johnt365

I went from stock pipes with turbo mufflers to 2.5 in pipes with an H and Straight through mufflers and I need help retuning. It feels like I have a miss now at low rpm. Off the line it is rougher.

My motor is a 355 with comp XE268H cam, 60cc performer heads and intake and Holley 600, with long tube headers. Its an automatic.

Before the new exhaust it was running very good. Timing was 14 initial 22 mechanical all in at 2500. Jets were 65.. A/F gauge read 13.2 at idle/cruise and 12.5 wot. I was in ported vac adv.

On the way home from the exhaust shop it was popping a lot and backfired some as well.

I have changed to 68 main jets and that helped the popping and backfire. I changed to manifold vac adv but have not driven it to test.

What has less back pressure done to me and what should I try to do to fix it?

cv67

what do you mean by rough off the line?
Hesitating? Bog?
Could something like a plug wire have been knocked off?

johnt365

By rough I mean the motor actually shakes the car and vibrates more than before. It smooths out as rpms increase.

Two more bits of info. The car has HEI ignition. And I swapped the transmission a couple weeks ago and went from a 2800 to a 2000 converter. Car has 3.55 rear with 200-4R trans.

I will test drive in the morning with results of manifold vacuum advance and check the plug wire connections and pull some plugs to inspect.

My fear is that the free flowing exhaust has had a negative impact on low end performance.

gcusmano74

I changed over to Magnaflows and 2 1/2 " pipes, plus an H pipe last year. It seemed very lean afterwards. I bought an A/F gauge to recalibrate the carb. Now it runs much better.
What kind of air/fuel numbers are you getting now? Use them as clues to adjust your mixture.
I am running about 12.5 on WOT, and from 15.5 to 14 on a cruise, depending on the speed. Seems to head slightly towards rich as the engine speed (and load) increase. Dunno how.
Idle is a bit unsteady. Dunno why. Can float between about 13.5 to 15. Starts out a bit rich and then leans out over time.
I have a Quadrajet on a '74 smallblock. #74 primary jets, #39 primary rods, DA secondary rods.
The improved exhaust flow is really going to help, but first you will have to retune your carb settings to the new situation. You have to embrace the idea that everything is part of a system. It all has to work together. Change only one thing at your peril.

cv67

The exh sizing imo is perfect dont think that hurt anything should gain everywhere.
Somethign going on somewhere else.

johnt365

Okay, that is what I need to hear. I have read so many theories on exhaust size and "too large kills low end torque" that I got stuck.

Plugs look dark brown. Tomorrow I will see what manifold vacuum does and get back with some Current A/F numbers.

Ibanez540r

Did the new exhaust include headers? Sounds like a plug wire could be loose, or a plug maybe got bumped and is cracked, etc.

cardo0

If the exh shop got to joy ride the car even for a few hours who knows how bad they drove it and at what RPM?
The reason i ask is that u said it was running very good before and i read u know how to tune your car.
If they over reved the car into vlv float the vlvs and springs could be ruined.

Sorry if i sound pessimistic but i know from experience what a small shop will do to a performance car without the owner in the car.
cardo0

jimboscarbs

sounds like you have a plug wire off or not seated all the way?

or as Ibanez540 suggested a cracked plug (thats my vote)

or something amiss in the ignition or valve train

Pull one wire at a time off until you find the dead cylinder

then work from there

Gas, Fire Compression

that is all it takes to make it run right

take one of those away and its down hill from there

The reason it seems to get richer is the volumetric efficiency decreases the faster your engine turns that is why the next statement will make sense after reading this one



after you get your issues worked out if you lean it up in the upper RPM ranges it will pull harder until the cam lays over...

Depending on the application I have run them as lean as 13.8
with a really good ignition you can run them leaner than that

Generally you will neve hurt a street driven motor because you never run it hard enough or long enough in the upper RPM ranges to hurt it


there is way more to this equation than this but this is the short of it I will try to dig up one of the old post I wrote aout troubleshooting caruburetors if I can find it I will post it back here.

just food for thought more often than not ignition problems are mistaken for carburetor problems

Are you still running the stock Hei module?

