Engine characteristics

[flattracker1]

So i was just curios about set-ups on an engine so lets get to some hypotheticals.

ENGINE - 434,12.5:1.oshc.stock carb and intake= tons of low end and mid not so much top....
So what would happen if you went 13.5:1 and bigger cams?? Would u lose the low end?? Would the higher comp. help on the bottom that the bigger cams are gonna take away?? Would you have to go bigger bore or even stroker to get some of the bottom end back??? Does the carb (fcr or non fcr 39-40mm) make a low end change or high end change??? What does the intake change (top end, low end)??? Length of intake??? Bigger valves??? Porting?? on a 2-stroke this can determine the high or low of the engine. what does it do for a 4-stroke?? How does the exhaust change the characteristics??


Thought this stuff would be good to now for the guys that dont now these questions. Would be a great sticky put in a better configuration of course...just food for thought for the guy that want the perfect engine for that specific style of riding...

[flattracker1]

nobody has any info to provide???

[beaverclever02]

This has been covered a lot in many threads, many times. You may just flip thru some old engine build threads, and see if you can find what your looking for threre. I don't think there will ever be a perfect motor for anybody..

[TuckerHardRide]

Let me ask you this. What do you want from it and what kind of riding do you do?

[flattracker1]

well this wasnt exactly for my engine specificly. this is just kinda way to get it all in 1 place. also all the different things you can do to these engines and what changes in the powerband it makes.

[CFM-Z440]

Originally Posted by flattracker1

So i was just curios about set-ups on an engine so lets get to some hypotheticals.

ENGINE - 434,12.5:1.oshc.stock carb and intake= tons of low end and mid not so much top.... It would be better all the way through with a bigger carb, but the power will still be more low to mid oriented. I have run a 13.5:1 440 with oshc's and in my opinion they are great for the trails and can work for mx (especially tight supercross style tracks), but the low end is a little excessive, and the power could be spread out a little more.

So what would happen if you went 13.5:1 and bigger cams?? Would u lose the low end?? More compression will let you get by with a little bigger cams with out losing as much low end over it if any. So if you only go up a little on the cam size, you could potentially have the same low end, and make more mid and top end. More compression without changing the cams will make a little more power everywhere in comparison to what it makes now, but then you need race fuel.

Would the higher comp. help on the bottom that the bigger cams are gonna take away?? Just answered that.

Would you have to go bigger bore or even stroker to get some of the bottom end back??? If you make a big change with your cam sizes, yes. 540/539 web's for example would lose a lot of low end and gain a bunch of upper mid to top in a 434 compared to oshc's, but in something like a 462 or 470, 540/539's would act more like the oshc's did in the 434. Especially if the intake porting is not opened up much.

Does the carb (fcr or non fcr 39-40mm) make a low end change or high end change??? The stock carb in the Z is losing you power even in a piped 400. The FCR will make a big gain in your 434, and it will be an improvement from nearly an idle all the way to the limiter. A larger non FCR carb will also be an improvement, but you will need to go larger to get the same flow as you would have with the FCR. The FCR, if tuned right, can be a very smooth carb. There's a myth out there that they are just for top end power or that they are light switch like. This is not true at all. I can still ride wheelies standing up, one handed, on my highly modded 470 with an FCR. It takes a little more thumb precision than it did with a stock Z, but I'd never go back to a BSR even if it was big enough.

What does the intake change (top end, low end)??? Length of intake??? Intake length has a surprising amount of effect on the engine's power band. The reason for this is because of the ram charging you can get from the intake's harmonics. For example, if you slap your hand over the end of a decent length of pipe, you hear a sound. This sound is a pressure wave bouncing back and fourth in the pipe.

The same thing happens in an intake when the valves slam shut. A pressure wave bounces back and fourth in the intake. Now at certain points in the rpm range, it will time out right so that the pressure wave is just getting back to the valves as the valves open. When this happens, they get the intake charge moving into the cylinder a little faster and contribute to cylinder filling. In the same way, at certain points in the rpm range, it will also negatively impact cylinder filling. The trick is to get it to make the improvements where you want them, and have the negative effects come into play when they aren't hurting things.

To make a long story short, with intake length tuning you can beat your brains out trying to figure out what length to use, but there are a hell of a lot of variables. In general, shorter will move the power higher in the rpms, and longer will move the power lower in the rpms. The best thing to do is try different lengths and see what works. I have always had my best luck with the Z, with intakes that are around 16" from the back of the valves to where the filter mounts. If you wanted to try changing this, go about 1" at a time. This is on a home made intake which is basically a chunk of 2-1/4" tubing, with a rubber hose coupling it to the carb. It's very similar to the velocity intake they sell all over ebay, but it's much cheaper to build yourself. This intake, back to back on the dyno, ran almost identically to the 05-06 trx450r FCI intake.

