Remember Eric, the neighbors car has Cats in the system, so bypassing them is free HP. I used "wing nuts" on mine (welded the bolts in so they were captive) and used some heavy gaskets.................but it wasn't pretty crawling under the car loosening the wing nuts and then tightening them again. Why not ask Mr. Roberts to help you design a system from the collectors on your shorty headers..............................I'm sure he would be glad to help you.
I'm not sure where the cutoff is of when open vs closed exhaust doesn't make a difference in et, but I can tell you that when raceway park ran a Friday night bracket program that was muffled only, that the best exhaust/muffler setup that I know of that my friends bolted on to be able to race was a loss of 2 tenths, I bolted on a set of 3 1/2 pipe just long enough to put a set of 3 1/2 race flowmasters to the backseat rise in the floor, with a set of turndowns, and I went from 10.60's and 70's to high 80's and 90's, just shy of three tenths. Im sure that there are better exhausts out there, but for the 10 of us or so that put exhaust on to legally run on Fridays, that what we found. these were mostly 10 and 11 second cars. Duane
How much do your street tires hurt you? I know guys running M&H, mcreary, and Hoosier Dot tires that run as well as guys with slicks, if you have street tires that you need to leave @2000rpm and walk it out so it doesn't spin, then I would say it isn't gonna help that much, and you should spend your money on good tires and suspension first. it never ends what to spend your money on, you just gotta figure what to spend it on first, and to spend enough in that area that you don't have to spend it again. because we all have done that at least once Duane
I'm still young so I'm fine with crawling under the car for a few years undoing wing nuts. Groberts101; how am I able to determine where to weld on the cut outs? I had a plan of 2.5 down pipe coming from the shortys to jus a little longer than the regular length of long tube and then weld them on there, but if you have some way to determine the best place to put them where they would flow the best I'm all ears... Extra power is extra power & if it's there I want it. As far as tuning the carb goes I planned on a wide band a/f gauge I could plug my laptop into and see the a/f ratio at rpm levels. If I pick up a couple tenths from this I'll be more than happy.
Don't have much time at the moment with the holidays and meals and such.. but I love this kind of design engineering stuff and would be happy to help however I can when I can sit back down to the computer later on. If it was warmer out right now(-14 below this morning) and you had the cash to drop ship me afew items I needed(I probably have all of it anyways.. but just saying).. I'd just mock up a set for you. Personally.. I prefer using only mandrel bent tubing and merging a notched radiius into the straight pipe rather than notching/cocking the angle of another straight pipe and creating sharp transitions/more turbulance. But.. exhaust gas/pressure has a good way of finding the path of least resistance when being allowed to bypass full length exhausts.. mufflers, bends and all.. so it's not so picky sometimes with only another horse or two coming out of that extra time and cash. I gladly spend it.. but not everyone cares to go for that last nearly immeasurable amount at all costs and it's easily understandable for a street/strip kind'a deal. As for length tuning.. that can be tricky and the old spray paint or temp marker trick doesn't always give accurate results. Gnerally speaking trial and error at the track is the only proper way to do it. Even using engine dyno's to tune collector lengths can leave some additional power on the table since they do not completely mimic a car running down the track either. Best to have the cutout as close as physicaly possible to the header collector and then tune the remainder of the length with a flanged pipe/extension coming off the cutouts flange. This design can also help flow slightly too since the gasses exiting off a flat flanges face can induce some shear/tumble as compared to flowing out the sharper end of a pipe. I've also seen a few well design street strip systems with capped cutouts that actually sandwich the block-off plate BETWEEN the cutout and the flanged extension pipe. Then all you do is unbolt the extension portion from the cutout.. remove the block-off plate in between.. and bolt the flanged extension back into place. Most seemed to be around 4-10 inches in length due to the cutouts needing to be placed further back just to clear the trans bell housing. There are several ways to tune for improvements with extensions and some are having to forego the optimum lengths coming off the collectors and shoot for the second wave pulse rather than the primary. Too short of an extension and you leave power on the table.. "just right" picks it up.. and slightly too long puts you back at square one or possibly drops it even further from your baseline power level. Then.. going even longer yet to take advantage of the second wave tuning brings it back up again.. even if to a lessor degree. On street car chassis with cookie cutter headr designs(far too tight to the trans bell housings).. we just have to take what we can get with the packaging restrictions. Still sufficient power to be had with proper test and tunes though.
