i'v got a '70 350/350 l46 w/ 11.0:1 compression. It's detonating pretty bad on pump gas. I've retarded the timing to where it'll barely idle and i still have pinging at low rpm's. Is it possible to run pump (premium) gas in these high compression engines? if not can anyone suggest a set of heads that would reduce the compression enough to accept pump gas and not lose any horses. I'm currently trying some 110 octane mix, but that stuff is not cheap, so please help!!

Thanks,
mag

 

11:1 is very high for pumped gas, try and octane boost, if that doesn't work if you can have someone "curve" your distributer, I don't mean a kit where you have no idea where the timing is, I mean have it put on a distribuscope so the advance doesn't come on to fast. Retarding is only changing your initial timing and not how the advance reacts at certain rpm, Your pushing your luck with 11:1, my friends 70 LT-1 has 11:1 and we went through a bunch of things to get it right.

 

might of some carbon build up adding to your problems, do you know how clean the cyl's are? If not you might try some Seafoam, water or ATF to clean the comb chambers out.

 

If you have some distributer springs laying around, just for some giggles, try a tighter set so your advance doesn't come on so fast.

 

You can run premium pump gas with no problems. It should at least help with the spark knock issue. Do you know for sure that your motor is 11:1 CR? As already posted, try stiffer advance springs. Retarding the timing as you've done also penalizes you at higher RPMs because the full advance will be less than optimum (around 36' BTDC).

 

are the bottles of octane boost a joke??

 

on an 11:1 motor??

 

I've been told elsewhere to use hot water to clean the carbon off the valves and pistons (down through the carb), do you use the seafoam or ATF the same way, or do you have to put it in the spark plug holes and let it sit etc???

 

Just got some seafoam and it states you can use a vacuum line to suck it out of the can or you can just pour down the carb

 

i wrote this hoping it would help
http://community.webtv.net/MATTGRU/pv
so far no takers, just my 2 VETTES

 

Disconnect your vacuum advance and plug it off. Do not use it at all.

Be positive that you are never running with a lean condition on your carb.

That should be enough to get you running OK. If not continue.

Change your cam timing. Retarding your cam timing should reduce your dynamic compression ratio by about 0.2.

Thicker head gasket. This can be a help or a hinderance. A thicker gasket will lower your comprssion ratio slightly at the expense of increasing your quinch area. This increased quinch area may promote even more pinging if it is excessive.

Good luck with it.

 

Pinging at low RPM probably is a timing curve, vacuum advance, and/or lean mixture problem.

Like the other guy said, disconnect your vacuum advance is THEE FIRST THING you want to do and will probably fix your problem. Also tune your carb to get good gas flow and metering at low RPM. Also you can play with distributor curbs with springs and such and a good timing light - heavier springs will delay the timing curve. That's probably the second thing you want to do if the vacuum advance didn't help.

Do a search on any of the above topics - timing (Lars has some excellent advice), carb adjustments, vacuum advance, to get more detailed information.

Octane boost for the most part is a joke. If none of the above work than it's time to go to the paint store and buy Toulene in 5 gallon containers.

 

GM markets a product called X-66 which is available at GM dealers and parts houses. Comes in spray can or liquid form, I use the liquid form myself. It's designed purpose is to eradicate all the carbon on your pistons and in the combustion chamber. Simple to use, pour into carb at a moderate rpm's. You want the car to be "stumbling" for a period of time until you finally stall the motor with the X-66. Wait about 20 minutes and restart the motor. You will see plenty of white smoke if you have bad carbon deposits in the chamber. Motor will clear itself out after few minutes of running. One of GM's better products, houstonvett

 

I have an L-46 with 10.4 C/R running Dart Iron Eagle heads and a tight quench thanks to a very thin gasket. It does not need any octane booster or reduced ignition timing but my pistons are flat topped where yours are small domed. That is where the problem is.

You could run some Edelbrock Performer heads with 70cc chambers. That would get the compression down and improve the performance. The aluminum material will help the detonation and the chambers are slightly larger to get the compression down. The shape of the chambers on the newer heads makes a lot of difference. Be sure to call Edelbrock and verify that the current chamber design will work with your pistons as I think they redesigned them and dome pistons might be an issue. The original design had no problems but you need to verify. I am not sure you want the very thin gaskets with those aluminum heads but I think the closer you can get the head to the piston, the better. You could shoot for around .035" to .045" or so. Any more than that and it probably does not help any more while any less is not safe.

Dart Iron Eagles come in 72cc chambers and 180cc intake ports. You could use them and a very thin steel shim gasket and it would work quite well. This is closer to what I am running. These heads do not have exhaust heat passages so they would eliminate the EFE early fuel evaporator heater under the carb. The better shaped chambers should help get rid of the detonation.

Just a few suggestions,

-Mark.

 

I don't think disconnectiog the vac advance is the way to go, this is a power device and on the street thats what you want. You can limit how much it gives but your better off limiting static than vacuum advance.

You have to retune the engine, change one thing something else changes, like your idle. Retard the timing and increase your idle with the idle screw on the carb, thats why its there. Not mixture screws.
There is no way around this. If it was possible to run 11.25:1 compression all the car makers wouldn't have stroked their moters and reduced CR. Low octane = low CR.
427 turned into low CR 454s
472s """ 500.
350-350s turned slowly into 350-145 hp

Also heat, mostly heat on a Vette, and load have a lot to do with when ping happens. You tune today at 70* and then drive into 80-90* and the ping comes in. If you can do anything to make the engine run cooler it will help somewhat.
The best is to make the CR so it runs on todays fuel, you lose HP but not as much as retarding timing. Modern heads run better with higher CRs but I don't think many are at this level if any.

 

It is possible to run this compression on pump gas. There is a great article in Hot Rod this month on this subject. Many of the new performance motors sre running near 11 to 1. I know they have all kinds of fancy computers and fuel injection but it is possible on an older motor also.

 

Disconnecting your vacuum advance will retard your timing which may get rid of the ping but it will hurt your performance since you'll lose that portion of your advance. IMHO vacuum advance is something of a misnomer since the purpose of it is to retard your timing under load when vacuum decreases. Make sure it is working like this (retarding your timing when you hit the gas). Maybe someone like Lars could recommend a can that will retard more easily if yours doesn't seem to be functioning properly for your setup.

 

You could also try running colder plugs. I think this is about the cheapest thing to try and some times it does help.

 

DISCONNECT THE VAC ADVANCE, if it cures your ping, get an adjustable vac advace from crane. it allows you to adjust when the advance comes in and total vac advance allowed.

keep your total timing at 36, put in heavier springs on your mechanical (they come with the crane vac advance)--my layman opinion is that you should be able to run pump gas. i run 10.3 compression and my motor WILL NOT ping(unless i run 60 deg tot mech timing).

 

Disconnect the vacuum advance TO DIAGNOSE THE PROBLEM. I think that's what most of us mean. Very quick and easy to do, takes all of 2 seconds, plug the vacuum hole on the carb. Once you identify the vacuum advance is the problem then get an adjustable can and limit the amount of advance it gives.