Bigger valves??? The biggest restriction in the intake port is the valves, no matter what you do. So the larger valves you can run without having them interfere with each other's flow, or having the combustion chamber interfere with their flow, the better. With the Z head, the chamber that's meant for a 90mm piston is your biggest dis-advantage. You can modify the chamber a little, but I don't know if going beyond +1mm valves would be an asset. The bigger the displacement, the bigger the need for the larger valves, and larger porting to compliment them.

Porting?? on a 2-stroke this can determine the high or low of the engine. what does it do for a 4-stroke?? Porting can have a huge effect on the way the engine runs. The stock Z head is really pretty decent. Some clean up will help though. The biggest places to gain power from porting are not from making the ports bigger to flow more air. The valves are still the biggest restriction remember? SO making the head utilize all the valve area, and designing it to flow well around the valve at all points of valve lift, are where all the power is made. Honestly, most of the improvements are made within 2" of the valve seat. The rest is pretty much for show. The short side radius also is a key factor in boosting air flow around the weaker side of the valve.

If a head is ported too big, low end power will be lost, and you won't gain up top. The reason is because the port velocity is lost. Port velocity is very, very important. You've got to think about how a long duration cam does what it does. The intake valves are open way beyond bdc. Like 50-60 crankshaft degrees past BDC. Know why? Because the inertia from the incoming intake charge is still pushing it through the intake port even though the piston is now moving UP. Now if the engine can only move air in as fast as the valves will let it, then what good does it do to have the overall port bigger? You're just throwing away port velocity which is the key to making this all work well.

Now on the other side of things. What about a port that is too small for the valve size? You will have a very wide powerband with this, but it will fall off a little earlier in the rpm range. The port velocity will be higher sooner which will overcome port reversion at a lower rpm (more low end power) Midrange should be up on power, but then the torque will drop off a little earlier than it would with bigger ports. This is a much smaller problem than ports that are too big.

How does the exhaust change the characteristics?? Remember how the intake tract had pulse waves bouncing back and fourth in it? The exhaust does too. Just like in the intake the timing of these waves can have a huge effect on the way the engine performs. The idea is to have negative pressure at the valve during valve overlap, so the negative pressure in the exhaust can actually help "suck" the fresh intake charge into the cylinder, and get the intake charge accelerating faster and moving into the cylinder faster. Just like the intake, this effect is going to be better at certain points in the rpm range, and can actually work against you at times. Again velocity is very important, and so is the overall length of the exhaust system.


Thought this stuff would be good to now for the guys that dont now these questions. Would be a great sticky put in a better configuration of course...just food for thought for the guy that want the perfect engine for that specific style of riding...

Let's have a discussion about this or whatever else you guys can come up with related to the overall performance of a 4 stroke, more specifically the Z engine. I will answer to the best of my ability and anyone else is welcome to step in as well.

[woodsrider129]

Hey CFM. In your 1st paragraph, you say that the bottom end with that setup is a little excessive, and the power could be spread out a little more. I'd like to know how. I plan to soon be running a similar setup and will be running a lot of tight woods.

[CFM-Z440]

Originally Posted by woodsrider129

Hey CFM. In your 1st paragraph, you say that the bottom end with that setup is a little excessive, and the power could be spread out a little more. I'd like to know how. I plan to soon be running a similar setup and will be running a lot of tight woods.

I used to have a 13.5:1 440 with oshc's and I loved that setup. I'd still recommend that for anyone with a 434-450 wanting good low to mid, but it lacked when it got above midrange. On the track, it was super punchy feeling till it got revved, then it just sort of lamed out. This was good for a lot of things, but some times would lose traction in the corner when you really needed it to be there so you could make the next jump. A more spread out power would be faster and more usable here.

So then I tried 288/540 web cams. It lost low end, mid wasn't quite as strong, but the high r's pulled a lot harder. On the track, it was very 2-stroke like. I had to keep it revved to do anything, and if I got in trouble, It didn't have the torque to do anything about it without using the clutch. I really didn't like these cams in comparison.

At that point, I decided to get experimental. I put the OSHC intake cam with the web 540 exhaust cam, and it was the best of both worlds in my opinion. Low end wasn't what it was with the oshc's, but it was close. Mid range was also very much like the oshc's, and the top end was almost like the 288/540. It was a great all around setup. Much faster on the track and worked well everywhere. It wasn't breaking the rear end loose under throttle as bad as the oshc's in the turns, and you could use the same gear longer and didn't have to shift as much, because it was still making good power. SO compared to the oshc's it was a little weaker down low, mid was around the same, maybe a touch weaker, but where the oshc's lame out and quit pulling, this setup kept pulling hard all the way to right before the limiter.

Now Oshc's are hard to find these days, and new style hot cams are even less aggressive. The Web 536 grind is very very close to the same thing as an old style hot cam. So now days if you want to duplicate the OSHC's, a 536/536 will be nearly the same, but with a little different LCL's. If you order them both at 106 LCL, they would be nearly identical to oshc's. (I believe Web will only set them at either 104 or 106)

Now if you want to copy what I had, just order a 536/540 grind from web, and you've pretty much got it.

