astrowrench
Member
When you go Bigger on the rear gear YOU LOSE top speed and gain acceleration.........I'm just being honest....
RPM's
- Thread starter rab3rd
- Start date
When you go Bigger on the rear gear YOU LOSE top speed and gain acceleration.........I'm just being honest....
Bruce Fowler
New member
As you go up in rear gear your top speed decreases and you have more RPM coming out of the corners. I dont type as fast as others.
Bruce
Bruce
Yeah, I was thinking backwards...
youngengines
Member
Ok, I lied.. Just a FYI, I did a buildup a while back starting with a out of the box predator. It picked up .6hp from just moving the timing up with a .080 cut timing key.
Here are results:
Stock.. 7.24hp.....3700rpm
tape blower housing up 7.41hp.....3850rpm
Header (.880, no muffler) 8.85hp.....4200rpm
air filter adapter
with .022, .036 jetting 9.39hp.....4150rpm
Advance timing (.080 cut key) 10.04hp.....4450rpm
Here are results:
Stock.. 7.24hp.....3700rpm
tape blower housing up 7.41hp.....3850rpm
Header (.880, no muffler) 8.85hp.....4200rpm
air filter adapter
with .022, .036 jetting 9.39hp.....4150rpm
Advance timing (.080 cut key) 10.04hp.....4450rpm
Thanks for coming back youngengines. Thats not an insignificant change. I ordered an 8 degree key just a few hours ago. I am intrigued by the idea of running with no key and setting the timing with a timing light and marker tape. I bought a piston stop and will be getting a degree wheel so I can start looking at timing with a timing light. The key is a non-tech item by the track rules so I can do what I want there. Too bad we cant run the raceseng timing setup, that looks sweet.
youngengines
Member
Ok, you are on the right track. I use a timing key, just because its easier for me. I check the timing with a light, and cut the key .006 for each degree I want to move it. Reinstall, and check again. Very seldom do I have to redo after that.
Here is an excerpt from an article I just read on timing. Interesting in regards to the low end loses if you advance too much. I would have assumed get a good timing and stick with it, not tune the timing to the track (assuming you travel to different tracks)
Probably the most commonly used ignition-tuning tool on the Briggs & Stratton 4stroke is the offset flywheel key. Every kart shop carries them and the savvy Briggs racer carries a selection to adjust the engine to the track. Basically, these little machined pieces replace the stock Briggs 1/8" X 1/8" flywheel key. Although they are sometimes marked in thousandths of an inch offset, more commonly they are identified by degrees of offset. In other words, how many degrees of crank rotation do they move the flywheel from the stock position. Most engine builders building methanol-burning Stock-class engines usually send their engines out the door with 5 degrees or so of offset. That amount of advance provides a reasonably safe performance increase without too many headaches for the less experienced or adventurous tuner. Part of that improved performance comes from starting the combustion process itself earlier and thus optimizing the point at which the combustion chamber pressure reaches its peak. Another part comes from the fact that the methanol/air mixture burns somewhat more slowly than a gasoline/air mixture for which the engine was designed. So you gas-classers out there take note; 2 or 3 degrees is a better place for you to start.
One thing to be careful of, silly as it seems, is that you offset the key the correct direction. You want the sparkplug to fire earlier in the crank rotation, when the piston is farther down in the cylinder on its way up. That means that, when viewed from the flywheel side, with the starter or starter nut removed, along with the washer, the keyway slot in the flywheel should be closer to top-dead-center than the keyway slot in the crankshaft, as the keyway rotates up toward the coil (Figure 4).
Ok out there, l see you rolling your eyes. But 1 have seen engines come in with the flywheels offset the wrong way. And believe me, retarding the spark 5 degrees or so won't do anything to help your performance.
Anyway, what happens when you move up or down from that 5 degree starting point? Offsetting the flywheel even more helps the engine achieve more complete combustion at higher RPMs. Starting the fire earlier, you know. But more combustion time in the cylinder means more heat absorbed in the block and head too. You'll gain top end but you'll probably need a bigger jet to keep the heat below the 380?390 degree upper limit. If you've blocked off part of the air intake on the fan shroud you may want to open that back up a bit. Most tuners aren't afraid to go up to 7 or 8 degrees, but few venture beyond there.
