I have owned a '79 CDV with a 425 for about three months now. It has 52,xxx miles on it, second owner car. The original owner insisted on running 93 octane in it, however I was wondering if this is necessary? It runs great and I was wondering if I'd see any detrimental effects if I were to switch back to 87?

Also, what's the best way to clean a Quadrajet without taking it apart?

Engine was designed to use 91 octane premium, so using regular might cause the engine to ping on light throttle. You can retard the timing but fuel mileage will be affected. If it were me, I would use mid grade 89. Best way to clean carb? use fuel system cleaner additive in fuel tank. To clean exterior of carb? use brake clean spray.

Engine has 8.2:1 compression.

I ran premium (93) in mine for awhile.... then after being convinced that unless an enigne needs the higher octane, the lower octane is actually better.... I switched to regular (87) and never had a single problem, no pinging or anything.

Worst that'll happen is you'll hear it ping.... so when you are low on gas just fill it halfway with regular, if it's fine, then you are good, if you hear pinging, then just fill it up form 1/2 to full with premium, then you know not to use regular in the future.

There is no real way to clean a carburator without removing it..... As for gas treatments, the only two that I have used with good results, and also stand behind, would be the Lucas fuel treatment, and Seafoam..... since the gas tank is big, maybe wait until you are down to 3/4 tank or so and empty a whole bottle of Seafoam into the gas tank, run it down till almost empty, then fill it up again and drive it.... Otherwsie gotta pull the carb and overhaul it.

I find it hard to believe GM would recommend 91 octane for that low compression engine unless it had some design flaw causing it to detonate. I never read my owners manual but always ran regular with no problems. As a racer, low compression + high octane fuel = low performance and carbon build up. Once an engine gets carboned up enough, then it will ping and require the higher octane. Every GM owners manual I ever did read said that premium fuel is an added expense that was not necessary. I realize Corvettes require premium and a lot of newer high compression cars and supercharged cars and Northstars require premium but the old fashioned 425 should not require any more than 87 octane. The old mechanic's term "killing it with kindness" comes to mind.

I've never seen much dirt or gunk in running carbs that I have taken apart. I only take them apart if they are leaking or accerator pump dies. Seafoam should be good enough for it.

I'm just about positive that with the compression drop for 71, all GM engines were designed for low octane.

I have a 1978 owners manual... it says "Use unleaded gasoline meeting federal government regulations.The federal goverment specifies the minimum octane number of unleaded gasoline." it says a little before that and a little after it.... but it says to use minimum octane fuel...

















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I have a '78 Cpe Deville that I drove daily from '92 till '04. While certainly no miles per gallon champ I pumped 87 octane into the tank for all that time w/no ill effects (pinging, etc.) As above, the compression is 8.2/1 and that in itself would suggest 87 octane.

If memory serves, the octane rating method was changed from RON? to something else or maybe vice versa. Regardless, my '78 loved 87 octane.

I have no written info re the '79 Cads but I do own the '77 Service Manual and the '78 supplement (which is used in tandem for the '78 year models). The "77 SM states the following: Engine fuel; ONLY unleaded fuel of At least 91 Research Octane. Refer to Owner's Manual for additional information. The Supplement has no new info re fuel. Soooo... it sounds confusing but it is! IMHO,shared w/ most of the above, 87 octane is the way to go.

I have a 76 500ci deville and get 13.8 mpg with 87 octane and no pinging. I have to agree with cadillac al if you use higher octane in a low comp engine you will get carbon buildup that will cause hot detonation and you will get pinging and run on. I have used seafoam to get rid of the carbon and was amazed at how effective it is. I fed it in directly through a vacuum tube and then let the engine sit for 15 minutes, blew out clouds of smoke for 10 minutes and then ran really well ((I had other damage so had to rebuild the engine anyway).

definitely stick to 87 octane!

cheers
Barri

Something to consider is the pressure in the combustion chambers in the running motor. Desktop Dynos 2000 or a similar program will let you model the predicted pressure ranges and show you the differences based on what cams you pick. A typical low (static) compression engine with a cam that has some overlap will have relatively low pressure #s (BMEP) and likely run fine on low-octane gas. I think I remember reading somewhere that it's when the BMEP (predicted) gets up to 160 or higher that you are starting to spec out a detonation-prone motor with your chosen parts combination. In the real-world: do what I did with my 77 CDV and the 425 carb'd motor in it and swap out a worn smogger cam for a torque cam like the MTS-3 (higher lift, shorter duration with hardly any overlap) and you will raise that pressure a bit. I get some slight pinging on regular now - didn't before - so I now run the mid-grade. It's worth it to me every time I crack open the throttle and feel the extra torque I now get - fun!

I ran mine on 87 octane.. I was always extremely surprised at how smooth this engine was...

Well wardog it sounds like you have a real cam in that motor now. Do you think your new cam really has shorter duration than stock? I would bet that with a real cam it won't require 20 degrees of initial timing anymore. A healthy motor shouldn't need that much initial timing anyway. If it were me I would do some experimenting with the timing. I have never played with desktop dyno, does that take ignition timing into account when calculating Pinging or whatever?

I just set my distributor for 36 degrees total centrifugal advance at 2500 rpm. No pinging and it runs GOOD! I'll have to try regular again once I burn the gas from this tankful...

I'm using 91 RON in my '75 and I'm still getting occasional run-on. I'm thinking about filling it with 95 and see what happens.