2 inch or 2.5 inch exhaust? - Impala Tech
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post #1 of 19 (permalink) Old 11-19-2009, 06:16 PM Thread Starter
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2 inch or 2.5 inch exhaust?

I need to get going on ordering the exhaust parts, since I will need them before too long. But I still can't decide which size exhaust system to use. I have 64 Impala sedan which started life with a 283 single exhaust and now a stroked 350 (383) engine that will be dressed up like a stock 327. If I go with the 2.5 inch exhaust, I will have to get new rams horn exhaust manifolds, and they don't seem to be a bargin. I make it out as right side 2.5 inch is 3797942 and Left side 2.5 inch is 3846563. Is that extra 1/2 inch really a big help ? And the Schafer's exhaust I am looking at goes down to 2 inches after the muffler anyways. Doing the Pi*r squared thing, a dual 2.5 has 9.8 sq inches vs the 2 inch duals having 6.28 sq inches of area in the pipe. As a sanity check, my 03 Silverado SS has a 3.5 inch final pipe, single exhaust which works out to 9.62 sq inches with a 364 (6 liter) engine.

1964 Impala 4 door sedan

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post #2 of 19 (permalink) Old 11-19-2009, 07:49 PM
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383 is still the same size displacement as the RB block 383 Chrysler (big block) that powered Road Runners and Cudas, so I would look more to the 2.5" exhaust pipes.

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post #3 of 19 (permalink) Old 11-19-2009, 09:26 PM
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2.5 if you have performance.
2.0 if you are stock.
And yes that 1/2 inch is a big difference.

Make: Chevrolet
Model: Impala
Year: 1971
Doors: 4
Motor: 400 V8
Tranny: TH350
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post #4 of 19 (permalink) Old 11-19-2009, 09:56 PM Thread Starter
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Darn, I was hoping you guys would talk me out of the 2.5 inch exhaust .

The stock size could be 2 or 2.5 depending on which HP 327 you had. Naturally, I want it to look stock and perform better

Do you think it is much of a factor that the 2.5 exhaust necks down to a 2 inch after the muffler? This is the way Schafer's makes it and the way the stock was.

And is there a better deal on 2.5 rams horns than the one's being sold for corvettes at around $225 each side? Some used ones in various condition seem to pop up on ebay once and a while.

1964 Impala 4 door sedan

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post #5 of 19 (permalink) Old 11-19-2009, 10:33 PM
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Hey David,
Did you look at Truck and Car shop for manifolds? Also, as I recall, most exhaust systems did reduce exiting the mufflers.
Gary
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post #6 of 19 (permalink) Old 11-19-2009, 10:57 PM
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Are you going to run dual exhaust? I know a lot of cars V8 models ran 2" to a Y pipe the 2.5 single to the muffler and 2" out the back at least that's the way most of my cars was. I guess you could but I think it would defeat the purpose of the 2.5 from the front going into 2" so just run the 2" if that is what you want.

Make: Chevrolet
Model: Impala
Year: 1971
Doors: 4
Motor: 400 V8
Tranny: TH350
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post #7 of 19 (permalink) Old 11-19-2009, 11:33 PM
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David,
I have heard the reproductions are good enough quality.
Here is a set of 2.5 inch ones for $150. That seems reasonable. I know I have seen them somewhere for a leven a little less than that. Check the vendors.

http://cgi.ebay.com/ebaymotors/Chevy...Q5fAccessories
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post #8 of 19 (permalink) Old 11-20-2009, 12:53 PM Thread Starter
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Unless my manifolds got switched around somewhere back when I did the engine swap from 283 to 350, I am pretty sure I have the correct manifolds on there now.
My existing 2 inch manifolds are left 3846559 and right 3747038.

Car and Truck shop wants $350 for what looks like the correct 2.5 inch left side manifold 58-10406 (NOS) but they don't bother to list the casting number. They also have a right side manifold that looks right, but is only 2 inch 65-10407.
Yes, this is going to be a dual exhaust. Originally, with the 283, it was a single exhaust, but I had a muffler shop put in a 2 inch dual system long ago. It is pretty well rusted out now, and a fairly poor job was done.
The Ebay repros have the wrong alternator mount cast in them, plus they are selling a pair of left hand sides as a "set". Which means you have an alternator bracket facing the firewall on the right side. Not only is it not correct looking, it is ugly and may hit the firewall. And I doubt they have the choke heater tube in the right side.