Unfortunately, that top-end increase comes at the expense of low end. And on track where getting off the slow corners is the difference between winning and losing, that's a poor trade. In cases like that, an experienced tuner reduce the flywheel offset to 4 or even 3 degrees to improve low RPM performance. Since the fuel/air mixture always burns, more or less, at the same speed, if you need the engine to be at its best at lower RPMs too much advance can move the peak combustion pressure back so early in the crank rotation that it actually resists the momentum of the crank and flywheel and hurts performance. Of course, to get the optimal fuel mixture at these lower RPMs, and to help get the heat up into the desired range, you may need a smaller jet if yon reduce the offset in the ignition. Closing off some of those air intakes in the shroud may help get the heat up too.
Probably the most commonly used ignition-tuning tool on the Briggs & Stratton 4stroke is the offset flywheel key. Every kart shop carries them and the savvy Briggs racer carries a selection to adjust the engine to the track. Basically, these little machined pieces replace the stock Briggs 1/8" X 1/8" flywheel key. Although they are sometimes marked in thousandths of an inch offset, more commonly they are identified by degrees of offset. In other words, how many degrees of crank rotation do they move the flywheel from the stock position. Most engine builders building methanol-burning Stock-class engines usually send their engines out the door with 5 degrees or so of offset. That amount of advance provides a reasonably safe performance increase without too many headaches for the less experienced or adventurous tuner. Part of that improved performance comes from starting the combustion process itself earlier and thus optimizing the point at which the combustion chamber pressure reaches its peak. Another part comes from the fact that the methanol/air mixture burns somewhat more slowly than a gasoline/air mixture for which the engine was designed. So you gas-classers out there take note; 2 or 3 degrees is a better place for you to start.
One thing to be careful of, silly as it seems, is that you offset the key the correct direction. You want the sparkplug to fire earlier in the crank rotation, when the piston is farther down in the cylinder on its way up. That means that, when viewed from the flywheel side, with the starter or starter nut removed, along with the washer, the keyway slot in the flywheel should be closer to top-dead-center than the keyway slot in the crankshaft, as the keyway rotates up toward the coil (Figure 4).
Ok out there, l see you rolling your eyes. But 1 have seen engines come in with the flywheels offset the wrong way. And believe me, retarding the spark 5 degrees or so won't do anything to help your performance.
Anyway, what happens when you move up or down from that 5 degree starting point? Offsetting the flywheel even more helps the engine achieve more complete combustion at higher RPMs. Starting the fire earlier, you know. But more combustion time in the cylinder means more heat absorbed in the block and head too. You'll gain top end but you'll probably need a bigger jet to keep the heat below the 380?390 degree upper limit. If you've blocked off part of the air intake on the fan shroud you may want to open that back up a bit. Most tuners aren't afraid to go up to 7 or 8 degrees, but few venture beyond there.
Unfortunately, that top-end increase comes at the expense of low end. And on track where getting off the slow corners is the difference between winning and losing, that's a poor trade. In cases like that, an experienced tuner reduce the flywheel offset to 4 or even 3 degrees to improve low RPM performance. Since the fuel/air mixture always burns, more or less, at the same speed, if you need the engine to be at its best at lower RPMs too much advance can move the peak combustion pressure back so early in the crank rotation that it actually resists the momentum of the crank and flywheel and hurts performance. Of course, to get the optimal fuel mixture at these lower RPMs, and to help get the heat up into the desired range, you may need a smaller jet if yon reduce the offset in the ignition. Closing off some of those air intakes in the shroud may help get the heat up too.
If u will listen to what young is telling you the guys that u are running against will start posting trying to figure out how to out run you. From the stock setting you have a long ways to go before its too much. Follow what young is willing to share there are very few guys that will shoot you straight and he is one of the few that will.
C&L_MOTORSPORTS
New member
yeah and the guys at the track won't shoot you straight on the gearing or set up usually. so don't believe everything you hear from your competition.
