bumping up the power some more

[boosted3g]

My trail quad is needeing a little more power and i was thinking of doing the following mods. Do you guys think the 12:1 is too much for a stock rod? Do you think with the added compression and bore i can get away with a stage 3 hc. My current set up is in my sig.

resleeve and 12:1 JE 426 piston
stage 3 hotcam
port head with 1mm over exhaust valves

[DementeD]

stage 3 hot cams are for stroker motors
so u will probably benefit more from the stage 2 u have..

as far as the rest idk really..i heard a mild port on the 400ex isnt really worth it since the head already flows well in stock form so i guess if u port do a race port

dang you already have a lot of performance mods. I wouldnt go past 11:1 unless you like running race gas and maybe blowng head gaskets i'm not sure. A port job would probally give you a power increas because you have a better flowing exhaust, intake with the 450r carb, and a stage 2 hotcam to let it breathe and flow more so I think I port job might be good. I wouldnt go stage 3 hotcam unless you have a stroker or something. If you want a ton of power do a 440 stroker.

[DementeD]

^hes running e85/93oct. mix so i think he could do 12:1

anyway..the thing im wondering is..

i see a lot of sites say that the true comp. of these "11:1" pistons only truly see around 10:1 even a little less..

so if thats the case then why couldnt u do 12-12.5:1 and truly see ~11:1..??
since these motors are suppose to be able to do 11:1 no problem...and is that 11:1 actual compression or is that running a "claimed" 11:1 piston which really isnt 11:1..


that might be really confusing.. :P

[krt400ex]

Originally posted by DementeD
^hes running e85/93oct. mix so i think he could do 12:1

anyway..the thing im wondering is..

i see a lot of sites say that the true comp. of these "11:1" pistons only truly see around 10:1 even a little less..

so if thats the case then why couldnt u do 12-12.5:1 and truly see ~11:1..??
since these motors are suppose to be able to do 11:1 no problem...and is that 11:1 actual compression or is that running a "claimed" 11:1 piston which really isnt 11:1..


that might be really confusing.. :P

alot of that depends on the cam. a cam that has more lift and longer duration will cause some loss of compression...

[DementeD]

ok so im still curious to see if..running a stage 2 hot cam + 12:1 comp. piston..well a claimed 12:1 comp piston...could run off 93oct.? since truly itd be around 11:1 maybe even less..



basically ...i need to figure out how u find the true compression of the piston in the bored cylinder..

i know for honda cars they had a compression calculator that would tell u the static and effective comp ratio..


so say maybe something that has a static of 12:1..would really only be an effective 11:1 which from what everyone says will work fine on pump gas..
its really confusing over all ..i guess i just wish the companies that make the pistons were 100% correct on comp. so u wouldnt have to figure all this stuff out..

The lift is how far the cam opens the valve. The duration is how long it leaves it open. The more lift and duration the more aggressive the cam. Big lift and duration numbers work best at high RPM while more mild numbers are best for lower RPM.

Also, the more lift and duration of the valves the more important it is to have a high(er) compression piston. This is because with high lift/duration there is more opportunity for air to escape through the valves as the piston begins its compression stroke. With an aggressive cam some dynamic compression (i.e. the actual measured psi of compression while the engine is cycling) can be lost. This must be made up for with a higher compression piston. An aggressive cam with a stock compression piston may actually perform worse than the stock cam because too much compression is being lost through the valves being opened so much.


cam: Intake lift; Exhaust lift; Intake duration; Exhaust duration

stock............... .316, .302, 234, 238

XR400.............. .326, .321, 240, 244
HRC................. .350, .339, 254, 256
GT Thunder....... .350, .345, 256, 256
WB Track.......... .350, .345, 256, 256
WB All Around.... .345, .335, 240, 248
WEB 450/451..... .350, .345, 256, 256
WEB 479........... .378, .378, 250, 250
WEB 463/9i....... .385, .370, 272, 270
Hotcams stg 1... .354, .354, 246, 246
Hotcams stg 2... .354, .350, 254, 248
HC stg 2 (old).... .348, .349, 254, 254
Hotcams stroker. .358, .354, 262, 257