According to my book, "Chevrolet by the numbers", the casting numbers (or copies of) that I need are :

Right side 2.5 inch for 1964 3797942

Left side 2.5 inch for 1964 3846563

1964 Impala 4 door sedan

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post #9 of 19 (permalink) Old 11-20-2009, 03:06 PM
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I went ahead and had duals made for my 64 wagon utilizing the 2.5" Rams and used 3.0" pipes all the way back at a local shop. Cost me $500.00 and it sure was worth it in added HP and Sound. Dont forget on 64 the drivers side receives the bolt that holds the alternator and is the hardest to find.
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post #10 of 19 (permalink) Old 11-20-2009, 04:26 PM
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Quote:
Originally Posted by dcairns View Post
Unless my manifolds got switched around somewhere back when I did the engine swap from 283 to 350, I am pretty sure I have the correct manifolds on there now.
My existing 2 inch manifolds are left 3846559 and right 3747038.

Car and Truck shop wants $350 for what looks like the correct 2.5 inch left side manifold 58-10406 (NOS) but they don't bother to list the casting number. They also have a right side manifold that looks right, but is only 2 inch 65-10407.
Yes, this is going to be a dual exhaust. Originally, with the 283, it was a single exhaust, but I had a muffler shop put in a 2 inch dual system long ago. It is pretty well rusted out now, and a fairly poor job was done.
The Ebay repros have the wrong alternator mount cast in them, plus they are selling a pair of left hand sides as a "set". Which means you have an alternator bracket facing the firewall on the right side. Not only is it not correct looking, it is ugly and may hit the firewall. And I doubt they have the choke heater tube in the right side.

According to my book, "Chevrolet by the numbers", the casting numbers (or copies of) that I need are :

Right side 2.5 inch for 1964 3797942

Left side 2.5 inch for 1964 3846563
Only ram's horn cast iron manifolds made in 2.5" pipe size are for the Corvette and the exhaust dumps right into your motor mounts (because there is a hole in the Corvette chassis that uses four corner (front and rear mounted) motor mounts instead of two mid mounts and a tranny mount.

Big Dave
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post #11 of 19 (permalink) Old 11-20-2009, 04:31 PM Thread Starter
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Found these two reproductions at a Corvette store

$255.00

http://www.corvette-paragon.com/p-35...e-rh-2-12.aspx



$228.00
http://www.corvette-paragon.com/p-35...d-lh-2-12.aspx


I was going to do stainless exhaust, but if I have to spend this much on the manifolds, I may go with the aluminized and just have to replace the exhaust in a decade or so.

1964 Impala 4 door sedan

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post #12 of 19 (permalink) Old 11-23-2009, 11:06 AM
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I have been told before that the 2.5 Rams book out to Corvette only, if this is true did they use the standard 2.0 for the 250 horse and the 300 horse 327s in Impalas in 1962-64? Or are the Books wrong? Or is it me?
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post #13 of 19 (permalink) Old 11-23-2009, 12:15 PM
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The only small block that had 2.5" inch exhaust was for the 375 and 350 horse 327 Corvettes, and 350 and 370 horse 350 cid small blocks. Because the ram's horns dump straight down they will run right into your motor mounts (there is only one casting as it fits both left and right sides by turning it around).

The full size Chevrolet and Chevelle ram's horns exhaust were made for a 2.25" inch pipe and have a 45 degree bend at the bottom (so there is a left hand and a right hand manifold). You can machine the 2.25" ram's horn to open it up to fit a 2.5" pipe but it will be a bit thinner. You can also port it just like a set of heads.

Sanderson makes a cast iron shorty block hugger header as an alternative to mild steel tubular headers if you like the quieter cast iron exhaust manifolds. Personally even a set of block hugger short tube headers are better than any cast iron part as they will segregate the exhaust pulses to restrict any reversion of exhaust gasses caused by two adjacent cylinders having an open exhaust valve (as in cylinders 5 & 7).

