Drop in Piston/Rod Options

[kjohannsen]

Hello All

New to the forum. Just purchased a 2015 GT with a 2.9 Whipple stage 2 SC. Car has just over 8K miles. Car idles and runs strong and does not use any oil, but when i inspected the oil in the catch can after putting some miles on it it had a strong smell of fuel. I changed the oil and the fuel dilution in the oil was back after putting about 20 miles on the car. I then noticed excessive blowby out of the oil fill. After doing a compression check i discovered cylinders 5 and 7 were at 120 PSI with all others at 225. Picked up a borescope and it looks like 5 and 7 pistons have broken ring lands. There is very slight scoring that i can see with the scope but need to pull it apart to really see whats going on. The damage pistons are clean on the broken side as well. i assume this is from the unburnt fuel washing down that side of the cylinder walls past the damaged rings

I am considering purchasing an aluminator short block and going with 9.5 compression. My other thought was just replacing the rods and pistons with a hone or going 10 over. Manley makes a forged rod and piston kit.. Was wondering if anyone on the forum has replaced just the pistons using the stock gen 2 rods and crank? Or went with new rods and pistons using the stock crank? Are the available rod and piston kits usable with a stock crank without balancing? Are there any forged pistons that are the same weight as the stock gen 2 pistons?

Thanks in advance for any feedback

Sponsored

 

[engineermike]

What fuel do you intend to optimize for?

 

[kjohannsen]

Current Lund Tune is supposed to be 93 pump gas. Has 3.625 pulley. I would consider E85 as its available close to me. But not sure if JMS BAP would work for E85. Would probably need a fuel return system..

 

[kjohannsen]

Links below to all cylinders top view and side view. Cylinders with low compression are #5 and #7


















Thanks

 

[K4fxd]

I see lots of signs of detonation.

Unless you are going to run E85 exclusively I'd get the aluminator at 9.5

My opinion only, I'd get a new tune shop.

 

[engineermike]

I was trying to find a tactful way to say what @K4fxd said.

 

[kjohannsen]

I have heard various opinions of Lund. However in this case i think it was the previous owner. Included in the parts that came with the car was a few different pulleys. When i called Lund they said the car was tuned for a 3.75 but the 3.625 that was on the car when i bought it was OK for 93 pump gas. On of the other pulleys was a 3.50. I would venture to say that 3.5 was on the car at some point and thats probably when the damage was done. 14 pounds of boost on 11:1 compression engine with pump gas not good.. I will be finding another tuner regardless.

I now need to figure out which way to go to reslove.. Thinking the 9.5:1 aluminator short block..
or would welcome any advise of replacement pistons that i could drop in considering the low miles on the car..

Thanks

 

[engineermike]

You could piece together a predator short block for about a grand over an aluminator. If you’re sticking with 93, I’m definitely a proponent of lowering the compression ratio. You don’t see many with gt500s talking about raising theirs and they make 850-900 rwhp on 93, something coyotes can only dream of.

From what I’ve can tell, Lund doesn’t like altering the timing vs boost relationship. So basically you get the same timing on 10 psi as 14. 14 can work at 11/1 or 12/1 as long as the tiking curve is right and all the oem protections are not neutered. Lund also doesn’t seem to like to tune for a low compression ratio. I tuned a vmp Odin car with the 9.5/1 aluminator and it does take some extra time to map out the torque, timing, and other things.

 

[kjohannsen]

Thanks Mike. Appreciate the help.
Yes I have heard that tuning a low compression motor is a bit more challenging to get right. Fuel curves to achieve better low end torque and drivability. But 9.5:1 seems the way to go with the whipple, and I will find a good local guy to tune.
I don’t plan on going anymore than 750 wheel so I would assume the Aluminator would be fine. Comes with a 2 year warranty too.. lol

 

[Cory S]

I had 2 short blocks that had ring land failures. Both were on gasoline with stock tight ring gaps. Over 700+whp on gas, is just too much heat/pressure on the thin piston tops with ring gaps as tight as .008".

The second one I used the block/crank again, and dropped in rods/pistons with .025" ring gap. A light hone is all we needed.

 

[K4fxd]

I had 2 short blocks that had ring land failures. Both were on gasoline with stock tight ring gaps.

What was the compression ratio?

 

[Cory S]

What was the compression ratio?

Stock 11:1

 

[K4fxd]

I believe the gt500 runs .008 ring gap. I wonder if the 9.5 compression allows this?

 

[Cory S]

I believe the gt500 runs .008 ring gap. I wonder if the 9.5 compression allows this?

I’ve yet to see an actual proof of ring gap measurement yet on those, although there’s probably some voodoo magic going on in other areas to allow this if it’s that tight. Additional Piston cooling, hot spot reduction etc.

 

[engineermike]

I can verify a stock predator ring gap is under .010. Bore clearance is .001 as well. The gt500 is not only 9.5/1 but also only runs about 16 deg timing. That doesn’t explain how the modified ones live though…

Sponsored