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Posted: Tue Aug 26, 2008 6:40 am Post subject: WHY HI-TEST IN BIG BLOCK?
Why is it that I have to run hi-octang gas in my big block? I know what happens when you don't but I don't know why? I made that mistake on my first tank full! _________________ SECOND PLACE IS THE FIRST LOSER
1968 El Camino S/S 396
Evidently the compression ratio is too high for regular or the combustion chambers have a high buildup of carbon creating some hot spots----if you are wanting to fix that problem you may have to change the heads to ones with largert combustion chambers to lower the compression---that depends on which heads you have now---or if its carbon buildup there is a product called seafoam that you run through the engine to clean up the carbon ----just have to have more information to be able to give a definitive answer..................Dan _________________ I don't think I have ever had a car that was "FINISHED"........unless it was totaled.
It's my understanding all BBC need hi-test, you have to run it straight from the factory or it ( tings) (spits) (sputters) and won't stop running after you turn the key off. Plus lost of preformance.
Is that true and if so why?
Being everything is factory stock.
Donny _________________ SECOND PLACE IS THE FIRST LOSER
1968 El Camino S/S 396
thats a load of hooey----needs for premium are based on compression and cam profile etc...octane boosters don't do anything for performance, as a matter of a fact they actually inhibit performance as they slow down the combustion process and displace some of the actual fuel that would normally be ingested if they weren't there....they do however, allow us to do things that do actually increase performance like raise compression ratio etc...the running on and preignition you are talking about is called dieseling --as in a diesel engine---the actual compression of the air/fuel mixture sets off the combustion process rather than the spark from the ignition system and CAN be caused by too much compression ratio or a buildup of carbon in the combustion chamber causing a hot spot that sets it off...unless you are having a problem with detonation or dieseling you are wasting money and losing performance by running more octane than you need----if the engine you have really needs it then by all means run it but if it doesn't need it---quit buying it, try running some Seafoam through it---just follow the directions on the can-- and try some 89 octane instead of the premium---if no problems there you can try the regular, is my advise...............Dan _________________ I don't think I have ever had a car that was "FINISHED"........unless it was totaled.
Necoa #: 10934
Location: 15(AZ,NM)
Joined: Feb 03, 2008
Posts: 457
Year: 1981
Model: El Camino
Posted: Thu Aug 28, 2008 5:24 pm Post subject:
You can pull timing to run the lower grade. Try some lower grade and see how it works for you. You may or may not have to make some adjustments to run it.
Mine is at 9.75:1 and in normal driving runs fine on mid grade. When I really get into it it pings when it really gets onto the cam.
By the way there is no carbon in it right now as I just went through it again a couple of thousand miles ago. _________________ 84 Elky with 455 Buick, Tremec TKO600 5 speed, 8.5 3:42 posi -
My Members Page Album
Another 3 to 9 years should finish it.
there are so many variables it is impossible to say that at a certain compression ratio you must step up the octane---it just depends ---its changed by piston dish shape and combustion chamber as well as the rod to stroke ration---a long stroke short rod engine with a lot of squish area between the piston and head can run just fine at 9 to 1 or even a bit higher on regular, but the same engine with a long rod, dished piston and large combustion chamber might need premium at the same compression ratio---normally anything approaching 10 to one is going to need more octane but the plus may get it done if the components in the assembly are octane friendly...........................Dan _________________ I don't think I have ever had a car that was "FINISHED"........unless it was totaled.
Necoa #: 6388
Location: 19(AK,ID,OR,WA)
Joined: Nov 03, 2005
Posts: 226
Year: 1970
Model: El Camino
Posted: Tue Sep 09, 2008 12:05 am Post subject:
Let's take a look at that 1968 El Camino SS396
It came with 10.25:1 rated compression ratio.
The L-35 was the 325 horse engine with a tiny cam
The L-34 was the 350 horse and featured a 'high lift' cam.
Back in the day, the formula was high compression + small cam + super gas to make a smooth idle and great low-end torque.
Take the good gas out of the equation and bad times for those old engines. Both of these engines have too much static compression as the Chevy Hyd cams are too small.
The solution of lowering the CR has been mentioned. Sure it works, but it takes away power.
A better power-making idea is to run a cam with more duration. The different cam timing prevents early cylinder-filling so reduces the low speed cranking compression.
(intake/exhaust)
The 'high lift' 350 horse cam had 214/218* @ .050" duration
Moving up to 218/228 helps some like the Summit 1302 cam
But what really works in a 396 with 10.25:1 is 225-230*
The 'street mechanical' cam has 242/242* which is plenty. Subtract 8* for lash....comperable to a hyd cam with 235*....just a bit much for great street manners in a 396... _________________ George Kettler
Lakewood, WA
68 Vette 427 Cp owner/builder the last 16 years
70 ElCo 350/Th350 cowl, buckets, console, 12-bolt, pdb. 15.1@91.25, budget 454 almost done
kettbo,
I absolutely agree with everything you said and if the engine is all original then your advise is spot-on.....I am betting that in the intervening 40 years that almost everything in that engine has been changed at least once---at least thats been my experience with big block chevies---so there is no way to tell for sure whats in it short of a complete teardown...there are ways to "whistle" a block (ask most modern machinist) to approximate the compression ratio but that still leaves the combustion chamber a mystery so experimenting is the only way---and a big cam with an automatic and without a stall converter isn't much fun either. Also a big cam and a loose converter equals "bye bye gas mileage" --bottom line is you have to bite the bullet and give the engine the fuel it needs..................Dan _________________ I don't think I have ever had a car that was "FINISHED"........unless it was totaled.
You guys gave me alot of great information. Very detailed, for that I am grateful. It don't get alot of miles on it so I don't have a problem paying the extra, I just wanted to know why. It will go through a tear down but it was so to me as a (fresh) 396 motor? All I know is it don't smoke,use oil, are make any nosies other than a nice deeep rumble. (Big Blocks Rock)
Thanks Again
Donny _________________ SECOND PLACE IS THE FIRST LOSER
1968 El Camino S/S 396
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