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What to do with a 343? |
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kellysguy 
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Joined: Mar/15/2012 Location: Mississippi Status: Offline Points: 745 |
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 Posted: Jul/09/2012 at 10:13pm |
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It's not pingig now and I run 87. I don't think the motor has ever been down so I'm assuming stock pistons and what I'm told is a comp 280 cam.
As far as the pistons sticking up, I got me a new belt sander.  I can get my old 360 back for about $100 for the long block. I was hoping I could use the crank and heads from that to help build my 343 to yield the same compression I have now but use the better flowing later heads. New pistons aren't an option. If I'm gonna buy new pistons it'll be for a 401. Can you mill pistons w/o having to rebalance?
Edited by kellysguy - Jul/09/2012 at 10:15pm |
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farna
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Moderator Lost Dealership Project Joined: Jul/08/2007 Location: United States Status: Offline Points: 7102 |
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Posted: Jul/10/2012 at 10:34am |
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You can mill the pistons as long as they are thick enough up top (should be for 0.020"). As long as the same amount is taken from each balance shouldn't be affected -- not enough for a street/mild performance engine anyway. You also have to take the same amount out of the dished area or compression will rise though. PHAT has the right info.
You said it runs well on regular fuel now? Are you SURE you have a factory 10.2:1 engine? The 2V engine was 9:1 (235 hp for 2V, 280 hp for 4V). If it's running well with no detonation (ping) at 10.2:1 you must have the timing turned way down. that works, but you're losing power. A 9:1 engine with the timing advanced as much as it will stand without pinging will produce about the same power as a 10.2:1 engine with the timing set the same way -- which will be a bit retarded from factory. You might have a 9:1 engine. A compression check will help determine what you have, but that takes the effect of the cam in consideration (dynamic compression). AMC didn't publish cylinder PSI until the late 70s or 80s. In 81 a 258 six with 8:1 compression was expected to produce 120-150 psi -- closer to 150 for a new engine. I'd expect a 10.2:1 343 to produce a bit more than 150 psi, but the cam might be reducing cylinder pressure, so it's not a real good test. If you're positive the valve covers are correct for the engine check the tag. 343/2V(9:1) would have an "S" as the 4th character, 343/4V(10.2) would have a "Z" (yes, that's the same as the 71+ 401). First number would be a 9, 1, or 2 (67/68/69). |
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Frank Swygert
American Motors Cars Magazine www.amc-mag.com |
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kellysguy
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Joined: Mar/15/2012 Location: Mississippi Status: Offline Points: 745 |
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Posted: Jul/10/2012 at 12:08pm |
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Not sure about anything, it was a factory 4bbl engine, well, at least ot came with a cast iron intake that was taken off of it. I'm going by the odometer, engine paint condition and amount of dirt on the engine. Paint condition and dirt are consistent w/ a 60K engine. It has 204P25 on the valve cover that came WITH the car but NOT ON the car.
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kellysguy
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Posted: Jul/10/2012 at 12:33pm |
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Seems they are 360 covers but I have 343 on the side of the block. I found something that says 7th vin and 4th of cover tag should match. My vin is T.
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kellysguy
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Posted: Jul/10/2012 at 12:34pm |
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Frank, the idea is to "raise" compression so I can run the later 58cc heads w/o loosing anything (hopefully)
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kellysguy
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Posted: Jul/10/2012 at 12:59pm |
That's not nessisarily true. With more compression you get more heat and pressure which makes the mixture easier to burn, therefore; requiring less time(ing) to light all of it. As long as it's lit off before you go into ATDC, you aren't losing any power. Better fuel is harder to light, thereby requiring more timing. No matter what fuel you run, it all has to be lit off by the same time (or degree of crank rotation). How much timing each type of fuel needs to get the job done by that time with a certain compression ratio is different. Power is made with compression, not timing as it all has to go off by the same time. Fuel and timing is a delievery system for compression, which dictates power levels. Granted, it what you look at first depends on who matches what, but it's still compression that makes power. The only reason to run alcohol is for the high compression levels. It's a very low BTU fuel which is why you have to burn twice as much (as well as keep it cool but that's another story) You are correct that you can loose power by improper timing, but you can't gain it. The thought being; anything less than optimal timing (when it SHOULD be going off) is a loss, not a gain. Max optimal timing per fuel type occurs before ping, right? Well, if I advance it and it pings, I'm already at optimal timing for this fuel and lose nothing unless I retard it from it's current setting. Now, if I change fuels, that's a whole nuther story.  All of this being said, I have no idea where timing is set at on this engine. It wasn't a concern as I needed to get the brakes and suspension straighten out first and it ran fine. While I have it out to weld the cracks in the shock towers...
