VetteLT4

i have 100 available out of a pump locally. would a lower horse 327 benefit from say a 50/50 mix of 91 and 100? even if i dont feel any performance difference, i dont mind doing it if its better for the engine.

SWCDuke

If it doesn't detonate on 91, mixing in a higher octane fuel does nothing.

Duke

VETTE1966

Related question---I have 327 / 300 1966 original motor. No detonation or performance issues but it does smell "gassy" some after I stop in my garage. I read where adding a lead additive might help--any thoughts?

Vetterodder

Some gas smell is normal on pre-emission cars due to vented tanks and open element air cleaners but make sure there are no leaks or fuel still dribbling in the carb after shut down. When they started putting MTB in fuel I used to gag on the smell in my garage after a fill-up. Once they stopped using that crap the smell went away.

Dyna

Duke is correct.

Dyna

SWCDuke

Yes, gasoline oxygenated with MTBE has a VERY pungent odor. Now that CA is using ethanol in place of MTBE, the gasoline hardly has any odor at all.

VettLT4 - Your car WORKS! Don't "fix" it with some snake oil gasoline additives, okay?

Duke

VetteLT4

definitely wont do it if you say not to. id like to understand why its not better though. i guess i have a misconception of higher octane = better for engine/higher horsepower.

now that i think about it i seem to recall you doing extensive write ups on how gas octanes work? ill do a search...

SWCDuke

Octane is a measure of a gasoline's detonation resistance - nothing else. If your normal gasoline doesn't allow detonation then high octane does..... NOTHING!!!

In fact, many regular grade fuels have slightly higher energy content (1-2 percent) than premiums, so using premium in an engine that only requires regular can cause a slight increase in fuel consumption due to the lower energy content of the higher octane premium.

Duke

jimdvan

This is an issue I've always been inerested in and misunderstood. SWCDuke, I have a 1970 4 bolt main 350 with 350 heads, Edelbrock 600 carb. I don't know what the pistons are but they each have 2 relief cuts. Am I wasting gas and money by running 91 octane? Could I get by with 87 or 89?

Jim

62fuelie

You're probably not wasting money with the 91. What you describe sounds like flat top pistons and these usually give about 10:1 compression. Starting in 71, there was an across the board compression reduction, down to 8.5 or so on most engines. The pistons were DISHED on these motors. If you have the "standard" 350 (350/300), the cam isn't too radical and the DYNAMIC COMPRESSION is probably higher than the 350/350 or the 350/370. These engines tend to need higher octane than the SHP engines. On the SHP engines, the valve overlap of the more agressive cams tends to lower the DYNAMIC compression, even though the STATIC compression is higher. :auto:

Matt Gruber

mine run great on 87 :cheers:

jimw2

What is the highest compression you would recommend with a 327 with a "151" cam?

62fuelie

Wel, personally, if I were building an engine from scratch for street use, I would go with flat top pistons which will yield around a 10.0:1 static compression. My 62 has domed pistons and runs ok on 92/3 octane, but it has an aftermarket cam that is at least as agressive as the original Duntov cam. In the early 60's Gm started installing double head gaskets to reduce the compression on SHP engines. They still advertised 11:1, but in reality it was less. The 151 is a great cam, but Duke Williams and others who are more knowledgeable are recommending the L82 cam instead. Apparently a little better torque curve.

All this said, I would use flat top cast or hypertechnic (sp?) pistons, an l82 cam, and the existing 461X or 462 heads, if I were constructing a good reliable, reasonably high performance engine for a street cruiser. :auto:

SWCDuke

10.5:1! If the deck and head haven't been cut since they left the factory and you use the replacement forged pistons and a composition headgasket, the CR will probably be no more than this. I always recommend that all the measurements be taken to compute actual CR when rebuilding an engine, and this begins with measuring the deck clearance BEFORE you remove the pistons on teardown.


"I have a 1970 4 bolt main 350 with 350 heads, Edelbrock 600 carb. I don't know what the pistons are but they each have 2 relief cuts. Am I wasting gas and money by running 91 octane? Could I get by with 87 or 89?"

Ifyou don't know enough about the installed engine to have a reasonable estimate of CR, you can experiment. Let the tank go to near empty and pump in a three gallons of a lower grade. If it detonates, fill it up with premium. If it does't detonate, let it go to empty again and pump in another three gallons of lower grade. Keep experimenting until you find a grade or blend that will operate detonation free.

Beginning in 1971 ALL GM engines, including the LT-1 were set up to run on 91 RON unleaded gas, and today's 87 PON has about the same detonation resistance.

Duke

62fuelie

Duke, would the domed pistons, (assuming around 10.5:1) actually make a noticable difference over the flat tops (assuming about 10:1)? Just curious, and of course I'm talking about an engine for a street cruiser, not a race car.
Ed :auto:

SWCDuke

No, I recommend the L-79 cam for L-79 rebuilds or to "upgrade" a 300 HP engine. I recommend the LT-1 cam in place of both the Duntov and 30-30 cam for solid lifter engine rebuilds. The LT-1 cam is also an option if someone wants to "upgrade" to solid lifters. The LT-1 cam will produce the greatest average torque in a street SB from 2000-6500.

The L-46/82 cam is essentially the L-79 can with about three or four degrees more duration, but it's retarded 4 degrees to 114/114 centerlines. If this cam is used in place of the L-79 cam on a 327 I would recommend advancing it 4 degrees so the centerlines will be 110/118 to match the L-79 centerlines, but it will have that bit more duration.

Duke

62fuelie

Duke,

Well, in that case, when and if I ever get around to building an engine for either car, I'll use the L79 cam I have been saving on the shelf for years. :D What about compression? Is the extra half point worth the extra expense and loose fitting forged pistons? Given that today's cast pistons are good enough to use in the LS1/LS6, I am of the belief that they offer better oil control and less blow-by. As always, your opinion is valued.

Ed :auto:

GCD1962

I have a 70-LT1 short block and Dart II heads in my 62 and agree that the LT1 Cam is excellent. I find it doesn't peak until about 7200 rpm. I use a few gallons (3-4) of Cam2 105 octane and the rest regular unleaded hi-test (sunoco 94 is near a station). Don't know if it's going to be any different this year with the new corn-gas that is manated in Connecticut.

Even on trip, after all the "good gas" is gone, the engine doesn't ping with Sunoco 94 unless a real heavy load is on the engine. I've had the LT1 shortblock in the car for over 32 years and even with 11 to 1 compression haven't had any problems

SWCDuke

With the a .025" deck height, .040" gasket, and a 64 cc cylinder head chamber, the difference in CR between the flat top 300 HP pistons and the SHP domed pistons the difference in CR will be about 1.2. This will reduce torque about 5 percent across the range.

The loose fit forged pistons also create a unique engine noise that will not be present with modern tight fit cast piston. Loose fit forged pistons might promote higher oil consumption, but it certainly won't be excessive.

I always recommend rebuilding vintage engines with the original type components to maintain their visceral character. A new Corvette is certainly faster than a vintage model and the new Corvette engines are much more powerful across the rev range, but they just don't have the "sound and feel" of a vintage SHP engine, especially one with a mechanical lifter camshaft.

Duke

62fuelie

Well, that makes a certain amount of sense. I guess if I were building an engine to rack up a lot of highway miles it might make sense to go to the cast pistons, but for general cruising the oil consumption issue isn't really all that big a deal. Hell, I may even break down and install a solid lifter cam. I seem to recall that you and/or John H. had a scheme to cold adjust the solids? Might be a future project. :auto: