My 383's performance

[Filthy Filbert]

Don't forget the switch from gross to net horse power ratings, and the fact that the 383 that made 335 was likely a prepped blue printed motor; not a slapped together production motor with much less compression. 

It's not uncommon for a 383 to be at 8:1 compression!

[B5fourspeed]

In 1970 the 383 was 9:5 to 1 compression.In 71 it was dropped to 9 to 1.

[70rag383]

I have a rebuilt numbers-matching stock 383 with 906 heads, Edelbrock carb, TTI headers, tremec 5-speed, 3.55 suregrip, 255/50R17 tires. Similar to others I find the performance a little underwhelming. I've never run the car in the quarter mile. Had the engine dyno'd at just over 300hp and 400ft-lbs of torque after it was rebuilt.  I can easily light up the tires and get rubber into 2nd gear however I feel like this engine should produce more. My Audi SQ5(an SUV!) with 340hp could probably walk my Challenger in a 1/4 mile race.

I have an engine guy (not the one who rebuilt the engine) who has worked on a lot of Mopars and he suspects the compression was not built to 9.5:1 specs and more like 8.0:1 which explains the lack of seat of your pants acceleration. He wants to re-do the pistons, stroke out the crank and put some better flowing heads. He says that will get me to 550hp.

[DeathProofCuda]

I have a rebuilt numbers-matching stock 383 with 906 heads, Edelbrock carb, TTI headers, tremec 5-speed, 3.55 suregrip, 255/50R17 tires. Similar to others I find the performance a little underwhelming. I've never run the car in the quarter mile. Had the engine dyno'd at just over 300hp and 400ft-lbs of torque after it was rebuilt.  I can easily light up the tires and get rubber into 2nd gear however I feel like this engine should produce more. My Audi SQ5(an SUV!) with 340hp could probably walk my Challenger in a 1/4 mile race.

I have an engine guy (not the one who rebuilt the engine) who has worked on a lot of Mopars and he suspects the compression was not built to 9.5:1 specs and more like 8.0:1 which explains the lack of seat of your pants acceleration. He wants to re-do the pistons, stroke out the crank and put some better flowing heads. He says that will get me to 550hp.

@70rag383 was that 300 hp on an engine dyno or chassis dyno?

[Mr Cuda]

I have an engine guy (not the one who rebuilt the engine) who has worked on a lot of Mopars and he suspects the compression was not built to 9.5:1 specs and more like 8.0:1 which explains the lack of seat of your pants acceleration. He wants to re-do the pistons, stroke out the crank and put some better flowing heads. He says that will get me to 550hp.

I get that not everyone wants spare stuff lying around,  but stroking a 383 would ruin the joy of a properly built 383. Big bore,  short stroke  big cam , and high rpm.
If you were going to stroke it,  you know the factory made this really great engine that is 440ci.

[70rag383]

@DeathProofCuda that was 300hp on an engine dyno.

[Mr Lee]

@MoparLeo  - No doubt a bigger gear in the rear would give it more snap but I don't want to trade off my highway cruising rpms of the 3.23.  Hindsight, I should've went with a 5 speed or OD trans .   

@DeathProofCuda  - Do you know what your compression ratio is with the Diamond pistons?  Just curious.

I wish I knew exactly what my compression ratio is but I feel like it's low.   And, more and more, it seems that with a low compression ratio it is almost a waste of time and money to invest in more bolt on parts.  Getting more air into the engine with an intake, heads etc is worthless unless you can squeeze it tight enough to get a big BANG!  If there's no squeeze, there's no bang!  The Bang is what drives that piston down hard and I have been slowly realizing that.

I talked to a guy who builds FAST stock appearing drag motors and he recommended I don't waste my time with thinner head gaskets etc etc. trying to build more compression... that the improvements will only be marginal...   That I should pull the motor and just build a stroker and I'm starting to think he's right.  But, money is an object.  I've already looked at 440 source.com at stroker kits.  And I've already bought a book on how to rebuild big block mopars.  Maybe one day I'll actually do it.  I've always wanted to rebuild an engine.  I think the inner workings are fascinating, I've just been too chicken to do it.  Have done a few small single cylinder engines but never a V8. 

