Waking up the low deck 400

[73Runner4spd]

I threw this up to gather a few ideas on what I should do. I have a 73 roadrunner with a 400 big block 4 speed all matching numbers. It currently has 915 heads with 3 angle valve job, a larger cam (nothing ridiculous), edelbrock intake and 1407 carb, factory manifolds into 2.5 inch exhaust. It has 8 3/4 rear with either 3.23s or 3.55 not sure. It goes alright but it doesn't seem to go like it should. I am having some problems with my clutch. I am not sure how bad that affects me, it feels like its slipping a bit. (Its a mcleod paddle clutch). Just looking for ideas to spruce it up a bit. Not looking to replace the roatating assembly right now, but would not be opposed to a nice set of eddy rpms or similar heads. I also have a set of 3.91s that I will throw in there when I do my clutch. Another thing, I have a question about, It use to lay posi all day not it only kicks a peg legger. Not too sure why but I havnt caught posi in a while. Thanks in advance about any advice.

[DartGT66]

The biggest problem with the 400 is the low compression ratio. Raising it to decent level will also make teh engine more responsive to performance modifications. However, it's pretty much undoable without changing pistons. The other less than perfect feature for a pretty heavy car like yours is the short stroke. A performance 400 tends to be a screamer, and in a heavy car it means that you should have a pretty steep rear gear ratio for best performance. We once had a built 400 that performed pretty well for what it was. It had the stock cast crank, factory 400 rods with ARP bolts and forged .030" over TRW2266F 440 pistons. The chamber of the 52 heads had to be clearanced since the pistons came slightly above the deck. Deep valve reliefs were cut, the heads were homeported and fitted with 2.14/.181 valves. The CR was slightly over 10:1. The cam was a solid roller comp cams, grind 306R. Intake was a Team G, 850 cfm race demon carb, and 2" priamry headers with 3" exhaust. This was in a '73 Valiant that was pretty light, 4.86 gears, SS springs and slicks. It run 10.6's at 128+ mph and the engine turned 7500 at the traps!

[73Runner4spd]

Yeah, I wasn't sure if you could bump up the compression with just the heads. I do believe that all the 400s in 73 and 74 came with forged cranks if they put a 4 speed behind. I could be wrong though. And those are some really low gears! lol that thing must have been screaming at 7500! thats awesome. .30 over 440 pistons equal the same bore as a 400? Thank you

[fox]

Just to add this, I had a 77 van with the 400/2bbl.

I changed it to a holley street master and a 650 holley spread bore carb and it was day to night different.
The 2bbl would not spin the tires while the holley set-up would fry them from 30 mph punch. Mile went up too, if you drove it decent.

But, I agree the pistons need changed to get any compression.

[73Runner4spd]

I have an edelbrock 750 on it right now, I have a feeling it has a lot to do with my clutch assembly. the engine winds out nice but it doesnt move as it sounds it should move if that makes any sense. And believe me, I have some funky noises coming from down there. At first I thought it was the throw out, but im starting to think that there is a problem with the way the pressure plate is engaging. If I hold the clutch in at any gear or speed and rev it up a bit. I hear a clunk and a winding noise. Even if I am in neutral with the clutch pedal in. However if I leave the clutch pedal alone in neutral it revs up just fine. Idk like I said before Im going to put a new clutch in it and probably go through the tranny and put the 3.91s in it so it gets up and goes better.

[John Kunkel]

Have you checked for free play at the clutch pedal and at the throwout fork. I prefer to set the pedal free play at the fork, the factory recommends 5/32". A lack of free play can make the clutch slip and also can allow the pressure plate fingers to over-travel and contact the clutch disc.

Using the closed-chamber 915 heads will raise the compression about half a point or better depending on the head gasket but the true compression was about a half point lower than advertised to start with so you only break out even. Compression is the key to performance.

You would be wise to find out the actual rear axle ratio, it might be taller than the 3.23/3.55 you think it is and that kills performance.

[cageman]

new higher compression pistons wake up the poor 400.

[fox]

Originally Posted by 73Runner4spd

I have an edelbrock 750 on it right now, I have a feeling it has a lot to do with my clutch assembly. the engine winds out nice but it doesnt move as it sounds it should move if that makes any sense. And believe me, I have some funky noises coming from down there. At first I thought it was the throw out, but im starting to think that there is a problem with the way the pressure plate is engaging. If I hold the clutch in at any gear or speed and rev it up a bit. I hear a clunk and a winding noise. Even if I am in neutral with the clutch pedal in. However if I leave the clutch pedal alone in neutral it revs up just fine. Idk like I said before Im going to put a new clutch in it and probably go through the tranny and put the 3.91s in it so it gets up and goes better.