If you are new and the fast guys have some experience, you should step up the rear gear and pick up some rpms. The fast guys can run that low of a rear gear because they have experience and can drive pretty smooth is my guess, while your a beginner and still make mistakes here and there, so if i were you, i would put a couple extra teeth on the rear and pick up some rpms.
Thank you very much guys for all the responses. You have certainly given me plenty to think about and play with. Right now Im a little more than a second off the front runners average lap times. The guys running in the top three consistantly run 50.1 second laps, while I am running 51.4 second laps. I was about a third of a lap behind first in my last race (third race weekend). I figure Im doing fairly well for only having raced three weekends (total of 6 races and 3 qualifiers). Ill tell you one thing for sure, this stuff is addictive!
Rob.
Rob.
what other racing won't tell you can fill volumes....what they will tell you is just enough to be competitive, but not enough that you out run them. learned that years ago....learn by watching, learn by asking and learn by doing. i also learned that racers will, by nature, bend the rules just to the point of breaking...but they will not tell you which way they bent them.....
i hate the trial and error method of finding out what works, but that's really the only way that your going to determine what is best for you. every racer on here will give you a starting point and suggestions, but almost none of them will send you thier notes and settings for you to duplicate. research on the web will usually give you a good base on what you need and then asking on here gives you a little extra to understand what they are talking about. although!! with that being said, different parts of the country have thier own way of doing things....what works down here in the south on a dirt track may be totally opposite of what works on dirt in spokane, washington. same with sets and set ups....what works in miami won't work in denver....
so i guess the bottom line is to take what you learn and test it...record everything you do and every change you make and then go back to square one and apply it to your needs on the track. one second on the track is a lot to make up. even running third, if the leaders are running 50.1 and your running 51.4, then you are always looking over your shoulder for that leader to lap you....
i hate the trial and error method of finding out what works, but that's really the only way that your going to determine what is best for you. every racer on here will give you a starting point and suggestions, but almost none of them will send you thier notes and settings for you to duplicate. research on the web will usually give you a good base on what you need and then asking on here gives you a little extra to understand what they are talking about. although!! with that being said, different parts of the country have thier own way of doing things....what works down here in the south on a dirt track may be totally opposite of what works on dirt in spokane, washington. same with sets and set ups....what works in miami won't work in denver....
so i guess the bottom line is to take what you learn and test it...record everything you do and every change you make and then go back to square one and apply it to your needs on the track. one second on the track is a lot to make up. even running third, if the leaders are running 50.1 and your running 51.4, then you are always looking over your shoulder for that leader to lap you....
And that's understandable. Cant give away ALL your secrets, at the same time its more fun if you have some competition. ;-)
It is not so much what one racer will tell another, it all comes down to experience. The best equipment and team in the world won't put you in the front that easily. The lead driver could switch karts with you and you probably will still be 1 sec off the pace. If it was that easy the tracks would only need to buy 1st place trophies about ten of them every week.
Sure, I fully understand that. There is no substitute for experience. At the same time if you arent getting the most out of ALL the components (engine, chassis, driver) you're leaving advantages in the pits. ;-)
alvin l nunley
Site Supporter
I never worried about telling people what they wanted to know. I really never had any secrets. I was always of the opinion that knowing what to do with that information was far more important.
And not that I ever thought of myself as a “great driver” but I was right up there close.
I know one thing; it’s rare that anyone learns to drive in less than a year. And I think you learn more on a practice day, with a stop watch, than you learn on race day.
And I know this; I would never go to a practice day, or a race day, without a stop watch, an air density gauge and a compression gauge. Few listen to me, but that’s my feelings.
Comments, Compliments, Criticisms and Questions always welcome.
And not that I ever thought of myself as a “great driver” but I was right up there close.
I know one thing; it’s rare that anyone learns to drive in less than a year. And I think you learn more on a practice day, with a stop watch, than you learn on race day.
And I know this; I would never go to a practice day, or a race day, without a stop watch, an air density gauge and a compression gauge. Few listen to me, but that’s my feelings.
Comments, Compliments, Criticisms and Questions always welcome.