The previous owner of my otherwise stock 66 L79 told me he had the engine rebuilt using Keith Black 10 to 1 compression ratio pistons (drive train is M21 with 3.7 rear end ratio). I assume that was because somebody told him the stock pistons wouldn't run on modern pump gas. If I replace these pistons with stock 11 to 1 pistons, would I feel the difference? Currently the car runs the best when advanced enough to give me a little chug on trailing throttle. When I tune this out via retarding the timing, it's not as snappy. Thanks.

 

Personality, with the gas that's available these days, I wouldn't even THINK of going 11 to 1 compression for ordinary street use. When these motors were first built octane ratings were MUCH higher so the stock 11 to 1 ratio was fine. I'm surprised you're even able to run at 10 to 1 without problems with detonation which, is possibly why you need to retard your timing to get it to run smoothly.

Gary

 

Even though Chevy says it was 11-1, they rarley made it that high, most were just under. These days ideal ratio if you have stock cam is 10 to 10.5, so you are good. If the car doesnt "feel" powerful to you, there are other issues. Do you know your actual compression, headgasket thickness, cc of heads?....etc
Boyan

 

As mentioned...the factory engines never made it to 11.0....and likely the replacement 10.0 pistons you have are not at 10.0 either. Most are slightly shorter than stock and if the deck wasn't whacked a good bit they are probably .025-.040 (some are even .060")down in the hole...and likely they used a .038-.040" composition head gasket instead of the .020-.022" factory shim gasket....and the valvejob likely sunk the valves somewhat...so if heads weren't milled or seats/valves worked to keep them at correct height...the heads cc larger than "nominal".

All that said..unless someone really did some decking on heads and block...you're likely in the 9.5 range at best.

You "can" run true 11.0-11.5 on the street with a lot of attention to detail and tuning...but not a lot of room for error. Compression is always good stuff..it helps idle vacuum as well as increased low speed driveability and pulling power that doesn't seem to be reflected by the 10-15 extra that might show on a dyno. But for something like you described...I'd keep the pistons you have (try to find out of they are forged or hypereutectics). If Hyper's be REAL careful of ANY detonation issues. Often you can't see it...but it will crack KB hyper pistons...and they're better than most. So don't get crazy with the total timing.

I'd rather see you run ful timing a little less compression than the other way around.

What cam is in it? That will make more difference than anything and can really make or break a low compression engine.

JIM

 

with the 70 LT-1 cam and stock 11-1 pistons you CAN run todays premium without destroying engine due to the cams overlap and such, I believe it has been spelled out thoroughly by SWCDuke more than once and since I have this engine never had a problem and owned since 90s. good luck and do as 427HotRod says and investigate internals.

 

It would make more torque across the board, especially at low/mid RPM. Would you FEEL it? I doubt it.
Yes, your engine would tolerate the above mentioned as-built stock 10.5:1 SCR on today's fuel.
You can advance the cam another 4 degrees for more snappy throttle response, but you will sacrifice some horsepower in the bargain. This tactic will build more cylinder pressure at low/mid RPM range.

 

My '66 L79 was rebuilt about 15 years ago when unleaded gas was not (widely) available in 93 octane form. The rebuilder suggested doing the valve mods and dropping the CR down to 10:1. He said he had never cc'd an L79 head that was much above 10.5:1 anyway.

The original L79 pistons were domed with hollowed out spaces for the valves; at 10:1, you probably have essentially a flat topped piston . The domed pistons are not quite as conducive to good fuel burn as are the flat tops, so some of what you lose in compression is going to be made up by a better burn.

Even though my engine was rebuilt long ago, the restoration was not completed until 4 months ago. I've not had an opportunity to try timing changes, but it seems to run pretty well.

 

If you tear into the engine just to raise the compression ratio, you'll feel it primarily in the wallet!

If it's worn to the point of needing a rebuild anyway, that's another matter. When my 327/300 is due, I figure on replacing the dished pistons now in it with approximately stock flat-tops.



Steve

 

Thanks all. The pistons are KD flat tops 30 over, cam is supposed to be factory spec for an L-79.

The spec sheet for these pistons says: Compression Ratio: 10.3:1 w/58cc Heads; 9.8:1 w/62cc Heads; 9.6:1 w/64cc; 8.4:1 w/76cc Heads.

