Hi Johnny Law, Mike Ham from Tubi sales, post 12 of this thread, says their collector is 2.13 inches OD, so bigger than the 2.00 inch OE collector. Beyond that, I think we're into the realm of custom exhausts and if I go this way, I want to get it right. I'm thinking a 2.5 inch, or at least 2.25 inch OD, is probably what's really needed for the collector and down stream. Primary diameter and length are still up in the air. What happens if a 2.5 or 2.25 inch collector is necked down to fit a 2 inch muffler inlet pipe? This would allow fitting of my existing muffler now and maximize any benefits of a large bore muffler system if fitted later. I need to come up with one more input set and ask both Mark and Vince to do one more analysis run. Bill
Hey Bill, I'm all for getting it right. Just never heard back and wanted to find out if this was going to be produced. Thanks.
"... my design change of opposing, helical gear angles, to cancel out axial loads, on the center transfer gear." Sorry to be the bearer of bad news Bill but think about the direction of the forces on the teeth-- opposing gear angles doubles the axial load on that middle gear.
Hi Derek, That's a great observation. Thank you. I got it wrong. Do you agree forces cancel when both helix angles are the same, like the single OE middle gear? The factory got it right. The stacked gears must have the same helix angle. Time for new drop gears. I may have to fit the Us spec 328 gears for now until I can get replacements. It may be worthwhile fitting up the current set just to see if there are other issues. Bill
I would do some calculations- torque, force, angle and see what the force will be on the thrust bearing and the cover/bell housing. My gut feeling is it will be ok. Will it push towards the cover or bell housing? How thick are the castings? What effect will a few thou distortion have?
Hi derekw, The drop gear kit manufacturer says give it a try.In reality, the cover is about one inch thicker than the 328 one, to accommodate the stacked gear train, so likely stiffer due to the taller section. This new cover required 10 longer, cover studs with the correct locking thread. It turns out the OE header system is really a 4 into 2 into 1 design on both banks. OE tube lengths I posted earlier are not correct. The takeaway is that the factory seems to have tried to broaden the torque range and improve fitment with this design and I concur. If I do a new design, I'm leaning towards a big bore design throughout. Do you think this will desensitize the effects of exhaust tuning (larger bore-->lower gas velocities/higher pressure-->reduced gas pulse amplitude?) and broaden the torque range, or less peaky if you will. Bill
Bill, the original exhaust manifold isn’t bad. Drive the car and start enjoying it— we’re not getting any younger. I’d rather have a few extra weeks enjoying the car than a few extra hp. You can always change it later once you have a better feel for what it may need.
A header that is larger than required will make less power everywhere. A header that is smaller than required will make more power up through midrange then fall off on top.....so as a rule you generally want to err on the small side.
Hi derekw, That is the backup plan. The car won't go back on the road till the spring now, so I have a several month window. Other things still need to be done like cleaning up the engine bay and replacing fuel/vapor lines. I hung the OE headers to get an idea of what the engine clearance looks like. Primaries in the 30+ inch range may be possible. This is a NOS rear header. It appears there is room for longer tubes toward the cam drive end of the engine. Image Unavailable, Please Login The front header. Image Unavailable, Please Login Sniffer tubes I made from stainless brake line tubing and a custom top bracket so tubes could be replaced individually. Image Unavailable, Please Login I need to add the grommet mounted bracket to tie it to the nearby bolt. It is well clear of the OE air cleaner and easy to get to. I think a simple silicone tubing cap will work to plug the ends when not being used for idle mixture tuning. Image Unavailable, Please Login Hi Mark, I apologize for my absence but I had to move out my 66 Lotus to make room to move the 308 in. A number of other issues need to be addressed on the 308 but with both the engine and car in house, it makes sense to consider a new design. I think we need a little parametric study to quantify the effects of larger vs smaller OD tubes. Everyone seems to agree, big collectors, up to 2.5 inch OD are good. I'd like to see this case run with 1.5, 1.62, and 1.75 inch primaries and see what the output looks like on your DM6. First, I'd like to ask Burns to run a couple of more design cases to establish lengths for a 4 into 1 design tuned to enhance torque the 3500-5000 rpm band. This is still above my normal driving range, but closer. The following is the proposed proposed Baseline input for a run on their tool. Any input before I submit it? Image Unavailable, Please Login Bill
