[bigsid21]

I added headers and flowmaster exhaust to my 68 302 and now i have a mojor flat spot in my low end acceleration. I dont think it backpressure bacause i didnt have the problem for a while when i was running open headers If someone has any ideas of what it could be please let me know. Thanx

 

[greggieatthebat]

Big exhaust always hurts low-end torque. You may have to play with your carb to get it right. Too much exhaust volume means that the exit velocity of the exhaust is lower, meaning you get less volumetric efficiency at low speeds.
You mention that you ran open-headers - did you have the same carb/intake/cam setup? Exhaust is a trade-off. Some restriction is ok if it's due to pipes that are a bit small (which keeps exhaust velocity up and helps empty the cylinder, increasing your volumetric efficiency at low speeds). Restriction in the form of mufflers is almost always bad (unless you have a properly tuned exhaust system).
My 302 (4v Holley, Performer 289 Edelbrock manifold, 1 5/8" headers, 2.5" dual exhaust/crossover, Dynomax? mufflers) has a low-end bog, but I haven't worked on my timing/mixture tuning at all. So far I've just run it as-is (as we installed it, out of the box). I'm not even using the vacuum advance on the distributor right now. There's a lot of power/throttle response/fuel economy to be had from an afternoon of tuning, if I ever get around to it!
I suggest doing a quick timing/carb tune to see if you can buy-back some of that low-end oomph.

 

[Royce Peterson]

Try turning the distributor clocwise about 1/16th turn, it sounds like you have timing retarded slightly. Keep turning the distributor in 1/16th increments until you get pinging or the car is hard to hot start. Hesitation is usually a timing issue. Be sure your ignition is working right before messing with the carb.

 

[bigsid21]

The problem was much worse untill i did check the points, timing, and carb. Before i did this the flat spot lasted much longer through first gear, but now that everything is tuned right it's running better but it still has the flat spot.

 

[Royce Peterson]

Did you set timing with a light or by turning it until it runs best?

 

[bigsid21]

I first check with the light to see about where it was, and then turned it without untill it sounded the best.

 

[leonbray]

Make sure you have the vacuum advance running of ported vacuum rather than straight off th manifold. Manifold vacuum will give too much initial advance and create flat spot

 

[bigsid21]

It was set up off the manifold to begin with, but we changed that and it did make a big difference. It's almost like the engine isn't making enough power to get the car off the line, but it's just fine at mid and high speeds.

 

[sixt9coug]

i still think the exhaust is causing that... when i had my 351W converted to duals with Flowmasters.. i lost some low end torque. i gained plenty more in the mid and high range but low end was off. with the single exhaust a little blip would get tire scratch, but it took more after the duals. i did notice that after teh headers went on, i gained some back. soem further tuning did make a difference, and while i had the spacer on the car.. it seemed to help it rev alot cleaner. my advice to you?? do what i plan... better rear end gearing....

 

[greggieatthebat]

BigSid- Is your setup otherwise stock (stock cam, stock torque converter the assumed auto tranny)? When I did my upgrades, my "expert" friend recommended a torque converter with a 2000 rpm stall, but after it was all together, he said he "couldn't even feel the cam". I think I know what he meant, but his calibration is skewed, as he's used to dragging big-block chevy II's (he used to race Ford's, which did help us a lot when we were initially getting the car fired).
Point of this ramble: cam selection, torque converter, and rear-gear all go together to put the oomph where you want it in the rpm curve, you may have to mess with all of the pieces.

 

[bigsid21]

everything in the car is stock, and my next project was to put a new cam in and change the rear end along with roller rockers and a new intake. I just wonder if the problem has to do with something other than these parts because i dont want to change all of those out and still have the flat spot. It's basically guess and check and that rings up the $$