Im missing something here. The engine build is right around 9.6 comp ratio with home ported heads, big valves and a TA310 cam, balanced ect Initial timing set at 8*(vac advance plugged, actually is not used at all at this point), total mechanical advance on the distributor is 25*(this I determined from dialing the timing light to 25 and right around 2000rpms its at zero on the timing tab), the springs for the mechanical advance are the stock springs(black ones! Supposed to hold it back until 4000 grand or so right?). I did polish and clean the weights and shaft on the distributor, just by hand, so it is freely working. Heres the problem: the slightest attempt to rapidly open the throttle causes major pinging. I can slowly run up through the gears and hold it at say 2000 rpms open the throttle and ping, ping, ping for a few seconds. At lower rpms it's much worse. Ive re torqued the manifold, sprayed carb cleaner around trying to find a leak, nothing. By the way it pulls 15 in HG at 850rpms. I thought lean carb since the break in carb is built for a stock 350. I put on the carb built for the motor and no change of condition. Could it be that cleaning up the mechanical advance is causing it to snap open way to fast? This is a four speed car so rmps do not lag. 94 octane gas. Anyone had something like this? I have searched the posts. Clearly I'm not seeing the trees because the forest is in the way! Pete
First thing I'd do is verify the correct position of the balancer mark in relation to cyl #1 TDC. If the outer ring has slipped you may not be seeing the right numbers. A piston stop is the best way to determine this. Devon
94 octane real gas or eth mix...not meaning E85, just 100% gasonline? Try backing the timing down and tuning by ear? Change when its colder or warmer air? Try different thermostat?
Bar, pump gas from the local Mobile station, so ya it' a mix. It is running slightly warm and we are in the 60's today. Maybe a 180 thermostat is needed. Lots of heat transfer from the ported heads. I do have access to Av Gas. I wonder if the compression calculation was off. Anyone know how high a ratio you can run on pump fuel?
You need to determine the total advance. From what you wrote above, all you know right now is that at 2000 RPM, your timing is 25*. That means, at 2000 RPM, you have 17* of mechanical advance + 8* initial for a total of 25* I am sure that the advance is going beyond that at higher RPM. That is why you need to get yourself some light springs. Install them, and slowly raise the engine speed while keeping the balancer mark in sight(use the dial back). When the mark stops moving, keep the RPM where it is and dial the mark to 0. Then you will know what your total advance is. Right now, you don't know.
Unless you measured everything, you can't calculate your true static compression. The engine doesn't run at the static compression ratio though. What determines whether you can run on pump gas is the dynamic compression. That is determined by the static compression and the cam you are running. When the piston is moving up the cylinder on the compression stroke, the intake valve is still open. No compression can build until the intake valve closes. The intake valve closing point will vary with the camshaft, and whether that cam is advanced or retarded. It is possible to run a lot of compression with a cam that closes the intake late, and still run on pump gas. So you see, static compression is only part of the picture.
Larry, gotcha on the compression issue:TU: My dwell/tach meter only goes to 2 grand so with the light springs I get 30* on the dial. The yellow springs in the crane kit are rated at 500 to start the advance and all in at 1600rpm. At 850 the timing mark was way advanced. I dialed 25 on the light to get it back in range. Are the weights on these 70-72 distributors the same? Maybe it has the wrong weights and this causes the advance to instantly snap open.
You don't need a tachometer at all. Put the yellow springs in. Forget the tachometer. Turn the dial to 30*, and point the light at the balancer. The mark will be below the timing tab. Slowly increase the RPM until the mark stops moving upwards. Those specs that Crane lists aren't accurate for every distributor. Keep increasing the RPM until you're sure the mark isn't moving up any more. That is the RPM you need to dial the light to 0. Then look at the dial. You may have a lot more total advance then you think.
