65 Fuelie - 11:1 compression - Octane Booster?

[Stuart F.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

The three "C's" : 1) Compression, 2) Carburetion, and 3) Camshaft = Performance.

Learned 50+ years ago, is still true today.

Stu Fox

[Joe C.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

I am not saying the engine will not run OK but I think you both are leaving lots of power on the table by not running the correct engineered compression ratio. Compression pressure (heat) is what vaporizes the fuel load, the gassed mixture is what makes combustion and proper combustion is what makes torque.

Low compression with a camshaft like that will make the low end manners even worse and sacrifice power at the top end. I am not aware of problems with detonation with these engines and the .125 pop up pistons.

The underlined clause is a very small part of the full story. The crux of it is:

Force = Pressure x area

Peak cylinder pressure is a function of quite a few factors, some of which are: efficiency of ring seal, concentricity of bore, stiffness of bore, static compression ratio, cam overlap, speed of cam flanks/bleed off rate, valve lift, intake valve closing angle..................... . Static compression ratio plays a very large part in this. Surface area of the pistons is constant, the greater the pressure bearing on that surface, the greater the down force exerted on the piston and therefore the con rod and finally the crankshaft.

Torque = force x radius arm

The greater the torque arm (1/2 the engines "stroke") for a given cylinder pressure, the greater will be its torque at any engine speed.

Work (horsepower) = torque (ft-lbs)/5252 (constant)

The higher an engine's torque peak, and the smaller it's drop off rate as the engine speed increases, the more work it is capable of doing as revs increase (the more "horse-power" that it makes)

[Brooks R.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

Hi Joe,

I know you folks are right. It is a compromise that breaks my heart just a little, as I remember running the 365hp version back in high school and it is not the same.

If I had easy access to 100+ octane fuel (and could afford it), I would have gone with 11:1. Just got a little conservative in my old age. Maybe it will help keep me from getting too many tickets for exhibition of speed.

[Rick G.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

Brooks,

Congratulations on your rebuild, it sounds like it has been well thought out and should serve you well. I am in he process of planning the rebuild of my 427 and since we live in the same general area, I would like to talk with your rebuilder. Can you please let me know who you used?

Thanks,
Rick Gower

[Monte M.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

You guys need to understand that we do understand that there is a difference. To tell the truth, A number of years ago when I built my built my big block car (a 69 back then) I regretted building it with all the power. I learned a hard lesson.

I like driving my cars. They are not trailer queens or just driven to shows, which is alright for some. But to be able to enjoy driving the car, it is a trade off. I would rather have a few less horsepower than to have the headache of having to figure out what I amgoing to do for fuel.

That is why I asked the question. What is working for the guys who have done each thing.
I no longer have the 69 because of the drivability.

My 63 might have 10's in it instead of 11's, but if it on the road a lot more because of it, perfect for ME.
If it works out well for me, my son can worry about what pistons he will put in it once I wear it out.

It is a fine line between originality, drivability, and horsepower. When I was younger, horsepower always won out. Not so much any longer.

Keep driving them guys. that is what they were built for.

[Stuart F.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

I left out one "C", and that being Cubic Inches. With that "C", you can get along without some or all of the other "C's", but it really only came into prominence in the late 50's. There were a few pretenders like the Chevy 348 and the Pontiac 389, but even they had trouble with a Chevy small block with the three "C's".

The engine in my 63 is an L-76 and has never been apart after a mere 44k on the odometer. The low mileage was not all by choice, like 7+ years in SEA during which a lot of the time the car was in storage. I too drove it as my only car for awhile after I returned, but once I had a family it became my weekend car and I limited to about 1000 miles a year. As long as I can get good gas and an Octane booster that allows me to run my factory compression and performance ignition timing, I'm happy as snot. It remains my defibulator.

Stu Fox

[George J.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

As it's been stated on this board many, many times before, using the stock pistons in these engines results in about a 10.25:1 compression ratio. I had mine physically measured and this is what it came out to be. This ratio and a 30-30 cam runs fine on pump gas at 92 ocatane.

George

[Joe C.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

As it's been stated on this board many, many times before, using the stock pistons in these engines results in about a 10.25:1 compression ratio. I had mine physically measured and this is what it came out to be. This ratio and a 30-30 cam runs fine on pump gas at 92 ocatane.

George

George,

That is very true! But still exceedingly and needlessly conservative.

Yes, the as built factory SCR was around 10.25 - 10.5:1 using pop-up pistons. The 30-30 cam has such sloooooooowwwwwwww ramps and such huge clearance ramps, that the bleed-off is unacceptable for a modern car, although back "in the day" it sounded mean and developed big power in the upper RPM range. The cam's "advertised" durations are 254/254 which are quite large, and if GM had published seat-to-seat durations (J604d), then they'd have been around 310/310!

When my engine had a 30-30 cam in it, I blueprinted the engine and designed in an honest (verified) 11.3:1 SCR. Running this engine on 93 octane pump high test and using a much more aggressive than factory spec. spark timing program, this engine never detonated, even when run under high loads and high ambient temperatures.

This approach can be taken with the LT1 cam, but to a slightly lesser extent. This approach can not be taken with the 097 cam.

