brausch

I am running AFR205 heads and cam on my stock bottom end C6. My current mods are:

-LG G5X1E cam
-Heads milled to 64cc
-0.040 cometic gaskets
-7.4 comp pushrods
-Fast 90/90 port matched to heads
-LG headers with cats
-ATI pulley
-stock exhaust
-Mail order tune

I did the install myself. I have been on a Superflow AWD dyno, and the car made 525hp (corrected at the wheels), exactly the same as my friends stock C6Z06 on the same dyno.

The car is beeng driven very hard, including WOT on the highway (I live in Germany . Everything was fine for about 10.000 miles. Total of 20.000 miles.

I had a little race last week. 95deg outsinde. Accelerated to 160mph, braked to a red light. Sat there for 5 minutes, raced again up to 160mph. On the next light, I heard a knocking sound from the engine.

After pulling the heads, we found a defective piston. It lost part of its ringland, and is completely cracked. The piston hit the head. I think this happened after it has cracked, the other pistons have some carbon buildup and no signs of contact with the heads. The piston was rocking in the bore, it has some marks in in it. I measured it with a bore gauge, it has two spots where it is +0.1mm and +0.18mm. It is possible that the bore is cracked, also.

Now I am thinking of building a forged 402 engine with an APS turbo kit. Maybe a block from W2W. My goal is a 800hp (engine) street car. It will be used for track days, also. Some questions:

-are the 205 heads ok for this? How much better would the 225 heads be for this application?
-can the 205 heads be reworked to 225 heads?
-the heads lost some material between the intake and exhaust seat. It is very similar on all cylinders. Is this a problem?
-The setup I have now has a very noisy valvetrain. I would prefer an emission friendly cam with soft lobes. Is this possible?

I would appreciate your help.

Best regards,
Bernd

67Firebird455

iTrader: (27)

Hey the top of that piston looks exactly like mine. Jeez, I can't believe it cracked like that though down to the pin.

1. The 205s are fine for that application, if you decide to run a big cam, the 225 may be better, but will likely lose some low end torque.

2. You can change the velocity/flow characteristics of the 205's to match the 225s with different cross sectionals and short side radius

3. No big deal

4. Are you running Comp R lifters? Your valvetrain should not be noisy unless there is slop in it (incorrect preload). You can get away from an XE-R profile, with a fair amount of duration (less than 230) and still be emission friendly, as long as you're running a good tune (IE - Dyno tune).

Good luck man, that is bad news....

MadSpeed

Id be willing to bet money the problems you've had are due to mail order tuning. That is most likely due to Detonation
You cant get a perefct tune without a dyno and NO two engines are alike they all require their own special tuning
I wish you the best of luck with your project
sounds like your going to have a bad **** ride when your done =)

Tony Mamo @ AFR

iTrader: (13)

The 205 heads would work well....especially if your a fan of good low speed part throttle TQ....the 225's would obviously make a little more midrange and topend power. If I had to guess I would say 10-15 addionaly @ peak. Not the end of the world by any means. I wouldn't bother paying to have someone rework the heads....you will invest money and potentially be no more ahead than you are right now. Better to sell your heads to someone else with a stock displacement and then step up to a new pair of proven 225's right out of the box. Again....assuming the 10-15 HP is worth all the trouble. The 205's you have would still make a great all around package on that displacement. You might consider opening up the chambers a little on the sides to compliment the larger bore motor....Low and midlift flow will improve dramatically without increasing the size of the runner. BUT....it must be done by someone experirenced with a grinder that preferably has alot of experience and the use of a flowbench at his disposal. I could email you a pic of a chamber I opened up so you could see the direction you need to go for best results.

The little bit of material missing between the intake and exhaust valve isnt a big deal but does show you might have had some detonation and/or a lean A/F ratio which likely caused it (and the cracking of your ringland I might add).

Noisy valvetrain was probably more of an issue with improper pushrod length....shoot for close to .100 lifter preload and I bet your valvetrain noise is much improved.

Good luck with the new combo....Im sure yuo will enjoy the added displacement and the extra TQ that always brings to the table.

brausch

Thank you very much for your replies.

I also think that the piston failed due to detonation. I had a look at the timing tables. The high octane tables are set to 28 deg between 2800 and 7000 rpm above 0.52g/s The low octane table is set to 25 deg. The air intake temp timing corrections are set to 0.

machinistone

Look at the ends of the top ring - is it shiny or dull-black? You may have butted the rings .

White_Hawk

iTrader: (3)

When you get it tuned again, make sure your tuner understands what you are going to be doing to the motor. I doubt many tuners have experience tuning cars making 0-160 pulls. That is probably a half mile or so at full throttle!

Sounds like a blast driving it full out on the autobahn though!

-Geoff

ls1perfcar?

iTrader: (10)

id check the heads also just to be safe make sure u didnt bend a valve.

brausch

The ends from the defective piston are half shiny, half black. I have looked at another piston, the ends are black. What does that mean?

Bernd