Modified EXHAUST MANIFOLDS...........

[Keith S.]

In the interests of deciding which road I will take when I buy my car (serious flight judging vs. a really nice second flight, but performance modified driver) I was wondering if any members here had used the services of any of the companies out there like Gessler (or done the work themselves) that internally modify exhaust manifolds (extrude honing and/or grinding) and then flow bench test them ?

Headers are one thing and I have nothing against them, but would maintaining the OEM appearance, with a set of properly modified BB manifolds, have similar flow qualities to a good set of headers, or be close enough that the HP variable was not enough to worry about, as I don't plan to be doing heads up racing on "Pinks" so an extra .20 of a second does not matter. Believing the advertising from companies that do this service is one thing, so I am naturally skeptical of course. Reading in Pete's L88 book that TRACO added 100+ HP to an L88 with machine work and headers, has me curious as to what difference a nicely hogged out set of OEM BB manifolds would have brought on a lesser engine that received the same treatment.

I am talking about a scenario where perhaps the original 454 in a top flight C3 becomes a 496 (or the 427 in a second flight C2 becomes a 499 using a Dart/Brodix block) and in the process the carb, heads, manifolds, and any other internals, etc, are optimized for a slightly different camshaft, yet the original parts and appearance is maintained to the degree that it can be, the aforementioned NOM block option excluded of course. For the sake of the question, it would be a 9-1 comp engine that spins out to <6000 RPM.

TIA for the feedback.

[Duke W.]

Re: Modified EXHAUST MANIFOLDS...........

You're reading too many hot rod books and magazines. I've said it a million times, and I'll say it again, road engines are COMPLETELY DIFFERENT ANIMALS THAN RACING ENGINES!

The L-88 from the plant with manifolds and mufflers probably made less top end power than the L-71. Why? Because it had a "racing camshaft" with very long duration and HUGE overlap that was designed specifically for a proper racing exhaust system - properly sized headers and 3.5 inch tailpipes exiting behind the driver (the rule for most racing sanctioning bodies). But it barely idled at 1500 and didn't make enough torque below 3000 to power a wheelchair.

A pleasant performing road engine should have at least 80 percent of peak torque at 2000, and 90 percent is better, especially with an automatic transmission.

Headers on SHP engines with muffers may, in some cases, offer a modest improvement in the upper third of the rev range, but they often lose power somewhere in the bottom half. Power is a zero sum gain. Changes like big cams and headers to improve the top usually lose something in the bottom end, and even modest exhaust system backpressure negates most of the negative wave dynamics compared to open exhaust. This is why you rarely see OE headers on cars designed for road use.

The BB manifolds are very efficient. At most they should be matched to the exhaust ports and head pipe connection to eliminate any overhang on the downstream side. They don't need to be "hogged out".

The biggest single improvement you can make to any OE engine configuration is to massage the heads. This will pick up about six to ten percent peak power with 500-1000 more useable revs without hurting the bottom end or altering the idle behaviour.

Post '70 OE big blocks, if rebuilt, should be done so with higher compression - about 10:1, which is worth 6-10 percent more power ACROSS THE ENTIRE REV RANGE.

Stroking is also an option, but all other things equal, a longer stroke engine will make about the same top end power at the same mean piston speed as a shorter stroke engine. So there is no improvement in top end power (where you spend 0.001 percent of your driving time), but average power across the entire rev range, which is what counts for a road engine is higher.

Do yourself a favor and find an unmolested car with an unmolested engine. Then if you want "more power" do some research. Some of us specialize in creating "cheater motors" - engines that look OE on the outside and behave just like OE at idle and in normal driving, but have a lot more power and make useable power well beyond the OE redline.

It's done with "invisible modifications" - massaged heads, higher compression, stroking, and sometimes a different OE camshaft or a custom designed camshaft because no aftermarket off-the-shelf camshaft will deliver the desired operating characteristics.

A good example is Mike McCaghs '57 283/250 HP FI engine . It's really a 338 with massaged heads, higher compression, and a custom camshaft that allows it to idle butter smooth at 450 in Drive (It's a Powerglide car), while making 90 percent of the 364 lb-ft gross torque peak (OE peak rating was 300) at 2000 and more top end power than the '61 283/315HP engine that had a lumpy Duntov cam.

Mike's car is a Duntov Award winner and the story was written up on both the car's history, restoration, and the engine system engineering and testing a few years ago in The Corvette Restorer. Buy the Restorer CD.

You've been member for a year, and we've been doing these shenanigans and documenting it here at the TDB and in The Corvette Restorer for over ten years. So far NCRS has not purchased an X-ray machine to see what the bad boys have been doing inside our engines that otherwise look and behave in normal driving like a Flint or Tonawanda built engine. The judges can only score what the can SEE!

