Off Roading Forums banner
1 - 20 of 29 Posts

·
Registered
Joined
·
672 Posts
Discussion Starter · #1 ·
All right, here I need some suggestions. I've noticed a lack in low end power (and many of you may remember me posting on this). I finally got into the shop to ask about a x-over pipe to link my duals. The muffler guy (which everyone tells me, "knows his stuff") said that the power loss isn't from a lack of equalization, it was from a lack in back pressure (i.e. too high flow intake & exhaust). He told me I could either add backpressure (dumb) or I could build up the motor to use the high flow. He suggested power chip or cam. I WAS thinking in the future I may add headers, but I'm guessing here that would be a bad thing!... Anyway, I've looked at jet chips and hypertech chips - they all say I have to use 91 or better octane. I'm thinking not so good. I asked him on this and he said I could find some chips that didn't need higher octane gas. TO MAKE IT SHORT, can you make any suggestions on chips, or cams, or anything else? Thanks in advance!
 
G

·
Well, how's the top end? If you have minimal backpressure it should breathe quite nicely at say 3500 rpm +

I don't remember your previous post, did this problem begin after installing a new, larger exhaust system?

I know too little backpressure can and will kill low end torque. I actually had a buddy just recently w/a BMW have a new system installed w/something like 3" pipe. It killed his power off the line, and he took it back and had another sytem installed w/2.5" or 2.75".

I know nothing about equalization, sorry.
 

·
Registered
Joined
·
6,842 Posts
i still dont buy the "backpressure helps torque" argument. having a pipe too big WILL kill low end power, but thats because you dont get the siphoning effect of the gases as they go down the tube because they arent packed in there tight enough to get suction. a cat or muffler will not help low end power, i dont think. to me, it seems that a 2" straight pipe should help the low-end torque tremendously, but at the cost of high rpm flow. a cat or muffler would restrict the high rpm flow (bad) but wouldnt add to the siphoning effect, so i think they are useless except for thier obvious uses. does that make sense? any exhaust experts on here to set me straight?
 

·
Registered
Joined
·
2,280 Posts
Looks like stock exhaust manifolds....
What size "duals" are you running, and what mufflers?
What arragement is coming out of the mufflers?
 

·
Registered
Joined
·
672 Posts
Discussion Starter · #5 ·
Starting at the intake...
K&N replacement filter
Helix throttle body spacer

Exhaust...
w/ Stock manifolds
and 2-1/2" pipe.
The only time they meet is at the O2 sensor.
2 high flow cats
2 "turbo" mufflers (don't know much about these, but they sound real nice)
2 2-1/2" exhaust tips at 45degrees.


I have NO power until 2100-2200 rpm. Then it takes off and is great until about 4000 rpm (except for pulling hills in OD, which is what I was trying to fix in the first place). It's the funniest thing, I can hold my foot at a spot on the gas pedal and it will creep up to 2100rpm then all of a sudden it gets louder/deeper and starts to accelerate at about twice (or more) than what it had been, all without me moving my foot a bit. Its almost like something "opens" up at that point. Also, flooring it at low rpm seems to hurt my acceleration more than it helps. Half throttle does better at that point. Man, when I got the bronco - it would suck you back in the chair, but had little top end.

Can anyone explain this, or tell me what I can upgrade to fix it? Thanks
 

·
Registered
Joined
·
2,508 Posts
Muddy, everybody that has posted is right. It is just a matter of scemantics. Call it back pressure, whatever, I like to think that what you are after is the proper size matchiing of the pipes, and other good components in good condition, that will yield the best results. In this case 2 2.5" pipes is too much for a stock 302 to develop good low end torque. Dual 2" or better yet a single 2.25 or 2.5" would work well.
 

·
Registered
Joined
·
1,034 Posts
I'm taking it that you have a mass air system. If so, my recommmendation for a cam would be a RV cam or Comp cams 4x4 extreme (not sure if they make one for the 302 though). Those cams would be your best bet for low end torque, but I don't know if they would be able to solve your problem by themselves.
 

·
Registered
Joined
·
672 Posts
Discussion Starter · #8 ·
Tell me more about these rv cams, please. Where do I go online to find/order them. About how much do they cost, and what all is required to install them? I know absolutly nothing about cams, except for the fact that I have one, right? Will this increase or decrease or not effect my gas mileage? I figure I'll go for the cam instead of downgrading my exhaust. And yes, I have the MAF system, which by now I figure is a good thing. Sorry for all the q's. And thanks for all the help!
 

·
Registered
Joined
·
1,034 Posts
I would talk with a reputable local shop about an rv cam. When I got my 302 stroked they slapped in a rv cam. I couldn't tell you anything about the specs except that it cost me $70, I should have asked for a spec sheet or something. A local shop should be able to help you choose a cam and then install it. All I did was tell them I wanted as much low end torque as possible and they hooked up the rv cam. Sorry, I don't have a part number or anything.
 

·
Registered
Joined
·
672 Posts
Discussion Starter · #11 ·
Do you think that it may not be working, or even worse may be hurting my low end?
 