Timing chain on correctly?marks lined up etc Its probably a timing issue as larry stated. Just making sure the mechanicals are put together the right way.:TU:
Larry, Chris, you both have gave me a lot to think about and to work on , thanks so much!! I put this cam in 4* advanced as per the TA cam card; the numbers were right on. I will post what ever the fix is. What keeps me perplexed is the rolling along at 2000 rpm(any gear on the four speed) punch it ( no secondary, no vac advance)and for the first few seconds there is lots of pinging. That should not happen at all.
I know you said you plugged the vac asvance and are not using it at all........but it sure sounds like it is getting vac. Another note....that 310 cam needs alot of gas , you could be getting a real lean mixture if you carb isnt set up right.
I know you tried 2 different carburetors, so it is unlikely that they are both lean, but you should check anyway. If they are Q-jets, check the power piston and make sure it isn't sticking. I also calculated the dynamic compression for a 350, 9.6:1 compression, with theTA 310 cam, on a 106* ICL (4* advanced). It came out to 7.51, so that is definitely not the problem. What spark plugs are you running? How hot is the engine getting? Vacuum seems good for that cam, so a vacuum leak is unlikely. I really want to know what ignition advance total the engine is maxxing out at. Some Buick distributors have 30+* of mechanical advance. If you are running 8* initial, you are at 38* plus for a total. What distributor are you running?
Like Larry says, get the timing right first then if you are still having major issues look at the idle mixture of the carb. What carb are you running? The reason I ask about the idle mixture is because you describe an issue from light throttle and low rpms where a lean air fuel ratio will result in your symptoms. I know you are not at idle while driving but the idle mixture settings make a huge effect on the transition from idle to part throttle.
I have almost the same combination. Little more compression but detonation all the same. I fought this for a few years but have found that tuning with a vac gauge (as mentioned in your earlier post at idle is the best way to get it right), next was a cooler plug. I went two steps cooler and used a 160* thermostat. The SBB is so touchy when discussing cylinder pressure and detonation. I think it has to do with the shape of the combustion chamber and size. A different type of piston may contribute as well due to differences in aluminum density in the tops of the pistons. I truely think the thermostat and cooler plugs will fix this problem.
Yes, I ran a 310 cam and it was a pain in the but....There is a lot of overlap on this cam and the low rpm manners are not very good. The 310 likes 10:1 and higher compression and is a good race cam, but lacks low rpm torque. The TA 212 was a way better street cam and the low rpm torque was much better than the 310.
As has already been mentioned, this is probably a timing issue. However, for what it's worth, I had fairly severe pinging similar in RPM range to what you're describing in my Apollo (stock 8.5:1 compression, 2-bbl), and after switching to 91 octane and using octane booster, it stopped. This was the first time I had to use 91 octane (highest pump gas octane we have here in CA) AND booster in an 8.5:1 compression car. An issue that I don't hear mentioned often in connection with finding causes of detonation is the effect of stroke length on detonation, for a given compression ratio. I have read in a few tech. articles over the years (Hot Rod, CarCraft, etc.) that engine builders have observed evidence that long-stroke engines are more detonation-prone than short-stroke engines with similar static compression and cam timing. At 3.85", the SBB has probably the longest stroke of any stock small block engine produced. I know it may seem like just "anecdotal evidence", but before owning Buicks, I had several '71 and later Olds 350-powered cars (same 8.5:1 compression, but with short 3.385" stroke) that ran strong with no pinging on 87 octane gas. Same story with a couple of 350 Chevys (3.48" stroke) and Mopar 318s (3.31" stroke)that I've had. If you eliminate "too much" timing, overheating, and vacuum leaks as potential causes of the pinging, then it seems like insufficient octane would have to be the culprit. I'm not a mechanical engineer, buy my personal experience seems to bear this out.
Distributor number: 1112080 built DEC 7th 1970(0M7) this is a 71 350 distributor, right? plugs AC R45TS gapped at .045, .005 more than the stock spec as per Pertronix recomendation. Carb on the car right is 7042240
Yes, 1971 distributor. Good one too. Only 14-18* of mechanical advance maximum. The carburetor is a 1972 455 800 CFM carburetor.
