It really depends on the type of engine, stock Vs custom and performance level. But for me a 6% difference really starts to mean something and getting around 10% or more means $$$$$$.

A simple compression test with funny numbers turns into more testing, wet compression then leak down to potentially see where the air is escaping. Valves, rings etc.

Funny story is I did a street bike motor for a guy this summer that drove me nearly insane. Short story was that after several different shops looked at this bike and could find nothing I got a kick at the cat even though it was a warranty issue. Well one of the plated cylinders had a casting flaw right at the highest point the rings would go to. You could only see it under higher magnification, Numerous people missed that detail and the manufacturer did not want to pay for any more engine work on this guys engine.
I got lucky by putting air to each cylinder with the rad cap off and found which cylinder was leaking. I almost did what the dealership did which was check everything for flatness then throw another head gasket at it. Something kept telling me to not do that and it paid off, this micro hole would bleed lots of air into the cooling system but at times it would seal itself off.

 

When I have performed leak down tests in my motors ,,
I do it with the piston at BDC,,middle,,TDC ,,as to check to see if cylinder bore is straight,,of course you have to take load off of valves ,, closed..
I believe are cylinders are nikasil plated as I try to plate all my bike builds with that,,tuff to hone,,a special hone is needed.....

 

AFAIK only most of the aircooled engines had nikasil. Ours are non-plated sleeves.

Nikasil is usually used in soft cylinder liners(aluminum, magnesium alloys etc..)

 

My past water cooled 944 Turbos had nikasil plating..

 

I am not here to argue with anyone. Life is far too short. My experience is worth exactly what it costs you.

Here is a picture of my 2001 stock 996tt rods. The car started and ran fine with the exception of a ticking sound that sounded like noisey tappets. We did compression checks and they were fine. After about a month of trouble shooting we tore down the motor. I might also add the sound would come and go while the car was being driven.

Later we found out the sound was the side of the rod brushing against the crank (note right upper side of far right hand rod). All six rods were bent. Maybe they were all bent about equally, but again the compression test didn't tell a thing in our case ...... we wish it had.

 

Ditto, I have had few bent rods on my turbo charged jap bikes in the past and it never showed up on a compression test.. Probably due to piston and wrist pin slop and rock.. Plus crank rod movement..

 

to the OP, maybe just leave your motor as is for now and try to achieve the 650 hp you're craving for via tuning? way more economical.

 

What hardware, tuning and HP did this happen at?

 

The rods bent back in 2004. KA was featured in May 2004 Excellence in an article by Dave Colman (Dave actually wrote the article in 11/2003) entitled .... We have lift off. Without going back over everything at the time just under 700 hp and north of 650 torque. Internals were stock.

The build started in July 2000 when I used Fun Care on Line, it was moved to Rennlist until July 2003 when I became a mod for 6speed. Here is the build from 7/2003. Almost 3900 replies and over 471,000 views.

https://www.6speedonline.com/forums/...-j-maring.html

 

Hey there are no absolutes, all these things are just another tool to use when you need them or want to. Sometimes they help you and sometimes they do not; they almost never hurt you.

 

I would think tuning back in 2003 compared to 2014 on this platform is a WORLD apart, I would venture to guess this played a part in your rod bends. But you definitely did push the limits on the internals, so I guess we have to thank you in a way for basically pioneering a platform.

 

Thanks for the kind words. The motor is currently complete. It is a 4.3L with not a single part that has not been reviewed, redesigned and manufactured. The designed redline is 8900 rpm. The motor is out of the car and a MOTEC M880 with all options is being fitted as well as a custom designed RAMP front LSD. If the motor has a weak link it is the piston rod ratio and the case which is the Porsche Spanish Case with machining done for GT1 3 index oil pump, boat tailing, shuffle pinning, 965 oil squirters, align drilled and special head/case bolt threads as well as being resized. Bottom line, if anything lets go, it will be the case. With the stroker we would have liked to use 133 mm rod length, but there simply was not enough deck height. By using NASCAR oval bearings, shortening the pistons, reducing the size of the wrist pin and offsetting the wrist pins to true bore centers we were able to extend the rod length by about 3 plus mm or an approx. 1.58-1 piston to rod ratio. She made just over 1000 bhp @ 1 bar on an engine dyno. She is designed for 2.8 bar. She is also fitted with a progressive 300 shot of nitrous (10 hp shot added every 1/10 of a sec.) Nitrous was not used on the above engine dyno test.

 

The beginning of the tuning era on these cars did see bent rods and head gasket failures,, even with less boost then we run today..

 

Wow Cjv.. Nice..

 

I have seen you mention these oval NASCAR bearings a handful of times. I haves search for them some but can't find them. I do understand the crank has to be ground for them. Where would I find these bearings?