15 replies to this topic

[Artful Dodger]

Hi,

just a quick question, what valve springs will i need if i run a ST 649 camshaft?

i know they have to be be matched to the cam to stop valve bounce etc but have no idea what ones i need:P


cheers!

[bmcecosse]

It's a much higher lift cam - so as long as you have double springs in there - they will be compressed more than normal = higher pressure. It will depend just how many revs you want to use, and what size valves are in the head ? Standard springs (in good condition) should be good for 7000 - but of course a 649 is only 'getting going' properly at 7000 !!

[Artful Dodger]

hi,

well, this is the thing, i know what i want, but dont know how to get it really

i want to use 35mm inlet and 30mm exhaust valves, will these be the best option? i dont want to offset the valve guides so is this the best sizes to use?

i am building the engine to be able to reach the rev range that the 649 is 'on cam' for, so the engine will rev higher than 7000, 8500 should be the red line

[bmcecosse]

Oh well - that's going to be an expensive engine build then! Everything will need to be fully lightened and balanced, and centre main strap fitted. And yes - you will need stronger springs for these revs. Ask the suppliers what they recommend. But you probably need 180 lb springs - make sure they don't go coil bound.......

[Artful Dodger]

yes, it is!!

been working my sock off around school ( and now college) and every penny has gone to this over two years, probably spent 200 pound not on minis in that two years!!

MED race crank, bladed, wedged and balanced, ultralight flywheel, back plate and then balanced as a unit. centre main strap, race bearings, uprated pushrods ( thinking about carbon ones?)

ok thanks. i have seen companys sell 240 lb springs, if a 649 cam only needs 180 lb what cam would need that higher poundage?

and whats coil bound? never heard that term

[bmcecosse]

I wouldn't worry about the push rods....... Coil bound -when the coils of the spring touch each other when the spring is compressed.....not good.

[Turbo Phil]

If you're planning on running it to 8,500 rpm the 180lb springs will not be suitable. You'll need something heavier. For the kind of spec you're looking at I use MED's race springs.

[Ethel]

I'm not sure you can still get carbon rods, but any weight you can save on the valve gear will reduce the spring poundage you need. Have you looked into using a less radical cam with higher ratio rockers?

[Artful Dodger]

so 200 lb or 240 lb springs will be the ones to get then. MED ones it is then, cheers!


hmm, shame about the carbon rods, spose they would have been mega bucks anyway..

i was going to run the 649 with higher ratio rockers at a later date anyway ( just want to get the engine running now), been talking to oldskool tuners about the 649 and they said it had a great profile but for out and out performance, it didnt really have enough lift. unfortunately i didnt ask them about what springs would be good for it.

[Sam Walters]

There are better cams out there than the 649.

The best valve springs ive found are swiftunes race springs. The only ones that stopped the valve in my 1293 bouncing above 7500rpm.

[Artful Dodger]

yeah, but you cant get them for free mate ;)

ok, il check them out, thanks! :)

[bmcecosse]

Too heavy springs are a BIG mistake - as are high lift rockers (especially with too heavy springs!) - but carry on - it's your engine........

[Artful Dodger]

i know that springs that ere too heavy are not a good idea, i know that they have to be correct fro the application and for the cam chosen, but turbophil said that the 180lb springs wont cope with 8500 rpm, and the engine can and will take that. its an out and out race engine.

but why are high lift rockers a bad idea?? i know that its better to put a cam in because you get the whole different profile of valve overlap instead of just more lift, but the cam im using needs a bit more lift to work to its full advantage.

[Turbo Phil]

You want to use the lightest poundage springs you can get away with, but if you're intending on spinning it to 8,500 then you're going to need something pretty strong.

[Artful Dodger]

is there any way of actually calculating it?? i can see you can get 180, 200, 240 lb springs but is it just a guessing game or is there any maths to it??

now looking at the cam profile it will peak at 8000, so the cam would be on its way out at 8500, bt still, judging by what you are saying im going to need something pretty strong..