L78 Dyno

[Furback]

I finished assembling my NOS L78 shortblock today. Put a pair of 69 840 heads on it and a 163 intake. The cam is original specs and solid. 3/8 pushrods and Crane race rockers. So other than the rockers the engine is exactly as an original would be. Standard everything. Also installed hardened valve seats and manley stainless valves. Comp cams springs. Should be dynode within a week. It will be interesting to see what kind of numbers the dyno sessions show.

[L78 Fred]

Let me know the numbers
I will be doing the same in the next 2 months
Fred

[Salvatore]

hardened valve seats really necessary?

[earntaz]

<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>

It's a toss up -- I've read pros/cons. If you will be running the livin' hell out of the motor for extended period of times -- hardened seats are a must. But if you're just cruzin' with occasional jumping on it you probably don't need to install.

Valved seats back in the day were induction hardened -- but if the valve seats are ground below this &quot;hardened surface&quot; erosion may occur.

I'm sure others will chime in on this ...

TAZ

[Salvatore]

doubt if either of these guys are going to run their car that hard. both will probably use a lead additive or some racing gas anyway. Don't like cutting into stuff that can cause problems later. Not necessary just like floating wrist pins arn't really necessary for street motors IMO.

[JRSully]

I put hardened seats in my 70 291 heads on the 70 L78, didn't want to deal with lousy gas issues and the engine gets hammered on regularly. My engine dyno'd at about 428hp at 5600 rpm, dead stock except roller rockers, pertronix and headers, orig intake, compression, 4557 holley, stock heads, nos 143 cam. You need somebody experienced to do the valve seats right, have to watch the water jackets

[earntaz]

Jim -- what did you do for valve springs? Stock LS-6?

TAZ

[JRSully]

Taz, I believe I used a very close to stock, dual valve spring. Consulted with Crane (roller rockers) and my engine builder, but I know they are very close to stock. All the hp is in by 6000-6500 on a stock L78 anyway, no need to rev it any higher than that, just asking for trouble. Roller rockers are the ticket on these engines (unless you need to have stock valve covers, but you can use a camouflage spacer)they really let the engine rev and are very durable. Valve train is the weak link on these engines. But mine (with this set up) has been bulletproof and I have adjusted the cam only twice in 6 or so years

[earntaz]

Copy that -- I agree with going crazy with valve spring pressure may cause hate a discontent ... just looking for trouble on a 95% street machice. I have roller rockers ...

Thanks -- TAZ

[L78Kiwi]

I am interested in hearing how you guys get around the high compression (11+/1 ratio)- low octane fuel problem. Do you modify the engine, such as install lower compression pistons or installer thicker head gasket, or do you run a fuel additive or have a source for higher octane fuel such as aviation fuel?
I have just pulled down the 427 (casting no.3963512, closed chamer heads, standard holley and winters manifold)I intend using in my 67. It was built as a race motor for a 68 vette and has not been run since the mid 70,s. On the whole it is in excellent condition and probably could be run as is....but it will need some modification for the street and modern fuels so if anybody has similar experience I would greatful for some advice.I have attached a photo of a piston.....anybodyseen pistons with this pattern embossed into the skirt before
http://www.yenko.net/ubbthreads/pics/usergals/2015/04/full-19248-24225-427_piston_08.04.15.jpg (http://www.yenko.net/ubbthreads/pics/usergals/2015/04/full-19248-24225-427_piston_08.04.15.jpg)

[Mr70]

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.

http://www.yenko.net/ubbthreads/pics/usergals/2015/04/full-458-24226-l78c.jpg

[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.

http://www.yenko.net/ubbthreads/pics/usergals/2015/04/full-458-24226-l78c.jpg
</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.

[L78steve]

I don't have any spark knock problems with either of 2 L78s running premium. 36 Degree total advance.

[Vortecpro]

I would never deface a set of 840 or 291 heads by installing exhaust seats unless they were sunk, and only then I would install custom made small ID seats machined to size along with a one piece solid bronze guide honed to size with a very precise valve job, which would run easily on unleaded gas.

[JRSully]

FWIW,My engine builder did note that a lot of L78/L72 engines that get rebuilt don't actually get to 11:1 when completed, for whatever reasons (head gaskets etc.) I know that my L78 will ping/detonate with straight pump gas (stock everything basically, timing all in at 3k) It seems to run best on a mix of Cam2/Avgas and 93 pump gas

[luzl78]

Love the smell of cam2!

[Salvatore]

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: JRSully</div><div class="ubbcode-body">FWIW,My engine builder did note that a lot of L78/L72 engines that get rebuilt don't actually get to 11:1 when completed, for whatever reasons (head gaskets etc.) I know that my L78 will ping/detonate with straight pump gas (stock everything basically, timing all in at 3k) It seems to run best on a mix of Cam2/Avgas and 93 pump gas </div></div>

Sounds like a good combo to me Sully.
That is a really good running car you have and you show it NOT much mercy.
Love it!

[JRSully]

You are correct Sammysan, It's gets wrung out regularly, but the relatively skinny 225 14s keep it from breaking anything for the most part

[Salvatore]

that is the way to do that Jim. smaller, skinny tires = smaller, little breakage.