Change to the smaller 2in exhaust manifolds in 66

[Tom P.]

Yes, I know this has probably been asked multiple times, but I've never seen an answer that made sense (of course, GM didn't always make sense ).
The 62-65 optional engines (small blocks) received 2 1/2in exhaust manifolds. Then, in 66 and later cars, ALLLLLL small blocks only had 2in manifolds. Why?
The front ex pipe is 2 1/2in on 66-later cars, but the first few inches was reduced down to 2in. Why not keep the manifolds 2 1/2in also?
One would think that the later hi-perf 350 engines would benefit from the larger 2 1/2in manifolds.

[Jack H.]

Re: Change to the smaller 2in exhaust manifolds in 66

I'll wager a guess...

There's a cost saving associated with only having one SB exhaust manifold (2-inch) to maintain tooling on and manage the inventory of. My guess is this was a cost reduction issue.

Further, designers discovered with computer modeling the net effect of 'necking down' the exhaust at its exit point from the manifold and marriage to the front pipe had a neglibable effect on performance IF the front pipe was swedged to rapidly increase in diameter from 2 to 2.5 inches...

[Philip A.]

Re: Change to the smaller 2in exhaust manifolds in 66

This is more of an observation, and not sure if it helps to answer. The base motor and "HP" 327/350hp motors were hydraulic lifter motors. The "SHP" small blocks were solid lifter motors. '65 was the last year for solid lift "SHP" small blocks (which had 2.5" exhaust).

[Clem Z.]

Re: Change to the smaller 2in exhaust manifolds in 66

the 2" pipes will increase the low end torque and will help starting out with a tall rear gear.

[Duke W.]

Re: Change to the smaller 2in exhaust manifolds in 66

No one really knows the answer and probably never will.

Jack may be closest, but they didn't do computer exhaust flow modeling in the mid-sixties. It's possible that the difference in dyno testing was minimal, so Chevrolet decided to standardize on the 2" size, which were used on all 283s, 327/250s, and 300HP/PG (through '65).

I disagree that they have any effect on low end torque. Manifolds don't allow wave dynamics harnessing, so it's just a matter of flow coefficient, and the 2" manifolds with a smooth transition to the larger 2.5" pipes won't cost anything at the bottom end and may not cost much at the top end, although I would not describe the diameter transition of most head pipes as smooth.

Friction is a function of Mach number, which is a function of gas temperature, and since exhaust gas temperature is very high at the manifold, the actual Mach number is lower than further down the exhaust system, so the 2" outlets probably don't cost much, but the quality of the transition to 2.5" pipe is probably the biggest issue.

Duke

[Joe L.]

Re: Change to the smaller 2in exhaust manifolds in 66

My speculation: 1966 was the first year for the air injection reactor system. So, from 1966 onward, there would have to be manifolds for use with AIR and without AIR. GM might not have wanted to re-tool the 2-1/2" manifolds to provide bosses for the AIR fittings. So, because they were offered primarily for Corvette applications and likely didn't very much affect performance, they decided to just drop the 2-1/2" manifolds.

[Tom P.]

Re: Change to the smaller 2in exhaust manifolds in 66

I really can't respond to any of the above answers.
First, I'm not an engineer.
Second, I've never had access to any of the testing equip/dynos that were possibly available within GM.
BUUUUUUUUUUUT, what I can positively say is that on anything larger than a healthy 350, a 2 1/2in exhaust system DOES IN FACT result in improved performance (fuel economy is not noticeable, but of course, on the 56, I could care less about fuel economy ).
For several years I have had a healthy SB400 in the 56. It started out with a FULL stock type 2in/ system. I always felt there was more to be gained with a 2 1/2in system. Once I finally tossed the 2in system and installed a full 2 1/2in system (including manifolds), it really woke up a sleepy 400!
It just never made sense to me that engines such as the 350/350 or the 70-72 LT1 only had 2in manifolds. It seems that on 66-later engines it would be tooooooooooo easy to install 2 1/2in manifolds and a pair of 63-65 2 1/2in head pipes. Theoretically, it seems that would be a bolt-on improvement for any 66-74 SB car. Right?