Big Dave
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post #14 of 19 (permalink) Old 11-23-2009, 12:52 PM
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....just thinking out loud regarding performance,...don't forget that you can have a cast-iron manifold extrude honed to get some measureably better flow out of them. (even if it isn't on par with headers....)
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post #15 of 19 (permalink) Old 11-23-2009, 03:22 PM
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Big Dave 1964 full sizeshop manual states."Two 327 cubic inch engines (RPO L-30 and L-74) are a futher development fo the 327 ...... Option L-30 has a WCFB or 4GC carburetor, hydraulic lifters and 2" exhaust, while option L-74 has an AFB aluminum carburetor, mechanical lifters and 2.5 " exhaust." both images of 327 show the same style rams and the ones on my 62 and 64 are 2.5 with straight dump over the MM lead pipes clear the frame and arms. I will have to get casting # for each as 62 doesnt have the bolt for the alternator as it has a bracket for generator.
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post #16 of 19 (permalink) Old 11-23-2009, 05:52 PM
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Guys,
The 327/300HP in the full size (Impala) had 2.5 in rams horn exhaust manifolds similar to the corvette through 1964. The 250 version and below had the 2' manifolds in 64.
No problem with interference on the center dumps with the X frames of 62-64. later 60's impalas used the angle dump on the drivers side.


64 2.5" LH manifold is #3846563 Two tapped holes for Alt Bracket
64 2.5" RH manifold is #3797942



I have the original 2.5" manifolds on my 64 327/300hp survivor.
They are out there but originals are getting very hard to come by. The replacements are fairly good though.

Here are some other 62-65 2.5 inch full size rams horn manifold part numbers

3797901 LH 62/63 327/200 with generator boss
3797902(1) RH 63 327/300 with AC
3797942 RH 62/63/64 w/o A/C

All of these were common with the corvette, none of these were passenger car only manifolds.

Last edited by Fossil_Fuel; 11-24-2009 at 12:39 AM. Reason: Added more 2.5" exhaust manifold numbers
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post #17 of 19 (permalink) Old 11-24-2009, 10:30 AM
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Thanks F_F I have been told so many times I have Corvette only manifolds, and can never seeme to find proof of # and use. My manifolds on my 62 and 64 300 hps are as you listed. Thanks for the confirmation.
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post #18 of 19 (permalink) Old 11-24-2009, 12:11 PM
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Tom,
Yes the "corvette only" engine parts is an old wives tale, just like every set of camel hump heads (491's) are corvette heads and every high HP 327 is a corvette motor.

The thing is that the high performance 327/300 HP engine in the Impala in those early years was common with one of the corvette engines. In 64 the 327/300 Full Size Chevrolet engine came with the Carter 3720 or 3721 AFB carburetor, 3844459 intake, 3782461 "corvette" heads, and 2.5' rams horn exhaust manifolds..All comon with Corvette.
All of these components are commonly sold at the swap meets as corvette items because they are common with the corvette (and therefore bring more money). But all of them are correct for the full size chevrolet also.

The impala engines are stamped with the full size assembly suffix so you can tell if it is really a corvette or full size application if the block has not been decked.

The majority of the 327's in full size are the 250 HP versions with the rochester carb, smaller exhaust manifolds, and heads common to the 283 so most people never really associate these together.
Another cool little known fact is that 64 Impalas fitted with the 327/300 HP got an extra body/frame brace at the rear axle so you can tell if the car was originally fitted with the High Perfomance 327 (or big block 409 of course).

Alan Colvins "Chevrolet By The Numbers" for 60-64 is a good reference for these engine parts.


Tom you wrote "AFB aluminum carburetor, mechanical lifters and 2.5 " exhaust..." The impala 327/300 HP has hydraulic lifters not mechannical, only the corvette had mechanical lifters in those years. I don't know why it would state that in the manual. I'll take a looK in mine.
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post #19 of 19 (permalink) Old 12-19-2009, 12:35 PM Thread Starter
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Yup, they fit just fine. Not a lot of room to spare around the 2.5 inch pipe, but it does make it through. On the left side manifold, the three bolts that clamp the header pipe onto the manifold, the forward most bolt is a bit of a pain (the one you can see a bit of near the steering shaft). the frame leaves no room for a socket wrench so it was an old fasioned manual wrench from above. Try not to notice the ugly engine, with the wrong valve covers and carb. That will get cleaned up... eventually





Comparing the "2 inch" vs the 2.5 inch exhaust manifolds. The difference seems quite a bit more than just a half inch.

1964 Impala 4 door sedan

My Bloggy Thing: http://www.impalas.net/forums/blog.php?u=2285
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