Engine question

[Kurt B.]

Re: 68 L79 engine 327 cid/350HP
This motor in 1968 had 11 to 1 compression and produced 350 horsepower.
I am having the motor rebuilt and am being told it will only have 10.25 to 1 compression and something less than 350 horsepower despite the fact that I have supplied all the original parts from GM, many which are new and took me years to find. 60 days and $3000.00 and I will get something less than what GM built in 1968. I know nothing about engine bulding. Why the difference? I would have thought it owuld be identical.
I am thoroughly disgusted and jaded with this 6 year restoration project that is becoming an abortion despite my efforts to do everything perfect and with factory correct parts.
How do the rest of you get your cars factory correct? I can't build an engine, I don't know how.

[Jim V.]

Re: Engine question

Not so bad Kurt. I took about six to do a 65 and thought the project would never end, but eventually it did; well, sort of. Anyhow don't sweat the Compression Ratio thing. Lowering the CR to 10:1 will allow you to run 93 octane pump gas. Higher ratios ofter require mixing in race gas, lead additives, or aviation gas. As far the factory correct stuff, NOWBODY is going to be tearing your engine apart for judging purposes. The short of it is have fun with the car! If you want factory APPEARANCE and can spend the time and money than that's fine too, just get the books, hang out here, join your local NCRS and pay your dues.
Good Luck

[Joe L.]

Re: Engine question

Kurt-----

If you used original pistons (or pistons with domes configured just like the originals), original shim-type head gaskets (or gaskets with equivalent thickness), and the original heads (or heads with similar 64 cc combustion chambers), your engine will have the same compression that it had originally. The slight amount of increase in combustion chamber volume (and consequent decrese in compression) which MAY have resulted from grinding of the valve seats will be more than off-set by surfacing of the cylinder heads (which will tend to reduce chamber volume and INCREASE compression). Also, if the engine was overbored, that will tend to INCREASE the compression ratio over the stock compression. So, I see no reason why your engine should have any less compression than it originally had. If the above conditions are met, it will likely have MORE compression than it originally had.

However, keep in mind that the factory-rated and advertised compression ratio is a NOMINAL figure. Because of various production variations, the ACTUAL figure may have varied by several tenths of a point. So, your engine may not have been a TRUE 11:1 to begin with.

[Duke W.]

Re: Engine question

Eventhough the advertised CR is 11:1 the actual CR is probably half a point less. This is because the decks were machined at greater than the 9.025" required to achieve close to the advertised CR.

If you don't deck the block or machine the heads and assemble everything with the OEM replacement pistons and a .040" composition gasket, you will probably be between 10 and 10.5:1, and with the L-79 cam it should just about run detonation free on 92 PON unleaded fuel.

You, or a competent machinest should be able to take the appropriate measurements to determine the ACTUAL CR on teardown and project the ACTUAL CR on buildup knowing the dimensions and specifications for the new parts.

If you do have any detonation, you can either fiddle with the timing, or dope the fuel with 10 to 25 percent high octane racing gasoline.

Duke

[Patrick H.]

Re: Engine question

I agree with Joe and Duke. When I rebuilt my 1971 350, we calculated that the compression ratio of the motor before the rebuild, as it came from GM, was close to 8.0 to 1 not the advertised 8.5 to 1. My new combintaion raised that to a real 8.8 to 1. Duke's suggestion that GM was often 1/2 point off is true to what I've seen as well.

I'd bet that you could hit 350hp with some carb and ignition tuning on a dyno with someone who knew his stuff. If your shop isn't running your engine on a dyno to back up their claims, then don't listen to them anyway. They figure you've got to put a lot more parts from Summit in there to hit 350 hp.

Patrick

[Duke W.]

Re: Engine question

GM had great dynos! They always seemed to show good power. 'Course, the PR guys liked nice round numbers, too. A box stock L-79 would probably show a little over 300 honest SAE gross horsepower on a laboratory dyno. With some blueprinting, pocket porting and port matching, and a bit of carb and ignition tuning, you can probably get close to advertised 350 gross.

Duke

[artarmstrong]

Re: Engine question

Actually Duke, they are STILL using the SAME dynos today, except the building is now called Powertrain. And yes they are great dynos. You would not believe the punishment those things did/do to an engine besides calculate gross horsepower.

Art

[Duke W.]

Re: Engine question

When I was at Pontiac in '68 I watched some dyno runs of the stillborn Pontiac tunnelport heads.

Since GM Powertrain is now headquarted in Pontiac, are those old dynos in the Pontiac Engineering building still being used.

I also remember seeing some complete XP-882 powertrains in dyno cells - three or four of them - at the Tech Center circa 1970 - 400 CID SB with the Toronado chain drive to the gearbox beside the engine and then through a bevel drive in the tailshaft to a driveshaft through the oil pan "window" to a Corvette axle. I really DID think that was the '73 Corvette drivetrain undergoing durability testing, but alas...

When I spent some time with Chuck Jordan a few years ago he finally nailed the coffin closed. When we got into a discussion of the XP-882 and the powertrains in the dyno cells that I saw, Chuck said: "Naw, that was just another one of Duntov's pipdreams."

Duke

[Wayne K.]

Re: Engine question

Kurt,

When I purchase my 67 L-79 I had to either retard the timing or add octane additive to keep it from pinging. When the motor was rebuild I had them put in the lower compression flat top pistons and now I can drive the car on 92 octane without any problems. I think you will be better off with the lower compression.

[Kurt B.]

Re: Engine question

Thanks to all for the informative responses.
Suddenly things don't look so bleak. I had visions of having a detuned, high priced Corvette that couldn't get out of it's own way.
Thank you all.
Kurt

[Jim T.]

Re: Engine question

Kurt my original 68 327/350 runs good on no lead premium 93 with no other additives. I set my timing at 10 degrees BTDC and don't lug the engine or usually use 4th gear below 40 mph with my close ratio M21 and 3:70 rear gears.

[artarmstrong]

Re: Engine question

Duke, I don't know about the dynos in Pontiac, I was referring to the dynos in the Chevrolet Engineering Center Test lab at the TC, which is currently (at least was as of a year ago) occupied by GM Powertrain.
Yes, Zora had lots of pipe dreams!
Art

[Mike McKown]

AMEN *NM*