I have the 420 Long rod kit. I raced a quy and he beat me in a sand drag. He was runing with stock bore. He told me to but my stock jugs back on. He said that you can make more power out of my stock jugs because you can rev the littler bore more than the big bore. He said he would do the work. I have already spent alot of money on the big bore long rod kit. If any one has any suggestions let me know.

 

Big bores will cause you to loose some revability. I do not like big bores without also going to a stroker crank. The big bore really doesnt help with the stock crank, you need the extra stroke to get those big pistons moving and doing what they are suposed to. I know a guy witht he same kit as yours and we wear him out all day long on our true 350'S let allone our big motors. Even with a stroker crank, I like to stay with the samller pistons because they rev harder and faster. Iam assuming trinity did this kit for you? Their porting leaves allot to be desired. There is no comparison with teh trinity bikes and the other bikes that we race with. They just arent on par. Trinity sent one of there 500's to "represent" them at our drags. They also sent 500$ for anyone to beat him. He had it on NO2 as well. well to cut to the chase, we all took turns humiliating him. He was nowhere as fast as anything down here. I'm not trying to burs your bubble or anything, I am just giving my experience. The big bore should atleast runas fast as a 350, it shouldnt slow you down.

 

Well the big bore sleeve will take away some of the transfer port area and that will give you LESS peak horsepower but the Long rods are good and give slightly more piston dwell on TDC and BDC . This will increase the time for the cylinder to fill up. Since you are only talking sand drags the smaller piston and stock sleeve might be the way to go. But drag race him again on a flat slick hardpack road. The broader powerband and superior torque that the larger piston gives and you will blow him away.

CT250R

 

This is interesting. The question I have is - On your 420 kit, is the motor fully ported as well? I am pretty sure that you can buy that kit with and without porting, at least from Trinity. If it was fully ported, then (assuming it was done right and is running well) you should be over 70 hp with it. LRD's 420 kit with pipes and carbs dynos around 75 hp, with a very wide powerband, but I believe it is a stroker + bore. Best numbers I have seen for a non-alcohol motor at 350cc's is about 70 hp max and it is quite peaky. On alcohol???? You tell me. How much more could you get?

What you are saying confuses me, unless there is something very important missing.

Regarding the other posts - I am wondering... Are you trying to tell me that more displacement does not equal more power??? Or is this confined only to a banshee motor at 350cc's when compared to a 420cc long rod bore only kit?? I just need some clarification. This is interesting to me since major engine work is on the near horizon.

 

Yes I have my 420 long rod kit fully ported. They say that the stock cylinders make more power. I have not yet turned it in to an aky but I am leaning towards it but don't know what way to go. I may leave it as is or soup up another one of my quads. I have a Blaster, and a 250r. The more info I get they say that my Banshee will make the most power. Brandon if you can give me a better direction PLEASE E-Mail me. I will sell one of my quads if I have to. Thanks for the help but I still don't know what to do.

 

If it were me, i would have the engine reported. If you send it to someone like ruddy kurtz at port magic I gaurantee it will be faster. There is no comparison between trinity and the other companies. I never liked trinity, but I get more negative everyday that i tlak to people. Before spending a bunch of money, have it ported by C&O or port magic. I am reall good friend and customer with bruce at c&o and if you talk to him tell him I sent you( Brandon carter from louisiana). He can flat out port as well as rudy kurtz at port magic. If this is not the problem, then I would say go to a 5 mm stroker crank. The pistons may be too big and heavy for the short banshee stroke. I suspect you are a victum of both scenario's. Alky will definetly wake it up, but get it right on gas first. What kind of pipes are you running? Toommey's I hope. Not many other pipes will rev high enough for a fully ported engine. Displacment is good, but it has too be done right and proportional. If I am correct you have 72mm pistons. That is HUGE! I have friends with 460's and they only run 68mm. Piston weight may very well be the down fall. The more you stroke and bore an engine the more rev you loose, but you gain hp. The hp you gain is more to the left. The 350 can be ported to a more higher rev and peaky band. A fully ported 350 is hard to ride for this reason. It takes a little effort to stay on the power band. I've been through allot of different set up's and I think the best for a larger diplacvment engine is small pistons and long stroke. The light pistons let the engine rev even harder because they are beeing driven with a long stroke. The cheapest way to find the problem is to try and report it first. Allot of your trouble will be right there.

 

Couldn't agree more about engine set-ups.....At very least a square motor, like lets say 72mm X 72mm. I've got a SAE Tech. paper that showed as the ratio of bore to stroke became greater power was influenced(for the worse in comparasion). I'm at work but will try to remember to look up what the best ratios that the paper found out worked best.

 

SAE 1999-01-33442 An experimental investigation in the effect of Bore/Stroke Ratio on a simple 2 stroke Cycle Engine.

The most important conclusions paraphrased here:

Engine power was at the greatest with bore to stroke ratios of 1.0 to 1.2

Efficiency of combustion tended to decrease with an increase in the bore to stroke ratio.

Mechanical efficiency had a tendency to increase as the bore to stroke ratio increased.

The BSFC (brake specific fuel comsumption)(Very important factor in predicting a motors power potential and longevity) tended to rise with the increase of the bore to stroke ratio.

So in a nut shell, purely from the scientific standpoint, a square motor(1.0 ratio), 80mm bore, 80mm stroke, or a slightly bore biased motor(1.2 ratio), 85mm bore, 71mm stroke. Make the best power over a wider range of RPM's and load conditions.

 

Thats some real good info Backcountry. I new from experience and just thinking aobut it that a square motor would be faster, but now I have written proof. The motor I am building now, I plan to make it as square as possible. I am probably going to wind up wiht a 61mm stroke and a 66 mm bore. That is going to be as square as I can get it because I do not want to go any bigger on the stroke. This ratio will be 1.08. Thats pretty close! I am looking to produce around 80hp with a wide band of power. This motor should be a round 420cc. That trinity 420 is a 1.3 ratio. Tahts not to bad, but it obviousely is not producing the 80hp I will. Actually, I have seen the trinuty 420 on a dyno. With alcohol and drag pipes it amde.....56hp! Trax 310 built a banshee for a friend and it made 58hp also, but it was a true 350 and on gas with toomeys! So you tell me????????

 

i just entered this discussion. it is time for a rebuild. who makes the best overall big bore kit as far as power and reliability are concerned? my current setup is twin 35mm keihins,T5 toomeys,reedspacers,k&n's,cool head,and fully ported