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  #1  
Old 09-20-2006, 10:38 AM
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Mutine Bullfrog Mutine Bullfrog is offline
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Help 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
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Old 09-20-2006, 11:44 AM
supernac supernac is offline
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Why did you go down on compression? Was it pinging on pump gas?
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Old 09-20-2006, 12:02 PM
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Lightbulb 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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Old 09-20-2006, 12:53 PM
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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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Old 09-20-2006, 04:31 PM
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Quote:
Originally Posted by supernac
Why did you go down on compression? Was it pinging on pump gas?
I haven't found anyone that makes a 10:1 for a 383. Would have to go with a custom for about $1,000.00.
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Old 09-20-2006, 04:36 PM
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Quote:
Originally Posted by Mutine Bullfrog
I haven't found anyone that makes a 10:1 for a 383. Would have to go with a custom for about $1,000.00.
That makes sense
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Old 09-20-2006, 04:39 PM
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Quote:
Originally Posted by dave571
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
That cam is identical grind as the stock cam. I had the 906 heads on and it ran worse. 3500 stall sounds a little high for this set up. I had a 750 on and it wasn't any better.

Also can't get the vacuum secondaries to open on the carbs. Probably should have mentioned that before.
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Old 09-20-2006, 04:43 PM
BJSracing BJSracing is offline
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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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Old 09-20-2006, 04:49 PM
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Quote:
Originally Posted by BJSracing
Always remember that the manufacturers give a rough comp. ratio. And they aren't always a little off from what reality is.
Yes I kind of knew that. I'm just wondering where I went wrong. The first think that came to my mind was the lower CR piston. Everything else seems to work for the 0-5500 RPM range I was aiming for. I could try a 2000 converter but how would that drive with this set up?
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Old 09-20-2006, 08:03 PM
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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  
Old 09-20-2006, 08:32 PM
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Quote:
Originally Posted by Mutine Bullfrog
I haven't found anyone that makes a 10:1 for a 383. Would have to go with a custom for about $1,000.00.
I have to agree with dave on the valves and cc of the chambers on your 516's. The cc's will be around 75-78,but how did you get the 1.81 exhaust valve seat cut in there? The water passage really clogs the 516's exhaust pocket,so putting an oversize exhaust valve really messed up your exhaust velocity(1.74's aren't even stock for 516's,but work good-those heads had 1.60 ex valves!). Even the weird split pattern cam you are running won't help that. Why so much duration on the exhaust and only a little more on the lift? I think a cam change will definately help.
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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  #12  
Old 09-21-2006, 12:31 AM
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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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Old 09-21-2006, 02:28 AM
DartGT66 DartGT66 is offline
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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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  #14  
Old 09-21-2006, 09:07 AM
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Quote:
Originally Posted by dave571
228, and 241 at 50 is identical to stock cam???????

On what?

There must be a misprint somewhere, or the specs must be aproximated.
Part No: P4452783, Advertised Duration: 268/284, @0.050": 228/241, O-Lap: 46, Center Line: 115, Lift: .450/.458, Basic RPM Idle-5800
Applications & Comments: Resto, 383/440 Magnum Super Commando Road Runner
From Mopar
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  #15  
Old 09-21-2006, 09:13 AM
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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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Old 09-21-2006, 10:56 AM
451Mopar 451Mopar is offline
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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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Old 09-21-2006, 03:11 PM
furz4444 furz4444 is offline
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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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Old 09-21-2006, 04:58 PM
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Quote:
Originally Posted by 451Mopar
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.
Actually it runs really rich. Ran rich with the 750 and 670 Holley and the 906 head. Still runs rich with the 670 and the 516 heads.

P.S. both sets of heads were milled to compensate for the gaskets.
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Old 09-21-2006, 05:03 PM
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Quote:
Originally Posted by furz4444
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?
This is the 2rd carb. When I first had the engine done I had a new 750 installed. Couldn't get the secondaries to open. Got a replacement 670 stock from year one rebuilt and still nothing.

The secondaries open fine manually on both. Vacuum line has volume but didn't measure it.
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Old 09-21-2006, 08:39 PM
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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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Old 09-21-2006, 09:38 PM
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Quote:
Originally Posted by furz4444
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?
Your at the same point I am. Vacume leak and/or carb. Going to check again for vacume leaks this weekend and if I have time i'll check the carb. I have another power valve in the original carb that worked fine prior to the rebuild.

Thanks
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Old 09-21-2006, 09:54 PM
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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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Old 09-21-2006, 10:01 PM
furz4444 furz4444 is offline
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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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Old 09-22-2006, 12:32 PM
451Mopar 451Mopar is offline
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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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Old 09-22-2006, 02:59 PM
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Quote:
Originally Posted by Mutine Bullfrog
Part No: P4452783, Advertised Duration: 268/284, @0.050": 228/241, O-Lap: 46, Center Line: 115, Lift: .450/.458, Basic RPM Idle-5800
Applications & Comments: Resto, 383/440 Magnum Super Commando Road Runner
From Mopar
Being a resto cam, doesn't make it a stock cam.

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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Old 09-22-2006, 07:41 PM
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exactly, how are you checking if the secondaries open?
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Old 09-24-2006, 10:20 AM
DartGT66 DartGT66 is offline
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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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Old 09-25-2006, 11:09 AM
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Quote:
Originally Posted by DartGT66
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.
Did the paper clip thing and not opening
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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