Engine Problems 350

I have a 1983 Chevy Pickup. With a 350 engine. Im having alot of problems keeping it from knocking and pinging. I have had to retard the timing so much that it hesitates and bogs down on me. Thats the only time it will not ping. I always use 93 octane, I've tried octane boosters and everything. I bought about a year ago. I don't know all that much but maybe you can help. It has the following:

350 LT1 motor
11.5:1 Dome pistons
400 crank
Crane .600 roller cam
Crane 1.5 roller rockers
600 Holley Carb.

Thats what I know, any help would be greatly appreciated,
Shane

Timing chain jumped a notch? Wrong Plugs? Increased compression ratio due to the dome pistons?

a 350 engine. Im having alot of problems keeping it from knocking and pinging.

11.5:1 Dome pistons
400 crank


11.5 to 1 pistons are way too high compression for pump gas.
Also, a 400 crank in a 350 gives you 383 cubic inches. This increase in cylinder volume boosts your compression ratio even further.

I have found that 10.5 to 1 is about as high as you want to go on pump gas.
To fix this, you need to install lower compression pistons, or cylinder heads with a larger chamber, or get used to buying octane booster by the case.

It sounds as if you have a valuable engine there. you might be able to sell the whole engine and use the $$ to build a more mild, lower compression 350 that is more suited to your needs.

i got a quick question for you... are you using the correct harmaonic balancer for your ignition system? Cuz i once bought an engine that i thought was so far out of timing, but really the idiot before me had swapped balancers. Instead of a side read balance, it had a top read balancer on it...lol
But really the engine was running fine..., just couldn't tune the timing.

I have a 1983 chev 1/2 tonn with a 350.
First off, if it Is a matter of compression Ratio & Fuel, you can resolve this by running high octane from the pump (supreme) which in canada is 91 points of octane on a good 15C day. Make sure you buy your fuel from a gas station that you know goes through a lot of premium fuel as premium fuel has a very short shelf-life. After approx. two weeks of sitting it can degrade to worse than 87 pt. octane(regular) gas. Also, try different gas stations and see if that makes a difference.. All gas stations have their own special 'mix' concentrated additive. Don't go to Mohawk because although they're fuel is better for the environment, it is worse for your engine's lifespan. What I would do to ensure High Octane Gas, If it is a gas issue in the first place is buy 3 SMP 'gas octane boosters' and add to Regular, Never Supreme. Because Supreme Gas already has additive in it, making it a saturated solution mixture. You cannot raise the octane points on supreme any more than it is, you'll just be wasting money. This is true. That is why regular gas and a 'performance' or 'octane booster' additive can 'boost' regular 87 pt octane fuel past that of the premium fuel you'd buy at your local pump. There are many magazine articles and studies done on fuels and aditives and fuel refiners/producers 'special blends'. So make sure you are running a True Premium fuel if you suspect your truck to be high compression, as this will fix your problem.
Second of all, when is the last time you've rebuilt your carb, or do you know if your Carb is running right? too rich can cause knocking, premature detonation, or just a slow boggy feel. This might be the hesitation problem.
Third, timing is pretty much a main factor for your pinging, or so I have played around with my truck.
Fourth, the electrical is always a possibility. I'm just throwing it in, so take an OHM meter and start testing resistivity. Check plugs and cap.
Fifth and final, It is a well known fact with 305/327/350/383/ and 'turbo400's' that from the factory they were buitl with weak bottom ends to put it directly. I have found 305's and 350's especially to knock when outside of their power range, before 2600 rpm - 3100 rpm depending on your cam setup. Chevy engine's will knock/clack and bogg if not set up right, But they still run forever =)

11.5:1 pistons designed for a 350 will produce about 13:1 in a 383. You would need about 120 octane fuel :) I think its available in bulk quantities for about $17 a gallon. :D

Different heads is the way to go here. Big chambers, aluminum construction, and then careful timing with 93-94 octane at least. At this point, you're not looking at stabbing in a distributor, you're looking at carefully modifying a stock distributor for some very custom curves. Shoot for 10:1 on aluminum heads or 9:1 with iron heads.

Also, is it an LT1 or an LT-1? Big difference. The LT1 is the 91-97 'vette, camaro, impala motor; EFI, knock sensors, etc. The LT-1 is a traditional small block that shares geometry with other SBCs.

Thanks for all the info. Yes it is an LT-1 crate engine approx 1970. It has a TH400 Tranny 3 speed auto with a B&M 3000 stall converter. It has accel HEI ignition. I brought it to a High Performance shop where I live. The mechanic stopped the pinging by retarding the timing and disconnecting the vacuum advance. But it doesn't run like it should. It wont even spin the tires. The carb has been rebuilt. It is a Holley 1850 600 cfm. It idles really rough, like lopy. Just the other day I had to replace the exhaust manifold gasket. It needs 2 on both sides. I checked the plugs and they have the correct ones. R45TS. Somethings cooking with it. I asked if it needed a bigger carb, like a double pumper, but the mechanic said it has the right size for the displacement. It already gets about 6 mpg. I can imagine a dp on their. And I've played around with the carb settings as much as I could. Sorry for writing so much, Im just new at this High Performance stuff.
Shane

Wow, that motor will definitely give you a rough introduction towards the temperment of performance engines. As curtis and some others have said, you're compression is EXTREMELY high.