I just reread that post again and now see you have shorties on here. Ouch! Long tubes would've given you more power up until peak torque and likely beyond too. Oh well.. we work with the parts we have and within the space we have to use them, I guess. I'm not sure how serious your build is here.. but I can give you some basic theory pointers to help you move in the right direction.. or we can get far more serious about spec'ing the exhaust to better tune what you have already bolted up with a program I use called pipemax. Never worked with numbers relating to shorty headers though.. so it may be a learning curve for me as well in that regard. Due to those headers more space constrained design elements.. I'm guessing you'll need upwards of 20-28 inches of additional collector extension to take advantage of proper wave tuning parameters. To give basic ideas and starting points here.. I need TONS of info for your engine specs all the way from air cleaner to tail pipes. And.. more importantly.. can you weld? Here's a fairly efficient style of design that many use for more serious street/strip cars. You'll notice right away that the dominant flow path is towards the cutouts rather than being biased towards the rear "street exhaust" section. Just depends on whether you want to live with reduced street flow.. or reduced race flow, is all. If you want other more realistic examples of what you'll lilkely have to live with under these cars tighter space constraints.. I can dig for a few other pic's of slightly improved cutout designs as well. The typical cookie cutter designs such as the ones from Hooker and others is very sharply transitioned on the interior and the flow losses can affect both primary(street) and secondary(race) systems. I've cut 2 apart to hand blend/smooth weld them out and you can actually cut your hand on the interior edges. After all that work.. I just make my own from scratch these days without costing much more. PS. if you have the smaller 2 1/2 inch collectors on those headers?(which is not all that bad because most don't realize that 3 inchers are usually too big for these little motors anyways) it may also be beneficial to incorporate some reverse cones coming off the cutouts exit. This helps with expansion and if designed properly can turn your smaller collector into somewhat of a venturi style collector that many of the merge collectors implement these days. Not sure if you want to really tear into your existing exhaust design too much at this point.. but was just another thought that came to mind.
i used this system on my car and we used the 3" version on mashori's. i located the cut outs on mine before the x. the cut out plug has an insert that goes into the bend to give it as easy and smooth of a turn as possible. on mashori's we put the cut outs after the x. i really dont know which way is better but we both notice a feelable gain in performance. these kits are really inexpensive for everything you get
The most effective location for cutouts is nearest the collector when they are merged into the design so that they act as collector extensions for the sake of proper pulse wave tuning. The harsh reality for any street cars is that it's pretty tough to package them into tighter quarters, especially like the ones we must deal with on these particular chassis.. so we have to settle for the 4th, 5th, and even possibly the 6th harmonic or greater. Generally speaking, the shorter the better due to frictional losses associated with pushing gasses down the interior of any pipe..especially those with cross sectional changes of bends.. but proper timing of the pulse at the back of the exhaust valve is the key to making the most of the scavenging affect and ultimately helping the intake velocity out in the process. That's why some ocassionally see losses when moving to a less than optimum length cutout length design from a fairly closely timed(usually by luck alone) half back setup. But the reality is that properly timed cutouts combined with additionally tuned extension pipes.. will always take the greatest advantage of pulse timing and reduced pumping losses associated with reaching the atmosphere more quickly. The rest is just unwanted weight and frictional loss mandated by space constraints and sound legality.
At the risk of sending this thread "off course": why do some people still say a particular car/engine "needs some backpressure"? I'm sure you've ears it before but every race car I've ever seen (without sound restrictions) runs open exhaust for no backpressure. It makes me wonder if these people have actually ever ran their car with open headers as we have done.
because they have an inadequate understanding of the dynamics involved with exhaust and intake relationships. You'll NEVER hear any highly competent or professional engine builder speak those words. Some of the misunderstanding is based on factual cases where additional backpressure actually helped a particular combination produce more power. But the reality is that those situations only come about because there was some crutching/compensation involved where camshaft timing/overlap and/or intake/exhaust runner tuning was not correct in the first place. Conversely, too much exhaust scavenging on an ill prepared/mismatched engine combo can also kill BMEP just as quickly though.. so it's all about proper component matching to make the most of any combo.
With just opening the cutouts my 331 picked up 5 hp at 6,800 and 4.5 tq 5,300 on the chassis dyno, but caused a leaner condition. So with cutouts open and more jet, may have picked up a couple more.
i just realized i didnt put the link and pics of the system i was referencing in my above post. that must have been confusing. heres system used on mashoris 3" http://www.jegs.com/i/Flowtech/387/53631/10002/-1 heres the one i used that is 2.5" http://www.jegs.com/i/Flowtech/387/53630/10002/-1 the pricing for these seemed really good for every thing you got. they even have these with electric cut outs in the kits
iowa When I was young we ran the same set up with removable duals and worked great. Boy those where the days, throw the tires in back an extra race fuel tank a few tools and a load of people and off to Cordova, what a blast
