L78 Dyno

[Mr70]

I'm running a dead stock 1970 L-78 with hardened valve seats,11-1 compression,R43T plugs & A.I.R.,with the factory timing settings in my Chevelle.
I use 100 octane unleaded gas found at certain stations around Chicago,with no pinging,hesitation or run on at all.

[VintageMusclecar]

L78Kiwi:

The skirts on that piston have been knurled to take up excessive piston to wall clearance. Standard practice back in the day, but <span style="font-weight: bold"><span style="text-decoration: underline">not</span></span> the way to fly today.

Am I seeing a 2 piece top ring there? Either way there's an awful lot of carbon between the top and 2nd ring, not a sign of good ring seal.

Measure the bores, I'm betting they're pretty worn and need to be machined. Be sure to have the block honed with a torque plate using the same gasket &amp; head fasteners the engine will use in service.

You can get away with 11-1 compression on 91+ octane pump gas, but it's a bit of a tightrope walk. Temps need to be kept at 160°, and timing needs to be kept at 36° or below, which probably leaves some power on the table.

With today's piston technology, you can special order pretty much anything you want in a piston. I would advise dropping the compression back down to 10.5 or less and move to a modern metric ring package to free up some power.

If you need any help with parts, let me know, I'll be glad to help.

[L78Kiwi]

Thanks for the advice, yes the top ring is a 2 piece. The bores have been measured. they are 5 thou over from standard. The piston clearance is 11 thou and there is about 1 thou in taper. The bore is not out of round.
What pistons would you suggest running with in terms of brand and profile. Given the bore dimension and condition what oversize would you suggest.....there is the usual 30 thou over but I am lead to believe and as you say you can order just about anything. Interesting point about the honing proceedure, does this mean honing with the cylinder head on? what is the reason for this type of honing?

[VintageMusclecar]

The bores are definitely past spec. It's evident the bores were worn before the last rebuild and rather than boring the block, they knurled the pistons and just honed the bores.

Not a good recipe for good ring seal and longevity.

I would bore &amp; hone the block .030&quot; and go with new pistons.

Rather than further derail John's thread, shoot me a PM or an email and we'll discuss things in further detail.

[earntaz]

Yep -- somewhere along the line the pistons were &quot;knurlized&quot;. This was a rebuild &quot;trick&quot; that would tighten up a piston in the bore to reduce piston slap. Looks like a cast piston also -- not an HP forged unit.

TAZ

[Kurt S]

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: [email protected]</div><div class="ubbcode-body">hardened valve seats really necessary? </div></div>
If it sees a sustained hi-output duty cycle. That would be a tow vehicle where it's running high temps for long periods of time.
Not for a street or race car.

[69 Post Sedan]

Around here.....we install hardened seats in just the exhaust valves as the intake doesn't need them.

Kurt

[bugsy]

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Mr70</div><div class="ubbcode-body">I'm running a dead stock 1970 L-78 with hardened valve seats,11-1 compression,R43T plugs &amp; A.I.R.,with the factory timing settings in my Chevelle.
I use 100 octane unleaded gas found at certain stations around Chicago,with no pinging,hesitation or run on at all.


</div></div> I like it.That car looks very clean. I have a 1970L78 block for sale. Date Sept. 1969 CKO STd. Bore

[owners2]

Nice 1970 L-78 !! I have a Charcoal Grey 1970 L-78... 70 SS Camaro's Rule..

[Mr70]

Thanks,Charcoal's a great color....some minor incorrectness in my engine photo,but that has all been fixed,since that photo was taken some 23 years ago.Heater hose,Radiator,A.I.R. tube,P/S hoses,etc.

bugsy,PM sent on your CKO block.