[Stuart F.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

Joe;

If it's not straying too far off this thread subject; what do you recommend for the 097 cam? I know the 63 L-76 is an odd ball, but mine has been shown to hold it's own against any other year L-76 or L-84, specially those with a final drive lower than 3.36 (higher numerically). Always seemed to be the right combination on the road, if not on paper.

Stu Fox

[William F.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

I have brought the following up in several previous threads regarding octane requirements/ detonation, but nobody has ever given a reply.RE: in a 1958 issue of Corvette News) in an article about tuning, the authors warn about setting the timing by ear, since they say there is a form of detonation that is detectible ONLY by special equipment and decreases ring life by 1/3. This goes against the oft repeated advice in these threads that "If you don't HEAR pinging, 93 octane is fine for the higher compression engines. I'd like to hear comments from someone qualified (engineer for ex.) to comment on this.
thanks

[Domenic T.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

I would have to say that the vette engine is and was the heart of the car and not keeping it the way it was engineered is changing the car.

Changing the compression WILL change the performance, sound, and originality.

I know fuel is a problem but nothing sounds better than a high compression engine running strong and not de-tuned.

We strive to keep things original even to the point of poor chrome, paint, finish, workmanship, rust and whatever.

Why tame the engine, that was one of the reasons we bought the cars.

I say pay the price and stay original ALL the way unless driving the car is not what we wanted to do.

The inside of the engine (parts) should look as original as the outside. The pistons are the engine.

Head work sounds fine, but why do that then steal the HP by changing the compression.


DOM

[Michael W.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

Hi Joe,

I know you folks are right. It is a compromise that breaks my heart just a little, as I remember running the 365hp version back in high school and it is not the same.

If I had easy access to 100+ octane fuel (and could afford it), I would have gone with 11:1. Just got a little conservative in my old age. Maybe it will help keep me from getting too many tickets for exhibition of speed.

I think the message is that the car would run just fine on 93 pump gas with stock compression. No need for 100 octane or anything similar.

[Monte M.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

The truth is, I have decided the take a 350 crank, grind it down to small journal specs. This gives you a longer stroke which equals a little more HP. Last night I drove a small block set-up that way. That was it. My decission is made. The thing ran really well.
I am going over to another guy house today that has built the same motor for his 63. Yesterdays car was built like a 340, but todays is an F. I. car like mine.
If it runs half as good as the car yesterday, I am moving on with the build.
I have heard a number of people tell me about this set-up, but once I drove it, I am a believer.
Monte

I will let you know how it goes.

[Joe C.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

Joe;

If it's not straying too far off this thread subject; what do you recommend for the 097 cam? I know the 63 L-76 is an odd ball, but mine has been shown to hold it's own against any other year L-76 or L-84, specially those with a final drive lower than 3.36 (higher numerically). Always seemed to be the right combination on the road, if not on paper.

Stu Fox

I'll assume that you're talking about static compression ratio.

Yes, the 1962-63 L76/L84 is an oddball. Why? Because the 097 cam was initially intended to be used with the smaller displacement 283, and in that application was already at the "ragged edge" of detonation. It was also specified to run .008/.018 lash with the 283. The specified SCR was (nominally) 11.25:1.

It is VERY important to realize that, all other things being equal, increased displacement results in increased DCR (dynamic compression ratio). So, a 327 with 11.25: SCR will have a higher DCR than a 283 (if you want the DCR's for any particular reason, I'd be happy to calculate them for you) built with the same 11.25:1 SCR. Remember that the 283 was built on the "ragged edge". When displacement was increased, the DCR had to be lowered, so GM specified the use of 2 thicknesses of steel shim head gaskets to effectively lower the SCR from 11.25:1 to 11.0:1 (nominally). Compounding the detonation issue was the fact that GM specified a wider intake valve lash to be used with the bigger engine and so the spec went from .008/.018 to .012/.018 for the 327. This change raised the idle vacuum, and addressed some idle vacuum issues that were occurring with the fuel injected engines. This had the added result of decreasing intake duration, which causes the intake valve to close earlier and raise DCR. The double head gasket "band-aid" was enough to quell detonation in the 097 equipped 327's, but underlines the point that there is almost NO margin for building in more static compression without encountering detonation.

Without a thorough understanding of engine design, a useful aid in evaluating engine compression is to look at valve events in relation one to the other. This chart is helpful:



The 097 cam was very docile as compared to the 30-30 and the LT1 because its durations were shorter by about 25 degrees and its valve lift was almost 0.010" smaller! That's the reason that a lot of people with 1962-63 L76/L84 engines are satisfied with the 3.36 axle ratio. I don't recommend trying this with '64-'65 L76/L84 engines as they are a V E R Y different beast from the 1962-63 version.

[Timothy B.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

Joe,

Is that how you built your 327 with the 12.3 static compression ratio. I sure wish everyone used the J604 valve timing points (.006 lobe/tappet lift) instead of the .050 lobe/tappet lift. With the approx 1.5 rocker ratio, the J604D is probably closer to calculate effective timing and static compression.

Just a FWIW, on my 327 .060 with flat tops and stock 929 cam the cranking compression pressure is 175 with a thick .038 head gasket and I have often thought how it would act with the steel GM thin head gasket and retarding the camshaft 4*. Idle will probably stay the same with a little extended power up top.