Four Stroke Tech's Mickey Dunlap Honda 400EX Peformance Tips

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 Honda 400EX engine 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.

4punksdad asks;

2 part question for you.
#1) What is the best way to clear up the cold blooded 400EX. Mine starts hard when its cold out, and it will foul a plug if not nursed to life just right. I have heard people suggest everything from needle adjustments to removing the choke. What do you suggest? (400ex with stock exhaust, drilled airbox lid, and a 155 main jet)
#2) Do you suggest a foam air filter, or the paper K&N type and why?

thanks for taking the time to answer our questions. I have learned a lot so far.

answer

For the most part, we usually only have to turn the fuel screw out 1 to 1-1/2 more turns than stock but usually we have installed a pipe and opened up the air box. You sound like you have it jetted a bit rich for just opening up the air box with a 155 main jet. It's difficult to give you perfect jetting specs without knowing your elevation, temperature, etc. Another thing that hinders starting is after-market black boxes and valve settings (usually your exhaust valves go closed). Although I like the K&N because it flows good, I suggest running an Outerwear on the K&N's at all times and in real dusty conditions a foam wrap around the K&N also. An after-market foam filter will work fine for all that you have done now. It's only when you build the motor up with cams, pipes and port & flow work will the air cleaner hinder you.

Jeff asks;

Hey, first of all, it is great that you are taking time to answer our questions. Now to the good stuff. I was wondering what long term affects does running my 400ex on the lake (in the winter, of course) have on the engine? I know that guys are upping the rev limiters, but is it going to do any long term damage running at high RPM for extended periods of time? Would studding my tires offset this? I really like bombing around on the ice racing track, and impressing the snowmobilers that things can actually turn on the ice, but if it is going to be the end of my fun prematurely, I'll have to just stick to the trails. Thank you so much for taking the time to answer my question.

answer

I am glad I can take the time to help give back to an industry that has been so good to me. Thanks for your encouragement. In most owner's manual, even on a stock engine, they warn against continuous high rpm use because it may lead to wearing out the engine prematurely. My only real experience of ice racing was in 1984 when I rode for Team Honda at Lake George and I found out real quick that you do not run straight 40 weight oil. We cold seized the motor in about 10-15 minutes because the wind chill factor would cool the front of the cylinder and the engine cases enough to make sludge out of the heavy oil. A multi-grade oil, even a synthetic such as 15/50 Mobile 1, would be advised for high rpm, high speed running. That's about the best advice I can give you except for stay on the trails - your motor will last longer!

oynot400 asks;

Mr. Dunlap,
After reading your answer to TREX400's question, I was wondering how you get the cylinder volume clearance to 0.040 inch on a 440 engine. I am sure that the head gasket must be different to accept the new sleeve. Also how can I check to see what my compression ratio really is?
Thanks,
oynot400

answer

Decking the cylinder is the proper way to set up your deck clearances. Most head gaskets will be about .040 thick so you try to deck the cylinder so that your piston comes up exactly to the top of the cylinder at top dead center. I suggested going to a thinner head gasket at about .030 (like an XR400 or Cometic's 3 piece steel head gaskets for the 440) and then not using the .020 thick base gasket and this would get you close without having to deck your cylinder. As far as figuring out what your compression ratio is when your engine is completely together, you need to measure the cc's of fluid it takes to fill your combustion chamber up to the bottom of the spark plug hole, make sure your engine is at top dead center and you tip your motor back and forth to make sure there are no air bubbles in the combustion chamber. For example, if you have a 440 motor and it takes 50cc's of fluid to fill it up, you would take 440 + 50 divided by 50 to give you 9.8 to 1 compression. To get a true 11:1 440, you will have to have a 44cc combustion chamber volume. Most so-called 11:1 pistons will have closer to 50cc's. You can get a syringe that measures in cc's at your local pharmacy and you can use just regular motor oil to fill your combustion chamber.

Eddie asks;

Just had a question reguarding the true benefit from having a 406,416 or 426 11:1 vs a Stock bore 11:1. What is the difference that you have seen between these and how much does race fuel effect the performance at this compression?
Thanks for your time-

answer

Boring a motor 80 over usually makes good benefits but nothing real big. If you plan on keeping your machine for a long time you will want to keep as many over-bores as you can. If you are truly running an 11:1 piston race gas is not necessary but the lead in the race gas will help cool your exhaust valves and gives better lubricating properties for better ring seal. The super-unleaded we get these days is usually mixed the methanol and has a high oxygen rating which makes it a high-performance gas in a way. At our local drag races in Bedford (where Cannondales are made), super-unleaded is illegal because it is a highly oxygenated fuel and it blows the meter right out.