Can anyone tell me what the chamber size is for stock L79 heads?

 

changing the camshaft to one designed for the compression you have now would be much better that building a new short block.

 

Stock heads were claimed to be 64cc
Be very careful when advancing timing, done properly you will run better, a little too much and you melt pistons or break piston rings.

Learn how to check total timing, most would agree 36 degree's total is about as far as you can go safely. The timing curve from idle to all-in will affect performance also but Total being correct will give max power and avoid detonation damage. Total timing would be the timing your engine see's above 3,000 RPM when your mechanical advance is fully advanced.
The vacuum advance will help with drive ability and all around performance but good power with no detonation damage is all about Total timing.

 

Roughly 62 to 63cc.
Those static compression ratio numbers are given when measured with the 0.040" composite head gasket. Using the original 0.015" metal shim style gasket will help to raise the SCR by maybe 0.3

I am torn between rebuilding my born with 67 L79 to OE specs
or
pulling and storing it then building up a 350 cid for playing with and not have to worry about the loss of the original engine. The new small block would be a real deal 11:.1 Speed Pro forged pistons (domed) and aluminum heads with 185cc intake ports and a roller cam and rockers, and headers too.
I guess I better get a 4:11 posi rear as well then


DonO

 

That is for the "small valve" 1.94/1.5 heads. The 2.02/1.6 valve heads that were installed on the L79 were 64-65 cc after the factory un-shrouded the bigger valves.

 

Bruce,

Use the following compression ratio calculator:
http://cochise.uia.net/pkelley2/DynamicCR.html
scroll to the bottom and click on the link.

If your deck(s) have never been surfaced, then your piston to deck clearance (although there was a significant amount of variation "back in the day") would be nominally, 0.025" although that could vary by as much as 0.015"!!!!!!!!!!!

Without giving you a tutorial, the important thing is DYNAMIC COMPRESSION RATIO. Search DCR on this website for more information.

I built a 327 with 11.35:1 SCR and 8.65 DCR with a cam whose intake valve closes fully (actually, by SAE J604d by 75 ABDC, virtually "seat-to-seat" valve timing) which runs detonation free on 93 octane (PON) pump high test gas.

By the way, my intake valve timing @ 75 ABDC is almost exactly the same as that of the L79 cam, which is 76 ABDC.*


*: My engine's "quench" is on the tight side at 0.032 +/- 0.002. Coolant temp and oil temp tightly controlled. Coolant temp @ 165 degrees, oil temp @ 190-220 degrees.

 

You are correct Joe.
I guess I was using numbers from replacement heads with the 2.02/1.60 valves.

DonO

 

You certainly can and should use the 11.0 L79 pistons if you are using an L79 cam. The reason that the L79 11.0 pistons were used instead of the flat top 10.5 pistons was to make up for the increased overlap of the L79 cam over that of the cam used in 300HP (10.5 pistons) engines. "Real" compression ration remained about the same. Yes, you are getting awfully close to detonation with today's 93 octane fuel.
My real world experience with an fresh L79 engine was occasional spark 'rattle' under light acceleration in 4th gear in the 1400-2200 RPM range caused by the spark timing advancing a bit too quickly using an L79 distributer vacuum canister which starts advancing the timing at 5Hg. I swapped the L79 vacuum canister for one for the 300HP engine that starts advancing the timing at 8Hg and the 'problem' went away. Total advance with either canister is the same the (8 deg. max). Max advance with the L79 canister is at 8Hgand and 16Hg with the 300HP canister. Changing the canister improved driveability, throttle response, eliminated any detonation and fuel mileage remained the same.
I hope this info and my personal experience were helpful.
Bob

 

IMHO octanes were not MUCH higher in the 60s. Today's octane is road octane which is research + motor /2. Back in the 60s Super was 100 research octane. Today super is 98-99 research octane and 87 motor octane or 93 road octane. It might have been a little higher but not MUCH. Motor octane was added when unleaded gasoline came out to make sure your motor didn't ping around town.

 

So I guess it was a mistake to have rebuilt the motor in my car with lower compression pistons, but not enough of a mistake to justify the cost of swapping them out if its not in need of a rebuild otherwise. Thanks all.