I have a 74 vette with a mild small block chevy. I would like to install side pipes and have been looking at obx which is a complete header and sidepipe system. My motor has 1 1/2 in tube headers now but the obx system, as well all the others I have found, all have 1 7/8 tubes and 4 inch collectors. According to all the research I have seen these tubes should be to large and cause a lack of torque and possible reversion. Has anybody else tried this setup? Is there some type of restriction in the muffler system that would elimimate these problems. I have some understanding of the principles involved here but don't understand why most of these manufacturers would not offer applications for lower hp engines if the larger tubes were only for racing apps. If these headers are to large does anyone have any ideas how to plumb an exhaust for performance and appearance to get rid of those big bulky mufflers hanging off the back of the car. Any help would be appreciated. I would really like a sidepipe system if it makes sense. Thanks, Greg

 

People have been putting aftermarket side pipes on small blocks with 1 7/8" tubes and 4" collectors for many decades without the issues you are concerned about. You have choices in mufflers from the highly restrictive reverse flow to the various free flow designs such as STS and Sweet-Thunder. Of more concern would be over carbureting or camming or wrong intake.

 

I have the Hooker headers with side pipes and run STS baffles. There may be some loss of torque at low rpms but I can't tell you what it is. Mine responds very well even with the 1 7/8" primaries. I also run a 2600 stall torque converter so this is probably helping me out some.
The motor is a mild 350 with a 270 duration cam on a 108 LSA. The holes on the exhaust manifold are smaller than the openings on the headers and help to control reversion somewhat. I suppose the exhaust manifold hole size will depend on the heads used, so this may not be the same for all applications.

 

Thanks for the response. Was clearance an issue anywhere with a larger tube? I can see this looking nice underhood but how does it look along the rocker? I have a 400 turbo and a stall converter. The real stall speed is about 2100. I run 3.55 rear gear. The motor is out of an old van. It has an intake and headers, that's about it. Runs good for what it is. Rear tranny and all the rest of the driveline is new. Quarter mile is mid 14's. I only run it once in a while. The motor has been in it for a long time. I quess one of these days i'll replace it with something a little more stout. Just curious about your setuup. Greg

 

I installed the obx side pipes with hooker max flow inserts on my 1980 this spring. I have a all stock L-48, it even had the original converter and exhaust pipes berfore I put the side pipes on.
I noticed a big inprovement in low and midrange performance after I installed the headers. But this could also be do to the fact that I removed the factory manifolds, A.I.R system and cat. converter.
The header instal was actually very easy and everything cleared in the engine compartment. I did have to notch the fiberglass behind the front drivers side wheel about 1/2". If you got to the hooker web site they have a install PDF for the hooker sidepipes and the install is the same for the OBX sidepipes. I also enlongated the second mounting hole on cylinders 1 and 2. It seems most of the threads ive read about these headers people have the same issue in that area.
The summit copper gaskets I first used did not work at all. The sealing bead on the gasket will be inside the header tubes and not against the header flange. http://www.summitracing.com/parts/sum-111406/overview/
I ended up using cheap mr gasket med race port header gaskets and havent had any leaks yet.

 

I got a quote for a set of obx for 650 bucks new with out baffles, I will try those and adjust as needed then go in for coating.It seems like most of these header side pipe kits seem to need some mods.at that price in stainless steel seems good to me .

 

Pipes that large for a mild 350 aren't ideal by any means, but they're not a problem. You didn't give specifics on the engine, but stock iron heads flow around 130 CFM on exhaust...which should be using a 1-1/2" diameter pipe.


Reversion isn't a significant concern but you're giving up a fair bit of the scavenging effect that delivers power and torque, possibly to the tune of 5-10% HP/TQ below 3,500.


I wouldn't sweat it - sidepipes are cool, the losses aren't terrible and you can always add more power later

 

I got my headers directly from jet hot coated and ready to go. Had no problems with fitment. The left side primary for #1 all but touches the steering box but does clear. Used stage 8 header bolts to keep bolts from backing out. Highly recommend the ceramic coating to keep under hood temps down. STS baffles claim better scavenge above 4000 rpm, personally I like the sound.
Looks great on the car. Get heat shields or you'll be branding you leg or a passengers. Your stall will probably work well with the 3.55 rear. Mine is a 3.08 with 2600 stall and works well.

 

I got mine from danny Woo info@sparktecmotorsports.com
He gave me a better price than the guys on ebay

 

The whole problem with side pipes is that they are NOT a tuned length power adder like full length tri-y or 4-1.

They are designed for looks! NOT Power They are just a free flowing exhaust with the right internal muffling. So they would be superior to cast iron manifolds and most shorty headers.

 

i have them on mine, if i left anything on the table power wise i don't miss it.
the more power you build in the less you will notice these "issues"

 

Thanks everybody, I appreciate the imput. I'll let you know what happens. Greg

 

You can use some math to figure out what size primary you need based on peak torque rpm of an engine or you can predict peak torque rpm based on primary size.
Max flow of the exhaust occurs at peak torque which is also the peak VE (volumetric efficiency) of your engine. This is what the primary needs to be sized to in order to accommodate the exhaust flow of the cylinder. Past peak torque exhaust flow falls off rapidly due to insufficient time to fill the cylinder completely. Peak torque will vary with the cam used.
So say your peak torque occurs at 4800 rpm and it's on a 350 CI engine.

