WHAT GAS IS A 340 MOTOR DESIGNED TO RUN ON ?

[cudaneil]

   Hi Guys,
                One thing i wasnt told by the previous owner of my 72 cuda,  is which grade of gas he was running it on !


                Was the 340 designed to run on unleaded gas................or leaded only.

                I have been using premium unleaded with a shot of lead additive.   Am i doing the right thing ?


                                                            Thanks,  Cudaneil.     

[73Chally]

Unless it's a high compression engine, you probably don't need the additive.  Have you tried running it on straight pump gas?  If it does knock you could probably play with the timing and tune to get it to run well.  Do you know from the PO what the compression is?

[426HEMI]

I have run mine on regular.  
The premium would be better though.  Just have to watch out for spark knock.  

[Chryco Psycho]

it entirely depends what was done internally , iif you have the timing set correctly & it doesn`t ping on regular there is no need for premium
 all of these engines are designed to run on leaded gas , we have no had that option for decades here but if hardened exhaust seats have been installed in the heads unleaded is ok too

[cudadave72]

I run premium unleaded in mine, however I have aftermarket heads with hardened seats. Still with a proper tune you should be fine with unleaded.

[cudaneil]

         Ok,  I have been on the vin decoder and the engine spec is:   340ci  v8  4bbl   HP  (LA MOTOR)

                 I know that the motor has been rebuilt in dec 2006  .030 oversize,but it doesnt say if the valve seats are hardened.

                I tried running it without the lead additive, but it started to cut out when in drive at idle speed  

[Chryco Psycho]

doesn`t sound fuel related more tuning or electrical

[cudadave72]

You would be safer just to run unleaded just in case the heads were updated in the rebuild. Lead will tear newer style stuff up!! If I remember correctly it was around 1972 when auto makers started designing engines for unleaded gas.

[71chally416]

I ran a set of old '70 340 heads on my 318 for probably 60,000 miles with no ill effects on the valve seats from using the cheapest unleaded gas I could find, but that was 15 years ago. It also had less than 8 to 1 comp.

[cudaneil]

   
  Does the HP in my engine code mean that its high compression then ?       

[426HEMI]

The 72 340 was lower compression stock so if someone did something to it then it would be higher compression.

[73Chally]

Since it has been rebuilt, the original specs will not tell you anything.  Do you have any info on what was done to the engine, other than the 0.030 over?  If the engine is cutting out at idle, that is a tuning issue.  First get it to run right (carb adjust/rebuild, timing, tune-up, etc), then worry about the gas, but most likely it will run with no problem on at least Premium.  Once you get it running well, try different grades to see what it likes best.

[cudaneil]

                     


            OK THANKS !       I will have a play with it, tune the carb  and check the timing.........................Got a timing light somewhere !   

[cudax34me]

just a thought......check with the previous owner about the rebuild specs.if it was punched out .030 but the pistons were replaced with the '72 spect ones it won't care what gas you feed it.'72 compression was in the 8's,

[jeryst]

I dont know about a 72, but I read some specs somewhere for my 73, and it said that it was designed to run on unleaded, unless it was being used for towing. So I dont know if that started in 73 or earlier.