Notes:

--Numbers are listed in inches and degrees of crank rotation (lift duration at approximately .050" lift).
--The Hotcams stg 2 old version is identified by "HCDG NO. 08" etched onto the cam--not sure what the current version says.
--White Brothers All Around is etched with "WB1870".
--White Brothers Track is etched with "WB1498".
--Notice that the WEB 450/451, GT Thunder, and WB Track are all the same grind.
--Honda part number for the HRC cam (it's actually an XR400 HRC cam) is 14000-NKK-000. I don't think they are still available from Honda.

The lift is how far the cam opens the valve. The duration is how long it leaves it open. The more lift and duration the more aggressive the cam. Big lift and duration numbers work best at high RPM while more mild numbers are best for lower RPM.

The more duration the intake valves have the more important it is to have a high(er) compression piston. This is because with lots of duration there is more opportunity for air to escape through the valves as the piston begins its compression stroke. Even on the stock cam, the intake valves are still open when the piston begins its compression stroke. With an aggressive cam some dynamic compression is lost through valves that stay open for a long time. This must be made up for with a higher compression piston. An aggressive cam with a stock compression piston may actually perform worse than the stock cam because too much compression is being lost through the valves being opened so much. I have personally seen this happen--dyno confirmed (although it wasn't a 400EX).

Here's a theoretical example to help in understanding this concept:

-bore: 85mm (stock)
-static compression ratio: 9.1:1 (stock)
-elevation: 1500ft
-intake valve closes: 40deg ABDC (After Bottom Dead Center) (stock at 1mm lift)

The resulting dynamic compression ratio is 8:1. Note that is a smaller figure than the static compression ratio we started with. Again, this is because the actual compression of the compression stroke doesn't really start until the intake valves close.

Now, if we increase the intake valve duration to 55deg ABDC and leave the static compression ratio at 9.1:1, we get a new dynamic compression ratio of 7.4:1! In order to get the dynamic compression ratio back to where we started (8:1) with the increased intake valve duration we would need a piston with almost 10:1 static compression. Just for the heck of it I looked at how 5500 ft of elevation changes things. At 5500ft with the 55deg ABDC spec you'd need a 10.8:1 static compression piston to yeild an 8:1 dynamic compression ratio.

Now, you might be thinking what's the point of adding a cam and a piston if doing both gets you the same dynamic compression ratio you started with?? Good question. The answer is that now you have the same dyanmic compression ratio you started with but also have the cam specs that will allow the engine to breath better. And in the real world, most piston choices are actually going to bump your dynamic compression beyond what it was stock even with the addition of a cam. A 10.5:1 piston with a cam (using the 55deg ABDC spec) would get you a dynamic compression ratio of 8.6:1--0.6 points higher than all stock.

I used a dynamic compression ratio calculator to get those figures. I guessed on certain required values because there's some information I don't have. So, the above numbers are not quantitatively exact. But the relationship between static compression, dynamic compression, and intake valve timing IS accurate.

[boosted3g]

The only thing that scares me with the stage 3 is the lobe centers. I get worried when they get that seperated and the fact that its such a big jump from a stage 2. My guess is that it will pull hard in the top end because of the 8-9 degrees more duration. In drag racing we view that as the peak power moving up about 8-900 rpms. I dont want to loose much more down low and hope that the compression can take care of that. Ill cc the engine and find out exactly what compression ratio i have but im aiming for 12:1. Id like to go to 13:1 but i dont trust the head gasket for the amount of riding i do. There are times i ride for 8 hours straight just trail riding and hill climbing.

[brian76708]

not to steel your forum but did you notice a big difference with the 11:1 piston and cam i was thinking about doing that setup

[boosted3g]

yes it was a big difference.

I called around today and got prices to do everything and in the long run its not worth touching the 400ex any more. The engine is getting pulled and a hybrid will be born. Anyone need an 07 engine thats looks and runs better than new? Ive built race cars but this will be my first quad build.