Duke

[Terry M.]

Re: Modified EXHAUST MANIFOLDS...........

The L-88 from the plant with manifolds and mufflers probably made less top end power than the L-71.

Duke

Gee, what a hoot that the factory supplied horsepower ratings for the L71 and L88 came out exactly that way. Of course no L88 owner or want-to-be-owner wants to believe those numbers, but that is another story for another time.

[Joe L.]

Re: Modified EXHAUST MANIFOLDS...........

In the interests of deciding which road I will take when I buy my car (serious flight judging vs. a really nice second flight, but performance modified driver) I was wondering if any members here had used the services of any of the companies out there like Gessler (or done the work themselves) that internally modify exhaust manifolds (extrude honing and/or grinding) and then flow bench test them ?

Headers are one thing and I have nothing against them, but would maintaining the OEM appearance, with a set of properly modified BB manifolds, have similar flow qualities to a good set of headers, or be close enough that the HP variable was not enough to worry about, as I don't plan to be doing heads up racing on "Pinks" so an extra .20 of a second does not matter. Believing the advertising from companies that do this service is one thing, so I am naturally skeptical of course. Reading in Pete's L88 book that TRACO added 100+ HP to an L88 with machine work and headers, has me curious as to what difference a nicely hogged out set of OEM BB manifolds would have brought on a lesser engine that received the same treatment.

I am talking about a scenario where perhaps the original 454 in a top flight C3 becomes a 496 (or the 427 in a second flight C2 becomes a 499 using a Dart/Brodix block) and in the process the carb, heads, manifolds, and any other internals, etc, are optimized for a slightly different camshaft, yet the original parts and appearance is maintained to the degree that it can be, the aforementioned NOM block option excluded of course. For the sake of the question, it would be a 9-1 comp engine that spins out to <6000 RPM.

TIA for the feedback.

Keith-----


I would use the Corvette big block manifolds as-is for a street application. I wouldn't even consider modifying them and I wouldn't even consider using headers on the street. Modifying the manifolds as you describe would almost certainly lead to cracked manifolds in short order.

[Dick W.]

Re: Modified EXHAUST MANIFOLDS...........

Gee, what a hoot that the factory supplied horsepower ratings for the L71 and L88 came out exactly that way. Of course no L88 owner or want-to-be-owner wants to believe those numbers, but that is another story for another time.

L/88 was somewhat subdued with 2" exhaust. Buttttt....factory headers and 4" side pipes, Katie bar the door! Totally different animal. Thur it 3,500 rpm's in first, second, or third and nail the throttle, you did not know which way you were going to go! The last time I drove mine with the headers, it tried to go through a doctors office when I caught third turning 6,500. . Ah, the good old days!

[Duke W.]

Re: Modified EXHAUST MANIFOLDS...........

Gee, what a hoot that the factory supplied horsepower ratings for the L71 and L88 came out exactly that way. Of course no L88 owner or want-to-be-owner wants to believe those numbers, but that is another story for another time.

It really has nothing to do with the OE ratings. The L-88 was rated on a lab dyno with manifolds and open exhaust with an evacuation fan like other OE factory installed engines, but the the rating was at 5000 revs, so it was probably not the peak, but the peak was probably not substantially better because of the extreme overlap cam that was designed for headers. Without headers there is large exhaust residual even at WOT, which limits VE to probably no more than 90 percent. Install in on the car with the best OE exhaust system that will generate about 6 psi backpressure at the top end and VE drops to 80-85 percent. The lower overlap L-72 cam won't lose as much with mufflers, which is why if you tested a L-72 back to back with an L-88 AS THEY WERE DELIVERED FROM THE PLANT, the difference would be small and the L-72 might actually be slightly better.

Now install a proper racing exhaust system and VE goes to about 105 percent without any other changes. Add a little head massaging and power goes to about 560 SAE gross at about 6500, but it still won't get out of its own way below 3000, barely idles at 1500, and is a cantankerous beast on the street regardless of the exhaust configuration because it's a RACING ENGINE, so all that counts is the upper third of the rev range. Put this configuration in a race car and under STP conditions the engine has the same output as on the lab dyno.

The interesting thing is that if you do the same to a L-72 your get about 500 SAE gross HP, so the L-88 only has about a ten percent advantage, but that advantage is probably across most of upper 30 percent of the rev range, so all other things equal on a two to three minute road racing course the L-88 will have about a two second advantage, which is huge even in a SCCA sprint race and devastating in a long distance race of two to three hours or more.

Duke