T.B.OILY asks;

hello...thanx for taking the time to help us at exrider's, get answer's to some troubling questions'.....here's my question..it's ganna be pretty long..i'm going try and be as descriptive as possible..... 1996 300EX....i'm having problems with my bike i think it's the rings but i'm not sure..i've gottn' all sorts of answers from several different forums all over the net...
I'm runnin 14t / 38t with 18/11/10 tire's on aluminum rims....(i'm thinking the small tires is what started all this)....one day i ran the bike hard for a streight streach ran through all 5 gears wide open just to see what kind of time i got in a certian distance...after i got turned around i looked down at my left leg and it was coverd with oil....i tracked the oil down to this rubber tube under the gas tank that goes up over the motor and back down into the head on the right side....took the seat and stuff off and found that the tube comes out of the head...and splits (tee's) and one end goes to what i described above (the left front side of the motor) and the other end goes to the back of the motor and points down at the ground by the swing-arm..(chain side) neither end of this hose has anything on them....they just end.....my question is..what made that oil shoot out like it did...it lost all but maybe a hair on the dip-stick.....is it my rings....what are these hoses for......i've riden it since all this happend and sometimes it will shoot out and sometimes it doesn't....like the other day.....ran it all day long sometimes hard sometimes not.....then right at the end of the day...i got on it real good....sounded like my rev-limiter kicked on.so i let off the gas and happend to look down at my leg......oil everywhere again.....(although not as much came out that time.just a little)....it's still reliable cranks easy and runns strong....seems just as powerful as before.....how should i go about getting this problem resolved....do you think it's the small tires..i was runnin the stock size before i got these...and had run these small ones before without any trouble.....i am about to get another set of rears.maybe 20's or 22's but these will still be my extra set on rims incase i get a flat......my motor is stock with a uni filter (clean) 4 1 inch holes in my lid..and rejetted for the D.G. pipe i got.....i can't think of anything else that would help you to answer this question any easier.or you may already know the answer....if you need anymore info...let me know so i can get this figured out before spring rolles around....also.....i ride my bike all the time should i stop untill i get this fixed.......
thanx again
T.B.OILY

answer Yes, your smaller tires let you run at a higher rpm and made your problem more prevalent. At high rpm's your rings are not sealing and they are letting hot gases blow past your rings into your crank case causing high crank case pressure and forcing oil out your breather tube. Without knowing more about your engine, you either have cylinder walls that are scored or the rings are just worn out and it's time to re-build it. If your motor hasn't started to burn oil yet, it soon will because you are washing the oil off your cylinder walls with the hot gases passing by the rings. When you tear it down you will also want to check out your intake valves on the 300EX. The stock valve springs doen't let the valves float but at high rpm when they hit on the seat they bounce and it causes them to prematurely wear out. You will notice the 45 degree angle will be cupped on the valves and they will need to be replaced. It sounds like you just need a good re-build and while you have it apart, you can usually build a little bit of performance into it with a higher compression piston for about the same cost.

fourplay asks;

I have a 2001 400ex with a T4 slip-on K&N jet kit 14 tooth front sprocket. I use the bike to race harescrambles. You mentioned in a previous post thast you used a silicone seal on the cylinder base, along with an XR400 gasket. Would it be benefitial to do this with a stock bore and piston?

Also, what is your opinion about advance keys for harescrambles? Would it benefit me to run a XR400 cam with a stock piston along with the modifications I have done? Any advise would be appreciated.
Thanks ...... fourplay

answer

Yes, you could put an XR400 head gasket and eliminate the base gasket to get higher compression with your stock piston. This will also give it a better quench area around the outside of the piston and give you a better burn rate across the top of the piston. As far as running a bigger cam with this for cross-country racing I wouldn't. I would like to see at least 11:1 compression before I went to a bigger cam. In cross-country racing we have put tachometers on our machines and most of the riding is done between 3,000 and 5,000 rpm's so to get through the woods as quick as possible, you need the most torque to move the machine quickly through the woods. If you run cross-country races that are more open then a cam may be beneficial. Most of the time we do not advance the timing except for on a motor that has a big cam in it and high compression. Motors like this tend to need more time to burn extra fuel.