Only one cylinder per primary needs to be considered, so 350 divided by 8 = 43.75 CI.
If you plug in the numbers into an equation you can come up with the proper primary size. My key board doesn't have several of the mathematical symbols so I'll have to spell out the equation.

The square area of the primary is = Cylinder CI multiplied by peak torque rpm divided by 88,200 (mathematical constant)

Then take square area (answer to previous equation) and multiply it by 1.273 (another constant) then take the square root of that and you get the cross section of the pipe needed.

(43.75 * 4800)
------------------- = 2.38 inches squared
88,200

2.3 * 1.273 = 3.0297

Square root of 3.0297 = 1.741"

This means for an engine that torque peaks at 4800 rpm the nearly right sized primary has an internal diameter of 1.741 inches or pratically speaking 1 3/4 inches.
This is the ID of the pipe. To get the OD of the pipe the gauge of the pipe needs to be factored in.
For a 16 guage steel primary the closest OD is 1.875" which gives you a 1.757" ID. Or if it was 14 gauge steel then the ID would be 1.725".
1.875 happens to be 1 7/8". So the side pipe primaries may not be as far off as one may think if you can get your peak torque to occur at 4800 rpm or maybe a little higher. That may not be practical for a less than peak torque performance kind of operation and it assumes 100% VE. Your ideal size primary may vary based on your application and VE%.

 

nice math for the diameter but, you left off the correct length. The correct length to the collector is what causes negative pressure in the other three pipes.

The larger the diameter of the primary requires a shorter length because of lower velocity

 

Formula is only useful when it's validated by experimental data. There are volumes of tests done with different primary sizes and lengths that provide good, practical and real-world guidance. VE is virtually never 100% with an engine making less than about 1.4 HP/CID or using forced induction.

1-7/8 primaries on a mild 350 are absolutely not the right size in the real world. Typically, SBC engine dyno headers are a beat up pair of 1-7/8 with sensors in them - I've watched these lose > 10% HP/TQ below 3,500 RPM on engines making around 1.2 HP/CID as validated by later corrected chassis dyno runs.

David Vizard provides an excellent chart for primary size based on exhaust flow, which is a far more realistic and practical measure and guide to primary size.

 

Only correctly designed side mount headers were Stahl headers! The tube length on the Hookers is so long that the pipe diameter kind of doesn't count same thing for collectors.

 

The only "correctly" designed headers including length and collector size are ones designed specifically for one engine for one application and confirmed with testing.
All others are only in the ballpark and maybe not even that if you are just making a WAG at it.
In either case the Hookers with side pipes are for looks and sound so no reason to get bent out of shape about their design. They are what they are.

 

So, if I'm understanding this math the larger the tube the less velocity and the longer the tube the less velocity so in a perfect world with a long tube header I would want a tube diameter that was less than the 1 1/2 in. tube that I currently have on the old shorty header. That being said, I had a 2 1/2 in exhaust with flowmaster mufflers on the old system that ran out the back of the car. It would seem that system would be much more restrictive than the side pipe system that I'm about to install. So, in a perfect world, from a performance perspective it sounds like I either would need a very small header tube or a very restrictive muffler to achieve any kind of velocity below 3500 rpm. But then again with a 300 hp engine or less if I only loose 10 percent of torgue and hp below 3500 a freer flowing exhaust above 3500 rpm might not be a bad thing. I understand that the smaller tube exhaust would still flow as much air as would be necessary at say 5000 rpm as the larger tube but at that point would the tube size even matter? Is it possible to tune this system with mufflers to achieve the correct velocity if this was a perfect world and time and money were no object? Greg

 

As with most things, there's a practical way to approach things here. Trying to get the last .0001% of performance is something that racers do...on the street, reasonable guidance based on practical experience will get you in the ballpark and that's more than good enough - especially if you're making less than 1.4 HP/CID.

Street performance exhaust design is pretty well understood - we don't have to talk much about "optimizing" and "tuning"...just making good choices that will produce reasonable results based on decades of practical, validated experience with SBC performance. Don't get caught up in math intended for race engines to make choices for a mild street engine.


Shorty headers are typically only slightly better than iron manifolds. They're lighter and hold less heat in the head but don't have a huge impact on power as they don't significantly improve scavenging.

Off-the-shelf long-tube headers work pretty well in most cases on the street unless you're running something really hot > 1.4 HP/CID. Practically speaking, it's pretty tough to find even a 1-1/2 primary tube full-length header; most are going to be at least 1-5/8". These will work absolutely perfectly with your mild 350 and significantly improve torque and HP everywhere except at the very top of the RPM range...and there will only give up a few percent over a larger header. Pair that with a decent exhaust system with a crossover and decent mufflers...and you're in about as good a shape as you can get on street exhaust. Tri-Y headers have demonstrated that they're better at low- and mid-range torque, with a small sacrifice at the top.

Certainly you can spend a bunch of money and have someone weld up something completely optimal and try running 3" tube, etc...but you'll have spent 4 figure money on what will ultimately be a relatively small gain...and not be optimal if you change your heads, cam, etc. later.

Buy a quality 1-5/8" header and connect it to your existing system, and at some point invest in a better downstream system with a good crossover and modern mufflers. It doesn't need to be more complex than that.

And, as noted by a couple of folks including me...if you want the sidepipes, get 'em

 

Summit has a Youtube video that discusses the same math to figure primary tube size. He sort of glosses over a lot but same method.


Wallace racing also has a calculator you can use for primary size and length that also incorporates exhaust valve events to get the length right, if you want to get more accurate.

http://www.wallaceracing.com/header_length.php