I tried that too, but the Holley had much better throttle response. That van ran so strong the wife didn't like it. She was the primary driver so I sold it all--30 years ago.
Plus it was used to haul our son who has MD and turning him over backwards showing off wasn't---, well cool!
I wish I still had it.

[73Runner4spd]

well I have been looking at new pistons, Keith Black forged seem to be the cheapest, but I hate going the cheapest route, so what you do you guys think about srp or mahle? I know Im using mahle on my 383 chevy build and they seem to be quality. Is it the same for mopar? And yes I agree about the Holley I prefer them over Edelbrock as well, Its just what was on the car when I got it. I will definitely check into the clutch play and find out what gears I have, should not be too har once the rear is in the air. Thanks guys I appreciate it.

[rampage_82]

For what you are wanting then the Keith Black Hypers will do fine. However if your are buying pistons anyways, then you will probably be boring it too, with new rings, bearings, etc... Why not just find an old 440 crank and LY rod assembly turn the Mains down to the 400 size, and grind the counterweights down? I know that you were not interested in messing with a new rotating assembly, but The KB Hyper pistons are going to be the same price, and it is basically the same amount of work to build a 451 as putting a 400 back stock is. Look at it this way... crank grinding is crank grinding. The rod resize is the same either way, and when you go with new pistons a balance job is always a good idea anyways.... Heck even the cast 440 crank would work just fine. Now I'm not a hater on the 383/400 stroke. Here are some pro's for the short stroke. The engine sizes are more appropriate to the lack of headflow that the stock heads have. Even with a stock bottom end and an aftermarket camshaft they will absolutely scream in the higher rpm ranges. Some of the cons. You will never have the off idle thump that the 440's or other RBs have. They are extremely sensitive to camshaft swaps, I.E. it is very easy to overcam one, and they are a bear to tune and live with if you get a cam that is a little too big.

[73Runner4spd]

Originally Posted by rampage_82

For what you are wanting then the Keith Black Hypers will do fine. However if your are buying pistons anyways, then you will probably be boring it too, with new rings, bearings, etc... Why not just find an old 440 crank and LY rod assembly turn the Mains down to the 400 size, and grind the counterweights down? I know that you were not interested in messing with a new rotating assembly, but The KB Hyper pistons are going to be the same price, and it is basically the same amount of work to build a 451 as putting a 400 back stock is. Look at it this way... crank grinding is crank grinding. The rod resize is the same either way, and when you go with new pistons a balance job is always a good idea anyways.... Heck even the cast 440 crank would work just fine. Now I'm not a hater on the 383/400 stroke. Here are some pro's for the short stroke. The engine sizes are more appropriate to the lack of headflow that the stock heads have. Even with a stock bottom end and an aftermarket camshaft they will absolutely scream in the higher rpm ranges. Some of the cons. You will never have the off idle thump that the 440's or other RBs have. They are extremely sensitive to camshaft swaps, I.E. it is very easy to overcam one, and they are a bear to tune and live with if you get a cam that is a little too big.

Yeah I was thinking about it, I dont know how much all the machine work would cost, but if Im gonna be a bear Im gonna be a grizzly if you know what I mean. I would probably build a 500 stroker. I have 3 400 blocks kicking around to work with. We built a 500 for my dads 73 using a 400 block and that thing is a monster... breaks loose in 4th at 50 with no problem. Unfortunately didnt really want to do this much to the motor, was just looking for a few quick ideas to make it a little more peppy...I was hoping to be able to just throw a set of aluminum heads on it with the cam I have and call it a day but if I got to change the rotating assy just to gain the compression Ill hold off and build one of the bare blocks I have. Im in the middle of building a 383 for my 97 z28 so it will have to wait a bit. Thanks for the info though! Unless the machine work to turn down a 440 crank and clearancing my block will be cheap? Never had to turn a crank down.

[73Runner4spd]

and its got 3.55 gears

[chirorod]

If the clutch is slipping, it doesn't matter how many ponies that motor makes. Won't get past the clutch. It feels like it slips, and it makes noise. Sounds pretty probable that clutch has problems.