[woodsrider129]

Thanks for the info. My problem being with the exhaust Webs, or most any aftermarket exhaust cam is they don't have the auto decomp. Wouldn't be a big deal, but a dead engine start is critical in XC. I need her to start up fast! What would be my options at this point then?

[CFM-Z440]

If you keep your compression down into the pump gas realm, it should be ok. A high compression engine will actually fire up faster as long as the starting system can turn it over fast enough. A decompressor is only going to have it cranking longer before firing. Instead of worrying about a decompressor, I would put a Warrior/Raptor 350 in it, and larger gauge starter cables. You might go through a starter clutch once every year or two, so I'd keep an extra one around, but it will pop to life much faster. That said, if the starter only has to turn it one revolution before it starts, the starter clutch should last longer too despite the extra stress.

My 470 is 12.5:1, and the stroker crank/large displacement, is amplifying the effects on the starter. I do have the large gauge starter cables, but I don't have the warrior battery (yet) and it pops to life immediately. The starter doesn't have any trouble turning it over unless it's really cold out, like 20F or less.

My 490 was 13:1 and also didn't have any trouble starting with the same stuff.

[flattracker1]

cfm, you should wright a book... ENGINE PERFORMANCE FOR DUMMY'S.... lmfao
this was exactly how i wanted it to be answered/discussed...thx cfm

[Pulido221]

Got a question (might have been asked before) I am in the market for an fcr. Is there a difference that I can feel or HP gain I'll feel between 39,40,41, bored 42, 43,44mm, straight bored, taper bored....Is bigger better? What will be the difference in performance if any between these and by looking at my sig, which will benefit me best? Oh, and I plan on getting a noss adaptor also.

[Hughesc99]

I second pulidos question. I want to get a noss adapter also and was also thinking about carb boring

[CFM-Z440]

Originally Posted by Pulido221

Got a question (might have been asked before) I am in the market for an fcr. Is there a difference that I can feel or HP gain I'll feel between 39,40,41, bored 42, 43,44mm, straight bored, taper bored....Is bigger better? What will be the difference in performance if any between these and by looking at my sig, which will benefit me best? Oh, and I plan on getting a noss adaptor also.

With your setup, I would go with either a 40 or 41. The stock Z intake port is only about 41mm anyhow, so to go bigger than that with out porting is just going to lower the velocity through the carb. As far as the difference from say like a 39 to a 41, I don't know that there will be much other than how it pulls way high in the rpm's (provided the tuning is equally good). Logic would tell you that the smaller one would make more torque down low in the rpm range, but with a slide keeping the velocity high where it matters to the carb, at partial throttle, I think an over carbed engine would just have more tendency to stall if you stab the throttle way down low in the rpm range. More accelerator pump shot would help this.

Just recently, I changed mine from a 40 to a 41 on my 470, and It is much more responsive now. It's better everywhere actually, but I already knew that a 40 was on the small side for this big of engine, especially with the amount of air I'm moving with 539+6/539 cams.

I haven't had one bored yet. I'm still running a 41FCR with the noss adapter on my 470, and I know I could use a little more carb. From what I understand taper boring is the way to go. Sredrum is the man for the job when it comes time to taper bore I'm told. I know he hangs out over on trx450r.org. This is the route I will go.

[Pulido221]

Originally Posted by CFM-Z440

With your setup, I would go with either a 40 or 41. The stock Z intake port is only about 41mm anyhow, so to go bigger than that with out porting is just going to lower the velocity through the carb. As far as the difference from say like a 39 to a 41, I don't know that there will be much other than how it pulls way high in the rpm's (provided the tuning is equally good). Logic would tell you that the smaller one would make more torque down low in the rpm range, but with a slide keeping the velocity high where it matters to the carb, at partial throttle, I think an over carbed engine would just have more tendency to stall if you stab the throttle way down low in the rpm range. More accelerator pump shot would help this.

Just recently, I changed mine from a 40 to a 41 on my 470, and It is much more responsive now. It's better everywhere actually, but I already knew that a 40 was on the small side for this big of engine, especially with the amount of air I'm moving with 539+6/539 cams.

I haven't had one bored yet. I'm still running a 41FCR with the noss adapter on my 470, and I know I could use a little more carb. From what I understand taper boring is the way to go. Sredrum is the man for the job when it comes time to taper bore I'm told. I know he hangs out over on trx450r.org. This is the route I will go.

Thanks for the info! I will be getting a fcr39 with noss adaptor and taper boring it to a 41. With my fci set up, it should rip This info gives me a stress free peice of mind not having to debate to myself on what fcr to get like i stressed out about what hauler paddle set up to get lol
Thanks again