This is more mental gymnastics at this point, so don't expect me to spend any time and money on it yet.
But it's something ive been wondering about. For a long minute.
Scenario: gas burning v8 truck. Used for truck stuff. My case is 90s gm, throttle body stuff, but id ASSUME this applies to all similar trucks.
My thought is opening up the exhaust tract. Banks did this on motorhome engines with success, with reports of improvements in gas mileage and power.
So, would it be worthwhile? Think headers, high flow cats, free flowing exhaust.
Would short tube, mid tube, or long tube headers be optimal? Single large, smaller dual exhaust? I've not seen dyno charts that focus on idle to 3500rpm, just stuff 3k up. Or really focused on area under the curve.
I may also be barking mad, and have spent far too much time alone in the shop.....
Mr_Asa
SuperDork
10/7/20 7:41 p.m.
It can definitely be worthwhile if you have a restrictive exhaust already. Before I went to duals with my truck I couldn't get it to downshift into OD at WOT. Passing was basically unheard of.
After I uncorked it I lost a little low end, but I gained a lot more top end. I was able to hit the governor on it once.
An engine is an engine. Make it breath better, and supply the right fuel for that flow capacity and you'll increase power. Whether you have power available to be unlocked is another matter.
Someone will know way better than me, but I think the ultimate TBI mods kind of covers the order of what should be done, but I think headers and y-pipe are pretty much next in line after them.
https://www.thirdgen.org/ultimatetbi/
https://www.thirdgen.org/ultimatetbi2/
Big issue with TBI is there is a limit to its adaptability until you need to chip it. At that point, I’d think it’d be better to save for an engine swap.
If it helps any I'm planning on spending more for factory manifolds on my K30 vs summit headers. I'm looking more at long term reliability vs. performance. It will be dual H pipe after the manifolds.
Michael, when running my '91 has Edelbrock emissions legal shorty headers (crossover pipe)....magnaflow convertor......Gibson 3" SS cat back exhaust .......with Edelbrock multipoint fuel injection manifold......throttle body stripped of injectors with TPS still intact and Edelbrocks' tune.
The y-pipe on the GMT400 trucks looks like low-flow junk. I don't know which exact parts I would use but something like shorty headers just to get a bit straighter shot out the ports out, decent cats, an H-pipe, and Hooker Aerochambers exiting in front of the rear wheels sounds about right in my mind. Nothing exotic and nothing overly large.
I know we have been talking so far mostly about gm trucks. Particularly the one i own.
Id like to generalize a bit more.
Tere was a comment by asa about losing some low end torque. Thats more or less opposite of what i want to do. Low ind torque, below 3k, is where the majority of truck stuff happens in my experience. Is there a way, with exhaust, to target that area? Asa, what happened to fuel economy?
Stampie: i agree wholeheartedly that manifolds are much more reliable and quiet than headers. I wonder if there are better manifolds out there for us?
I feel like I've heard that the Vortec exhaust manifolds flow better than the TBI (and earlier) ones. Therefore, I've been collecting them for future projects (truck type projects...they won't fit in a G-body well). Can anyone back that up or is it just a tale?
In reply to ClemSparks :
If its true, that just jumped to the short list....
I had a set of long tubes (Hedmans I think?) on the TBI 305 in my truck for a few months before the LS went in. Hooked into the 3" catback that came on the truck with a catted Y pipe I built with some 2.5" tubing I had laying around and a stamped Magnaflow Y section bought from Summit. The headers definitely made a noticeable difference on the butt dyno across the entire rev range. Passing on a two lane road became an actual possibility if properly planned (keep in mind, we're still talking <200hp in a 2+ ton vehicle...), whereas before that activity was positively suicidal.
If you're interested, I've still got the parts and would let them go for a get them out of my garage price.
As far as exhaust diameter is concerned, I did some quickie calculations in a spreadsheet back when I was getting ready to do the exhaust for my RX7. Basically, what I came up with was a 3" single should support 370hp, 3.5" single 502hp, and 2.5" duals 513hp. That's all based on some math I found on the internet and probably never saved the source material for, so take it for what you will.
TBIchips.com ......some light reading when you got no time.......my go to source for TBI
if its a 90s GM product your exhaust is probably trashed by now anyways, replacing the whole thing would be a good benefit.
ported manifolds or long tubes seem to be the way to go if you are in search of torque. Shorties and mid length headers only ever seem to effect high rpm efficiency. Single vs dual exhaust really depends on merge points a tubing size. For trucks I usually prefer long tubes, 2-2 1/2” duals with an X pipe. Part of this is simply that you have near endless space under them though.
I will say this. Having owned two TBI trucks, the exhaust isn't the cork. Headers and a cat back will help, but not much. 5 hp maybe. You'll get more SOTP feel from the sound than the actual power.
TBIs use low pressure fuel nozzles that squirt liquid fuel in a throttle body that the googles will tell you flows 600 cfm... which it does, rated at a much higher pressure drop and without the nozzles. If you rate it with the the big nozzles in place and at 1.5" Hg, it flows 390 at best. It then goes through a perfunctory intake into GMs worst-flowing head ever. Even debatably worse than the heads on the 265 from the 50s. They are cast with swirl vanes in the ports that they say were there to help emissions and make a complete burn, but in honesty they were there to try to vaporize the liquid fuel that was injected by the nozzles. Then they head to the valves which are operated by a really tiny flat-tappet cam. 186/191 IIRC. They go into a cylinder with dished pistons and low compression. Overall, they are perfectly tuned for the 180-200 hp they make, and the factory exhaust is more than capable of carrying the spent gasses. Suffice it to say, the exhaust isn't where you'll find magic.
