Mopar 383 HP performance problem

[jamesroney]

Help. I am working on my 1970 'cuda with the 383 / 4 speed. This has been an on-going problem since the restoration several years ago.

The car runs sluggish and hot. I restored it using the original Holley 4bbl, and 906 heads on a standard bore 383. 26 inch radiator, 3.55 gears, Accurate Exhaust. Compression is right at 140 per cylinder, +/-2%.

It never ran right. It just gets hot (220F on a manual gage) Will only create 18 in-Hg of vacuum. AFR sits right at 13.8. So I thought it must be the camshaft, or cam timing. So I changed the cam to a lower profile HP cam. The I change the cam again to a 2bbl "L" code cam. Same result. So I changed the intake from factory to a Weiand. Then changed the heads to Edelbrock aluminum. Then changed the ignition to Mopar Electronic. Then changed the carburetor to a known good Holley 600, then tried an AFB. Then changed to an FITech EFI. Same or similar results in all case. Base timing has been set everywhere from 8 BTDC to 18 BTDC Vacuum advance is functional, an mechanical advance is also working.

I drove a stock 1970 Challenger 340 for comparison, and it ran amazingly better. I'm not talking performance. Just drive-ability and cruise. It's very strange.

But I noticed that the exhaust sounded kind of raspy and whistle-ly. I got my IR pyrometer to check for exhaust temp. I was surprised to see that my cast iron exhaust manifold surface temperature is sitting over 600 degrees F. I made the same measurement on my Bronco, and the highest temp was about 450. So I thought that was strange. I'm starting to think that my resonator / muffler is blocked, or baffled or restricted.

Can SOMEONE factory HP manifolds and a temperature gun PLEASE tell me what your under-hood manifold temperature looks like when the engine is idling at operating temperature?

Any opinions are welcome.

 

[Ricks72Chlgr440]

Help. I am working on my 1970 'cuda with the 383 / 4 speed. This has been an on-going problem since the restoration several years ago.

The car runs sluggish and hot. I restored it using the original Holley 4bbl, and 906 heads on a standard bore 383. 26 inch radiator, 3.55 gears, Accurate Exhaust. Compression is right at 140 per cylinder, +/-2%.

It never ran right. It just gets hot (220F on a manual gage) Will only create 18 in-Hg of vacuum. AFR sits right at 13.8. So I thought it must be the camshaft, or cam timing. So I changed the cam to a lower profile HP cam. The I change the cam again to a 2bbl "L" code cam. Same result. So I changed the intake from factory to a Weiand. Then changed the heads to Edelbrock aluminum. Then changed the ignition to Mopar Electronic. Then changed the carburetor to a known good Holley 600, then tried an AFB. Then changed to an FITech EFI. Same or similar results in all case. Base timing has been set everywhere from 8 BTDC to 18 BTDC Vacuum advance is functional, an mechanical advance is also working.

I drove a stock 1970 Challenger 340 for comparison, and it ran amazingly better. I'm not talking performance. Just drive-ability and cruise. It's very strange.

But I noticed that the exhaust sounded kind of raspy and whistle-ly. I got my IR pyrometer to check for exhaust temp. I was surprised to see that my cast iron exhaust manifold surface temperature is sitting over 600 degrees F. I made the same measurement on my Bronco, and the highest temp was about 450. So I thought that was strange. I'm starting to think that my resonator / muffler is blocked, or baffled or restricted.

Can SOMEONE factory HP manifolds and a temperature gun PLEASE tell me what your under-hood manifold temperature looks like when the engine is idling at operating temperature?

Any opinions are welcome.

When you installed the cam, did you degree it or did you install it dot-to-dot? Did you use the same cam gearset with all of those cams? If so, with that much heat in the exhaust sounds like the cam timing is really retarded.

 

[jamesroney]

When you installed the cam, did you degree it or did you install it dot-to-dot? Did you use the same cam gearset with all of those cams? If so, with that much heat in the exhaust sounds like the cam timing is really retarded.

I think I DID use the same gearset. That is a great observation. Thank you for the insight. Easy enough to slap a degree wheel on it and verify valve events. I did put a piston stop in it to verify TDC on the damper, and timing marks.

But a retarded cam, and then compensating by advancing ignition is a distinct possibility. Thanks.

 

[Deathproofcuda]

Have you checked to confirm whether the manifold heat control valve on the passenger side exhaust manifold is open?

