View Poll Results: Primary size for street 347 for quick spool

1 1/2''

1 5/8''

33.33%

1 3/4''

50.00%

Stepped 1 5/8''-1 3/4''

16.67%

Voters: 6. You may not vote on this poll

347 tube header primary size

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Oct 22, 2009 | 10:46 PM

lazylongboarder

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347 tube header primary size

What primary size is optimal for a street driven 347 with twins. Looking for 750 rwhp with meth. I'm just about to buy a "U-weld" kit off egay and wanted to get my FI build going. I'd like as early spool as possible. QUICKEST SPOOL POSSIBLE.

Can stepped primaries be beneficial? My current headers are 1 5/8'' hooker shorties i'm planning on cutting up and using the flanges off them.

http://cgi.ebay.com/ebaymotors/1-3-4...Q5fAccessories

I'm new to FI and wanted to ask the experts.

BTW...no, i won't be using truck manifolds. I need the turbos to sit up by the valve covers and don't have the space for them.

Thanks

Oct 23, 2009 | 08:28 AM

Phil99vette

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1 5/8". I used 1 3/4" on a 438 and had great results.

Oct 23, 2009 | 09:08 AM

Shearer

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1-5/8". 1.5" will make for great response but will become pretty restrictive higher up in the power band.

Oct 23, 2009 | 11:41 AM

lazylongboarder

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Quote:

Originally Posted by Shearer

1-5/8". 1.5" will make for great response but will become pretty restrictive higher up in the power band.

I would think 1.5" would be great for a street 347 and certainly flow 750 rwhp, truck manifolds are obviously proven, what are the primary's and collector's size on those? Collector should be 2.5, right?

1 5/8'' should be the popular vote imo. That's what i voted even though i might do 1.5''. I would do 1 3/4'' only if i was looking for 900+ rwhp.

Keep it coming. Thanks guys.

Oct 23, 2009 | 12:51 PM

Josh@KY-Turbo

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1 3/4" for street strip.
1 7/8 or 2" for full tilt race cars.

Oct 23, 2009 | 01:25 PM

Shawn @ VA Speed

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for a small inch motor 1 5/8 is perfect,i wouldn't go to a 1 3/4 unless it was atleast 400cid

Oct 23, 2009 | 08:47 PM

lazylongboarder

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Great. My current headers are hooker shorty 1 5/8'' with a 3'' collector. I'll probably cut these up to save some dough.

Other opinions welcome.

If 1.5'' will give me better spool without killing top end i'll do it. But it sounds like 1 5/8" are the winner. I just can't get over how stock manifolds run >9's consistently on these boards and the primaries on them can't be bigger than 1 1/2''.

Has anyone used 1 1/2'' tube primaries on a F/I LS motor?

Thanks

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