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Help with 383 Super Commando rebuild
Looking for some help with my recent engine rebuild. I have a 383 super commando, A/T in a 1969 Sport Fury convertible. The engine had been rebuilt at a shop and I seem to have lost a lot of power. Had a problem with the shop so I never go a build sheet (long story). I asked for the engine to be basically stock. I do know that it is 30 over and was told the pistons are 9:1 (stock was 10:1). I decided to rebuild the top to see if I could get some power back. I installed the following
Cam: Mopar P4452783, 228/241@0.050”, 115 CL, .450/.458 lift Heads: 516 closed chamber 2.14X1.81 Stainless Valves, 86-88cc (I have the stock 906 heads) Intake: Mopar M1 duel plane (have the stock cast Iron intake) Carb: Stock Holley 670 rebuilt Exhaust: Stock HP manafolds and duel exhaust All else is stock except for an aluminum Water pump. The car has A/C, Power disc brakes, stock 3.23 rear, auto trans, stock tire size. Did a compression test and found 134, 141, 139, 145, 138, 145, 141, 141 in order form 1 to 8. Stock is 150 for both 10:1 Super commando and 9.2:1 commando engines. The car runs like a dog. I estimate 250 HP down from the advertised 330 HP stock. I’d like the car to run with close to 400 HP or at least 350HP. What would you guy suggest to obtain this? Do I need to bump up the compression? New pistons? I don’t want to go to big with the cam and I’d like to keep the stock converter mostly city street driving. I’m up for anything including dropping the 440 block I have in the garage (would like to keep it stock only 1,400 made). I just have to get this thing running properly. Thanks |
#2
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Why did you go down on compression? Was it pinging on pump gas?
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Thinking This Out
What size carb do you have and have you checked the timing? Also why the drop in compression?
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Valves are too big. 516's will cc at around 78 cc. I know the closed heads I ahve done were.
carb is a little small buyt shouldn't hurt much Cam is too big for a heavy car, unless you bumped the stall to 3500, and the gears toi 391's. As for compression, with the closed heads, if the piston is zero deck, and a .040 gasket, then you are a litttle less than 10:1. If the piston is down the hole much, it drops off really fast. With the 906's you will need a domed piston to get 10:1. Your static compression test is good. Look at the spread, not the total value. I've done 10:1 motors with about that much. Your spread is a little high but ok. certainly not enough to cost you any power. As for keeping the stock converter, and doing mods like this, you won't see the gains you hope for. Tuned porperly, the motor and combo you already have will make the hp you want, BUT it will be completely unusable with a stock converter in a heavy car. It will perform worse, which is what you have seen |
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Also can't get the vacuum secondaries to open on the carbs. Probably should have mentioned that before. |
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Always remember that the manufacturers give a rough comp. ratio. And they are always a little off from what reality is.
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I would get the secondaries working on the carb and it should make plenty of power. One other thing to check is the exhaust manifold valve sometimes they get stuck and that will kill performance.
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#11
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You can make the compression ratio higher several ways. Most piston manufacturers require you mill your block to get the right deck height(parallel decking is good to add here). Butwhat Dave said is also true. Get your pistons to zero deck. Pistons are either dished,flat,or domed. For closed chambers,go with a flat top piston. Pistons have a compression height,or distance. This is measured from the center of the wrist-pin to the top of the piston(to the flat). You can probably find a piston that has a little more CD than what you have. You can also mill the block and/or heads to raise your CR. All you have to worry about here is your pushrod length(it'll change) and your intake manifold fitting properly(a good machine shop will mill both sides of the heads-cc and intake side). Good luck Shaun |
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228, and 241 at 50 is identical to stock cam???????
On what? There must be a misprint somewhere, or the specs must be aproximated. |
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It's what mopar performance advertises their stock replacement magnum cam to be, which I kind of doubpt. Anyway, I don't see any major mistakes in this set up, it should run decently. Unless your true CR is VERY low, 8:1 or less. Or, if there is something wrong with the cam, cam timing, ignition or carb?
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Applications & Comments: Resto, 383/440 Magnum Super Commando Road Runner From Mopar |
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Any Ideas on why the secondaries don't open. wouldn't open with the 670 or the 750 holley. I tries different springs (vacum secondaries). Would Cam timeing effect this? Vacum leak (I tested and found none)?
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Engine Combo dosent look too bad, and compression seems to be OK for the combination. I would check the fuel supply (pressure/volume/float levels, etc.) I'm guessing your not getting much fuel, and the engine is running lean.
OR, the "rebuilt" carb was not built correctly. If you have another carb, you could try swaping a different one one. |
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I would suspect the carb rebuild to be the problem. Make sure the secondarys will open manually and are not hanging up on the base gasket. Open the throtle and move the secondary lever by hand, it should operate relitavly easy. Also you might want to remove the vacume assy and make sure you get movement when vacume is applied. You could have a bad diapharam. Could also have a pluged passage in the carb body. Did this carb work well before it was rebuilt?
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P.S. both sets of heads were milled to compensate for the gaskets. |
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The secondaries open fine manually on both. Vacuum line has volume but didn't measure it. |
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I will assume you have checked float level and are running standard mechanical pump. What kind of manifold vacume do you have at idle? If you have a vacume leak power valve could be open all the time. Also if you have had backfires power valve could be ruptured and leaking. Is your choke open all the way when the engine is warmed up?
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Thanks |
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I had a low compression 440 in a 74 roadrunner I had years ago. Had the same cam and had around 17" of vacume at idle when not in gear with the timing set to 10 BTDC. Pretty heavy car but ran high 14's with headars and closed exhaust and street tires of the day with 3.23 gears also. Your set-up should run pretty well once you get the problem figured out.
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#23
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Sorry thought you were running 440 not 383. The 383 in my 70 roadrunner with the same cam had about 18" vacume at free idle. Was running a 3310 780 holley on it with stock intake and manifolds.
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From your description, I don't really see anything wrong with your combination, and because the compression test seems decent, I would think the cam was installed correctly? At least where the engine should run OK.
This leaves the Ignition, fuel and exhaust. So, check that there is nothing backing up the exhaust, like the heat riser sticking, or something plugging the exhaust. I already mentioned the fuel system and carb, so maybe re-check the ignition system and firing order. Several years back, my friend did a tune up and changed his spark plug wires with "new" performance wires. After messing with the carb tring to figure why the car ran like crap, we put the old plug wires back on, and the car ran great. |
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In any case cam timing may eb worth checking. How do you know the secondaries aren't opening? On a vac secondary carb, they will not open just by cracking the throttle. There needs to be a load on the motor to bring the air demand up. Your problem could be they are opening too fast, and causing a bog. very common, and can be corrected with a stiffer spring in the secondary diaphram. |
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exactly, how are you checking if the secondaries open?
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An easy way to check that is ot put a paper clip to teh opening rod fo the secondaries right next to the vacuum housing and take the car to a spin. If the paper clip has come down on hte rod, the secondaries have opened, if not, they haven't.
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#28
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Wires and plugs have been changes a number of times. no change Distributer was upgraded to Electronic. no change. Exhaust was changed. no change. Heat riser is open in the manifold. Going to check for Vacume leaks next. I'm thinking I have a vacume leak which is causing the power valve to stay open all the time and flooding the carb. |
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