[Joe L.]

Re: Change to the smaller 2in exhaust manifolds in 66

I really can't respond to any of the above answers.
First, I'm not an engineer.
Second, I've never had access to any of the testing equip/dynos that were possibly available within GM.
BUUUUUUUUUUUT, what I can positively say is that on anything larger than a healthy 350, a 2 1/2in exhaust system DOES IN FACT result in improved performance (fuel economy is not noticeable, but of course, on the 56, I could care less about fuel economy ).
For several years I have had a healthy SB400 in the 56. It started out with a FULL stock type 2in/ system. I always felt there was more to be gained with a 2 1/2in system. Once I finally tossed the 2in system and installed a full 2 1/2in system (including manifolds), it really woke up a sleepy 400!
It just never made sense to me that engines such as the 350/350 or the 70-72 LT1 only had 2in manifolds. It seems that on 66-later engines it would be tooooooooooo easy to install 2 1/2in manifolds and a pair of 63-65 2 1/2in head pipes. Theoretically, it seems that would be a bolt-on improvement for any 66-74 SB car. Right?

Tom-----


It's easy unless the car has an air injection reactor system and one wants to maintain it.

[Duke W.]

Re: Change to the smaller 2in exhaust manifolds in 66

I really can't respond to any of the above answers.
First, I'm not an engineer.
Second, I've never had access to any of the testing equip/dynos that were possibly available within GM.
BUUUUUUUUUUUT, what I can positively say is that on anything larger than a healthy 350, a 2 1/2in exhaust system DOES IN FACT result in improved performance (fuel economy is not noticeable, but of course, on the 56, I could care less about fuel economy ).
For several years I have had a healthy SB400 in the 56. It started out with a FULL stock type 2in/ system. I always felt there was more to be gained with a 2 1/2in system. Once I finally tossed the 2in system and installed a full 2 1/2in system (including manifolds), it really woke up a sleepy 400!
It just never made sense to me that engines such as the 350/350 or the 70-72 LT1 only had 2in manifolds. It seems that on 66-later engines it would be tooooooooooo easy to install 2 1/2in manifolds and a pair of 63-65 2 1/2in head pipes. Theoretically, it seems that would be a bolt-on improvement for any 66-74 SB car. Right?

On a high power output engine, regardless of displacement, there WILL be a difference between 2" manifolds/pipes and 2.5" manifolds/pipes, but I thought your question was the difference between 2 and 2.5" manifolds with 2.5" pipes.

The latter will show much less difference than the former, and is probably measureable, but probably only 1-2 percent.

My data shows about 3 psi backpressure at close to 300 SAE corrected RWHP with the 2.5" manifolds/pipes.

The only way to specifically determine the difference would be to do back to back testing on a high output road engine (250-300 SAE corrected RWHP) with both 2" and 2.5" systems connected to a 2.5" exhaust system.

As I said before, I don't think there would be much difference if the head pipe had a nice smooth flare from 2" to 2.5", but most I have seen don't look very good from a flow perspective, and that's were most of the losses probably occur.

Duke

[Norris W.]

Re: Change to the smaller 2in exhaust manifolds in 66

Since this discussion started with the "WHY" instead of the end result, it might also be worth throwing into the mix that in '69 (I don't know what they did in '68............. never owned one) the 427's INCLUDING the SHP and HD (L88) had the two inch pipes all the way from the manifolds, whether side exhaust or under car. Both were necked down right off the manifolds. Another thing hard to understand is "WHY" did they run 2 1/2 exhaust pipes from manifolds to mufflers on the 70 LS6 Chevelle, and then choke it with 2" tailpipes and resonators? And let's not forget the generic low rise intake on the 70 Chevelle with LS6 that required a spacer to make the air cleaner work with the cowl hood and was rumored to have cost a solid 30 HP. Where was the old L78 intake? My best guess is that instead of any engineering reasons, some new hotshot college boy in the bean counting department figured they could save a coupla' bucks per car by downsizing the pipes. Equally unexplainable was the selection of the cast iron intake on the '69 350/350, an otherwise great engine, when they could have used the Z28 style intake that had been around forever and similar to what was used later on the LT1. Bean counters and college boys................ how many compromises have we been dealt because of bean counters and college boys?

[Clem Z.]

Re: Change to the smaller 2in exhaust manifolds in 66

i was told when i asked one of the GM engineers about the 70 454 chevelle that low manifold was used because that manifold was also to be used in a corvette with the SHP BBC engine that was listed but never was produced. that manifold would allow the carb and air cleaner to fit under a corvette hood. i changed out the manifold on my new chevelle as soon as i got the car home.

[Joe L.]

Re: Change to the smaller 2in exhaust manifolds in 66

i was told when i asked one of the GM engineers about the 70 454 chevelle that low manifold was used because that manifold was also to be used in a corvette with the SHP BBC engine that was listed but never was produced. that manifold would allow the carb and air cleaner to fit under a corvette hood. i changed out the manifold on my new chevelle as soon as i got the car home.

clem-----


Yes, I'm sure the manifold was originally designed for Corvette use. In fact, it's the only Holley flange, rectangular port big block manifold that GM EVER made that would fit under a 1968-72 Corvette big block hood.

[Norris W.]

Re: Change to the smaller 2in exhaust manifolds in 66

i was told when i asked one of the GM engineers about the 70 454 chevelle that low manifold was used because that manifold was also to be used in a corvette with the SHP BBC engine that was listed but never was produced. that manifold would allow the carb and air cleaner to fit under a corvette hood. i changed out the manifold on my new chevelle as soon as i got the car home.

I'm sure that's true, BUT............. again WHY? They had the alum. high rise intake that'd been around since '65 and it was a good performer. If they'd simply used it on the Chevelle they wouldn't have had to "rig" the cowl air cleaner with the piece of stove pipe to space it up to the hood. Also, since the LS6 option didn't come out in the Chevelle until after the first of the year, long after the front office had killed the option for the Corvette it becomes even less explainable unless purchasing had ordered up a zillion low rise intakes and they had to use 'em somewhere. Even the early year 396/375 (402 actually) in the Chevelle had that cheezy intake and that engine was never an option on the Corvette, although maybe the intake was required to stick it in a Camaro. I'm tellin' ya', it's college boyz gone wild. They're out to change and ruin the world.......... and they're comin' to take me away ha ha ho ho he he.

[Tom P.]

Re: Change to the smaller 2in exhaust manifolds in 66

--------------------------------------------- I'm tellin' ya', it's college boyz gone wild. They're out to change and ruin the world.......... and they're comin' to take me away ha ha ho ho he he.

I agree--------------to a point. I'm an OLD college boy (man), just recently got my MS in Aerospace Admin, and I STILL prefer the added expense of the better performance parts on my engines. If it don't fit, I MAKE it fit. My 70 Chevelle received the hi-rise alum intake (LT1 manifold on a SB400) under the CI hood and NO spacer is required. Looks and functions just great! UNFORTUNATELY, There is no such thing (never was) as 2 1/2in ex manifolds for a SB to fit a Chevelle/Camaro, thus headers are required. I REALLY wish Chev had made 2 1/2in log manifolds.

[Clem Z.]

Re: Change to the smaller 2in exhaust manifolds in 66

I'm sure that's true, BUT............. again WHY? They had the alum. high rise intake that'd been around since '65 and it was a good performer. If they'd simply used it on the Chevelle they wouldn't have had to "rig" the cowl air cleaner with the piece of stove pipe to space it up to the hood. Also, since the LS6 option didn't come out in the Chevelle until after the first of the year, long after the front office had killed the option for the Corvette it becomes even less explainable unless purchasing had ordered up a zillion low rise intakes and they had to use 'em somewhere. Even the early year 396/375 (402 actually) in the Chevelle had that cheezy intake and that engine was never an option on the Corvette, although maybe the intake was required to stick it in a Camaro.

i would guess all the smog testing was done with that manifold and to change it out all the tests would have to be run again if they change the manifold. the 375 HP 396 chevelles used the high rise manifold from the start