Standard 383 vs 383 roadrunner engine

[CompSyn]

This topic has been discused at length over at moparts. The factory compression rating was wildly optimistic. A 68-69 383HP was in reality around 9.2:1. A non-HP down in the 8s. This is one reason that if one isn't attentive to this and stabs in a popular oversized cam; the performance becomes really underwhelming.

 

[CompSyn]

I just happened to by a Charger with a non matching two barrel 69 383 in it.
So, I put a 440 in the Charger had the 383 rebuilt.
I feel it was a good price too.
That engine will go in my 69 RR and I'll save the matching engine like that.
I'll get around to doing that some day.
It just made sense to keep it looking right.
I'm only going to go so fast anyway.

I was considering a similar idea with mothballing my numbers matching 383 and building an alternate 383 for my road runner. Ultimately I decided against it. If I don't put the numbers matching engine back in now, if the engine isn't bolted to the car, there is a chance it might not ever happen. Many, many unintended circumstances for this.

 

[droptop]

This topic has been discused at length over at moparts. The factory compression rating was wildly optimistic. A 68-69 383HP was in reality around 9.2:1. A non-HP down in the 8s. This is one reason that if one isn't attentive to this and stabs in a popular oversized cam; the performance becomes really underwhelming.

What is the reasoning behind the compression ratio being almost a whole number lower than the advertised ratio by Chrysler?

 

[CompSyn]

What is the reasoning behind the compression ratio being almost a whole number lower than the advertised ratio by Chrysler?

You'd have to ask old Chrysler that. I'd assume the reason was because it was the height the horsepower wars so fudging the numbers was advantageous for sales.

 

[69hemibeep]

I would think the opposite for insurance purposes, all the manufacturers lied about horse power. The Hemi for instance rated at 425 was closer to 466 according to a factory dyno sheet that leaked out.

 

[CompSyn]

I would think the opposite for insurance purposes, all the manufacturers lied about horse power. The Hemi for instance rated at 425 was closer to 466 according to a factory dyno sheet that leaked out.

True, but the advertised horsepower and and compression ratio ratings are not exactly the same thing.

Remember, the base 383 Road Runner was marketed primary to the younger folks on a budget during the golden era of drag racing. Folks wanted to know they had a solid foundation for day-two race mods, like a bigger camshaft and headers. So here it is, the Road Runner with wait for it... 9.2:1 compression. No, not quite. Let's round it up to 10:1. Now were talking, it's a race ready pedigree.

 

[69hemibeep]

Guess I kinda blurred the lines, now back to compression ratios LOL

 

[droptop]

What is the name of the thread of this discussion on the 'parts? I did a search and did not come up with anything. My curiosity is getting the best of me because I have never heard of this before now.

 

[CompSyn]

What is the name of the thread of this discussion on the 'parts? I did a search and did not come up with anything. My curiosity is getting the best of me because I have never heard of this before now.

Sure I can help.

The guy with the 383 info is moparts.com user: JohnRR

I don't have the links on hand at the moment but I saved some of his comments on the topic as follows:

"It's a warmed up number, the real factory compression ratio was 9.2 at best.

To clear up a few things, that have been posted a hundred times at least, the stock 68-69 383 HP piston has a CH of 1.932, on a factory blueprint deck of 9.980 it sits .0025-.003 in the hole. If you build a 69 383HP to NHRA BLUEPRINT you get close to 11.0 compression, the NHRA spec has the piston .021 ABOVE the deck and it uses a 79.5 CC head chamber volume, NEITHER is factory actual build spec. I have an untouched 56K mile 69 383HP in my 69 GTS Dart and this is how it , and another 69 383HP I have on an engine stand, are built as were others I have disassembled with original pistons and uncut decks."

 

[Terry Hauck]

That is a TON of excellent information. Thanks

 

[CompSyn]

That is a TON of excellent information. Thanks

Yeah, that’s what I thought as well.

Now go look at the common on-the-shelf pistons available for the 383 today. Yep! None of them come close to the measured 1.932 Compression Distance, at least that I’ve seen.

This is why when I build my 383, I’ll be springing for some semi-custom Diamond or CP pistons in order to get a true 68-69 replacement piston. They’ll be expensive but I’m after a more accurate representation of a 69 383 HP.

 

[CompSyn]

What is the name of the thread of this discussion on the 'parts? I did a search and did not come up with anything. My curiosity is getting the best of me because I have never heard of this before now.

I just had a little time to look for one of the moparts threads. There's a few other threads. I'll post them when I find them...

Help me with 383 upgrade -> http://board.moparts.org/ubbthreads/ubbthreads.php/topics/210645/5.html

"It takes more than a zero deck piston to get compression in a stock stroke 383 , the small bore is the killer. Doing the math on factory built 383's you BARELY get 9.3 compression out of a CLAIMED 10.0 factory spec , the pistons have to be poking out of the deck .020 AND the heads cut down to 79.5 vs the as delivered 90-92 cc. or you have to stick a 10cc - 12 cc dome on the piston which gets in the way of proper flame travel ." - post #210645

 

[ACME A12]

the small bore is the killer.

Well, that's a curious statement... A 383 has the same bore size as a 426 @ 4.25". Not exactly what I would call "small"...not for a stock bore diameter anyway...

 

[Terry Hauck]

I just happened to by a Charger with a non matching two barrel 69 383 in it.
So, I put a 440 in the Charger had the 383 rebuilt.
I feel it was a good price too.
That engine will go in my 69 RR and I'll save the matching engine like that.
I'll get around to doing that some day.
It just made sense to keep it looking right.
I'm only going to go so fast anyway.

Yes. The cars are cool enough now. But if we were 20 it would be about how fast we could go. The good old days.

 

[ricks_RR]

pictures would be good

 

[Russ69Runner]

Ah shove a turbo on it or nitrous and it will run like a bat out of hell. Yea been watching Street Outlaw's.

 

[Terry Hauck]

I have a 69 hp 383 out of a runner, been sitting 30+ years, in Ohio.
If interested let me know here.
The heads might need some work, but it'll run like a camp when you get it on the road. I'll be moving soon, 1 year 1-1/2, I have no use, you can get it on the cheap.

Angrymike, Where did you go? Do you still have the 383?