Good Luck, Jim

ezobens

Something happened at the shop-
You shouldn't have such a drastic change in engine behavior after a simple exhaust change.
You could have gone from a 1.5" single exhaust to full 2.5" duals and not experience the issues you are having.
Something got botched somewhere...

johnt365

This is all good to hear. The car had the headers before, we just replaced pipe and mufflers. I stayed with the car the entire time and was under it with them too. Whatever happened is going to be my fault somehow. Its raining a bit this morning...

MotorHead

Finally but the ol wive tale will never go away, once you build a motor correctly you should have more HP than TQ this is how it all started, especially on rebuild with old smog engines that has more TQ than HP. Once you rebuilt it right the HP and Tq will both go up with the HP higher than the TQ adding to the reasoning that I must have lost TQ, did you lose any TQ, no.

A Corvette should never be left alone in a shop unless you talk to the shop owner and tell him no one drives the Vette, especially just for an exhaust job. Then I put a yellow sticky of the mileage directly on speedo glass, nothing worse than going to pick up your car and seeing go by you at 100mph with an 18 year old kid driving it.

Happened with my '70 Cuda and it has never happened again. Could have been what happened with yours, some kid got his hands on your keys and f'ed your Vette up, changing exhaust isn't going to do what you described.

The Oil Changers shop near me where I take my daily driver told the employees they were not allowed to drive the customers cars any more from the front around to the back, which should take 30 seconds, my Turbo PT Cruiser showed up 5 minutes later, they take them out to the road and beat on them and then take them to back where they enter for the oil change

I got the clutch replaced many years ago in a transmission shop, told them no one drives it, even to test the clutch. They can set it close enough that I drove it home and fine tuned the clutch setting there

cv67

happened with my DD yrs ago getting tires put on guy was burning rubber in it. Went "shining" on him.


Took my Camino in to get the tires balanced told Hector hes not pulling it in anywhere.....So he stood in front of the rack crouched down watching while I nosed it in to the rack. asked him if I had room to clear the arms....yes yes boss!....So i pull forwards and BANG! Car hung up on them sitting on my brand new headers. I dont even want to see if they are flattened or leaking
Boy was I fn pissed...if he had done it dear god.

When I was youger worked at a lot of shops.

Dont leave your car there if at all possible is all I can say

garygnu

maybe put 66 jets up front ,and try some new plugs.what was your vacuum reading before the new exhaust ?whats your vacuum reading now in in gear with your foot on the brake ?mite need a different power valve .

cv67

I bet hes got an ignition/grounding issue of some sort.

Think the welding coulda done something to his ign where hes not getting full voltage or made something wacky? (ballast resistor?!)

Id leave the carb alone for now

johnt365

Vacuum is around the same. Tomorrow I will try to isolate by pulling plug wires one at a time and I will reinstall my old HEI dist too.

I was there the whole time they were working on the car watching. I think the welding messing up the ignition is a good idea and I also fear a failed cam.

Thanks for the ideas.

LastC3AZ

I'm certain you will see some minor differences in low end torque with the bigger pipe ... but 2.5" should take care of you fairly well .... some concern with the head size ... but again you should be able to tune around it ,... all of this said ... should not be acting like it is just with this change .... you;ve definately got something else going on .... as stated above .... would certainly do an overview of all things related to ignition .... mite also check you grounds underneath as well .... you mite have pulled and/or burned something ......

sly vette

I'm guessin' an ignition problem. How would your cam go bad with a simple exhaust mod???

johnt365

I swapped out another HEI module and went for a ride. It seemed smoother at low rpm and had good power of the line (burn out at stop light). I have run out of day light and tomorrow need to try and get the carb tuned .

I hope the module fixes it.

RIJVETTES

Your problem sounds like the ignition system should be checked first, carb second.