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Discussion Starter · #1 ·
hi all,

I have a GM 350 crate engine in my '67. I currently have 1 3/4" headers going back to dual 2 1/2" exhaust. I think I have too much flow, robbing me of low end torque/performance. Should I replace the headers with 1.5" shorties into dual 2" exhaust?
 

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1 3/4" is a little big for a mild traditional350 , but it won,t be a huge difference. Long tube traditionally makes more low end than shortie headers. Advanced designs like tri-y can pull a little more low range power.
If you have a good exhaust now, I,d look into other factors such as tuning & regearing before downsizing exhaust.
 

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way to much unless you are gona run in the 5500 to7500 rpm range.street driven non race cars especially 350 auto cars stay in the 1 1/2 to 1 5/8 primary tube the small tube headers are good to about 6500. 1 3/4 are for big inch and race engines:nanawrench:just like running too big a cam or carb on a street engine you will loose scavanging at low speed
 

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Factory cast exhaust manifolds or factory headers such as the shorties they put on small block Corvettes in the 80's were 1 1/4" to 1 3/8" I.D. tubes in general. 1 3/4" is more than your engine needs, especially if it is a crate motor or similar. What kind of cam is in it? Carb size and intake? Rear end gear ratio? It is important to tune all the parts to the application. The headers may be robbing you of a little low-end torque but the combination of parts will contribute to the issue.
 

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Discussion Starter · #5 ·
Thanks for the responses, guys!

The cam is what came with the GM/Goodwrench motor, I think it's pretty mild. Edelbrock dual-plane (2701) intake manifold, had an Edelbrock 600cfm (1406) carb but am replacing that next week with a Quadrajet 750cfm. Tranny is TH350.

I'm thinking 1.5" headers? Or should I wait until the QJet goes in to decide?
 

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The headers you have will never warrant the primary size of 1 3/4". I would go with the 1 1/2". As JLcustoms pointed out, Tri-Y headers are also better for off the line (low-end torque) due to the unequal length of the runners. "Tuned" headers - long, equal length tubes are better for mid-hi range RPM's. You may even find eventually that the 2 1/2" exhaust is too free-flowing and might take that down to 2 1/4" or even 2" for the combination you have. One step at a time . . .

One of my original Elkys - an 82 - I set up with a crate 350, THM 350 trans. Edelbrock and 600 Holley carb. I notched the frame to accept early 70's Chevy pickup ram-horn exhaust manifolds and had 2 1/4" duals. Off the line it was great - gas mileage was pretty decent for a daily driver.

For a street car, a 750 CFM carb is usually more than you need. If you are worried about gas mileage there will be a big difference from a 600 to a 750. A 650 might be a good compromise - all depends upon the use. Mild cam will be an improvement over the crate cam.
 

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Keep you r foot out of the quadrajet and it will give you the best of both worlds. the nice thing about them they will only supply what the engine can use.
 

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Discussion Starter · #12 ·
Okay, I just found my invoice for the engine build parts from Summit from 2005...they're actually 1 5/8" Summit headers...why would my low end torque be so anemic with 3.18 rear gearing?:dontknow:
 

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Tuning? Check all the basics--- timing , may need to recurve distributor , plugs , plug gap, Vacuum leaks, etc. Are you sure you have a 3.18 gear?
How old is crate engine? also retarted cam timing will drop yuor low end power.
 

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I don't think 1-5/8" headers would have that much of an impact by themselves if you are feeling a low-end performance hit. Ditto on the tune/timing suggestion above.
 

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set the timing to about 10 to 12 degrees and around 32 to 36 total.if you have you vacuum advance hooked to manifold vacuumit will dump your advance when you crack the throttle and make it doggy especially with such a short gear.what trans do you have if its a powerglide with thier 1.7 first gear they have to be tunned correctly . a 630 /650 qjet will give you a lot better throtle response than any carb out there.the edlbrocks are a pain to tune if off.hook your vaacuum advance to a ported sourse so when you open the throttle it adds timeing not take it away
 

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Discussion Starter · #16 ·
TH350 tranny, had rear axles re-done a few years back and the guy said "3.18 rear," HEI distributor, cam/crank/heads which came standard with the Goodwrench 350/260.

Quadrajets were only made in 750 and 800cfm, right?

Can you give an example of a "ported source" to attach the vacuum to?
 

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the ported vacuum is above throttle plate vacuum.on a qjet it is the one sticking out from pas side front corner.depending on the vintage carb there are several ported nipples some have one to the right of the fuel inlet.on any other carb look for a nipple with no vacuum at idle :nanawrench:
 

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Discussion Starter · #18 ·
Got the quadrajet :)

Will be running Summit's SUM-G9001-9 1 5/8" headers. Not using the crappy paper gaskets so I ordered Mr Gasket copper header gaskets. The Jeg's guy told me to order the Mr Gasket 7152G with 1.63" round ports but then the Summit guys say I need the Mr Gasket 7150 with the 1.5" round ports...which is correct for these headers??:dontknow:
 

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Discussion Starter · #19 ·
update: those Summit 1 5/8" headers work great with the Mr Gasket 7152G 1.63" gaskets!

The Qjet will NOT mount to the Edelbrock 2701 intake. There is NO adapter, their square bore carb to spread bore manifold adapter doesn't work if you flip it upside down (despite assurances from a local mechanic and a Jeg's tech that it would work.)

I'm waiting to see whether I can swap my QJet for a Holley 600cfm (the cheapest solution.) I don't really want to deal with an intake manifold swap but I may be able to justify the expense just in MPG savings alone (gas is $4.30/gallon here.) But I don't want to have to do that....
 
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To be honest,you might wanna think about keeping that Q-jet. The Q-jet has small primaries and huge secondaries while the Holley has the opposite. I'm running a Q-jet right now and it gets great mileage on my 383. It needs a rebuild but it can only get better when it does...
 
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