Also, you're running a cam with (roughly) 300-305 advertised duration? Thats killing the trucks streetability.

Yeah... maybe I should just buy it from you :D It would be tough, but I would learn to live with it. :D

Your Accel ignition is a good one, but its by no means ready to run out of the box. The advertised advance curve is rarely correct. Some experimentation with a Sun ignition tester would do wonders. You'll probably want 34-36 degrees total and you want as much as possible at idle to help smooth it out. I might suggest (until you get your combo settled in) running manifold vacuum to the vac advance canister. That will be all wrong for MPGs, but it will help you get the advance you need at idle and less at high loads. Figure out the actual advance the mechanical offers, then set it so it provides a total of 34-36. If your mechanical offers 20 crank degrees, set the initial to 14 degrees at idle. Sounds like a lot, but then you can adjust it within an acceptable range.

6mpg with that combo in a truck and an untuned ignition curve is pretty impressive. I would have expected 3 or 4. I only get 14 from my mild 454 in a car with an ignition curve that took brazing, shimming, and tuning for 8 hours.

The plugs your running are great. They run cool which will help prevent detonation. Smart choice.

I still suggest getting that compression down with different heads. Until then we will just be covering up for the proper state of tune. The cam you're using is very hefty but well tuned for that compression. The two together really require much more than pump gas. You can burn pump gas in it, but the ignition curve you have to use (as you've discovered) will have to be so slow that power will suffer big time. Your 500-horse 350 is probably only making 300 hp since we're covering up the problem superficially. If you run the proper gas or drop the compression, you could have a 450-hp 350 that will run on the street gas at the pump.

Thanks for all the help everyone. As you know I mentioned I was new at this stuff. Alot of people tell me that it's not really streetable. Im not sure what they mean by this though. I checked the timing and it's at 8 degrees at idle. But when the mechanic worked on it he disconnected the vacuum advance and told me to leave it that way. The detonation is gone but it is running real crappy. I go to step on the gas and it almost stalls out on me, I have to let up on the gas pedal it order for it not to stall.

It's Idling at 800 RPM's right now. But real rough. It doesn't smooth out until about 1200 RPM's. I know the mechanic made adjustments to the carb as well. It has the Holley 1850 model 600 cfm. which was upgraded to a metering block to replace the metering plate. He put in higher jets in the carb as well as a new accelerator pump. He also replaced the valve cover gaskets. The mechanic said that who ever owned this put some money into the engine. It has crane gold roller rockers. Crane double roller timing chain. quick lock stud girdles. And the cam is a crane oller .630 lift with 288 int. duration and 296 exh.

When someone says "streetable" it refers to several things, but in a nutshell, high-hp (naturally aspirated) engines require cams with long durations like yours. The longer the valves stay open (longer durations) the more air the engine can ingest and the more power it can make. At idle when air requirements are very small, the cam is still giving it huge gulps of air and it runs very rough. There are other factors involved, but the huge gulps of air mean the engine is tuned to run at higher RPMs. Higher RPMs leads to higher HP. Conversely, small cams tune the engine to run at low RPMs. Smooth idle, but it can't breathe enough to make power up high in the RPM band. For this reason, your engine doesn't make as much power until you get up in the revs. Great for a race engine that spends its whole time revving anyway, but for a street engine that spends much of its time stopping and starting, it can be a little more frustrating using an engine that doesn't make its peak power until 6000 rpms.

The other main factor in streetability is vacuum. For some it doesn't matter, but some options like cruise control, power brakes, cowl induction scoops, EGR valves, charcoal canister purges, and climate control are all controlled carefully by vacuum from the engine. Lopey big cams don't allow the engine to make good vacuum and all of those creature comforts and emissions systems have to be set aside.

The other reason you're engine is so hard to tune is because carburetors run on vacuum. A nice smooth idling cam gives the carb a clean steady vacuum signal; clean flow of air at idle = steady fuel metering. Lopey idling cams give it peaky and unsteady vacuum signals making carb tuning very difficult and unforgiving.

Higher compression as you've already found out requires high octane fuel. Low compression engines can use cheap fuel and aggressive or even sloppy ignition curves with a great deal of lattitude keeping it away from detonation. Higher compression engines are a little more picky about their ignition advances which is why your mechanic's solution of just ditching the vacuum advance has left you with poor performance. Its also important to note that (while any detonation is bad) detonation in a high-compression engine is more damaging than in a lower compression engine.

Find a real live honest to goodness race shop tuner. Someone who builds race engines in their sleep, but owns a few hot street cars. They will know the ignition and fuel requirements your engine needs and be able to help set it up. With an engine like this one, we're beyond the "mechanic" stage. We need a full-on engine master.

Curtis73 has given you some good information, and I'd like you to try something and if it works I'll give you a solution to your problem.
I'd like you to get 2 gals. of toluene (paint thinner) and mix it 50-50 with your gas. If it stops the pinging try advancing your timing till you get the ping, then back it off 2 degrees. If this works let me know then we'll go the next step.