1978 bronco- rebuilt c6, regeared with 4.10's. running 31 inch super swampers in local mud races(street stock class) The engine- 400 bored .030, TRW forged pistons in stock configuration, ARP rod bolts, fully balanced, aussie cylinder heads with all new hardware including stainless valves and roller rockers, factory exhaust manifolds with dual exhaust and glasspacks, factory intake with EGR passage removed from bottom of intake and holes welded closed, autolite 2v carb that has venturis bored out and set up to use holley jets. The reason for the factory intake and exhaust manifolds is for rule requirements in the street stock class, this truck does not get driven on the road- its not even liscensed or registered. Have been tweaking on this engine for a couple of weeks now, and although its not what I would call a dog, it is certainly not very impressive!! especially considering the 4.10 gears!! I am using a Comp 268 cam due to the generous throttle aplication nature of the mud racing, but I am coming to the conclusion that I have chosen the wrong cam for this package!! I have a few more things to play with to try to get some more power out of this, but I dont reallistically see me getting the power I envisioned before the trucks first race of the season this weekend!! I have driven two other vehicles that had STOCK 400's in them that in my opinion had more punch than this one!!! Is there something I have overlooked here??? I wont have the available time to swap out the cam before this weekend, so thats out for now. But after this weekend is done I was thinking of two options--1 swap in a different cam, or 2- try a 3000 stall (roughly) converter I have sitting on the shelf. I want to try the converter, but common sense is telling me the cam is my real solution. any suggestions, experiences or comments????

 

If you are looking for low end torque, you have the wrong cam. You need a shorter duration cam or one with and earlier Intake Valve Closing angle. A Comp Cams 255DEH is a popular one. It has an early IVC angle, but the longer duration exhaust helps supply a flat torque to 3000 RPM.

I hope that you can use racing fuel. With 10:1 CR and and early IVC, your DCR will be too high for pump fuel.

Crane makes a nice Hydraulic Roller cam grind #529721 that will really boost your low end torque. However it will take more money and more work to use a HR cam.

You didn't mention any head work. You should do what you can to improve the head flow. even at low RPM improving head flow improves VE, and better VE means more torque.

 

If this bronco would have been planned for a daily driver or a trailrider or tow rig, I would have gone with the low end torque cam for shure. But seeing as how it was for mud racing, there will be a significant amount of time spent in the upper RPM's too!! I just am thinking that my existing cam just cant live up to its potential while trying to breathe through factory intake and exhaust manifolds. Would a low end torque cam be more of a team player with the stock components?? Actually the engine is currently running on pump gas--a mixture of 5 year old stuff that was in the tank when I hauled it out of the weeds and some "fresh" fuel put in last summer!! While it does ping somewhat, I dont think its any worse than my dads 79 that he bought new and drove that way for about 12 years with no signs of doing any damage, and this one has forged pistons. Besides, I should have the majority of the existing fuel used up by sat morning and we will stop and fill it with premium (91 octane here) and maybe a bottle or two of octane booster to be shure. My machinist told me he cleaned up the bowls a bit also, but it seems to me the biggest obstacle is the factory induction and exhaust.

 

The pinging will be much worse with a 255DEH cam than it is with the 268H cam. I don't think that a 255DEH cam would run very good with 91 Octane fuel and 10:1 CR.

You are right, your problem is breathing. With the 268 cam you are giving up a lot of low end torque, but you don't get any high RPM performance due to restrictions in the intake and exhaust.

A stall converter will not help without fixing the breathing problem. I have heard of people who have modified their stock intake and exhaust manifolds to reduce the restrictions.

DD2K shows your torque rolling off sharply after 3000 RPM. That is due to restrictions in your intake and exhaust. If you can't fix that, then you should try to maximize your torque below 3000 RPM. A hydraulic roller cam is your best bet. You can get more lift and faster ramp rates to give you more low end torque and maintain more power at higher RPM.

You will certainly need racing fuel with 10:1 CR and any short duration cam.

 

The are a couple of other ways to improve the high RPM performance of your motor. One simple one is to retard your cam by 4 degrees. This is the factory setting for a stock cam. it will also help the pinging.

Another is a dual pattern cam. A 265DEH will give you about the same low end power, but will improve the high end, but will not have a large effect on detonation. Retarding this cam will also help.

A dual pattern solid lifter or hydraulic roller with about 224 intake degrees @0.050" and 232 exhaust degrees @0.050" and about 0.550" lift on 110 degree centers should help.

 

Bronco had it's first race yesterday- it did pretty well on the trips back and fourth, but really struggled in the thick sticky clay of the turn-arounds. Actually, most trucks in that class were having problems fighting the deep stuff in the turn-arounds. Suprisingly, when it did lose some ground because of the turn-arounds, it managed to pull the other truck back in most of the time on the trip back to the finish line!! decided to put in the low end torque cam- most likely the Comp255. Will just run race fuel if I have to- I run it in my other mud racers anyway. If my dad decides to start driving that bronco on the street then we will just put a set of standard heads on it!!

 

If you can improve the breathing, by porting the heads, you can run a 255DEH cam and get performance at higher RPM. Also anything that you can do to your intake and exhaust manifolds to make them flow better will help. You can open and polish your exhaust manifold as much as possible. Don't open up or polish your intake manifold, but round off any corners and any sharp edges or flashing.

Let your engine flow take care of the Higher RPMs, let the cam be as efficient as possible at low RPM.

 

I will dig up another set of exh. manifolds that I have in the back of a parts truck and study them to see what I can do and check out the intake while I have the engine apart for the cam swap. Thanx for the replies. I will repost when the engine is recammed for results.