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I would expect a 2.5" Y-pipe to be a significant restriction for a single exhaust at 300rwhp levels. Even going to a single 3" would net a change in area from 4.91" to 7.07": a 44% improvement in flow capacity!
Max power AFR ratio's are in the 12.5 to 12.8 range with gasoline, stoich is 14.7 but that will not produce max power. Make sure you understand your tuning program, tunercat shows max timing in the timing table and that includes the base timing of 6 degrees. Not all tuning files with tunerpro has a few different tuning files available and some show the total timing and some only show the advance amount without the base being displayed. you would want to make sure your is displaying the total advance. 29 degrees seems too low for max power, I would expect 32 to 36 degrees to be the sweet spot.
Hedman 68487 or 68447. I have the 68487 headers. Has that special coating. Don’t forget to get a collector with an o2 bung. Mine didn’t come with the CARB EO sticker. Ended up emailing hedman for one.
nice, what is that like 290 hp or so it looks like? not bad for rear wheel power, how much you go into that set up? it looks like you run out of steam fast at around 4500, typical .. you should consider a holley track warrior carb for some serious power
So I have my timing coming in at 34 at 2200 rpms. My AFR starts at around 12.5 and leans out at the top end around 13.2. I still have my timing at 29 past 4800 to avoid knock retard. Definitely feel more go power. Tires want to break loose easily. If I had more time I would have played with all this before my dyno run. But have been working a lot so time hasn’t been on my side and that dyno run was by appointment. I have no knock counts, I’m tempted to try max timing of 36 maybe.
nice, what is that like 290 hp or so it looks like? not bad for rear wheel power, how much you go into that set up?
Yeah 296 hp. Wow I never thought about till now. Heads, cam, superram was bought in three parts upper, lower, and throttle body. $4000. That’s not including when I rebuilt the block 9-10 years ago. Probably a $5000-$6000 engine. I know my tune was not right. I’m darn sure it has to be over 300 hp now. Next time I can spare it I’ll put it on there again.
Last edited by kitttransam; May 2, 2019 at 07:46 PM.
I had the same problem in Massachusetts until they stopped emission testing pre OBD2 cars.
I have a supercharged ZZ4 crate engine and with a stock type Dynomax exhaust I measured over 15 psi of backpressure at the right oxygen sensor fitting, full throttle before shifts.
After a long discussion at a referee test station, he said yes to Carb EQ headers and he didn't care much about the rest of the exhaust, but the converter had to be stock (or legal stock replacement) and in the stock location.
Put on headers and fabricated two 3 inch pipes. Took a 6 inch pipe and walked it thru my shop press with a 3 inch square die inside to make an oval pipe. Removed the heat shield and cut the converter front and back to match the oval pipe and reinstalled the shield. Ran that oval pipe back to mate up to the stock type y-pipe and Dynomax mufflers. Measured about 6 psi of back pressure at the collector, at the shifts with this and passed emissions for about ten+ years. The converter was still to restrictive but the pipes helped a lot.
Now I have dual 3 inch bullet converters and dual 3 inch exhausts back to 2.5 inch Magnaflow mufflers, and barely register 1 psi at full throttle shifts.