[73Runner4spd]

Originally Posted by chirorod

If the clutch is slipping, it doesn't matter how many ponies that motor makes. Won't get past the clutch. It feels like it slips, and it makes noise. Sounds pretty probable that clutch has problems.

lol I know this... already got a new one to go in, just want to do a a few things at once.
dont know what that afro thing is supposed to mean but I put it up for shits and giggles... haha

[73Runner4spd]

Sorry to wake up a thread that has been dead for a few months but rummiging through craigslist the other day I found a 383 with a steel crank...not sure if its a factory steel crank or an aftermarket but it also has what he said were pop up pistons, I'm assuming they're just domed instead of flat tops. He didn't give me any info on the pistons them selves bore or deck height but he said he ran 906 heads without having any work done to them. I didn't think you could run unleaded gas on the 906 heads...correct me if I'm wrong. He claimed it had 11 to 1 compression with those heads. Sounds like it would be a screamer but I just don't know enough about it. I'm going to look at it tomorrow night. Now I know with my lt1 383 stroker motor I can get away with 12 to 1 compression on pump gas. 11 to 1 on a big block mopar seems a little steep to me. Are there any good afermarket aluminum open chambered heads available for these. I'm assuming the 88cc mmeans open and the 84ccs are closed. Yet again correct me if I'm wrong. Just don't want to buy a boat anchor but he's only asking 800 for it and it seems reasonable. Thanks

[rampage_82]

To get 11 to 1 compression with that short of a stroke and open chamber heads, would take a pretty serious dome. I think that the Eddy RPM's have the open chamber, and they are a very good head for a 383... BTW I'm running 12cc dome Speed-pro (TRW copy) pistons and 906 heads checked out to be 85cc chambers, with .050 head gaskets in my 440. The compression ratio came out to be 10.87 to 1, and I run it all day long on super unleaded with no issues (Comp Street Roller 308R). I think that my cam bleeds off a lot of cylinder pressure at lower rpms...

[73Runner4spd]

I wish he had more info on the pistons so I could tell how many cc's the pistons were. Will this affect the heads I can use at all?. I have never worked with dome pistons before so I am a little new to this haha. And you haven't done anything related to putting hardened seats on the 906's?

[cudabob496]

Originally Posted by 73Runner4spd

I wish he had more info on the pistons so I could tell how many cc's the pistons were. Will this affect the heads I can use at all?. I have never worked with dome pistons before so I am a little new to this haha. And you haven't done anything related to putting hardened seats on the 906's?

Putting in hardened seats is not that important. Didn't have them on my built 440 with 915 heads. Heads were ported with bigger valves, and not having hardened seats caused no problems.

[73Runner4spd]

So I ended up picking up the motor for $700. The pistons had some carbon build up but they are dome pistons claimed to have 11 to 1 compression. It has a mopar purple came in it. I believe it to be the .484. The 906 heads it came with could def use a valve job But that is ok as I am probably going to end up putting Stealth aluminum heads on it. The cam appears to be brand new but I will still probably swap it out if other cams would respond better to the Stealth heads and the compression. With that being said, do you think those heads would mate well with the pistons I have? and which cam would mate best with those heads? I will also be putting a carb on it similar to the carb the guys from mopar magazine used to obtain 455 hp and 456 ftlbs on a relatively stock 383 with bolts ons and cam. I have a set of hooker comps going on it as well already. Thanks guys. I feel like such an idiot when it comes for big block mopars. To much research going into my sbc build -_____-

[bulldog426]

now with some dome pistons i know you can't use close chamber heads because the dome will hit the head... and the 88cc edelbrocks are open chamber and the 84cc are closed

[73Runner4spd]

Originally Posted by bulldog426

now with some dome pistons i know you can't use close chamber heads because the dome will hit the head... and the 88cc edelbrocks are open chamber and the 84cc are closed

Do you by chance know what size the chambers are on stock 906 heads? I would like to be able to gauge about what I should have for compression for the 88cc edelbrocks. Also I took the pan off this morning and noticed that the crank had been cut so I'm assuming this is a little 438 stroker and I'm not quite sure how I feel about that. Regardless I'm going to have the crank magnufluxed.

[Mroldfart2u]

Best i could find is appx 90cc stock. (not having been milled)

[73Runner4spd]

Ok, here we go, here's another beginner question. I pulled the pan off and it looks like the counter weights were ground down a little bit. Along with some serious drilled holes. I'm assuming its drilled for balancing. So what I'm not sure of is would they have done this to a 383 crank or is it possible that its a 440 crank? Thanks