TedH

How would 1 3/4" headers compare to 1 5/8" headers? Is that extra 1/8" ID THAT big of a difference (read, lose torque/power)?

I've freshened the 350 (.020" overbore) with a solid top end kit over the last few years. Wouldn't it 'like' that extra 1/8" ID?

Am asking as I picked up a killer set of stainless headers couple years back and had them Jet-Hot coated inside/out to boot. My hedman's are showing their age and due to replace in the next year; along with a fresh header-back exhaust.

toobroketoretire

The larger 1-3/4" headers may be beneficial on a 400+ cubic inch small block that operates at a very high rpm.

jb78L-82

Based on your build stats with .020, roller cam .509, and those Dart Iron Eagles 180's which I have heard flow pretty well, I would go with 1 3/4 headers versus 1 5/8's on a typical stock engine. With my built L-82 355 with Roller cam .525/.525, AFR 180's, and 10.2:1 compression, the builder who did my bottom end suggests 1 3/4 inch headers for maximum power. I discussed the 1 5.8 inch headers and he recommends the 1 3/4 headers based on all the engines he has built AND dyno tested on his Mustang dyno in his shop...that is pretty decent experience to me.

Kubs

1-5/8" headers are good on a stock to pretty mild 350 if someone is just looking for a little extra edge but not going too crazy.

cv67

1-5/8 are plenty for what you have

gkull

Primary diameter is only part of the header science. Primary length and collector dia. and type are also included.

I would install the nice new headers and let the cyber space geniuses debate it.

REELAV8R

I think the less back pressure you have the better off you are as headers go. Header back is important too. No good to have 1 3/4" headers and then restrict it with a stockish muffler or catalytic converter.

cardo0

Tube dia determines the rpm of peak torque. Larger dia = higher rpm at peak torque. Now is that the same rpm for a 350" motor as a 383" motor. No. If u use 2" dia pipes on an 283" motor will it ever limit peak torque rpm? No again.

Silvertone

Lingenfelter says if you build big on the intake side then build big on the exhaust side, and vice versa. I predict you'll feel a very slight loss of torque in the useful rpm range driving around on the street. That's due to lower velocity in the air column, and I am assuming you already have low restriction pipes and mufflers. The diameter difference is equal to 17% more area. I hope that turns out not to matter much. If you want them, I'd get them. It would be a good excuse to build bigger later.

Do you have a local dyno shop that can measure before and after? Nothing better than hearing about your actual experiences.

BOOT77

Too big hurts power worse than too small for exhaust. If you drive the car 1 5/8 and if you have a 5000 stall 1 3/4.

MotorHead

2 in. primaries would be good for that. Is there any difference between a 600HP small block and a 600HP big block beside the size of the block and heads ? My 427ci AFR 227's small block heads outflow many aftermarket performance big block heads.

They are both air pumps and no one would suggest 2 inch primaries were too big for a 600HP big block. So since they don't make 2 inch pri. for a small block I have 1 7/8 in which I had to design and prtototype.

To the OP the 1 3/4" headers will work fine, I had them on my warmed over 355ci and made boatloads of torque

gkull

360 ci sprint cars run 2 1/8 or 2 1/4 inch headers on their spread port heads with a short primary length. Where a 1 7/8th or 2 inch because of high gas velocity would require longer primaries to be tuned for a certain rpm.

You also have to look at the exhaust port size. If you have a 2 inch head exhaust port 1 5/8th headers are going to just choke the motor.

Bigger headers than the exhaust head port give some good anti reversion qualities.

BOOT77

I'll add, some cylinder heads exhaust ports are large/poor matched and need a larger primary to avoid obstruction, but I prefer to weld and match/taper the flange than use too large a primary on a street car. The 180 ports shouldn't be too big and Dart may even have a header size they recommend. IE world sportsman II's suggest a 1 3/4 with the square port size it needs about that large of a circle on common headers to not obstruct, but I've used 1 1/2 that were matched with great results. If they are available I suggest you should get headers with flanges that match your port shape.

Match is mostly why you'll get a wide variety of answers, because some were poorly matched and they had to go larger to get better but not best results.

Silvertone

Yep, here is a before and after. These 1 5/8" flow just great on a 486hp 383.





Before, round port.





After, D-port. Welded more metal at the top of the tubes to flange, then follow your airflow zen with a grinder.

BOOT77

^^^^^ Great example ^^^^^

Flow is highest at the top corner of the outer wall of port usually.

htown81vette

Probably stick with the 1 5/8 for a mild build, you'll get more low/mid range torque too. But if you think you might go wild with mods go 1 3/4. I have 1 3/4" on my 408" LS motor, and been told I really needed 1 7/8". But I ring it out to 7200 rpms sometimes, seems to do ok with the 1 3/4".

mitch.1972vette

I'm fixing to buy a set of full length tubes for a 383 build in the 500 hp range. All my research on the net lends the understanding that the 1 5/8" primaries are the best choice for the operating range of my street motor. If you intend to spend 70 - 100% of your driving time (with HP in excess of 500) between 4,000 and 7,000 rpm then you should go for the 1 3/4" primaries. If your driving experience will be mostly in the 3,000 - 6,000 range then consider the 1 5/8" primaries. My rev limit will be in the 7,000 range and I will make power near that point.... the smaller tubes generate that seat of your britches power below 6,000rmp and that is the wonderful tire smoking pull-your-eyeballs-back-in-their-socket power range I enjoy the most...lol....JMO....I like the American Racing SS Headers...

htown81vette

Those are good headers

gkull

header science is the way to really determine what you need. Exhaust CFM is a factor and you can go these tables. So look up your after market head flow numbers.

http://www.hotrod.com/how-to/engine/...-size-headers/

http://www.wallaceracing.com/header_length.php

MajD

Do you have a peak torque RPM target your trying to meet? The primary pipe diameter helps to set at what RPM peak torque occurs. Ideally, it should be matched with the peak torque of the intake. For a 350 running 1.75" diameter primaries, peak torque occurs at approximately 4,850 rpm. For 1 5/8" (1.625") primaries, peak torque occurs at about 4,200 rpm.

How much low end torque you lose by going to the larger pipe depends on many other things. If your engine was designed to run often in the 4000+ rpm range, then I'd go with the larger header. If it sees 3500+, then I'd probably sell the larger headers and buy a nice set of 1 5/8" tuned long tube headers instead. If you see 3000+, then you may want to consider an even smaller header. Horsepower sells cars, torque moves them.

Hope this helps.

MajD