Welcome to the first edition of "ask Mickey Dunlap".. I'd like to start by thanking him for taking time out of his busy schedule at Four Stroke Tech to answer some questions for us!

I know that one of the best things one can do to get more out of a stock 400ex is to outfit it with a aftermarket Pipe. I kind of understand why this works on a two stroke however, I would like to know how this adds horsepower to a four stroke. Thanks for any input

answer; This is a very good question because it's one of my "pet peeves" when somebody asks what pipe gives the most horsepower for a 400EX for instance. On a stock machine, you can build 10 different pipes and not really see more than 1 HP between all of them but as you build a motor the right pipe can make all the difference. The reason being, head pipe diameter can adjust your torque up and down, head pipe length will adjust where your power comes in at and the muffler system or megaphone on the end of that tuned length will also make a difference. Each rider has his own style of riding - one may ride the machine like a 125 always wide open and in a high rpm mode and another may ride like an open class rider short shifting and using the torque of the motor. The pipe can be adjusted to your riding style, the cam opening and closing and the size of the motor by changing head pipe size, length and whether or not you have a megaphone. Then you also have the question of whether you are running a spark arrestor with discs or screens. The discs tend to flow more; the screens usually kill the power pretty bad on a performance motor. With the disc-type exhaust systems (SuperTrapp, White Brothers, FMF, etc.), you can adjust back pressure for your riding style also. More discs giving less back pressure and more top end usually; less discs creating more back pressure giving more low end and throttle response. My point is, one pipe may work for one person and not for another. If you are building an engine you need to talk to your engine builder, discuss what type of riding you are doing and your riding style. I hope this answered your question without getting technical. I like to keep things simple.

Does modifying the rev box or using a White Bros. rev (cdi) box increase performance on a 400ex? I have the 416 kit with an XR cam; would it help then? Also, my quad won't start when cold if using the WB rev box. What's up with that? Thanks for your help!

answer;

I believe the White Brothers black box and other companies use XR250 boxes which will have a slightly different advance curve for a smaller, higher reving engine. Personally, I have not seen that big a difference between this and a stock box that has been modified and the rev limiter completely removed. One of the reasons I might not have seen a difference is because I build bigger 500cc motors that would not benefit from the different advance curve and if we do change the timing, it's done on the fly wheel key. The XR250 boxes need the engine to spin faster at start up to trigger the ignition coil. When it's cold out or when the engine is not jetted correctly, this will add to the problem. With a stock box that has been modified, this is not a problem. In cross-country racing we want the engine to start as quick as possible because of the dead engine starts and the hole shot is extremely important - this is one more reason why we use the stock box. I can't answer your question whether the 416 with an XR cam will make a difference or not. If it does, it's very little.

I would like to know your opion on what setup should i use to start racing mx this year??? I am looking at 416 and was wondering as far as what kind of cam and brand of piston I should run?? Also should I harden the rocker arms, put new valve spring kit, and a new cam chain that wouldn't break form all the new stress that is put on it??? Also will the stock 400ex clutch stand up to more acceleration and torque??

As of the chassis part, what type of shocks, a arms, axle and swing arm do personally recommend for motocross racing?

answer;

If it's your first year out racing, concentrating on a good handling machine so you are able to ride to the full potential of your motor would be at the top of my list. All the horsepower in the world won't do you any good if you can't ride it to it's full potential and be consistent throughout the moto. Most riders should spend more time training than worrying about how fast their motor is. So to answer your question, on the motor making everything match will be number 1 priority. A 416cc motor with a good mid-range cam would be a good start. I don't have any preferences on brand. Building reliability into your motor means you should harden your rocker arms to ensure that you don't wipe out a cam. I also recommend an oil cooler system to help with the longevity of your motor and with a good cam you will definitely need valve springs. As far as a heavy duty cam chain, our cross-country riders put more hours on a machine than the average moto-crosser and we haven't had any problems with the cam chain although we replace them once a year to be on the safe side. You also will need to modify your black box to get rid of the rev limiter. Heavy duty clutch springs are a must but be careful what you buy - some of the leading clutch people sell a spring that's no stronger than stock. Like I said before, depending on your level of riding, this should give you good power gains for your money and get you the experience to know whether HP is an issue without spending a whole lot of money. At most local tracks, if you can ride this machine as hard as it will go, you can win on it.

As far as what our team runs, we are running Houser control arms and Elka shocks. There are several companies that build good stuff. These people, however, have given us great service and sometimes that's more important.

How will shaving the flywheel on a 400EX affect the perfromance in low and hi rpm operation?

answer;

Lightening the fly wheel will give quicker acceleration but will not affect high RPM operation. I have heard people saying they get one pound off of this fly wheel - I don't think that is quite true. We don't usually lighten our fly wheels for cross-country racing. Mostly because we are running 500cc motors that are stroked and this makes them extremely quick already and lightening the fly wheel may cause it to stall out easier on a big motor. It won't hurt anything on a stock motor or a mildly built motor but the gains will be minimal.

