock fuel system with helper pump and an FMU. I usually don't like FMUs since they try to meet the upgraded fuel requirements by simply increasing the fuel pressure an on the stock sized fuel lines flow can actually be adversely affected if the pressure is set too high. I would also consider upgrading the fuel system if it is in your budget. We changed the lines and everything to larger capacity to feed the engine with the blower. Although you are only using 6PSI I like to have a better fuel system than 5/16" feed lines and 1/4" return lines since they are marginal on a 300 RWHP NA vehicle and you should always try to build headroom into the fuel system of a blown vehicle.
I would also look into a programmable chip for your EEC VI processor. The one we use has USB and is capable of data logging so we can be driving along and read everything on the laptop. I know the manufaturer here in Houston personally and can help you procure one if necessary. Although they run about $500 they are worth their weight in gold. We picked up over 75 HP on the dyno with the chip after we tuned it properly. You can also fatten up the fuel curve or take a little timing out as necessary to control detonation. As I said, we made over 500 RWHP with a blower and much more boost than you on iron heads with no detonation at all and I feel you should be fine if you address the real problem which is not too much CR. This chip we use also allowed a friend of mine and myself to shave over half a second off his 1/4 mile times with tuning alone on a NA '94 Cobra.
Good luck, and if I can help let me know.
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