Author Topic: possibly a dumb question (looking for extra bottom end) (Read 7001 times)

Vroomy · « **on:** April 28, 2014, 05:57:43 AM »

hi guys ive had a real good ride on my stock 88 motor with CT pipe, vforce3 reeds, 36mm airstriker and no airbox. Loved it through the trails and dunes, revs real nice. Only thing im not happy with is the bottom end, would really like to increase the low end power for steep dunes and popping the front end up etc. Only thing i dont want to lose the rev ability so is this gonna be possible? also dont want to make it too thirsty either as we do quite long rides. Any advice would be great

Bigred · « **Reply #1 on:** April 28, 2014, 08:15:20 AM »

Pail turner type 6 pipe will make the power come on sooner or ur looking at more compression/race fuel or a stroker crank or if your cyl isnt ported a low to mid port would help

Jerry Hall · « **Reply #2 on:** April 28, 2014, 10:57:44 AM »

Quote from: Vroomy;32564

hi guys ive had a real good ride on my stock 88 motor with CT pipe, vforce3 reeds, 36mm airstriker and no airbox. Loved it through the trails and dunes, revs real nice. Only thing im not happy with is the bottom end, would really like to increase the low end power for steep dunes and popping the front end up etc. Only thing i dont want to lose the rev ability so is this gonna be possible? also dont want to make it too thirsty either as we do quite long rides. Any advice would be great

The exhaust pipe design has the largest influence on the shape of the power curve. Borrow an arm load of pipes and try them on you bike and you will feel what I am talking about. You may find that a different pipe may give you what you are looking for.

Adding compression just lowers the exhaust temperature and makes the engine think that you installed a lower RPM pipe. Adding compression basically moves the whole power curve to a lower RPM at the expense of loosing a similar amount of RPM at the higher RPM.

If you lower the cylinder (ports) it will also move the whole power curve to a lower RPM but also reduce top end power.

I read a lot of stories on the internet about shops offering porting that drastically improves low end. I have not found that removing material in the ports will help the low end but have the opposite effect. Some shops also putty up the ports in their quest for more low end. Be cautious of the shops that offer low end porting. I see a lot of messes and it usually reduces the engines ability to rev and make power at the top end. The ports can be butchered so that the power is reduced enough in the mid and top end that it does not hit hard giving the impression that it has more low end but the dyno most often shows that power is reduced everywhere.

The_Steve_Man · « **Reply #3 on:** April 28, 2014, 11:34:52 AM »

A pipe will make a huge difference. For example. I have a cylinder ported low mid power and have a CT pipe (it is on an atc but it applies ) . The powerband hits hard and the power is definitely in the mid range. I switched to a fmf and it gave me more bottom end and took away from the top. ATC pipes are more difficult to find and try different ones.
Like Jerry said just try different pipes.

Vroomy · « **Reply #4 on:** April 28, 2014, 05:58:10 PM »

ok thanks for the info, so which pipe would be a good start for the power curve im looking for?

pinned250r · « **Reply #5 on:** April 28, 2014, 06:55:47 PM »

I think the TRX5 is a pretty good all around pipe, but it is definitely good on the top end. But maybe someone else can chime in on the 5, i feel like ive seen it on mx applications. [MENTION=201]Burns363R[/MENTION] ?

Jerry Hall · « **Reply #6 on:** April 28, 2014, 07:02:49 PM »

Quote from: Vroomy;32620

ok thanks for the info, so which pipe would be a good start for the power curve im looking for?

The most difficult part of my job is trying to meet the expectations of my customers. One customer may say he wants more top end. Another guy wants more low end. Saying more top end or more low end is very vague. Everyone has a different expectation of what they want the power to feel like.

If I know the power curve of a particular engine and someone wants 5 more Hp at 7000 RPM I can usually tell them if it is possible or not. If some one wants 3 more HP 500 RPM after the power peak, I can say no problem, I have already been there and know exactly what to do to satisfy that request........ or I may say we will have to do some testing and see if we can make it happen. I hope you can see where I am going with this because if someone can put numbers on things it makes it a lot easier to know exactly what the target is and if I can hit that target.

I think that a lot of engine builders also like to use the terms low end, mid-range, and top end because it gives them an out if they do not meet the expectations of their customer because everyone has a different definition of those terms.

