Should the #4 piston look like this?
 

Greetings,

Engine builder and mechanic telling me that this is caused by detonation at high RPM. Found rings broken on #5 cylinder, but piston does not look like this one.

Engine is a '78 Euro 3.0L with 3.2 cylinders and pistons built in 2009. JE 9.5:1 pistons. Currently the 964 profile cams and PMO carburetors (was CIS until a year ago). Lightweight track only car that sees 6,800 RPM (since the carbs install) a couple times each lap in 4th gear. Carbs recently tuned on dyno and set a little rich. Track is within 1 mile for the dyno station. No wide band....I know I know....

So now what? A costly rebuild for sure. GE60 Cams with correct 9.5:1 pistons? heavy springs such as Eibach pro series? Would like to stay with pump fuel for obvious economic reasons. Builders are saying "yes"...this is possible on pump fuel.

I stick to busy stations that sell Chevron or Shell. I suppose it possible I bought a stale batch. Are these octane boosters for real or are they just the typical marketing BS. If so, which one is best.

Also....what is best wide band to purchase and the best configuration for a carburetor engine with discrete headers

Thanks

http://forums.pelicanparts.com/uploa...1364337164.jpg

Looks like you we're tuned way rich with too much timing advance.

Stock distributor?

Easy to do the build you want, you need the carb specific pistons, race specific valve springs and a custom cam set up. What size headers?

Measurement is key, deck height and valve clearances. It is best to disassemble and measure, machine if needed and measure again, then order your pistons. If you are machining the heads then do not expect to end up with advertised compression ratios with off-the-shelf pistons.

Definitely need a wideband on there to know what's happening. It's possible your carbs are not tuned properly up top and combined with aggressive timing can destruct a piston in such a fashion. Just as over-fueling can cause ringland failure, you may also be lean at high rpm and covering it with soot at lower rpm. Could be a combination of causes instead of just 1. A wideband is a good start as is monitoring the ignition advance on the dyno to ensure it is as desired and safe. Given the failure at the top of the piston I would be more inclined to believe you're lean up top or detonating (nickles in a coffee can sound).

Why the thought of changing cams, using stiffer valve springs and installing "proper" pistons? 964 cam profile and JE 9.5 pistons are not a bad combination. It's very similar to running 964 cams on 3.2 Motronic pistons (9.3 comp ratio), which is a common modification on the Carreras.

Those 9.5 pistons you have are a suitable choice for your application- carbs and single plug on pump gas.

AEM makes a nice wideband that isn't overly expensive. I have one and it's worked well for me to dial in my carbs (USA Weber 46 on a 3.2SS). You want the O2 sensor installed nearest as possible to the header collector, just downstream of it. Put a bung on both headers so you can check both banks of cylinders.

[QUOTE][Why the thought of changing cams, using stiffer valve springs and installing "proper" pistons? 964 cam profile and JE 9.5 pistons are not a bad combination. It's very similar to running 964 cams on 3.2 Motronic pistons (9.3 comp ratio), which is a common modification on the Carreras./QUOTE]

Great question and one I ask of the "experts". Many say with carbs, move away from fuel injected cams and domed pistons. Current configuration is 219/194 at the rear wheels. I need a new set of pistons no matter what.....40 or 60 cams will be about $600 - $800. This engine is track only so it lives between 5,200 RPM and 6,800 RPM. The "race" springs are another $200. So what does this $1,000 get me? 10 HP? 20 HP? Or a complete waste of money?

We are thinking the damage started with CIS. Its a very long story, but we ended up diagnosing clogged injectors. The engine was run on dyno and and a few test laps in this condition. Perhaps it took 4 weekends to get to the point that damage was visible. While that piston looks bad, the engine purred right along with no noise and still made good power. It was the oil smoke on decel that gave it away. Porsche engines are quite amazing in the regard.

A wide band is most definitely in the budget. We'll add the bungs to each header collector.

I believe you are on the right track then as that would make most sense looking at that piston. It's really hard to detonate all-motor when rich as the extra fuel cools the air and works in an opposite manner. You would have to be way, way off with ignition timing to achieve that result.

Well the domed pistons like yours are exactly what I used for my engine. Their design is intended for carbs, MFI, or ITB EFI. Agree the cam swap would be beneficial to the carbs. But look closely at your choice of cam and be sure they actually need higher rate springs. High rate springs give you that extra level of protection, especially with high lift cams. But they also steal a bit of horsepower due to their higher rate.

Stock springs can be reliably used if the ramp angles of the cam profile allow for it. For instance Grady Clay has mentioned on numerous occasions that he uses stock springs in his 8500+ RPM motors. However, he also uses forged rockers with lash caps & lightweight valve train components because he believes there is a lot to be gained there. For our relatively mild engines, race springs are not always a must. Especially if you keep it under 7000.

If it is a detonation issue, you might consider going twin plug.
-C

I imagine those lightweight components are at considerable cost with short replacement intervals. In today's money, an engine as described is probably north of $50K

So! I gain a little power with the 60 cams and lose some or all with the stiffer springs? Can anyone estimate power gain from 60 cams and in range in the power band (thinking 10 hp - 12 hp at the top end) along with power loss to stiffer springs? Anyone using stock springs with 60 cams?