 

[Chryco Psycho]

Everything is in normal range by my experience , 18' of Vacuum is great
600* at the manifold is good , considering 1800+ in the combustion chambers
I have seen cams ground 11* off , not checking ? degreeing the cam is a big mistake , I personally have had excellent results with Lunati VooDoo cam grinds .

 

[DetMatt1]

He’s tried 3 different cams though?

 

[Chryco Psycho]

What I meant to add is the timing gear could could easily be machined 11* or more off

 

[jamesroney]

Have you checked to confirm whether the manifold heat control valve on the passenger side exhaust manifold is open?

Yes, and that's a great first thing to check. The heat riser manifold valve has been deleted via acetylene torch to the butterfly.

 

[jamesroney]

Everything is in normal range by my experience , 18' of Vacuum is great
600* at the manifold is good , considering 1800+ in the combustion chambers
I have seen cams ground 11* off , not checking ? degreeing the cam is a big mistake , I personally have had excellent results with Lunati VooDoo cam grinds .

Thanks for the comment. I was looking for some first hand experience with the 383. Are you saying that a pedestrian L-code 2V-383 in a C-body should not be able to reach "normal" vacuum of 20-22 inches at idle?

I'm also puzzled at your exhaust temperature observation. 1800+ in the combustion chamber is a theoretical number (and likely very accurate) that cannot be measured by an optical pyrometer. Just like 600 in the manifold might be different than the 600 shown at the surface of the manifold. I'm sure that if I installed Exhaust Gas Temperature sensor in the flow...I would get a completely different set of temperatures.

So are you saying that when you shoot a cast iron manifold on a fully warmed 383 with an optical temp gun...you would expect to see 600 degrees? That's what I was hoping to learn.

 

[jamesroney]

Have you checked to confirm whether the manifold heat control valve on the passenger side exhaust manifold is open?

Just to ensure, this morning I removed the passenger side exhaust pipe and verified that I did indeed delete the butterfly.

I also added a pressure / vacuum gage to the exhaust O2 port to measure back pressure. At idle, it is a fairly constant 1 psi. At 2K RPM, it becomes unstable, and the needle bounces all over the place, but the middle of the needle sits right at 1 psi.

 

[Deathproofcuda]

@jamesroney

I also have a 70 Cuda 383 4-speed car. I'm running the factory 906 heads, stock intake with Holley 650 DP, and stock exhaust manifolds with full exhaust (mufflers and resonators from Accurate Exhaust), 26 radiator, so should be pretty comparable to your setup. I plan to have it out this weekend, so will try to remember to shoot the manifolds with my IR thermometer after it's warmed up.

I did look back at some of my tuning notes and have one entry where I recorded exhaust manifold temp, it says approximately 550 degrees F. That same entry lists a water pump surface temp of 201 degrees (measured with an IR thermometer).

 

[Deathproofcuda]

@jamesroney

I also have a 70 Cuda 383 4-speed car. I'm running the factory 906 heads, stock intake with Holley 650 DP, and stock exhaust manifolds with full exhaust (mufflers and resonators from Accurate Exhaust), 26 radiator, so should be pretty comparable to your setup. I plan to have it out this weekend, so will try to remember to shoot the manifolds with my IR thermometer after it's warmed up.

I did look back at some of my tuning notes and have one entry where I recorded exhaust manifold temp, it says approximately 550 degrees F. That same entry lists a water pump surface temp of 201 degrees (measured with an IR thermometer).

Checked my car out this morning after returning from a cars-n-coffee meet up. Using my IR thermometer I saw varying temps ranging from 450 to well over 600 degrees on each side. Hottest point seemed to be where the runner from the third cylinder back (5 and 6) comes into the main trunk of the manifold. Surface temp of water pump was just under 190 degrees. Outside temp was probably around 75 degrees.

 

[moparlee]

Checked my car out this morning after returning from a cars-n-coffee meet up. Using my IR thermometer I saw varying temps ranging from 450 to well over 600 degrees on each side. Hottest point seemed to be where the runner from the third cylinder back (5 and 6) comes into the main trunk of the manifold. Surface temp of water pump was just under 190 degrees. Outside temp was probably around 75 degrees.

Good information. What was the reading on the temp gauge?

 

[Deathproofcuda]

Good information. What was the reading on the temp gauge?

I don't have an added temp gauge, just the standard one equipped in the rally dash, which does not have any degree readings. It has a "C" and and "H" and an interior band to show the normal operating range. It was pointing toward the higher end of the interior band. Example pic in the ebay link below.

NEW Mopar Temperature Gauge 1970-74 E-Body Rallye Dash | eBay