Wildcardracing Posted April 2, 2012 Report Share Posted April 2, 2012 is there a point to where you LOSE power if the exhaust is taking to high? i know of a stock cylinder 4mil on alky running 208 ex/135 trans and it runs HARD!!! im not sure i want to take a cub that high most say to leave trans at 130 but thats stock on a cub so that cant be the best power numbers on the trans Big thing to keep in mind is where the pipe wants to operate at it's best efficiency with each cylinder design. Your pipe of choice will really dictate where the exhaust and transfer timing need to be to get the most of your setup. Going too high on the exhaust will begin to raise and narrow the power curve resulting in less torque backup which will slow you down when you shift. At some point you will have such a short power stroke you will begin to loose power and have issues starting the engine. Raising the tranfers too much will result in a motor that pulls strong in the midrange and signs off early. Stock cylinders are limited on ways to get more time area and therefore will need to be raised more to make big power. If you've not ported a cub before your best bet is to clean up the casting flaws and match all of the windows/timings then concentrate on dome design. You'll find your biggest gains in the domes. -Brandon Quote Link to comment Share on other sites More sharing options...
stroking Posted April 3, 2012 Author Report Share Posted April 3, 2012 (edited) any pictures of a alky dome vs standard ? i have many different domes but im not sure what a (ALKY) dome looks like. i also notice that when the exhaust timing is higher that 200 the compression needs to be high also to get it to rev clean and pull really strong i have a cub with 204 ex and 210psi that runs high 5's in 1/8mile but when i dropped the compression..(165psi) the ET when up in the 6's...could it be that when the ex timing is high the compression also needs to be high ??? Edited April 3, 2012 by stroking Quote Link to comment Share on other sites More sharing options...
PassionRE Posted April 11, 2012 Report Share Posted April 11, 2012 Jim @ Passion likes a shit ton of exhaust on his setups. True....But not always a ton of duration. is there a point to where you LOSE power if the exhaust is taking to high? MOST DEFINATELY!!!! Effective drag race engines ,with the exception of 1/4 mile asphalt motors require, the most violent power delivery possible. This ensures peak acceleration, no bog, and the ability to carry gear and tire. This is made possible by the highest possible blowdown pressures at just prior to rated pipe tuned length rpm.....preferrably you want the max blowdown pressures(blowdown pressure by design at desired peak hp output rpm)..to happen approx. 150-300 rpm after tuned length rpm. Blowdown pressures are the regulated by the ratio of transfer total area vs. ex total area...not just the difference in duration heights. The main reason for choosing duration vs. area wisely is effective displacement.(degrees of power stroke before the exhaust port opens). Every 2 cycle engine has a max HP capability based on displacement and rpm where effective displacement is concerned. If you exceed the exhaust duration to the point that effective duration efficiency rpm is exceeded (based on displacment/rpm )you will lose some power, but most importantly, you will lose the abiliity to violently carry gear and tire. You may still make good hp numbers, but the the tq. numbers will suffer badly.....and you just gave up 3 bike lengths in an actual race. The one exception to this is the use of "fly weight riders", or for that matter any means of less drag coefficient whether it be bike/rider package weight, or operation enviroment drag ...such as deep sand vs. asphault ... this changes everything...every engine has to be designed specifically for the intended enviroment, and package weight and tuned length of the pipe...these are the main concerns where duration is considered. 1/4 milers have the room to stretch their legs so big over-rev past rated tuned lenghth of the pipe is less constrictive. The opposite is true in deep sand ,with "normal "size riiders. Big top end motors will get killed by big tq. number engines at near the same peak hp ratings....thus "violent power delivery" saves the day. As far as duration numbers are concerned on transfers and exhaust ports, the most effficient design will vary on casting constraints, displacement, proper tuned length/max rated efficiency of your port layout, not to mention compressiion ratio and combustion efficiency. Every exhaust port and transfer for that matter , regardless of shape and duration, has an exact rpm of peak efficiency. The trick is to know exactly where it is in the design process...even a dyno cannot reveal this most critical information. Using the tuned length spec of the expansion chamber as the guideline , and then matching the max efficiency rpm of transfer and exhaust ports (using desired compression ratio), to falll extremely close to this number, is going to tell you where to set the durations at. And to throw one more wrench in the gears....miss the proper area/duration calculations on the transfer ports and you've just decreased the effects that your expansion chamber was designed around. There is NO magic exhaust/transfer,blowdown duration numbers that are predictable across the board for a variety of engines with even slightly different displacements,strokes,peak HP rated rpms, or intended uses and enviroments. Each engine is a case by case delema if earth shattering performance is the goal. If you take all of these factors in consideration, and a few more not mentioned, in advance< YES, the PERFECT numbers can be calculated.....Bottom line .....absolutely rediculous calculations are required from the very beginning of the build....Jim Quote Link to comment Share on other sites More sharing options...
registered user Posted April 12, 2012 Report Share Posted April 12, 2012 i didnt see it mentioned but something else that should be a fairly high priority when building a drag race engine is the type of tranny (override or non override) and the ratios of each gear. my honda was having a problem pulling 5th in the rpm range the engine was designed for. changing sprockets wasnt a option because it would throw the speed of the other 4 gears out of whack, so i put in a different 5th gear that shortened the gap between 4/5 so the rpm doesnt drop near as much. little shit like that can make alot of difference. if anybody messes with something other than banshee like honda or kawi or ktm etc etc youll find the trannys have different ratios from one bike to the next. a typical drag port job may not work that well on other engines with wider ratio trannys so you might have to port everything for alittle extra torque Quote Link to comment Share on other sites More sharing options...
stroking Posted April 12, 2012 Author Report Share Posted April 12, 2012 sound like i need to send you a set of cubs cylinders to get me roll'in !!! we only pavement race 1/8mile Quote Link to comment Share on other sites More sharing options...
PassionRE Posted April 12, 2012 Report Share Posted April 12, 2012 i didnt see it mentioned but something else that should be a fairly high priority when building a drag race engine is the type of tranny (override or non override) and the ratios of each gear. my honda was having a problem pulling 5th in the rpm range the engine was designed for. changing sprockets wasnt a option because it would throw the speed of the other 4 gears out of whack, so i put in a different 5th gear that shortened the gap between 4/5 so the rpm doesnt drop near as much. little shit like that can make alot of difference. if anybody messes with something other than banshee like honda or kawi or ktm etc etc youll find the trannys have different ratios from one bike to the next. a typical drag port job may not work that well on other engines with wider ratio trannys so you might have to port everything for alittle extra torque Good point...thats why peak HP only design concepts are going to be found lacking by someone with different circumstances. Your much better off concentrating on max tq....you can always add gear to make up for over-rev and rpm....Jim Quote Link to comment Share on other sites More sharing options...
Wildcardracing Posted April 12, 2012 Report Share Posted April 12, 2012 For most racing applications it has been my experience that torque wins the race. Torque=acceleration. Peak hp #'s look great on a dyno and are cool to talk about but that's all they really are.....numbers. -Brandon Quote Link to comment Share on other sites More sharing options...
SuperMex Posted April 12, 2012 Report Share Posted April 12, 2012 I do love me some TORQUE!!!! Nice input here fellas I learn something new everyday. Quote Link to comment Share on other sites More sharing options...
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