383 Horsepower/Torque predictions
- Thread starter BGarff
- Start date
70bigblockdodge
Old Man with a Hat
9.2 is the ratio they designed it to have if everything is machined to minimums, however most of the machining is not brought down to these numbers. The only way to get your compression ratio is to cc the heads (check with measured amount of water) then convert that to cubic inches them add in volume of head gasket plus volume piston is down from the deck compared to same volume plus the stroke of the piston at BDC. If you know your numbers cc, head gasket thickness, how far down in the cylinder the piston is at tdc you can find calculators all over the web. So it's not 10:1 maybe its more who cares run it learn about it and report back here we all love to talk about cars, that is why we are on here.
It's hard to believe, but back when there were still dinosaurs, I was a teenager once.
You really have to stop figuring things based on what they advertised back then. Any values like that were calculated using optimal measurements. In production, the values changed to allow for manufacturing tolerances etc. The true compression ratio was less.
Just to give an example, back when we "blueprinted" 440 engines to race in NHRA stock class, the piston was supposed to have a .027" deck height. That was the factory spec and what the advertised compression ratio was. In reality, it took milling .020 off the engine deck to come close to this.
So... your advertised 9.2:1 compression ratio is probably closer to 8.5:1. Maybe even less. Now you take that engine apart and pitch the steel head gasket that's .015" compressed (IIRC) and replace it with a composite gasket that is at least twice the thickness and you've dropped your ratio down even more.
If your machinist had CC'd the heads, measured the deck height and figured in the head gasket thickness and told you you had 10:1 ratio, I'd believe him.... But it sounds like he cut the head and gave you a figure to make you happy.
Here's the good news.... Having a real 10:1 ratio on the street is more of a PITA then it's worth. Pump gas will vary in octane from load to load and station to station. Here again, don't believe the ratings on the pump. You will want a car that won't knock itself to death on hot days anyway. .
Anyway... bolt the car together and run it... That's the only way you are going to know how it runs. We can tell you what to do until the cows come home and you aren't going to make any changes (read spend $$) based on our ramblings.
I think you have to actually do it and get a base line... Maybe you'll be happy the way it is... It will go rumpty rump and sound good in the parking lot. If you decide to go a different direction, let us know and we'll help.
65Fury440
Senior Member
If you try looking for an article Steve Dulcich did year ago for one of the mags, I think he rebult a 383 "Magnum" to factory specs, and it was 335 as advertised, while a 440 that was advertised at 375 put out 360. I suffer from CRS, so google could probably take you to some reading of interest.
I don't remember if he rebuilt them to advertised specs, or as they come off the line though...
I don't remember if he rebuilt them to advertised specs, or as they come off the line though...
commando1
Old Man with a Hat
Gerald Morris
Senior Member
In the interest of propagating helpful data to all deserving good folk, let us refer to some accurate information w.r.t. the cylinder heads. IFF, as Master BGarf stated his heads are original to that 383, then the serial number should be 2406516, the "516" heads. I got the listing from our friends at Mopar Casting Numbers - MyMopar.com - Mopar Part Numners . This will at least give us a start at knowing what to expect, given the information supplied by Young Master BGarf, once we allow for the probably deliberate inaccuracy from Chrysler Corp ca 1966 on the subject.
The valve sizes should be correct as given. Now, are there any reliable data regarding the volume of the head per cylinder? I see figures like 86-88 cc, which is pretty voluminous. Since the heads under discussion were milled, I second the suggestion that some water or other fluid be used to estimate their new volume. The 516, being closed chamber is actually better for low end torque on a low power setup such as the 1965 2 bbl 383, which I happen to be a happy owner of myself.
If I were building my 383 for power, I think I would spend my $$ on some aluminum, after market heads with larger valves, already seated for modern unleaded octaneless (modern gas is heptane and hexane, with VERY LITTLE real octane). Rather than pursue high compression, which is expensive in every way, above all at the pump, I would advise low compression with the best gaskets available and a modest turbocharger or super charger. There was a relatively recent article about how to get good power from a 383 with "bolt-on" additions only. Try Googling it. Since I'm not quite so enamoured with burning good rubber at stop lights as IU was 30 or 40 yrs past, I look at this subject with a little more detachment now. I hope that leads to better thinking for all concerned.
Gerald Morris
Senior Member
Read this article by Richard Ehrenberg:
Low-Buck Bolt-On Upgrades from Mopar Action
Here's a nice bit on a 383, "Budget build" by Steve Dulcich:
Chrysler 383 Engine Buildup From The Bottom Up - Mopar Muscle Magazine
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Landyacht
Active Member
Doesn't look like a very good combination to me. You have much cam for your engine displacement if you plan on running it in a street driven C-body. Unless you plan on building your 383 into a 496 stroker I would stay away from the Comp XE 284H and maybee go with the Comp XE268H. I would also ditch the single plane intake and go with the Edelbrock Performer RPM intake.You also need the proper valve springs to go with whatever cam you are using so that you do not float a valve while getting on the throttle. If you milled your heads for more compression I would do a clay test especially if you're running a high lift cam with stock pistons without valve reliefs. Milling your heads will add extra preload on your lifters so I would suggest getting some adjustable rockers or a set of custom pushrods to compensate for the difference. As for horse power gains Increasing compression , changing your intake and cam aren't going to be all that massive. I would estimate maybe a 50 or 60 hp gain at best. If you want to make serious power porting your existing heads or going with a set of aftermarket aluminum heads is the way to go.
Landyacht
Active Member
The only way you will achieve those numbers is with some cylinder head porting or a good set of aftermarket aluminum heads. Performer RPMs are an excellent choice and they come with a variety of combustion chamber volumes and open or close chamber designs. They are a little bit pricey compared to some of the other heads out there such as the 440 Source Stealth heads or the Sidewinder heads but are of much better quality.
