i have a 1995 formula lt1...the car is currently in the garage..waiting to be worked on. I have a ati 8 psi procharger,with all the basic bolt ons....can the stock pistons handle 500 hp? just wondering if..the pistons..should be changed as well..i...car has 40k miles on it...15 of which were blown..if the pistons do need to go...any anyone has recommendations..as to which pistons..to get id appreciate.it..thanks







Steve

 

Believe it or not stock pistons can handle allot power. (Under perfect conditions and nooooo detonation). But the only problem is that if you drive a car everyday (like I do) you will know that pump gas is not created equal and you will run into bad gas from time to time or other problems etc. (heat soak, worn out ignition components fuel pump etc.)
The problem with Hyper pistons is that they are very brittle and will not tolerate detonation or any other problems when you compare it with a forged piston. I believe about 90% of all stock piston failures with supercharged, nos, and turbos is detonation. (Due to lean, tuning issues, bad pump gas etc)

If you choose to run stock pistons I would recommend that your combo be on the richer side and be very conservative with timing. This is not the ideal tune for the max power but, your pistons and connecting rods will thank you with longer then normal service life and will help to avoid an early rebuild

I would say 450 hp is about where you want to be if you want the motor to last for any long period of time
I have seen guys who have taken it up a bit but the motor did not last more then 20k before failure. But as they say in the forced induction world be prepared for a rebuild in your future.

 

I agree with "Maldo" with one addition. The best way to conceptualize the limits of stock pistons in this context is to think of boost as being the limitation, not hp. People who stay in the 6-7lb range and observe the other cautions Maldo cited seem to do ok. More than 6-7lbs of boost, and "bye-bye" pistons.

Rich Krause

 

Rich, I have been pondering the same question lately, and was curious as to what happens past 6-7 psi. For example, the general consensus is that with 8 - 9 psi, you get about 4 months out of them. Isn't it the ringlands that usually go? I would attribute that to pre-ignition. That would mean the tuning is the culprit. People used to think that a stock Honda LS motor could only "handle" 6 psi before the motor let go. However, by getting rid of the FMU and running a standalone, or programmable stock ecu, people have gone well past 6 psi, and are making in the 300+ range on stock bottom ends. I have 3 pistons here from Hondas that people blew the motors in. All of them have cracked or broken ringlands, and the tops of the pistons are TOTALLY pitted. Isn't that a sign of detonation, which could overheat the chamber, leading to detonation induced pre-ignition? Each of the cars were tuned "safely" on a dyno, with a conservative a/f ratio and timing. However, using an FMU, I would speculate that you are getting an overly rich condition on some cylinders and and overly lean condition on others, which is why the a/f comes out correct. EGT monitoring would have been better, but I don't have that kind of $$. Even Chevrolet found that the stock ecotec motor could handle around 280 HP before the rods became to weak for the power. I know its a "useless risk" but if you chuck the fmu and run properly sized injectors, couldn't you realisticly see upwards of 450 - 500 RWHP on a stock LT1. Also, I would use power levels as opposed to boost pressure, because 8 psi on a D1 is alot more volume than 8 psi on a P600. Then throw in a NOVI 2000 with 8 psi and you have even more. So, from your experience, what causes piston failures?

 

You bring up a good point about an EGT. The only thing now is to find which cylinder in an LT1 runs the leanest under boost. In a N/A application I think the #1 cylinder is actually the leanest. But under boost your going to have a totally different distribution of air. I would imagine under boost the cylinder the furthest back would be the lean one. So if you probe either #7 or # 8 and tune with those cylinders you probably could get away with running more boost.

Jose

 

Not 9 pounds of boost, that is for sure. That was my first try back in 1995. It broke 3 pistons awhile later, all on the weak ring-land area. If your going to bolt on a blower or run nitrous, the bottom line is that forged pistons are highly recommended. Even on N/A for anything above 400 HP.

 

EDS, did you have your car tuned with bigger injectors and no fmu? That is the main question that hasn't been answered? How conservative of a tune did you do? What a/f did you run and how much max timing? Did you tune timing on the dyno or just bolt on a 9 psi intercooled kit and fmu? I think that would make all the difference.

 

I was running 475 rwhp on 8lbs. boost with my last motor. I had TRW Forged flat tops which were fine when I pulled them out. The problem was the stock rods and crank were doing some twisting and tearing up the bearings, The crankshaft was also cracked. Personally I don't think the bottom end can handle over 400 rwhp without some problems.

 

When I first installed the supercharger, the fmu and inline fuel pump were used with stock injectors. The tuning was stock also.
It never really ran correctly under full boost, the computer kept taking like 8-9 deg timing out no matter what I did. I eventually purchased a smart spark to take some timing out and the motor finally quit, spraying oil all over the bottom of the car. It lasted maybe 10K miles with the blower on it.

 

I admire your attempts to reason this out! I really can't say if it's detonation or simply cylinder pressures that are too high for the weak stock piston design, or both. As far as individual cylinders being too lean, I doubt this happens with a typical bloweer setup, which is (by design) pig rich. That's how I run mine! Support for this idea is the observation that there is not pattern for which pistons go first - I have seen all different combinations.

No matter what, detonation will kill the pistons. So avoiding it is a high priority. And if each cylinder is making max power, the whole motor is also. Given this and that the stock PCM can trim individual cylinders, I am strongly considering having some EGT probe bungs welded into my headers. The dyno where I tune has EGT probes, so this will give the data needed to find out what the fuel distribution is for an LT1 under boost and to tune for it. The only thing holding me back is $$$. My project is already way out of hand and over budget for this year. If I do it, I will post results here.

Rich Krause

 

This question has been ask many times but I never posted my experience until now.

When I was still running my 350 with stock internals, I was running 10# (seen on the gauge so don't know exactly what is in the engine) aftercooled boost for almost two years at 476 rwhp dyno tuned without a problem for 30K miles. Granted, I also have AFR heads that dropped my CR to about 9.2, cam, 1.5 RR, 1000CFM TB, Eldebrock shorties, etc .... The A/F was around 11.5 and stock timing.

Finally my head gasket blew up after three years of boost. When we open up the engine, the rings and pistons looked fine. We poured transmission fluid into the chambers. All the chambers held the fluid for many days except one cylinder that allowed the fluid to run through the rings overnight.

cheers