Exhaust Backpressure- 3" vs. 2 1/2"

[Bob Eyres]

In case you still believe that old theory that "you need a little backpressure in that exhaust system to get the most out of your engine", check out this Hot Rod Network video of back to back dyno runs on a Hi-Po 454:

http://www.hotrod.com/news/1605-engi...-inch-exhaust/

They also address the issue of variation in overall weight.

[Hog]

Excellent comparison, boy are we spoiled nowadays. Engine dynos with data graphs and overlays that can be pulled up in seconds. A lot different than it was done back when these Mark 4 BBC's were being developed.
It would have been interesting to see the difference between open headers with the 18" collector extensions compared with the same open headers without the extensions.
Dropping the exhaust on any header equipped vehicle I have owned has always been greeted by a massive drop in low rpm torque, but sure screams up top. But my examples were usually low duration hydraulic roller cams set up for low-mid rpm torque.


It would have been nice to see a set of Corvette style BBC manifolds tried as well. The manifolds/Camaro exhaust on the 1969 ZL-1 Camaro with the all aluminum ZL1 427 BBC brought the 500-535hp output of the ZL1 427 down to about 275hp as per testing done at the Tonawanda plant. Chev knew that the buyers of these cars were going drag racing and would be ripping the stock Camaro manifolds and exhaust off of them anyways, so why design a complete new exhaust for only 69 cars.

I'm sure the 2 or 3(depending who you're talking to) ZL=1 Corvettes stock exhaust would cut into the power a bunch as well, although there would be more incentive to do a better exhaust for the Vettes, as there was the ZL-1 cars, and the 20 L88's for 67, 80 for 68 and 116 for 1969.

Good thread on the more rare BBC Chev cars with famous RPO's(now-back when new, very few even knew what RPO ZL1 or L88 entailed)
http://www.zr1.net/forum/showthread....hlight=ZL1+L88

[Mystic ZR-1]

BBC...
blah blah blah....
You could get a Big Block in a school bus!
(I'll bet they had a 3" exhaust too...)
But they never picked up the little darlings in an LT5
powered bus... not even the "Short Bus" that a few of us rode...
😄 😄 😄

[BigJohn]

Originally Posted by Mystic ZR-1

BBC...
blah blah blah....
You could get a Big Block in a school bus!
(I'll bet they had a 3" exhaust too...)
But they never picked up the little darlings in an LT5
powered bus... not even the "Short Bus" that a few of us rode...
😄 😄 😄

School Bus ; you could put an LT5 in one if you wanted to.
You can find them in street rods, impalas, pickup trucks, 55 & 57 Chevys and boats!

[Mystic ZR-1]

Originally Posted by BigJohn

School Bus ; you could put an LT5 in one if you wanted to.
You can find them in street rods, impalas, pickup trucks, 55 & 57 Chevys and boats!

But the General never put 'em there!

[Bob Eyres]

Originally Posted by BigJohn

You can find them in street rods, impalas, pickup trucks, 55 & 57 Chevys and boats!

Not to change the subject, but..it's my thread so, did you ever wonder why the hell you would put an LT5 in anything but a C4 ZR-1?

And the only answer could be aesthetics right? It just looks better than any other Chevy engine.
If I had any one of those vehicles the only reason I would have to put an LT5 in it would be looks.

So, why not to put an LT5 in anything other than a C4 ZR-1?

- Expensive - If you can find one. No new crate motors available, just NOS.

- Heavy - Way heavier than any modern Chevy engine of comparable horsepower.

- More difficult to install - nobody has a plug and play CCU setup do they?

- Replacement Parts availability - Sure, you can get parts, but availability, and price, is nothing like it is for LS, or other crate motors.

- Not easy, or cheap to modify - For the price of a Haibeck 500hp upgrade, you can buy or build a motor with WAY more performance.

Bottom line, you gotta really love the LT5...and I do.

[BigJohn]

Originally Posted by Bob Eyres

Not to change the subject, but..it's my thread so, did you ever wonder why the hell you would put an LT5 in anything but a C4 ZR-1?

And the only answer could be aesthetics right? It just looks better than any other Chevy engine.
If I had any one of those vehicles the only reason I would have to put an LT5 in it would be looks.

So, why not to put an LT5 in anything other than a C4 ZR-1?

- Expensive - If you can find one. No new crate motors available, just NOS.

- Heavy - Way heavier than any modern Chevy engine of comparable horsepower.

- More difficult to install - nobody has a plug and play CCU setup do they?

- Replacement Parts availability - Sure, you can get parts, but availability, and price, is nothing like it is for LS, or other crate motors.

- Not easy, or cheap to modify - For the price of a Haibeck 500hp upgrade, you can buy or build a motor with WAY more performance.

Bottom line, you gotta really love the LT5...and I do.

Yes, the looks, the rarity and dumb money!
With many ZR-1's being wrecked and stripped; the LT5 and it's computer are available.

[RussMcB]

Originally Posted by Bob Eyres

- More difficult to install - nobody has a plug and play CCU setup do they

CCU = Engine Computer? Yes, Megasquirt has one now.

[Hib Halverson]

Originally Posted by Hog

Excellent comparison, boy are we spoiled nowadays. (snip)

Mr. Hog gets a Beacon of Reality Award for that spot-in statement!

It would have been nice to see a set of Corvette style BBC manifolds tried as well. The manifolds/Camaro exhaust on the 1969 ZL-1 Camaro with the all aluminum ZL1 427 BBC brought the 500-535hp output of the ZL1 427 down to about 275hp as per testing done at the Tonawanda plant. Chev knew that the buyers of these cars were going drag racing and would be ripping the stock Camaro manifolds and exhaust off of them anyways, so why design a complete new exhaust for only 69 cars.

Point well taken but some of the supporting numbers puzzle me. Using the rating systems of that period, ZL1s made either 560-hp (if you believe unofficial "real" numbers) or 430-hp (if you believe the "official" power rating. No question that when a stock Camaro exhaust was attached there'd be a big loss, but a loss of more than 50%? Not sure I buy that. What testing done at Tonawanda are you talking about? I didn't think there were any dyno cells at Tonawanda back then which could accomodate a full OE exhaust system. Can you post the numbers and the testing method used? Or at least post the source for your info.

I'm sure the 2 or 3 (depending who you're talking to) ZL=1 Corvettes stock exhaust would cut into the power a bunch as well, although there would be more incentive to do a better exhaust for the Vettes, as there was the ZL-1 cars, and the 20 L88's for 67, 80 for 68 and 116 for 1969.

C2/C3s with L88s or ZL1s used the same exhaust system as other Big Blocks, ie: the pretty darn good Corvette BB exhaust manifolds, 2.5-in pipes and the bigger mufflers.

[Bob Eyres]

Originally Posted by Hib Halverson

C2/C3s with L88s or ZL1s used the same exhaust system as other Big Blocks, ie: the pretty darn good Corvette BB exhaust manifolds, 2.5-in pipes and the bigger mufflers.

Yes, they were delivered with that exhaust system. But the race cars Zora intended them for almost invariably used tube headers, and "off the road" side pipe configuration seen on factory supported L-88's.
I doubt if any part of those systems measured 2 1/2", except maybe the header downtubes.

A side issue- Where did they get that 430hp. figure? It's generally thought that it was pulled out of the air and put in the order form to coax the buyer into ordering the 3 carb 435hp. street version instead of the L88 race motor.
But did they "rate it" by going down the dyno rpm scale until they found the number 430, and rate it at that rpm, instead of where it actually peaked?