How do you feel about porting and when do you feel it is necessary? When would you recommend full race porting? Do you recommend play porting(cleanup port) and when?

Also:

What oil do you recommend and what do you use on your race machines? Do you use oil based, semi-synthetic, or full synthetic?

answer;

Depending on what model you have, port and flow work can benefit most motors. The 300EX actually has a 250X head on it which has small valves even for a 250 let alone anything bigger. Port and flow work on this machine, even with stock size valves, will benefit greatly. When you go to port the head on it and you find it has bad intake valves (which most will) it would benefit you to go to a big valve port and flow for an extra $100 or so (plus valves which you have to buy anyway) from a reputable engine builder. The 400EX on the other hand has very little gain in power from doing a mild port job with standard valves on a 416 motor or smaller. You won't see a difference until you get into a 440 engine or bigger going to a big valve head on this one. For most trail riders, the stock head will work fine. If you run in the higher RPM's and have a bigger carberator then the big valve head will benefit greatly.

I see a lot of motors run on everything from 10/30 automotive oil to full synthetics and straight weight performance oils. I personally like a straight 40 weight oil most of the time except for on frozen lake beds where the wind chill factor can cause the oil to thicken up and cause the engine to cold seize. I experienced this the first time I raced for Team Honda at Lake George back in 1983. I have seen full synthetics (Mobile One 15/50, Amsoil 15/50 and Red Line 15/50) work very well with motors that run OEM clutch plates and heavy duty clutch springs. Mike Penland used to run it in his 435 Wolverine with an automatic clutch with no problems. I stay away from multi-grade 10/40's and 20/50's because the polimers it takes to change the oil from the light weight to the heavier tends to brake down the film strength and not give as good of protection as the straight weight oil. We personally run a Shaffer's straight 40 oil (same thing I used when I raced for Powroll) but you can use Valvoline or Kendall and it will be easier to acquire from your local auto parts store. Most of the time you will have to order in a case of it unless you are in an area that has a lot of auto racing and then they may stock it.

My names is Paul and wanted to get a 440 kit for my 2002 400EX, I've been wondering what all is involved in the "FULL TREATMENT " I mean valves rockers springs, the piston and well the whole top end. Also what is the highest compression ratio I could get but still keep my all ready reliable engine. I already have a white bors R-4 pipe, a edelbrock carb and the REV kit, with 18inch turf tamers on the rear with a 16 tooth sprocket. I'm really looking to get a 440 kit before my season starts.

answer;

11:1 compression is about as high as you want to run on pump gas and this is as about as high as I would go for longevity with the stock rod. The most important thing when buying a 440 kit is to get your deck height right so you have a good quench area around the outside of the piston to keep it running cool and to have a good flame travel across the top of the piston for an efficient burn rate. Make sure the cam matches the type of riding you are doing and the compression ratio you are running. Heavy duty valve springs and hardened rocker arms are a must with any good performance cam. For racing a big valve head will be beneficial especially with your big carberator. For longevity, an extra oil cooler will be a plus.

next to a slip-on silencer, jet change and air box mod which would be the best hp for the dollar: high compression piston (416cc or less, no big bores) or a new cam grind??

answer;

Thank you for your question. I prefer to build a motor that pulls quick and hard through the RPM range and the piston with higher compression will build more cylinder pressure which develops torque and will rev quicker and run faster through the RPM range. On most after-market pistons, the pin to deck height is the same as stock. You should have a minimum of .040 clearance between the piston and head with everything torqued down. Right now the piston is about .043 too low and you are not getting a true compression ratio as advertised by most piston companies. For instance, a 440 11:1 flat top piston advertised is really only about 9.75:1 if you measured the volume correctly. In order to get the deck heights better, I use Honda HT silicone seal on the cylinder base and an XR400 gasket if you are running an 80 over piston or smaller. This will get you close to what your deck height should be. Keeping the squish clearance down to .040 gives a better quench area around the outside of the piston keeping it cool and less chance for detonation. You will be able to run an 11:1 motor on pump gas in most cases and have a more efficient flame travel and quicker burn rate across the top of the piston. I have read on the forums here how everyone thinks a 416 is faster and revs quicker than a 440 kit with the same compression. I think most people are not running as high a compression ratio with the 440 kits as they think they are and when you put a cam with it, you end up with lower cylinder pressure making the machine rev slower than the higher compression 416 motor. Just my thoughts anyway. Without talking to each individual, it's hard to figure out why people think the 416 is faster than the 440. Any time you put a longer duration cam in, you have less time to build cylinder pressure. This is why it's important to put a higher compression piston in when you install a longer duration cam. You can get more torque the higher you go on the compression ration and your cam will give you the mid-range to top-end without losing low-end.