Hi,

got my engine done:

350 sbc, 4 bolt
Holley 750er Double Pumper
Weiand Team G Single Plane Intake Manifold
Dart Pro 1 Alu Heads (mild ported, 2.05er und 1.6er Valves, Titanium Valve Springs
Harland Sharp Aluminum Roller Rockers (1.5)
Comp Cams mechanic roller Cam
Valve Lift .549 und .574
Duration@50 244 und 255
Lobe 109
diales in 4 degrees advanced
Keith Blach hypereutectic Pistons
Compression 10.5 : 1
Mallory Unillite Distributor
Accel 300+ Wires and Coil

with this exhaust:

Hooker Shorty Headers 1 5/8
after Headers all 3" single with carsound cat and hooker muffler.

ET 13.4@111mph


Now I have sold the complete exhaust and want to go to a true dual setup.
The car was crazy loud with the single pipe and I also think it was a big restriction.

The new setup will contain:
new headers
dual 2.5"
no cats
2 ultra flo mufflers 2.5" in/out

Now my question is, what size primaries do I need? I've searched a lot, but didn't find some answers.

Will 1 5/8 be to small, or won't be that much difference between them and 1 3/4.

It's also annoying, that there are only the LT from hooker and the expensive slp's left in 1 3/4. Ok, and also the weld on kit from hedman...

Now what do you think, 1 5/8 or 1 3/4 with my combo?
Will the new setup make a huge difference over the single exhaust?

Would be nice to hear from you.

Simon

 

I would go with Hooker 1 3/4 long tubes with your combo. Hedman 1 5/8 LT's would work well too and are cheaper. I've used the Hedmans and gotten into the mid 11's, I just switched over to the Hooker LT's this winter. I think the 1 3/4 LT's would be best matched to your combo considering the duration and lift of the cam especially.
I used to have a single 3in exhaust with 2 1/2in cutouts after each header(a couple years ago when I ran mid 12's). I would normally run .2-.3 sec quicker with the cutouts open so I definately think you will see some improvement switching to the 2 1/2in duals.

 

I think there was an article in either car craft or chevy high performance that compared the two, 1 5/8 and 1 3/4. the 1 5/8 long tube headers performed better on a built 350 in terms of rpm range and the torque curve. I can try to find it if you would like.

 

This header chart is from an article by David Vizard. By the way SLP now says to use 1 3/4 inch for a 350 and 1 5/8" for a 305.

Notice that the pipe is ID. The lower line is for a mild motor. The 2nd line is for a high performance street motor and the top line is for a race motor.

 

Heres that article. I guess it was the really big headers but the testing was also done on a 406ci chevy.

The Big Tube Test

This is a great example of how larger headers affect the power curve. The larger 1 7/8-inch headers lost a tremendous 47 lb-ft of torque at the bottom of the curve. By 3,400 rpm, the big headers were actually making a little more than the smaller 1 5/8-inch headers. This may be a hiccup with the smaller header power curve that could be addressed with jetting or timing had we spent more time on tuning. By 4,400 rpm, the smaller headers were making as much as 33 lb-ft more torque than the larger headers. Then by 5,600 up through 6,600, the larger headers took over and made as much as 26 more horsepower at 6,600 rpm.

If you look at the average power curve numbers, you can see that the larger headers actually made slightly less average power. That's because the horsepower gain at the top did not completely make up for the torque loss at the bottom of the curve. So what's the bottom line here? If this engine is going into a light car with a manual trans and a deep rear gear, or at least an automatic with a stall speed of no less than 3,600 rpm, the larger headers might be a slight advantage. The important point here is that a 1 3/4-inch header is probably the answer, since it would improve the low-speed power while not sacrificing nearly as much horsepower at the top end.

Another point worth mentioning is that what we're seeing here with these two primary-pipe-diameter power curves is not really as much about power loss or gains as it is about moving the curve around where the engine makes its best power. Had we tested a set of 1 ¾-inch headers, we're confident we would have witnessed an average power increase over the other two primary pipe diameters. This merely reinforces the idea that engines operate best when outfitted with the right set of parts that complement the rest of the engine package.

Test 7: A 406ci small-block Chevy outfitted with a set of Dart Pro 1 heads, a healthy mechanical-roller camshaft, a set of Hedman 1 5/8-inch-diameter primary-pipe four-into-one headers, and a 2 ½-inch exhaust using a pair of Borla XR-1 mufflers.
Test 8: The same engine outfitted with a set of Hedman 1 7/8-inch-diameter primary-pipe four-into-one headers with the same Borla exhaust system.

From http://www.carcraft.com/techarticles...st/index1.html

 

Two thoughts. One, that cam is way HUGE. 2nd is I assume you went 4 degrees advanced to try and tame the cam. Check my times with a cam with 32 degrees less duration on the intake side. Another question. How many cc's are the intake runners on the Dart Pro 1 heads?