Well, a couple things. Collectors are complicated and there really is no such thing as a 2.5" collector....I see people talk about in those terms but its not real. By basic physics/math the input to the collector is the equivalent diameter of the combined are of the tube entering it. So when you change from 1.5 tubes to 1.625 tube you have by definition changed the size of the collector. Then ,and I tried to explain this already, the exit on modern headers is designed as with a choke followed by a cone that can expand to any size you like to match whatever you're running for exhaust system.....where 2.5" seems about right, no harm in going big with any of that. If you don't have a choke in the design then the ENTIRE exhaust system right to the tail pipe tips becomes the collector...depending on what type of muffler, a glasspack is collector, an open chamber type may be able to terminate the waves. Adding a choke means you don't need to play this guessing game, the collector ends at the choke and the collector "size is the area of the pipes coming in and the area of the choke going out and the distance between them.....so you see that saying you want a 2.5" collector doesn't really mean anything? I don't have the DM6 key back yet, I can ask for it but I can also just tell you the answer because its always the same result the smaller the tubes the better the low end but the sooner the top end starts to roll over. As you go from 1.5 to 1.625 to 1.75 the power will drop below about 6k and a dip will start to get bigger and bigger in the 4-5k range but the top will creep higher and by 1.875 you have the dyno test headers that builder like because they make great peak numbers. the 1.5" will probably make about 5hp less at peak as a reference and I ran that design for you already so you can pencil that onto you dyno sheet then the other options will be in between those lines. Collector length is pretty important (hence the choke so it can be better controlled) but changing primary length has less effect than you might think...I used to be all about " the tube lengths MUST match" to now I'm certain a couple inches of mismatch means nothing so its better to use the largest bend radii you can fit and don't worry that it caused tube length errors. Tri-Y options always produce the flattest torque curve....ALWAYS. I don't think I've ever seen a chase where step headers didn't make the most peak HP or were the worst in the midrange. Standard 4-1 work pretty well and are the cheapest so they are the most common. So if you say best average torque 1k-8k you get tri-y. If you say best torque 3500/5k there is a pretty good chance you'll get 4-1 with the collector length tuned specifically for that range (the collector length info is not on the design burns sends out so you need to buy the collectors from them). Asking for both I'm not sure...probably tri-y....generally anything driven on the street wants tri-y so that is the design I would/did do for you but burns software works different and picks the best for the specific condition entered and ignores everything else hence which can result in sever collateral damage like the dyno headers caused and burns step design would have caused. Think long and hard about that best average vs best result at a specific rpm...for a street car I think I'd suggest best average torque 2k- 6k might get you what you're after, a nice driving car that will still make peak hp up around 8k or so. 1k is idle and I can't imaging you ever actually drive below 2k or maybe 1800 so I would include that in the design requirement as it just confuses things and there is very little that can be done that low without really destroying the top, think 1" or 1.25" primaries when you are talking about 1000 rpm. The "BEST" is about what compromised make the most sense for you personally. Since this is a street car you plan to drive I'd strongly recommend a tryi-Y with 1.375 or maybe, MAYBE 1.5" primaries and have a car that drives nice and isn't a bugger to get the carbs tuned right....and if you really also want a higher peak number add a 50hp nitros kit so you still have a car that drives nice. there is a reason the factory chose the design they did and a modern design with a choke on the final collector will will make that design work even better. My second choice would be burns 4-1, it will make maybe 5hp more on top than the tri-y and isn't too bad down low but I would even consider going any larger on the primaries as the dip that starts to form really buggers the mixture when you have carbs which makes the dip get bigger faster than my software predicts as the software unless I go in manually and guess at the mixture changes. Good luck!