Because the TBI engines are a completely matched package, there is no one magic bullet that will uncork it. It kinda takes a whole comprehensive approach.
I have "unplugged" multiple exhaust systems on vehicles I have owned with headers and free flowing exhausts and have never been disappointed with the results. This included several pickup trucks and automobiles.
My dad put headers and dual exhaust plus a 4 barrel manifold w/Holley on his 1967 Ford F-100 w/352 engine. This really woke up that engine.
My favorite was in 1978 when I took a severely stopped up 302 V-8 out of a smog gear laden 1974 Ford Torino, removed intake and exhaust manifold, 2 barrel carburetor,distributor, fuel pump, and then bolted on an Edlebrock hi-rise intake, Holly 4-barrel and fuel pump, headers with dual exhaust Hush Thrush mufflers, Mallory duel point distributor w/ massive coil and dropped it into my 1965 Mustang Fastback. I am pretty sure the bolt on/unplugging added 75 hp or more to that smog slug.
In dealing with headers, very general rules: short/large primaries favor higher RPM. Longer/smaller primaries favor lower RPM. Just like with an intake, it's about peak velocity and where it reaches that velocity... and how it matches your torque peak. Other general rule - even the worst header is usually better than the best manifold.
If I were doing headers on a TBI, I would use 1-5/8" or 1-3/4" primaries, full length, 2.5" collector, and run a 2.5" single pipe. The TBI is done by 4500 rpms, so bigger would just slow velocity and be not as well matched.
The aforementioned Y pipe is a big bottleneck on these, but there are higher flow options out there. Seems like a possible easy button would be Vortec manifolds and exhaust system as one of the big improvements was no Y pipe. It is dual exhaust all the way to the muffler.
I have not seen what the Vortec manifold outlets look like, but the TBI manifolds have a TINY outlet if my memory serves.
I agree with Curtis about the TBI being a 'matched' setup to an extent, but at the same time I was very surprised how much of a difference going to a super cheap exhaust (scratch and dent long tube Flowtech headers, dual high flow ebay cats, dual inlet/single out no-name gigantic RV "turbo style" muffler, and 2.5" piping. I had done the 'ultimate' TBI mods before the exhaust though, so maybe that gave me a little more headroom to support the exhaust, but helped across the board.
That truck (2wd, manual, 5.7, 3.08 rear end) always surprised me with getting consistently about 23 mpg empty after the TBI mods and exhaust.
In reply to Dusterbd13-michael (Forum Supporter) :
Seems like Curtis is a resident expert on TBI and his advice seems extremely solid. Are you really tied to keeping the TBI? Not the most optimal system, pretty antiquated at this point, and going to a good aftermarket or OE Quadrajet carb could be a much better solution to improving the performance of the truck overall.
The guts of the longblock still are not the best, but how much money do you want to throw at it? A better bumpstick, lifters, and springs won't cost too much and would greatly improve things from stock. But then the heads and low compression are holding you back and you are pretty deep down the rabbit hole at that point.
Small primary long tubes into an appropriately designed collector and 2.5 inch single from there back should give you what you are looking for in terms of exhaust flow and improving low end performance. Catch the right deal from Summit, Jegs, Ebay, etc. on the headers and you could get out pretty inexpensively.
Mr_Asa
SuperDork
10/8/20 10:06 a.m.
In reply to Dusterbd13-michael (Forum Supporter) :
Well, now I could pass people, and comfortably cruise above 70, so it suffered. I dont know that I ever tried to drive with mileage in mind after.
In reply to Dusterbd13-michael (Forum Supporter) :
One correction. Tubular exhaust manifolds such as Short tube, block hugger, etc are not "headers". A actual header is designed from the camshaft and low RPM engines such as you are describing require very long pipes.
I'm a big fan of shorties, in 1 5/8" flavor, decent y-pipe, and a single 2.5" rest of the way as straight as possible and not crimp bent.
What I am not a fan of is long tubes that require you to torture yourself to install, replace plugs on, or require miles of some wrap to avoid burning plug wires. And good luck on that.
If you want "all out" power, you'd need to dump the top end and start again. IMO.
If you feel a loss of torque on the butt dyno, you probably moved the torque a bit higher in the rpm range, more than any actual loss.
If you have the access, I know Engine Masters have done a bunch of comparisons in regards to different designs, and while not a high output engine some of the results may provide information as well.
Headers leak, crack, whatever. For a vehicle that is used a few times a month and you just want to get in, start it, use it, and put it away with never an issue I wouldn't install headers. Everything after the manifolds could be done better though. But even then, I doubt you'd see any improvement under the curve. Possibly a small increase in economy, but probably not enough to justify the cost.
The flip side. I've come to realize that the better a vehicle sounds, the more I enjoy it. Easily justifies a few hundo for good sound. Like a good stereo. YMMV.