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billd
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Posted: Jul/10/2012 at 1:33pm |
?? No, not quite......... >>As long as it's lit off before you go into ATDC, you aren't losing any power.<< Wrong - at least the way it's worded here.......... You want PEAK combustion pressures to happen at roughly 14 to 16 degrees after TDC. You need to time the spark event to make sure the fuel has ignited and burned to the point where it is producing peak pressures at that point. Depending on compression and A/F ratio, the mix may burn slower or faster. Lean burns slow, low compression burns slower as a general rule if all else is equal. Does "lit off" mean the spark event that triggers the fire? if that's the case, then the statement "as long as it's lit off........" is quite wrong, as if you trigger the fire at 1 degree BTDC, and it is a slow burning mixture, then peak pressures will happen well after that 14 degrees and you lose power. If you trigger the fire or light it at 15 degrees before TDC and the pressure peaks out as the piston is hitting TDC, you are pushing back down on it too soon, or if the peak pressures take place at 2 degrees ATDC, then you've blown the charge before there's enough leverage to produce the best mechanical advantage. >>Better fuel is harder to light, thereby requiring more timing<< No again - at least how it's worded. Better fuel - higher octane resists self-ignition. Timing the spark sooner won't light it easier, it will just light it sooner. It will burn at the same RATE once the fire is lit - the burn rate and power output or BTUs will be the same. It will resist self-ignition as the flame front approaches the unburned charge remaining and compresses it into a corner, where lower octane would tend to be ignited simply by the heat and pressure, causing detonation. Advancing the timing won't do any more than light it sooner. Has nothing to do with how hard it is to ignite via spark. You change timing to compensate for the SPEED at which it burns, not the difficulty in lighting it. It's urban legend that higher octane fuel burns SLOWER - it does NOT. It simply resists self-ignition better than lower octane. The RATE of burn is still a matter of A/F ratio, compression, the chamber design, etc. You can advance timing for higher octane not because you NEED to, but because you can, and often it gains a bit of power if your engine "pings" and you need to set timing back to compensate. Otherwise, there's no BTU output difference. >>Max optimal timing per fuel type occurs before ping, right? << Nope - another urban legend. sorry, not sure where this comes from, but it's not right. Ping has 0 to do with it. Advancing to ping, then just backing off is shade-tree stuff with no basis in fact. In fact, it can be quite damaging, or actually LOSE you power. Maybe the engine doesn't ping easily, so you keep advancing the timing and force a ping, then just back it off - you could well be causing the peak combustion pressures to take place well before that 14-16 degree mark, losing power - or your engine may be prone to pinging, so you back the timing off, meaning the charge hits peak pressure well after the 14-16 degrees, when it's past the top mechanical advantage point. I just had to reply to this as there's so much mis-information, some simply wrong - incorrect. this one - >>Max optimal timing per fuel type occurs before ping, right?<< is way off the mark - octane does NOT change burn rate, and that statement assumes it does in order to be even close. we've debunked that more than once, and even recently as I recall as a young fellow with a Jeep had a conversation going on this very thing. |
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kellysguy
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Joined: Mar/15/2012 Location: Mississippi Status: Offline Points: 745 |
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Posted: Jul/10/2012 at 2:55pm |
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Bill, you're splitting hairs and not following the train of thought nor the spirit in which it's written. I'm not talking to a novice, therefore I don't have to spell everything out. Lit off mean completel burn ( in most cases, but then again I don't have to explain that to Frank as he's quite capable of following my train of thought...even if I change my usage.) Everything is indeed factual as you just so explained (more or less but you got some detail wrong). Bottom line, you know EXACTLY what I'm talking about and even acknowledge "that word differently it is correct". Frank isn't an idiot, therefore I don't need to spell anything out to him or speak to him as if to a child.
Materials with higher flash points are indeed harder to light which is why the resist self detonation.  Higher flash points require more heat to light...which means more heat is needed, ergo; you can not light both materials at the lowest flash point. Can you light diesel with a spark plug? Why, beacuse it's just as easy as gasoline? Basic rule of thumb, the more BTU's the harder to light and slower to burn. ( Admittedly, VERY basic and VERY crude, doesn't apply to all circumstances) Heat is energy. To say two different material require two different levels of energy to ignite and then say one doesn't need any more energy than the other to burn is literally insane. Are you saying all spark is the same temperature? Why do you think there is a whole performance ignition industry? Why else do you think engines run poorly with weak spark? So, why then, if it all burns at the same rate(speaking of gasoline) and needs to go off ( burn completely, as in all gone, no more, bye bye) by the aforementioned degrees of timing you cited...would you want to increase timing further, "because you can"? As you stated ( and I did too) it all need to be lit off, ( burnt all up, no more fuely, all gone bye bye) by a certain point. Advance timing "because you can" to light before that point is useless. So, it either better fuel needs to be all burnt by the degrees you state and needs extra time to do it OR, you increase timing "because you can" and make more power...but it can't be both....as you have already so well explained Now, if you say, you need to run higher octane so you can advance timing to the aforementioned majic window then YES, now you have something. You do it to get to a point, not because you can and cetainly not at or past the point of detonation. What race are we tuning for, the Junkyard 500? Bottom line, peak power always occurs before detonation because a broken engine can't produce anything...no matter what the timing is set at....and I shouldn't have to explain that.... not even to a child This is fun. I could do this all day. Edited by kellysguy - Jul/10/2012 at 3:24pm |
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kellysguy
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Posted: Jul/10/2012 at 3:08pm |
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You see William, I know exactly what you are talking about too.
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kellysguy
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Posted: Jul/10/2012 at 5:52pm |
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Now, back to our regularly scheduled program...
I've been thinking how impossible it would be to do 500hp for $500 with this engine then I got to thinking, I wonder if I could do 400hp for $400? I already have a cam, intake,carb and headers, could I realistically get 400 hp with the 360 crank, milled stock pistons (by me, no cost) and later model dogleg heads? I can get the long block for $100 and do all of the other work myself. Can it be done? I'm at what, 325 now maybe? Edited by kellysguy - Jul/10/2012 at 5:55pm |
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farna wrote: