66 350 hp /rebuild

[Jim S.]

Well guys its come to that time of the restoration when the motor gets done.
My question is this;should I reduce the compression so as to run on todays gas as many have suggested.....or as others have stated, never mess with the original specs at the risk of reducing its value,and use octane booster or av-gas.
Let me hear your thoughts.
Thank You in advance,
Jim Schwering

[Ed Jennings]

Re: 66 350 hp /rebuild

My 67 327/350 and 62 327/360 both run on ordinary premium with no issues. The engines more prone to problems are like the 327/300 which still has moderately high compression and a mild cam. The 327/350 has enough valve overlap that the dynamic compression isn't all that high. Do like GM did and use two head gaskets.

[Duke W.]

Re: 66 350 hp /rebuild

With a .040" composition head gasket and no cutting of the block deck or heads the actual CR should be no higher than 10.5:1, and it should be okay on premium with no more than a bit of timing adjustment. If it runs okay now on premium, then there's your answer. You should measure the deck clearance before you remove the pistons, so you can compute the actual CR and determine a good head gasket thickness for the buildup.

If you want more power have the heads massaged. Be sure you have the late model small bearing rods. If they are the early type, replace them for sure. They are weak. E-mail me if you don't know how to identify the early versus late small bearing rods, and I'll send you a side by side picture.

Everything else OE or OE equivalent replacement parts. No need to use a bunch of hot rod parts. The L-79 is a great engine as is!

Duke

[Mark #28455]

Re: 66 350 hp /rebuild

Although adding a second head gasket will lower the compression ratio, it actually increases the chance of detonation as the quench area of the chamber is most effective when the gap is about .035 to .040. If the gap goes above about .050 then there is no effective quench.

You should also consider how much you intend to drive and the driving conditions. If you live in a hot area and plan to do a lot of longer trips you may want to drop the compression to 9:1 or 9.5:1. Hot engines are more likely to detonate also so consider a 180 degree thermostat. If you plan to drive a lot, also consider the 9:1 ratio as you will likely be able to run 89 octane gas (a lot cheaper in the long run).

Good luck!

[Duke W.]

Re: 66 350 hp /rebuild

According to Taylor an increase in detonation resistance at a give CR does not improve until the quench clearance is equal to or less than .005 times the cylinder bore, which for a SB would be .020" - much less than Chevrolet's recommended minimum of .035".

The are many owners of original SHP/FI engines that are running pump premium with no significant detonation, and most of these engines were originally built with two .015" steel shim gaskets. A .040" composition gasket will decrease the compression a bit from original, and the real compression ratio is a long way from the advertised 11-11.25:1. There's no point reducing the CR to less than 10:1 unless you wnat to run regular unleaded fuel. The '71 LT-1 had an advertised CR of 9:1 and it was designed to run on REGULAR unleaded!!!

I don't know where this "quench clearance more that..." will increase detonation story got started, but the available research data going back the the 1930s does not support it.

BTW, overlap has little to do with how much compression an engine can run on a given octane fuel. The big determining factor is the point that the inlet valve closes. This point and the static CR determine the "dynamic CR". The later the inlet valve closes the lower the dynamic CR for a given static CR and SHP cams have late closing inlet valves. The Duntov and L-79 are about the same, LT-1 a bit later, and 30-30 later, still.

Duke

[Loren L.]

For as little as you will drive it, I'll get

you into Deer Valley airport for some "low-lead 104" - go for the popups and listen to the sound and feel the heat.......

[Michael H.]

Re: For as little as you will drive it, I'll get

Agree 100% with Lundburg. I don't understand all this 9.0-1 comp ratio nonsense. A little juice in the tank and a slight timing adjustment and yer right back to sounding the way it did when we all fell in love with these cars in the 60's! Leave the original 097 or 346 cam in it and enjoy the sweet sound.

[Rich G.]

Re: 66 350 hp /rebuild

My 66 L79 is a standard rebuild. It runs fine on pump premium. It has about 3500 miles and 3 years on it since overhaul. I do sometimes put a little 100LL aviation gas in it, but I honestly don't notice a difference. It just smells better.

Rich Giannotti

[Jim T.]

Re: 66 350 hp /rebuild

Jim my 68 327/350 L79 started its life back in 68 on AMOCO premium and now has right at 100K miles on it burning premium pump gas with no problems.

[Mark #28455]

Re: 66 350 hp /rebuild

Hey Duke,
On a similar thread you noted that with high compression you may need to add up to 25% 100LL avgas to get the motor to run properly! Are you sure this owner really wants to get trapped into that hassle? How much power do you really lose by dropping the compression a half of a point? I bet you could more that make up for it with cam timing.
I have several Corvettes and I can assure you that my favorite one to use as a "driver" has 9:1 compression and I can buy gas for it anywhere. It all goes back to what you're REALLY going to use the car for. There's a huge difference between a true daily driver and a car that only gets driven on weekends when the weather is nice.
As far as the quench distances, I was referring to not using two composition gaskets to lower the compression. My advice regarding the distances was taken from the Keith Black pistons web site. I would personally still recommend a maximum compression of 9.5:1 for a true daily driver with 35 year old combustion chamber design.

[Duke W.]

Re: 66 350 hp /rebuild

Very few engine builders whether the owners or professionals seem to bother with making the measurements to compute actual compression ratio, so few know the ACTUAL CR they are running. If the CR is close to Chevrolet's advertised values on SHP engines, which can only be obtained with block decking and a thin head gasket, then some high octane fuel may be required to blend in with pump premium. Big blocks are more "octane hungry" than small blocks, and will be more likely to require something higher than available at normal gas stations.

The anecdotal evidence indicates that OE machined decks on SHP/FI engines and double shim gaskets or a composition gasket will operate satisfactorily on pump premium with little more than some timing adjustment. Such a configuration yields actual static CR between about 10.25 and 10.5:1.

Don't know how Keith Black arrived at his conclusion, but Taylor's book has a complete description of the test set up used and lots of data to back up his conclusion.

Duke

[Clem Z.]

Re: 66 350 hp /rebuild

duke from what i have been able to find out from the pro engine builders is that fuel get trapped in that area instead of being "squeezed" into the combustion chamber and burns at a different rate and causes the detonation. i have found that on race engines were you are not allowed to exceed a certain CR building them with a small a deck clearance as possible,using tight fitting pistons in the bores to prevent "rock" at TDC,and the deck clearance as close as .015 make more HP than a engine built with the same CR but .035/.040 deck. getting rid of all the sharp edges,smoothing the chamber and the piston top goes a long way also to prevent detonation