G

·
i seem to have the same problem as you, no low end but it sure flys above 2400 rpm i have a 2.75 cat back from gibson with evertything else pretty much stock. the truck acted this way since i purchased it so i did the normal tune up =no change....found out they had a new ecu so i forked out the money =holds od better on hills but did not help the low end. then the converter started to whine, i thought yes finally the culprit! new converter and trans =no change, i did get a chance to talk with a tune up tech at ford and he said he'ed lay money on my 7&8 sparkplug wirs were cross arching"?" i haven't had a chance to change the routeing but he said that it has to be exactly as the diagram shows or it will cross arch. maybe this will help you. when i get time i'm going to make sure i did it right, but i usually fallow directions pretty well.
 
G

·
What he's talking about with "cross-arcing" is that the electrical "pulse" (for lack of a better term) that's going though the wire from your distributor is bleeding through the insulation on both wires, and causing the 7th or 8th plug to fire out-of-turn (although, if i remember right, the 7th and 8th cylinders are on the opposite sides of the engine...) and with a weaker-than-normal charge (basic inductance...you get 2 wires close together and flow electricity through one, you get less charge from the source wire, and a slight charge from the non-sourced wire) although this shouldn't be happening with a set of good plug wires.....
Just my thoughts...any one to correct mis-conceptions?
 

·
Registered
Joined
·
398 Posts
The 7 and 8 cylinders are next to each other,drivers side last 2.They fire after each other.This is what causes the problem when the wires are touching or not routed correctly.

Billy
 

·
Registered
Joined
·
2,508 Posts
I would stop trying to put blame somewhere it is not. Do the wires, BBilly just told you where they are and then report back. A stock 302 with a single? 2.75" pipe is probably too much. Duals, way too much. Everyone keeps buying into all the hype that Gibson and others spew out. Bet you have great power after 2500-3000rpm, but look what you lost.
 

·
Registered
Joined
·
672 Posts
Discussion Starter · #17 ·
So then, what do WE do to fix this problem? In other words, what do we add to be able to take advantage of the high flow exhaust? Is a cam the best fix? Or would it be to our advantage to decrease our exhaust
? Thanks for all the help!
 
G

·
Re: Wrote another book

First, you need to figure out a goal for the engine. What power band do you want it to run in. Low end grunt, High RPM power, etc... Once that is figured out, you can then get the right components. All of the different systems for an engine (induction, valve train, exhaust, combuston) have specefic working ranges in the RPM band and they need to match. If you put a monster cam into a factory engine, its not going to run good because the intake manifold has a working range of idle-5000 RPM, but the cam has a working range of 3000-6500 RPM. Your manifold is done before the cam even starts and that is a problem.

Exhaust is the same. Pipe diameter plays a big role in what working range the exhaust system is in. Small pipe diameters work in lower RPM bands to the small size keeps the exhaust gas velocity higher. The problem lies in that When the high RPM power comes around, the pipe is just physically too small and cannot flow enough. Now you slap a big exhaust system on, it will g\be great for mega top end power, but there is little velocity for low RPM use and the gasses will stagnate in the pipe and cause a back up.

Other things play a part as well. Valve sizes and head port size. Big valves will work better for high RPM, but small valves will flow better and smoother at low RPM. Small, long head ports and intake runners work better for low RPM use because of that gas velocity thing like with the exhaust. Cam timing in relation to the crank will also change things around. An advanced cam will move the power band lower in the engine's RPM range and a retarded cam will move the power band higher. This is not to be confused with ignition timing.

Basically, I just wanted to show that there is a bigger picture than just putting parts on and making power. You got to know what parts to put on. If you are looking a particular part, study the specs of it and see if it's working range fits the goal for your engine. If anybody has questions, let me know and I'll go into as much detail as you want.
 

·
Registered
Joined
·
672 Posts
Discussion Starter · #19 ·
Re: Wrote another book

All right, I want some grunt off the line and SOME top end. Probably a power band of 1500-4000rpm (-5000rpm preferably, but I can't be greedy). Really, what I would like on low end is to be able to at least bark the back 2 (I dream of squalling all 4, but I don't see how that's gonna happen without a motor swap).

Now, if I slap a supercharger on, will that be the layman's quick (but not cheap) fix? Or would I need headers, flat heads, bigger valves, etc?
 
G

·
Re: Wrote another book

When you start huffing an engine, turns it into a different ball game.

A good basic cam with little or no overlap works the best. A lot of the bigger cams have too much over lap and while they work, a lot of the efficiency of the supercharger is lost. Stock cams work pretty well as a starting point and most manufacturers make a cam specifically for supercharged engines. Up your exhaust system size since there will be so much more exhaust gas to get rid of. A supercharger typically adds about 1.5 times the cylinder volume. I personnaly wouldn't worry about bigger valve sizes, but do get a good three or four angle valve job and port/polish the heads. Gasket match intake and heads as well. This will keep the supercharger efficiency up. With good ignition and fuel control, you will see plenty of power.

Just to satisfy everybody, you can get more wild, but that comes at a price. How fast can you afford to go?
 
1 - 20 of 29 Posts
Top