300 motor mods
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Junior User
Joined: Apr 2000
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300 motor mods
I have a 1980 F-150 with the 300 straight six. It has a lousy ONE barrel carb, and a very restrictive exhaust manifold, also it has the four speed with granny low. I want to put a small 4-barrel setup on it, cam, and full exhaust. This truck will mostly be used off road. Has anyone done this and is it worth the $1000 dollars that it is going to cost me??
Fleet Mechanic
Joined: Nov 1999
Posts: 1,415
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From: Lower 48
300 motor mods
I think if I were doing it I'd go with a split exhaust off of a newer 6 and go with dual exhaust. JC Whitney list a 4 bbl intake for those engines. I would also stay with around a 390 4 bbl. Check out the tech section. There's a van article that covers this.
Trail Rider
Trail Rider
Junior User
Joined: Jun 1999
Posts: 76
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From: Vail, Az
300 motor mods
The article in the tech section is one of mine.
The intake manifold in the JC whitney catalog is a Offenhauser dual plane intake. There are 2 sets of runners. One low speed (small diameter) for the the primaries and a high speed (large diameter) for the secondaries. Being in a van I ran into problems you should not run into. One of which is fitting headers in there. I have a set of Clifford (non emision) headers for a 300 I-6 that will fit a pickup (I think) but not my Van (frame rails are too narrow).
The low speed runners are smaller in diameter which will increase the speed of your air/fuel mixture. This helps bottom end. The High speed runners are larger which will allow more (volume) air/fuel to get to the cylinders. I am only using the low speed runners. I have a night and day difference in power than my old 1 barrel did. I can tell when I get to 3000rpms. At 3000rpms I am trying to force too much air/fuel through the low speed runners. You will not run into this problem if you use a 4 barrel carb. I had to use a 2 barrel holley since a 4 barrel would put the 2nd float bowl into the firewall. I am going to change to a Quadrajet and use that as my 4 barrel carb since the float bowl is on the sides and not the front/back.
As for an exhaust... get rid of the stock exhaust if you can. The only reason I kept mine is I had to make emision equipemnt. If the headers would have worked I would have had o2 sensor connections brazed onto the header to allow my Air injection and EGR to work. Since they didn't fit and I didn't have the time/money to buy a split exhuast manifold I just used my old one. The hardest part is still getting the intake/exhaust connection to seal (just like you do with the stock intake).
If you want some more power (low end grunt) I would put in some type of RV cam. The 300 motor is a torque motor.. not rally HP.. and most off-roading is torque.. not HP (except mud)..
One thing is for sure.. no matter what you do.. the 300 I-6 is a rock of a motor.. When I blew the head gaskette between cylinders 2 & 3 I had to drive 130 miles on the freeway.. It would barely idle.. but I could maintain 70 on the freeway.. Just to give you an idea how bad it was.. my exhaust charge was coming OUT the carb at idle..
If you have any specific questions on the Offy intake/Carb etc.. just let me know..
Mark
mark@legendz.com
http://mark.legendz.com
Off-Road & Motorcycle Information, Pictures, and Movies
The intake manifold in the JC whitney catalog is a Offenhauser dual plane intake. There are 2 sets of runners. One low speed (small diameter) for the the primaries and a high speed (large diameter) for the secondaries. Being in a van I ran into problems you should not run into. One of which is fitting headers in there. I have a set of Clifford (non emision) headers for a 300 I-6 that will fit a pickup (I think) but not my Van (frame rails are too narrow).
The low speed runners are smaller in diameter which will increase the speed of your air/fuel mixture. This helps bottom end. The High speed runners are larger which will allow more (volume) air/fuel to get to the cylinders. I am only using the low speed runners. I have a night and day difference in power than my old 1 barrel did. I can tell when I get to 3000rpms. At 3000rpms I am trying to force too much air/fuel through the low speed runners. You will not run into this problem if you use a 4 barrel carb. I had to use a 2 barrel holley since a 4 barrel would put the 2nd float bowl into the firewall. I am going to change to a Quadrajet and use that as my 4 barrel carb since the float bowl is on the sides and not the front/back.
As for an exhaust... get rid of the stock exhaust if you can. The only reason I kept mine is I had to make emision equipemnt. If the headers would have worked I would have had o2 sensor connections brazed onto the header to allow my Air injection and EGR to work. Since they didn't fit and I didn't have the time/money to buy a split exhuast manifold I just used my old one. The hardest part is still getting the intake/exhaust connection to seal (just like you do with the stock intake).
If you want some more power (low end grunt) I would put in some type of RV cam. The 300 motor is a torque motor.. not rally HP.. and most off-roading is torque.. not HP (except mud)..
One thing is for sure.. no matter what you do.. the 300 I-6 is a rock of a motor.. When I blew the head gaskette between cylinders 2 & 3 I had to drive 130 miles on the freeway.. It would barely idle.. but I could maintain 70 on the freeway.. Just to give you an idea how bad it was.. my exhaust charge was coming OUT the carb at idle..
If you have any specific questions on the Offy intake/Carb etc.. just let me know..
Mark
mark@legendz.com
http://mark.legendz.com
Off-Road & Motorcycle Information, Pictures, and Movies
