Exhaust Replacement

[rpounds]

Took the '75 down to the local muffler guru today. My vette had true duals with 2 1/2" tubes all the way to Flowmasters at the back. However, since this is California and we have extremely strict emissions regulations, I had to put on a Y head pipe into a Catalytic converter back into a Y to the dual Flowmasters. $350 later, it is all taken care of, including a straight through design Cat.

Funny thing is, now it has MORE power than with my original true duals. Yes, it is a little quieter . . . naturally. However, the power increase is not an illusion brought on by less noise. It really does have more low end torque than before the switch.

I haven't really sat down to think about all of the whys and wherefores . . . so maybe someone will chime in here and tell me what you think the difference can be attributed to.

 

[KenSny]

I've never heard anyone say they have more power with the 2-1-2 and cat system installed after true duals.....

When I pulled off my 2-1-2 system it had an aftermarket cat on it, so it probably wasn't as restrictive as the original. Anyhow, I put on 2-1/2" duals from the OEM manifolds back to the OEM mufflers. When I got it back on the road, it really did perform better, by a lot.....

Maybe something was plugged in your dual system causing it to be more restrictive than it should have been?

BTW: did you re-tune the carb? It will probably need the mixture adjusted to compensate for the new exhaust.

 

[rpounds]

Ken,

I had the carb re-tuned after installing a cam and intake manifold. My mid-range (1500-3000 RPM) showed a marked improvement over the stock engine. The bog when the secondaries kicked in was completely gone. However, with the high differential ratio (3.08:1) it still left a lot to be desired off idle. It would 'break 'em loose' under full throttle off idle, but it certainly was not a neck snapper. The off idle performance is where the exhaust really seemed to do the trick. The cat that I had installed is a straight through design. From it's appearance, I would really be surprised if it creates much in the way of back pressure. From the cat back, it has all 3" tubing to the Flowmasters. So, I am assuming that the back pressure compared to the 2" duals is not that much different. The rule there would be that when you double the size of a pipe, you increase the flow by a factor of 4. I have read that using a cross over pipe or an 'X' design can actually increase the scavenging of the exhaust system. That may be the case here, considering that the cat is a 2 into 1 design. I suppose that it would work something like a venturi as is commonly used in products that are designed to create vacuum in industrial applications. I'm not absolutely sure what the answer is . . . but I do know that I am enjoying the increased power and response!!

 

[KenSny]

What brand was the Catalytic converter?

Yes, it's suprising what another 1/2" in dia. will yield in total cross-section area of an exhaust pipe....