Displacement vs Compression

[junior636]

I am just curious what some of the theories are on this. I am currently building a drag car and no matter who you talk to, anyone will tell you there is no replacement for displacement. Seems to be quite the opposite opinions when talking about these small engines. I hear a lot in the 400ex forum here that going to a bigger bore doesn't help at all or isn't worth it. How can that be true? I understand upping compression raises hp but how can an 85mm 12.1 be the same or just slightly less significant than an 89mm with 12.1? I need to rebuild my 400 and cannot decide which way to go. Lets put aside any heating issues, and just assume that isn't a factor. I would love to here some opinions.

[CJM]

Of course a larger bore will yield better results, but were not talking boring a 350 out to a 396 here. Even then doing so wont gain you insane power either. Thats why so many have forced induction, NOS and whatnot ON TOP of ported/polished heads and a hi comp setup.

You also only have 1 cyl and piston vs 6 or 8. EX: Friend of mine has a stang cobra, it basically uses the same block as the regular stang 4.6L with one big twist-higher compression piston and a better flowing head. This is why in a race vs a reg GT they will always win, it has that much more power.

I can take a 350 block and spank a even a mildly worked 427 block if I were to build my 350 right. Its been done time and time again too.

The fact of the matter is the EX motor is a 397cc air cooled engine. If you up the bore size to 440-aka 435 that is indeed quite a boost. HOWEVER almost all of the time people buy those 440 kits, the piston is the same old lame compression as stock.

The main reason people dont do a 440 is b/c of the heat problems. Sure it makes good power but the heat problems are the bain of existence (sorry I had to go there).

When were talking about boring over slightly, say 402, 407, 416, 426. Your not creating enough displacement to really matter. I did a 416, friend of mine did a 407 same mods as me but I have a stage 2 HC and he has a stage 1. We are dang near pretty much the same. Mine is only slightly faster and I weigh 100lbs less than him.

If such a large bore was viable and worth it, wouldnt all quads now have 600+ cc engines? The reason why is b/c you can just take a 450 motor, slap a hi comp piston in it with the right head and spank that 600cc engine. IE Raptor vs 450.

This thread may also be of some interest.
Piston Compression Discussion (http://www.exriders.com/vbb/showthread.php?s=&threadid=487458&highlight=high+compression)

[junior636]

That was a good read, I remember that thread now that I see it again. Im just torn, I have decided on going 12:1, just cant decide what bore size. If I go with the 89mm I am going to do a larger oil cooler and spal fan, if I stay with 85mm I think im just going to add a fan to the stock cooler. Im proabably not going to race too much but do ride mostly mx so I would like the most power. Im kind of leaning towards the 85mm though just because it will be cheaper and safer and like I said im not racing anymore just out having fun, but....then theres always that side of me that wants things to be as fast as possible :devil:

[junior636]

I forgot to add that I have a fully ported head, Web 479 cam, full exhaust, aftermarket cdi, and a 450r carb on the way that should be here any day now for when the rebuild happens.

[Honda5]

We raced the 400exs for several years and none of the big bore or strokers could run with our 13.5:1 with a little off head. They would get almost a full year out of them if they were built correctly. If you want to get that hard pull from the corners just add a FCR and cut the flywheel and you will be amazed.
I like the higher compression and a mid to top end cam.

[odog]

here's the problem with bigbores most people that have them dont change anything else just the bore.so natural you will feel a big difference in the first 3gears then the power just falls off.if you change every thing else around the bike with the bore it will be a night and day difference.i probley road 10 different exs and to be honest only about 3 off them stud out.all of them had different mods only one was completely stock.3off them were 440s only 1 of the 440s were truely fast.all three of them were torque but only one of them had good top end speed the other 2 of them were so horrible on top end that when you shifted to 4th and 5th gear it was no pull at all.i rode a fast 426 that was completely modded with the exception of a carb it was very quick.and i rode a 406 that was high compression and slip on pipe and kn filter its fast and goes through the gears and pulls pretty good that was the 3rd fastest off them all.but as you know my ex is the fastest one i ever rode.the bad thing about it is with all the work that i have done.it didnt truely stand completely out of the bunch until i added that fcr carb.so to make a long story short more displacement is better but i would go with a higher compression if you dont plan on doing all the little things around the engine to compensate for those extra cc's.

[junior636]

Well I guess thats another thing, I have an FCR 39 but by popular opinion here the 450 carb is supposed to be better?

[junior636]

Who makes a 13.5:1 piston? I was planning on running VP110 mixed 50/50 with 93. Would that be enough to run on a 13.5:1 piston?

[CJM]

Not to many makers make that type of piston. 12:1-12.5:1 is pretty much where your gonna be at.

You could do the sparks 12.5 kit. its a 416 12.5:1.

I would only run race gas, its not worth it if it blows.

[odog]

Originally posted by junior636
Well I guess thats another thing, I have an FCR 39 but by popular opinion here the 450 carb is supposed to be better? well the 450r carb isnt better its cheaper to get so i guess you can say for the money its the better option but for perfomance ill put my money on the 39mm fcr any day.

[CJM]

Originally posted by odog
well the 450r carb isnt better its cheaper to get so i guess you can say for the money its the better option but for perfomance ill put my money on the 39mm fcr any day.

Id agree the power is better, but its not an insane difference. VS the EX carb it is tho.

I dont like FCR carbs, I find them annoying to tune properly. Also i find whatever it is-when a bike gets hot they also dont like to start. My ex and 450r starts everytime, friends FCR 450s take FOREVER sometimes!

[Honda5]

The FCR is a much better carb. Do the simple mods and you will be good to go. The 04/05 is to big and mellow for a 400. JE was the only one making a 13,5 piston back when we raced them. You will need the good studs or you will the stock ones. I don't know that cam you have or the ignition your using but 108 should be good.

[CJM]

^ Do you have a keihin 04-05 450r carb on yours? My 400 would really beg to differ.

The FCR is better no doubt, but like I said a total pain to tune and get setup right.

[Honda5]

I had them on 3 of our 400exs and I only had good luck with the one on my wife's 460 but it still didnt run with the smaller higher compression motors on 39fcr. I don't run them on my 04/05 450rs either you just can't get that strong hit from the bottom. Setting them up takes a little but if you Get the R&D bowl or merge adjustable leak jet and the merge spring you will be good to go.