300hp will never happen out of a 3.9

 

What transmission do you have? Auto? 42RE?

I think you said you had the 8.25" rear as well.

 

42RE, 8.25 but I plan on a shift kit and upgrading the rear end to a 9" with 5 lug.

 

OK, I was wondering if you'd considered an upgrade.

 

won't happen NA without custom pistons/rods/crank. 300 HP from a 239CI motor is 1.25 HP per CI, equiv to 400 from a 318 or 450 from a 360.

Per my post above, your $3,000 in mods will only get you to 215 HP.

With fully ported heads, 11.5:1 compression, a custom single-plane manifold, and .515 of valve lift, you're only talking 249 HP at 5500 RPM.

Now if you call Bullet racing cams, and ask very a VERY aggressive cam (that won't be very street friendly), you could hit 301 HP on the money at 6,000-6,500 RPM, if the crank held together. If you switch the truck to pure race and put it on methanol, the claim is 320 @ 6500.

Here's a way to look at it: There are tons of 318 pistons out there. I'm sure you can get a raised-dome high compression piston made to fit the magnum heads and still have .200 of valve-piston clearance. I highly doubt it will be cheap, though.

 

I don't know if you've already told me but What size valves should I run?

 

The valves have to fit the heads - you can't just put in bigger valves. The seats have to match. There is only so much that can be done to a stock head.

 

Your putting a ton of money into an engine that's not designed to take it and all these changes will make your engine vulnerable to failure compared to stock OEM.

It may run like crap as the PCM will not understand all the modifications you are planning.

I suggest rethinking this


A suggestion only

 

I can match the valve seats to the valves.

 

I don't think there is room for bigger valves. It's a very tiny combustion chamber.

edit- I just looked it up. The biggest valves that will fit the 3.9/5.2/318 head is a 1.88 intake and 1.60 exhaust, which is the stock size for a 360, and plus 1 tenth per valve over stock (1.78/1.50).

Read this. The 3.9 is a 318 minus 2 holes. They only got 330 from a 318, you'll be hard pressed to get 300 from 6 holes. http://www.buschis-place.de/mopar_31...erformance.htm

Per Wes Garret: