The high performance 327's in the C2's had 2.5" ram-horn exhaust manifolds. Anybody know why they went to 2" later on and for the C3's? I am considering replacing my stock '69 manifolds with the earlier 2.5's so I can run 2.5" pipes all the way back.
Also, anyone had any bad experiences or good doing this?
Thanks

 

I had a 69 350/350 that had the 2.5” manifolds on it when I bought it. I don’t remember any issues other than I think the drivers side had a provision for a different alternator setup that the C3 didn’t need. That was years ago so I’m sure others can clarify this.

 

garkahn60
I put 2.5" ramhorns on my 68. I used 64-65 downpipes from the ramhorns to the regular 2.5" system for a 1968 SBC. No problem really. I have since modified the pipes to go around a 200-4r and below the crossmember but you can eyeball the original installation process at this link. https://www.corvetteforum.com/forums...post1598185200

 

I did this on my 74.
Went from 280 hp, 315 tq.
To 300 hp and 330 tq,
It is all to the good!

 

Good information!

 

If you are going to run manifolds....the 64-65 2.5" units are the only thing to use.......
Not one engine that GM dyno'd will make it's advertised power with the stock manifolds and exhaust......GM screwed the pooch on this bad.....the LT-1 had a 2" outlet with a flared out 2.5" exhaust back to a muffler with 2" outlet.......
I recently did a 357ci small block with TF DH175 heads and hyd. roller cam.......we used the 65 2.5" manifolds and Corvette Central downpipes and full exhaust.....with a pair of Magnaflows......the car ran much better than I anticipated....but left some power on the table....I would say to the tune of 25-30hp.....this is a 430hp setup through headers.....

Jebby

 

Interestingly if I'm not mistaken, Donny Brass who runs a 327/350 HP '65 in the FAST series as a pure stocker actually said he went slightly quicker with 2" over 2.5's. Sounds counter intuitive for sure and If I was building one I'd use 2.5". Donny runs low/mid 12's in his car.

JIM

 

This is true, on the dyno Donny stated his car picked up six horses with the 2” manifolds over the 2 1/2” manifolds.

 

While I find that interesting, I also find it hard to believe.
I personally would like to see that repeated.
Generally speaking a restriction in the exhaust should cause a loss, not a gain.
None the less, No one has really answered the original question, Why did Chevy do this???

 

In my opinion it's more than just manifold out/collector size. For example Castheads claims that a y-pipe will make better ave power with manifolds, dyno tested they said. And I agree with that.

Still with countless combos some things will effect others differently but I still think the majority a 2.5" will work better. I have some of the reproduction 2.5" for my C3 and even if they cost me a bit over LT headers, I'll make up for it in other areas.

Another thought is half-truths, for example I know someone who use to say that a certain part almost always lost HP on the dyno But he would leave out how the car was faster on the track because dyno's don't move! Nothing wrong with dyno's and just like a flowbench they are a useful tool to a point But ultimately the road/track is what has the final say.

GM & others manufacturers purposefully reduced power, they claimed emissions But the fact is people use to be able to walk/drop out of highschool and go right into high paying factory jobs. Buy a nice high powered car and then wrap themselves around a tree drinking on the weekend. So power had to go away and now that safety has become fool proof almost and cost more for those cars, it's back! Seriously go watch youtube and you can see kids doing stupid stuff in a bmw w/o seats belts n hit a wall, then walk away. Most imported cars were higher power in countries where commoners couldn't afford them.

 

Yeah....I find it very hard to believe too....and I stand behind my original statement as someone who has seen the swap performed many times....and drove the car afterward....

The only real reason I can think is money....GM simply just put one manifold on everything....

I had both on the floor last year and compared the size difference....it is pretty staggering where the two tubes come from the outside to meet the center....huge volume increase....
My father performed the 2.5" swap on every Vette he ever had....even both of his 77's"....it was a very popular thing to do....

Jebby

 

The Venturi Effect can be difficult to grasp because one might expect pressure to always increase when air is pushed through a more resricted pipe. However, the consequent increase in velocity is sometimes greater than any potential increase in pressure and so a net increase in gas velocity and a net decrease in gas pressure is realized.

In other words, depending on hp output, bigger exhaust pipes sometimes helps but in other applications can actually slow exhaust flow somewhat.

 

I find it extremely hard to believe.

 

I believe it WAS answered. Consolation of parts by GM the did not think there was any longer a need for 2 1/2” SB manifolds.

 

In a venturi, the pressure at or in the restriction will reduce compared to the pressure pushing the exhaust through that smallest restriction. But, the pressure at the cylinders required to push the exhaust through that restriction will not reduce. You can only force the same flow through a smaller pipe by using more pressure.

On the other hand, the velocity increase could help with scavenging the cylinders.

 

That would be my guess. The Corvette was the only model that used the 2 1/2" ram horns, and only on the higher horse engines. By dropping the larger manifolds, they reduced inventory, and simplified production by hanging the same manifolds on every engine. They probably figured out that there wasn't that much of a difference in performance either.

The introduction of big blocks, as the highest performance optional engines, probably had something to do with it too, but in the end it probably came down to money and costs. Almost every change GM made usually was based on building cars quicker and cheaper. This is not a knock on GM. If you can save .25 cents on every car you build, without effecting build quality or operation, that's a huge savings when spread across thousands or millions of cars.

 

I think they also assumed that most people would install long tube headers on these cars.......the 69' Camaro 302 had long tube headers as a dealer installed option and were delivered with them in the trunk......
Another interesting fact is that ALL C3's fater 69' came with 2" exhaust....save for the L-88, LS-6, and L-71/L-89 cars.......my original 69' 427/400 had 2" exhaust on it and was so terrible that when I removed it for a 2.5" chambered system it was like someone stuffed another engine in it......I had to adjust the idle screws to compensate......I was going to rejet it but ended up selling the car.......it probably added over 50 horsepower just about everywhere.......only thing I can figure is, like the 2" manifold deal, is that they saved a $1 on each car.......
Good ol' Generous Motors...LOL.

Jebby

 

Actually, by 69 Chevrolet no longer offered headers for the Z/28, headers were only available with the 67 and 68 Z/28.

In 67 and 68, the Z/28 could be ordered with a set of headers for installation after delivery, a cowl induction air cleaner for dealer or owner installation or both. Unlike the 69 cowl induction hood, the 67/68 offered a cowl air cleaner with duct work that attached it to the firewall above the heater box (and requiring a hole to be cut in the firewall), which drew air in through the cowl fresh air vents at the base of the windshield. The 68 Camaro Assembly Manual shows the headers being "cancelled" around April 68. The GM headers were manufactured by Kustom Headers for GM, though I think they may also been sourced from Stahl. For 69, the headers could still be ordered through Chevrolet Parts, but could not be ordered as part of a factory new car order. This was also the case with the Cross Ram 2x4 set up, it was strictly a service replacement part.

When it was first offered, I don't think Chevrolet expected to sell many Z/28's. Like the L-88, it was only offered so that Chevrolet could build enough of them to make the engine legal for Trans Am racing. Headers were probably offered to make it easier for someone to build a race car from the Z/28. By late 68, Chevrolet probably realized that most people buying Z/28's were not buying them to race, and the headers were dropped. Also, while I've never seen any production numbers for the headers (or the cowl air cleaner), I don't think they were very popular options. For 68 the Z/28 package was about $400, but almost doubled to $780 when the headers were ordered, making the headers a $380 option, far more then headers from a speed shop at the time.

Back to the ram horns, the fact that only one engine in 66 and 67, the 327/350, would have come with the 2 1/2" ram horns, probably figured into the death of the larger manifolds too. From 62-65, anywhere from 2 to 4 different engines each year used the larger manifolds, making it easier to justify producing two different manifolds. The coming emission regulations also may have figured into the equation. Once A.I.R. started being needed on some engines, that increased the number of different manifolds needed.

As I said before, in the end I'm sure it was a dollar and cents decision, made by GM's bean counters.