Compression ratio L-79 350HP compared to 300HP

[Daniel D.]

[Chris D.]

Re: Compression ratio L-79 350HP compared to 300HP

Daniel,
The piston is different between the two engines. 300 horse engine uses cast flat-tops rather than the forged dome piston in there now. There's also a small difference in combustion chamber volume since the 350 has the larger 2.02 valve and the accompanying relieve machining to unshroud the valve. But that's a minor delta.

That gasket thickness is right for an original.

[Daniel D.]

Re: Compression ratio L-79 350HP compared to 300HP

Thanks Chris. That clears it up. I was getting a little overwhelmed, but your reply helps a lot.
Dan

[Richard G.]

Re: Compression ratio L-79 350HP compared to 300HP

I would be careful on my piston choice. I would use the flat top piston but would recommend checking the deck height. Many (if not most) aftermarket piston manufactures drop the deck height from factory specifications. They are attempting to take into account the small number of engines that have had the block decked. This keeps the replacement piston from extending above the deck. However, it also drops the compression, on a block without decking, as the piston height will be close to .020 lower than the original factory specification. Read the fine print! Order the standard deck height or you can custom machine the piston tops for a consistence deck height.

[Duke W.]

Re: Compression ratio L-79 350HP compared to 300HP

I’m in the process of rebuilding a 1965 L-79 350HP. My question is, what causes the difference in compression ratio between the 350HP (advertised 11:1?) engine and the 300 (advertised 10.25:1?)? From searching TDB, I see that piston L2166 is the recommended replacement for the 350HP, but this piston also seems to be recommended for the 300 (unless I’m mistaken). If the pistons are the same, what causes the different CR … is it the heads, or possibly head gasket? My engine does not have detonation issues now, so I would like to stay with about the same compression ratio. I’ve read Duke’s “Compression Ratio Explained” and see that a true compression ratio of about 10.25:1 works well with the L79 (or CS179R) cam.

One other question … what is the thickness of the factory head gasket on the 1965 L-79? I haven’t pulled my heads yet, but the gaskets appear to be about .018” when compared to a feeler gauge. I’m wondering if these are possibly the original head gaskets. The engine only has about 50,000 miles, so it is possible the heads have not been off before.

Thanks.

The OE piston is forged, same as the L-76 and L-84 with a net dome volume of 5.3 cc. The 300 HP piston is cast and has four valve notches with a net "dish" of about 5 cc. It has four because the piston pin of offset to the major thrust surface by .060". This allows the same piston to be used on both sides. The 461 L-79 head chamber is nominally about 64 cc, a couple more than the 300 HP version due to the unshrouding cut for the bigger inlet valve.

The L-2166 pistons are expensive, and may yield too high CR without a thick head gasket. It probably doesn't detonate now because of the extremely lazy centrifugal curve. You are better off dropping the CR a bit so you can run a more aggressive curve. The .018" gasket is very likely original. Consider the KB156 and 157 pistons. Run them through the calculator. The are considerably less expensive. Put the savings into better rods since your OE rods are the weak first design.

With only 50K miles the engine probably has plenty of life. What are you rebuilding it? If it's consuming oil, it probably just needs new valve seals.

Duke

[Daniel D.]

[Domenic T.]

Re: Compression ratio L-79 350HP compared to 300HP

Daniel,
If you change the pistons to flat top you no longer have a 350 HP engine. The 350 HP was designed with the dome. Your engine would be more like a 325 HP. We are so careful about numbers like the trim tag and paint color, but change the HEART of the corvette by replacing the original parts design with another! It's bad enough that we have to de-tune to use to days fuel. SO, we keep all we can see (numbers ETC.) and change what we cannot see. If you wanted a 350 HP engine I would keep it a 350 HP, .018 gasket and all. By the way when I was rebuilding the engines there was a SS head gasket available thru marine parts for boats.

Dom

Dom

[Ray K.]

Re: Compression ratio L-79 350HP compared to 300HP

Daniel,

I agree with Domenic's suggestion that you retain the original domed pistons which are rated at the 11: 1 compression ratio. I would absolutely do this if it were my engine. If you are so inclined use a composition head gasket of about .042, GM offers one ( #10105117 ) plus Fel Pro and Victor . The GM steel shim gasket is #3830711 about .022 before compression and is still available. The stainless head gasket was #3916336 but I believe it is no longer available.

Ray.

[Dick W.]

Re: Compression ratio L-79 350HP compared to 300HP

Believe the number you want for a 350 hp is L2166NF Speed Pro

[Richard G.]