Can you put some numbers on it like "I would like for the power to hit 300 RPM sooner" or I would like it to have the power come on at the same RPM as an xxxxxxxxx Pipe.

Burns363R · « **Reply #7 on:** April 28, 2014, 07:57:36 PM »

The trx5 is a great all around pipe. Strong in thw mid range with good over rev which I like. The pipe has a good bottom end as well. Its a very broad pipe.

Vroomy · « **Reply #8 on:** May 01, 2014, 05:11:49 AM »

hey Jerry im not real great with figures and stuff with motors all i know is the power seems come on strong later than i was hoping for. Spoke to Allen at CT before i bought this pipe and i was meant to bump up comp with the CR head gasket but was having leaking problems with it so changed back to stock style gasket. Not sure if that will make a big difference? also read a bit about having a square motor (bore same size as stroke length) would a 310 kit be a good improvement? how do they rev? it would make the motor square 72mm stroke/ 72mm bore

aberegg05 · « **Reply #9 on:** May 01, 2014, 12:27:57 PM »

A good square motor I like is the 76mm stroke and 76mm bore. Which is the 344.

Jerry Hall · « **Reply #10 on:** May 02, 2014, 01:54:35 AM »

Quote from: Vroomy;32808

hey Jerry im not real great with figures and stuff with motors all i know is the power seems come on strong later than i was hoping for. Spoke to Allen at CT before i bought this pipe and i was meant to bump up comp with the CR head gasket but was having leaking problems with it so changed back to stock style gasket. Not sure if that will make a big difference? also read a bit about having a square motor (bore same size as stroke length) would a 310 kit be a good improvement? how do they rev? it would make the motor square 72mm stroke/ 72mm bore

There is not really that much magic in a square engine. If you are designing an engine from scratch it would be a good place to start. If you already have an engine I would not chase that rabbit at this time. Time and money will be better spent getting the ports and a pipe working togther

Raising the compression a little will shift the power curve to a lower RPM on most two-stroke engines.

Adding a spacer ring inside the pipe flange will also shift the power curve to a lower RPM on most two-stroke engines.

Advancing the ignition timing on a stock ignition will shift the power curve to a lower RPM on most two-stroke engines.

Making the carburetor a little rich will sometimes shift the power curve to a lower RPM on most two-stroke engines.

The list can go on and on as far as changes that can be made to move the power curve to a lower RPM. When any of the above changes are made it usually affects other parts of the power curve and may actually narrow the power band. (it may make the power come on 300 RPM sooner but take 500 RPM off the top end.)

A well developed engine has all aspects of an engine working together and will often make a lot of power over a wide RPM range. It takes a lot of testing to get all those components or design aspects of an engine really dialed in so that it will beat up on larger displacement engines that are not well developed. Just adding displacement in a quest for more power is often a sign of an inexperienced engine builder. Getting the most out of what you have will show who knows there stuff.

Vroomy · « **Reply #11 on:** May 02, 2014, 02:30:18 AM »

Yeah I'm hearing you, as we got the jetting on the carb correct I noticed the powerband got narrower. Ultimately I'm wanting that broad stock style power but more of it. Not sure whether to mess with pipes? Gonna cost a lot and as you said everything needs to match or I'm chasing my tail.

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Onski326 · « **Reply #12 on:** May 02, 2014, 08:52:02 AM »

Get the jug ported by a reputable shop. There's a handful around I keep hearing about that all seem to do a great job.
I messed around with pipes and they were good for tuning what I had. Got my jug ported and bam! Huge difference. Then messed with the pipes again to fine tune it. TRX5/6, CT National, Sparks MX, Paul Turner 6 are all good woods/bottom end pipes.

udontknowme · « **Reply #13 on:** May 02, 2014, 07:53:13 PM »

if you read some other sites alot of people have jumped on the bandwagon of cutting down a cr250 jug .030" in a quest for more power at lower rpm. im highly skeptical that it works as ive never seen anybody post legitimate proof other than a butt dyno. maybe it does work slightly but i can almost bet a million dollars it knocks down the midrange and top end because the reduction of transfer area. most people dont realize when you do stuff like that it almost certainly affects something else, usually for the bad

Victork · « **Reply #14 on:** May 03, 2014, 01:51:23 AM »

Like it was said already, porting is gonna make you a happy happy man! Just make sure you have your plan laid out like what carb/pipe youll use, your exact power goals like you stated, and use a builder whos good.