Need som advice on a 400

[troy.70R/T]

I recently purchased a 68 Charger R/T with no motor or trans. I just want to make a driver out of the car for now while I finish my 70 R/T.  The questions I have are. Does any one make a motor mount conversion kit to allow me to bolt the 1975 400 up to my 68 440 K-frame?  The motor came out of a Plymouth wagon it is a 400 2 barrel. It ran good when I bought the wagon 10 years ago. I have no experience with 400's I was just wondering what I could do to get some good power out of the motor with out dumping alot of cash into it. I plan on beating the hell out of this motor until it blows. All I am planning so far is a set of headers and a 750 vacuum secondary Holley carb. Should I put in a diffrent cam? I am not sure of the gear ratio in the rear yet but I was going to put some 4:10 in the rear. I am also using the trans from the same car. Any advice or direction to some good links would be appreciated. Thanks Troy

[idahogrumpy]

Just put the 68-71 motor mounts on the 400 and it will drop in. use the 400 torque converter, or one that has the correct balance for the 400. Grumpy

[troy.70R/T]

thankyou I appreciate the information.

[idahogrumpy]

Also 4:10 gears might be a little steep for the street with that 400 with out a deep oil pan or hemi pan and other internal oil pressure circuit mods.  Build it like a 383. Cam, Carb, etc.
Grumpy

[troy.70R/T]

any idea how many RPM's I would be running at 70 mph with the 4:10's?

[Ghoste]

I have 4:10's with 28 inch tall tires and IIRC 70mph is just north of 3000 rpm. (2750 is 55mph or thereabouts)

[Sublime/Sixpack]

any idea how many RPM's I would be running at 70 mph with the 4:10's?

Before I switched from 4.10 gears to 3.54 gears my '70 Charger (with everything factory original) the tach read right close to 4000 rpm when the speedo indicated 70mph if I remember correctly (its been 15 yrs. since I went to the 3.54 gears, but I'm pretty sure about those numbers).

[motorcitydak]

Lets say your rear tires measure out at 28 inches tall, they have a circumference of about 88 inches.
70mph is roughly 73920in/min
Your tires have to spin 840rpm
assuming that you are using a good trans that will lock up and actually give you a 1:1 3rd gear you will be spinning 3,444rpm at the crank

[troy.70R/T]

Well the trans that I am using is out of the 75 wagon as well. So would 3:91 gears be better? Any idea how many horse a 75 400 was rated at? it is a factory 2 barrel but I was going to put a 4 barrel on it. I will probably only be crusing this car in town and if I do go anywhere I will probably not be driving over 70 MPH for more than an hour. Ghoste are you happy with your 4:10 gears?

[b5blue]

Daily driver I run 3.23 and love it. 

[troy.70R/T]

Do you think a 75 400 will have enough balls to win some races wiht 3:23 geas??  I plan on running the motor hard (Having alot of fun with it)

[bordin34]

A 74 400-2 has 185hp so you should have about the same or less.

[troy.70R/T]

Well thats not very much Any ideas on how much I will have if I have a 750 Holley four barrel carb a set of headers and dual exhaust. any sugestions for a cam swap? or am I wasting my time and money with this 400?

[motorcitydak]

3.91 would be at 3281rpm at 70mph
3.23 would be at 2713rpm at 70

[troy.70R/T]

Would you post the formuls that you used to figure that out. Please

[bordin34]

Try this site, a member on here made it i think.
http://vexer.com/68rt/speed.html

[troy.70R/T]

Wow real cool. Wish I knew how to do that kind of stuff. Thanks for the link

[motorcitydak]

Believing in that whole "Teach a man to fish" thing...

I am sure that there is a fancy formula out there somewhere, but I do not know it. I just do it all by simple math. First, get the height of your tire in inches. You then need to know the tires diameter. You can do this by wrapping the tape measure around the tire itself or do the math on it. The tires circumfrence gives you the distance that it will travel in 1 revolution. In the case of a 28 inch tire, that is 88 inches around.You get that by the simple formula of pi(3.14) x diameter = circumfrence

Because RPM is obviously in increments of 1 minute you have to then change miles per hour into a distance travelled per minite. Since we measured the circumfrence of the tire in inches, you need that to be inches travelled in one minute.

In this case it was 70mph. Divide that by 60 (minutes per hour). You get roughly 1.166666 miles covered in 1 minute. Then you have to change that into inches per minute. Multiply that number by 5,280 (feet per mile) and then take that number and multiply it by 12 (inches per foot). You will end up with 73920 inches per minute.

Knowing that your tire is 88 inches around, you divide 73,920 with 88. That gives you 840 which is your tire RPM. You then just take that 840 and multiply it by whatever your axle ratio is. That gives you the RPM of your driveshaft which should be your engine RPM also provided that your transmission is not slipping.

[Ghoste]

Of course, the old non-lockup 727's always slip a little.  I double checked mine yesterday and it was just over 3500 rpm at 70 mph which falls right in line with the figures motorcitydak has given you.

[Cooter]

A 1975 400?  Best Bang for your buck?  Screw on a Big Shot plate system, bump the timing back about 6 degrees and set it for kill.....In a 3900 LB B-Body, you might run in the 12's on a 300 Shot, till those cast piston's give up the ghost...

[b5blue]

And keep in mind it's torque off the line and H.P. top end. 

[troy.70R/T]

Well thanks for the explination motorcitydak now I can figure it out if something ever happens to that handy model that someone created. And thanks for checking your car to see if it was accurate Ghoste.

[Ghoste]

No thanks needed, it was more a case of me having the car out anyway and paying attention to the instruments at that speed for a change.  But always happy to go 70mph to help out a fellow Charger owner. 

[troy.70R/T]

Well thnaks anyway. Well I need to start another post. I have the opportunity to trade some stuff for a 67 383 2 barrel.   Thanks again for all the help. Troy