@Nepagarage  - I am measuring vacuum from the manifold side of the carb.  I will post a video.... 

[Nepagarage]

Obviously we don't know the specs on your cam but from my experience that is way too fluttery. I'm used to seeing steady vacuum that barely moves the needle on engines I've built. Just my two cents for what it's worth

[Mr Cuda]

I think you know that something is wrong with the valve train,  as you mentioned it in your first post.
Valves are way too tight or massively over advanced camshaft.
I'd blame valve adjustment.
Maybe just shims under the rocker shaft for a test.

[DeathProofCuda]

I had my block zero decked and also had the heads shaved (maybe .010") which reduced the combustion chamber volume a bit.  With a .020" steel shim gasket I think I'm right around the blue print spec of 9.5:1 for a 70 383 HP.

[MoparLeo]

Download a service manual. They are in the resources section o0f this site and are free. It will give you some vacuum info. Double pumper carbs are overkill for the street. Just uses more gas. Street carbs were always single pump. As said low, fluctuating vacuum usually indicates a valve train issue. The worst thing you can do during a diagnostics test it to just start replacing parts. Resist the urge, you usually create more problems. Manifold vacuum is what you need, not ported. So be careful where you hook your vacuum line to. Best is directly to the intake manifold. No question about being correct. A/C or Power brake cars have intake manifold vacuum ports that are ideal to connect to for your vacuum gauge.

[DeathProofCuda]

I talked to a guy who builds FAST stock appearing drag motors and he recommended I don't waste my time with thinner head gaskets etc etc. trying to build more compression... that the improvements will only be marginal...   That I should pull the motor and just build a stroker and I'm starting to think he's right.  But, money is an object.  I've already looked at 440 source.com at stroker kits.  And I've already bought a book on how to rebuild big block mopars.  Maybe one day I'll actually do it.  I've always wanted to rebuild an engine.  I think the inner workings are fascinating, I've just been too chicken to do it.  Have done a few small single cylinder engines but never a V8. 

Instead of going all-in on the stroker right away, maybe take it in steps.  Tear into what you've got and figure out the valve train issue, pull the cam and figure out what it is, and then reinstall it (or something else) using a degree wheel.  Figure out how far your pistons are in the hole.  Maybe your valve train issue will turn into a new set of heads that happen to be closed chambered, leading to better compression.  You'll learn a ton and all this work will give you the confidence to build the stroker later if you still want to.  The new heads will fit perfectly on the future stroker, so you'll have spread the pain of those costs out over some time.

[HP_Cuda]

If you truly have a lower compression motor you may have a simple way to generate power.

Put a roots or a centrifugal on there and have some real fun.

First, I would figure out the issues with your compression test first. Btw, your AFR to begin with on the dyno pull was way lean to start with and then leveled out where it should be.

[FF376]

Personally I don't think its probably very far off from where it's supposed to be. Shaky vacuum might be some valve issues but it runs well just seemingly low on power. I had a relatively stock 383 in a cuda once and it was a mid to high 14 second car. I added a small Comp 270H cam, Carter AFB and a used Edelbrock Performer intake and it had good low end but still was a 14 second car.. I would think if the cam was advanced or retarded it would be a more negative on how it drives?

[Chryco Psycho]

There is a Lot to this , 383s work very well but everything has to be right .
First the 383 has virtually the same stroke as the 318/340 so they do not make a ton of torque , so they depend more on HP / RPM .
You need to have max compression at 10:1 at least , the cam has to build compression or you need to have a loose converter to get up into the powerband before any load is applied , loading the engine at too low RPM will kill performance . You need to match the intake & cam rpm but for most street applications the Holley Street Dom is one of the best , you need 2x the CI minimum for carb size so 750 -800 CFM .
You really need to know what the cam is or select a well matched cam not an unknown one .
Building a 436 stroker is a no brainer , now you have a 440 with 8 lbs less rotating mass so it revs easily & you get the TQ of the longer stroke .
Let me know how i an help .