Re: Compression ratio L-79 350HP compared to 300HP

This question is for Duke:
Since the 60's we have found out what works and what doesn't. A lot of the factory stuff has changed as they made a lot of changes to reduce emissions and up power levels. The thinner and higher rings are one example. The lack of domes on the pistons is another. The trend is to use flat top pistons to promote flame propagation and reduce the head volume to up the compression. Of course, they also have the advantage of aluminum heads and modern combustion chambers with high swirl. I know higher compression results in a more efficient motor. But so, does a small quench area and flat top pistons. I realize the dome isn't very large, say compared to the 302 domes for instance. However, if one doesn't consider the "as built" part of the equation, what would make the best power using pump gas using the 350 HP camshaft? One could mill the heads to get to the 11.25 compression ratio. I also understand the compression ratios were about as accurate as the HP rating of the day.
Rick

[Duke W.]

Re: Compression ratio L-79 350HP compared to 300HP

This question is for Duke:Since the 60's we have found out what works and what doesn't. A lot of the factory stuff has changed as they made a lot of changes to reduce emissions and up power levels. The thinner and higher rings are one example. The lack of domes on the pistons is another. The trend is to use flat top pistons to promote flame propagation and reduce the head volume to up the compression. Of course, they also have the advantage of aluminum heads and modern combustion chambers with high swirl. I know higher compression results in a more efficient motor. But so, does a small quench area and flat top pistons. I realize the dome isn't very large, say compared to the 302 domes for instance. However, if one doesn't consider the "as built" part of the equation, what would make the best power using pump gas using the 350 HP camshaft? One could mill the heads to get to the 11.25 compression ratio. I also understand the compression ratios were about as accurate as the HP rating of the day.Rick

There's no magic formula to determine the optimum compression ratio... too many variables beginning with available fuel octane. Actual compression ratio from the plant was up to 0.5 lower than advertised due to increasing finished deck height as the broach tools wore. The only way to know the true CR is to measure deck clearance on teardown, cc the heads and run this and all the other necessary data through an accurate CR calculator. If the engine didn't detonate as built, whether original or rebuilt, then it will probably won't detonate if assembled with the same CR.Detonation was a problem even back in the sixties.

In '62 GM got a lot of detonation complaints on 340/360 HP engines, which had an advertised CR of 11.25:1. The field fix was to pull the heads and reinstall with two OE .018" head gaskets that dropped the true CR by about half a point. This was also implemented in mid-'62 production and continued through '63.In my tuning seminar I listed recommended maximum CRs for all OE engine configurations, and I'll add that those values are with current 93 PON fuel. As a rule of thumb, reduce by 0.1 for every octane point below 93.

The bottom line is that it comes down to the individual engine, available fuel octane, altitude, maximum ambient temperature, and the spark advance map.My philosophy is to be a bit conservative on CR so the typically lazy OE spark advance map can be made more aggressive. This means more power in the lower operating range, which is a lot more useful than up near the redline, and an aggressive map will also yield better fuel economy.

A good choice for SHP/FI small blocks is the KB 157 piston that has a 0.5 cc net dome. This will reduce actual CR by about half a point compared to the L2166, so a thin head gasket can be used, especially if the decks are higher than nominal. That's why deck clearance MUST be measured prior to block disassembly in order to start the compression ratio management process. Another advantage of the KB pistons is that they cost less than half the L2166. If fact, you can buy a set of high strength con rods AND KB pistons for about the cost of a L2166 set.

I'd much rather have bullet proof rods than forged pistons so I don't have to worry about an old inferior design rod breaking and punching through the block.I always invite those who are rebuilding their engines to call me to provide all the specifics. Sometimes it takes more than one call for the owner to get all the necessary data, but once it's it hand I can recommend a "not to exceed" value, and none that I have ever recommended resulted a detonation problem.

The higher the CR the more efficient the engine, which means "more power", but going even a tenth or two too high can result in detonation, which will require either high octane "race gas" or a very suboptimal spark advance map. It's not worth it. Selecting a conservative CR that will allow an aggressive spark advance map will make for a much better engine in normal vintage car service than too high a CR that requires expensive fuel and a suboptimal spark advance map.

Duke

[Domenic T.]

Re: Compression ratio L-79 350HP compared to 300HP

Duke,
I worked on those engines back then and we found that all ethyl (hi test was not the same octane. Lots of cheating was going on and there were companies that would do a random test on the octane. One former station owner said that red dye was put in low test (regular) so the could make more money selling it as ethyl. He was the service manager at the Chevy dealership I worked at. We never added a gasket and the customer was told to change gas stations till the problem stopped.
I found the detonation to be a problem at different gas stations. Always had grease on my distributor gasket so I could retard when that happened. There was lots of cheating then, enough to have pop up tests done.
By the way, I decked and milled using flat top pistons and got the same CR on a engine and found that it ran better. The CR was as equal to the dome as we could calculate. Also did some port work and put the larger valves in the 461 castings to equal heads.

Dom

Dom

[Duke W.]

[William F.]

Re: Compression ratio L-79 350HP compared to 300HP

Can someone submit photos of 350 hp L79 domed pistons and the flat top 300 hp pistons for comparison?
thanks

[Richard G.]

Re: Compression ratio L-79 350HP compared to 300HP



I don't have any pictures except the one of an original L76 340HP piston.
Others may know if they were held over to the 350HP L79.