A quiz

[Fiatracer]

Hmmmmm, 13.7:1 to 14+, that would be some serious compression. The compression is rising as the torque builds. You need a big bang to drive that piston down. A bing bang requires big hot spark and the right octane and all that other stuff...

Sounds like that at 6500 RPM something is happening to the spark and fuel.

[pheyden]

MItch - You are getting warmer in your analysis. Take the three items you mention (spark, octane, other stuff) and drill down deeper.

[Fiatracer]

Hmmm...
1) octane - Iooking at some of your previous technical documentation (spreadsheet) on octaine ratings:
Static CR Dyn CR** Min. AKI
11:01 7.79:1 93
12:01 8.46:1 102
13:01 9.14:1 106
14:01 9.82:1 110+

the VE of this combination was around 114-119% at peak torque. Even if this is optimistic, and it is really only 110%, the it would mean that the static compression of 12.5:1 would at peak torque be 13.7:1. IF, on the other hand, the static compression were higher than the 12.5:1 claimed and were instead 13:1, then taking into account elevated VE this would compute to 14.4:1.

I would question what fuel octane is being currently run through the engine. My 1st answer would be that the octane rating is not high enough for the compression.

2) spark - as you raise compression it becomes more difficult to ignite the combustible gasses in the cylinder. Is the spark happening at the correct moment. Is the coil putting out enough juice for the spark plugs (assumed to be properly gapped and the right temperature range). I believe I read somewhere that higher octane fuel requires a shorter or longer duration spark?

Still scratching the head

[RaceSport]

Surely if the compression /R is that high then without the correct fuel it would show signs of detonation on the plug/s if not audible on the rolling road.
14.1 seems almighty high to me in this country of 98 octane max. Most people here go for a 11.1!
It is somewhat in the region of diesel fuel ignition!! Who needs a spark plug?
Assuming the compression is there to start with!
Anyhow if thats what it is and alls well elsewhere. I would have suspicion of the fuel ratio and flame travel with a notion of exhaust scavenging.
I do not know the Colombo and Birianna camshaft or its timing figures. Are we sure it matches the charactistics of the engine ...Like where does it start to work and finish..If someone said to me my car peaks out at 6500 then loses power,id be inclined to think that the camshaft is not wild enough,or the exhaust valve clearances are getting tight at full chat!

The symptons merely state a power drop off at 6500 without misfire,so to say the spark plugs are wrongly graded would be out.
I can only think the fault lies within the camshaft!

On a different note...How do you get such a compression ratio with a hemi head. The piston crown must be very convex in shape!

[pheyden]

David,

I was only "intimating" that unless one physically checked and measured the compression ratio to confirm that it was 12.5:1, it could just as easily be 10:1 or 14:1. You just do not know. NOW if the engine is in detonation, with an open exhaust you would likely not hear it. If you on a rolling road, and the motor was behind you with an open exhaust, then you definitely would not hear it.

Yes the dome on the piston is LARGE.

TCR Piston.jpg (51.1 KiB) Viewed 74 times

TCR piston cross section.jpg (76.9 KiB) Viewed 74 times

In addition, the 98 octane fuel that you refer to, is in reality not even 98 octane. In Europe they publish the RON octane number on the pump. Race fuel is almost ALWAYS rated as the average of the RON (Research Octane Method) and MON (Motor Octane Method). This is also referred to as the AKI (Anti-Knock Index) number. As the MON number is always 10 below the RON, this makes the AKI number for 98 octane European pump fuel 93. This is low enough that it should not reliably support more than 10-10.5:1 compression.

In the 60s and 70s we had fuel that had AKI numbers of between 100-104, so it was possible to run 12.5:1 compression reliably. Before I forget, this was also LEADED fuel, and tetraethyl-lead was also an anti-knock agent. Yes, it was up there, and it produced good power, but it required GOOD fuel. BTW - Sunoco 114 race fuel would have a RON rating of 118 and a MON rating or 108, and it will support 13.5:1 compression with reasonable anti-knock ability.

It is therefore my considered opinion, backed up by years of experience, that you cannot support anything over about 10.5:1 on European pump fuel rated by the RON method at 98-100 octane.

[RaceSport]

Well some time has passed since this thread opened..So what is the outcome?

[pheyden]

I have not definitively heard what was decided by the owner, but the things at the top of the list were:

1. Camshaft was mistimed
2. Defectively made camshaft
3. Compression was higher than estimated

Paul