V-12 Engine into 308 Build, over on Grassroots

Going to sound crazy but.... Rod stretch? I don't recall the rods you're running.. what's the weight Delta between the pistons normally on the rods vs the ones you're running?
I know.. Numbers numbers..

It's an interesting problem. Either you've got the cam timing way out, can't imagine you'd be that off, or theres more going on with the thermal and structural stresses

 

I'm not sure i understand the question....the clearance I just measured in 0.045" and if i torqued the head it would probably be closed to 0.040. I had targeted 0.050", so not right but hitting at modest rpm/temp? I just don't understand why. .....

 

What are the spring pressures, seat and over the nose

 

Yeah, that's what I was getting at, the closest they come to each other lest they kiss.
Ok, rule of thumb, I don't like getting below these values:
Intake 0.080"
Exhaust 0.100"
Thermal expansion and stretch of the rods.
0.040" that's really tight.

 

Thinking a bit more.. wiggle and rock. The intake valve is wiggling in it's guide and the head will be obviously wiggling around, the piston is also going to have rock. Those two combined will eat away those clearances.. if/when they touch those motions are very prone to breaking the head.. As we've seen.

Just my thoughts/guesses..

 

Just a touch thicker head gasket will give a bit more clearance, but you'll of course lose a bit of compression

 

Seat is about 55#, Intake nose about 185#, exhaust nose about 170#.

 

Right, a quick google and out pops .080/.100 are the recommended clearance numbers. Point. But that can't be right for every engine ever made and when I dig a little deeper and I start to find what I'd expect....smaller engines run small clearances but no real formulas I've found. Motorcycle guys (4v sport bike type builds) are more like 050/080 which is where I got my targets from....a target I missed it seems.

My piston to head clearance is 0.035" with a 0.050" compress thickness head gasket....or it was, I shimmed the liners so its probably a touch more now? That is a pretty hard stop on the piston up at/near TDC and no evidence of contact there but that could eat up most of my valve to piston clearance.

Thermal stuff...the block expands more than the rod so piston clearance should increase. The head heating moves the cam away from the piston. The valves heating are all that's left and is they expand more than lash that would show up as lost hp and poor hot leakdown and doing some math I see where the recommended lash numbers come from with the goal to avoid 0 bucket to cam clearance.

Clearly something is wrong and just adding a lot more clearance would probably fix it, I just wish I understood what exactly is happening.

 

How much does clearance change if you check it reverse rotation? (if you have manual tensioners for timing belts)

Maybe on rapid decel the inertia of the cams advances them slightly? (though I think not likely due to valvetrain load), maybe something to check.

 

Oh...I like where you're going here! 5degree of slop would be 25-30 thou and chain stretch is at least that. I think you solved the mystery

 

That would be more of a torsional twist effecting the opposite end of the drive side though wouldn't it? Should be able to see/feel the cam ratchet if it's loose enough.

Could be happening at temp though. Aluminum block and head engines get loosey goosey at temp. Seems counter intuitive but I've read many a debate that AL engines have increased lash at operating temp. It doesn't actually get smaller. Which I think.. Working the complex movement out mentally.. Would support your thought.. Increased base circle degrees more cam movement under inertia...
Didn't the old air cooled porches run really really tight clearances..
I'm going to go dig up my engine engineering design text books...

 

I'm not thinking it's enough to see slop in the system, just that if the valve clearances are on the tight side and the cam ramps aggressive even a slight advancement of the cam on decel (lots of inertia in the cam weight at rpm) might close that up.

 

Yes I forgot it was timing chains. How much movement does the tensioner allow? Maybe would help to mechanically limit it.

 

A kick back during cranking is a full reverse. a 6-7 degree change and the valves would hit.

Also there is a LOT of chain stretch, enough that I need to be very carful to keep the system in motion to the check point, no bumping it a touch or its wrong.

I think you got it.

 

Good point..esp with chain drive.
I've got marks cam profile, very similar to one I use on some builds. The ramp rate is at the top of mild aggressive compared to what many would call a hot cam.

Inertial advance would bugger up his clearances for sure.

 

Yeah think so. Totally forgot you're running chains. Any way to take up the slack?

 

There is a tensioner, its a bolt pushing on a shoe that pushes on the chain....if it moves while the engine is running its not much. The chain itself though stretches, a lot. an amazing amount really. And its not direct to the crank and way shorter like the motorcycle I pulled my clearance numbers from, there is a gear drive with lash and very real chain stretch. My valves are too tight for the engine I have.

 

The drive should be like the V8, thrust (float) in the gears is where you'll get the most "lash" movement. Crank gear should be pinned by the pulley, so any little timing movement would be in the other floating gears. I'm not overly familiar with the 12, they run the Belleville washer to take up that thrust?

There's got to be a way to crunch numbers and calc out the stretch and account for most of it id gather.

Or, or.. just add a whole bunch of gears and go with geared timing.. Think of the 'whine' at speed!

Here you go a starter pic!
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And all straight cut gears!

 

If you're gonna scream, might as well SCREAM!