Carbs and Cams (Split from "heat proof wrapping" thread)

[graham bahr]

Originally Posted by dtype38

.
Spookily enough, I fitted a much higher flow head to my car (same cams, no turbo) and wrapped my manifold and exahust. I got masses more mid range torque but no more top end power..... ?

most likely is that the gains came through increased air flow through the head, but the cams wont allow better breathing at high rpm

[dtype38]

Graham, you could well be right. I have quite short duration cams (285 deg/11mm lift) which give me good mid range pull. That's working really well for me on track so I'm loath to lengthen the cams and risk trading that away for the sake of top end power (there are folks I race against claiming a lot more power with humungous cams but it doesn't seem to convert to good lap times :-)). Anyway, The rolling road people, plus what I've read both suggest that my carbs are too small, so I'm gonna try increasing them before going for more cam...... errr sorry for the diverstion. Back to heat wrap..........................

[Denis Bassom]

285 deg duration!!!!! Isn't that just a fast road cam?

Oh the pleasures of a decent capacity engine in a relatively light car.

Anybody tried the heat coatings on a dyno on something with a lot more cam, say 320 degrees?

[Tim Falce]

Originally Posted by Denis Bassom

285 deg duration!!!!! Isn't that just a fast road cam?

Oh the pleasures of a decent capacity engine in a relatively light car.

Anybody tried the heat coatings on a dyno on something with a lot more cam, say 320 degrees?

Not even fast road, mine are 308 deg duration with 11mm lift and are only classed as fast road.
dtype, are you sure they are only 285, the amount of fuel you pump through the exhaust I would have thought they are more like 360 degrees

[Al Weyman]

Yes I have a mild(ish) cam in the yellow chevy and that has 310

[dtype38]

Correction: The cams are 285 deg, but because I use 10thou valve guide clearance I only actually get 277deg of valve operation. Do you think its a bit short??

[Al Weyman]

Well I don't know the engine dtype but mine could definitely use more cam, it is a hydraulic stick as well. Now the solid roller cam I have in the black chevy that would make your eyes water, I will dig out the specs tomorrow at the shop if I remember.

[Chris Y]

I use a standard road cam, and even I think 285 is short

[Denis Bassom]

Having said that, I do use fuel injection which can mask some of the more negative points of a long duration cam on a small capacity engine.

Would have still thought you could get away with something around 295/300 without it losing too much mid range, particularly if it was fairly conservatively timed.

[graham bahr]

285 is a very mild cam for such a big engine, in a 1000cc engine a 285 is very racy but a cam effectively gets " milder " the bigger your engine gets.

i would certainly agree with dennis in that you should be able to use a 300 degree cam with no ill effects, infact i've just used a 300 cam in a roadgoing opel manta engine and thats very drivable thoughout the rev range, if your cars a pure racer i would of thought that 320 degrees would be closer to the mark, infact thinking about it i used to use a 336 degree cam in my 2.0 8v beemer

[graham bahr]

d type, what are you running carb wise?

[Al Weyman]

I just checked the specs on my solid roller cam in the black car and wonder if we are geting confused here (well I am at least) maybe Graham can shed some light if I give the specs. Would it be because this is measured at a higher lob lift then dTypes as on the face of it, it looks lower duration still and I assure you it is a very powerful cammy engine.

Cam spec at .50" Lobe lift

Intake opens: 25deg BTDC
Intake closes: 54 deg ABDC
Lobe center: 105 deg
Lobe duration: 260 deg
Lobe lift: .390"
Gross valve lift: .585"
Rocker ratio: 1.5

Exhaust opens: 66 deg BBDC
Exhaust closes: 24 deg ABDC
Lobe center: 110 deg
Lift duration: 270 deg
Lobe lift: .410"
Gross valve lift: .615
Rocker ratio: 1.5

Sorry forgot the duration on the exhaust.

[graham bahr]

AL, looking at the raw figures your cam looks mild,

however i notice the duration figures are at 0.050 lobe lift, most cams are quoted at 0.040 VALVE lift

which is a double whammy, as both the increase lift and the position of the measurement will make the duration figs low, i would say taking both into account your duration figures would need about 40-50 degrees added to them to get a better comparison with other cams.

the real factor which ultimately dictates how hot a cam is is a rarely quoted figure and thats lift on overlap or TDC

[Al Weyman]

I may be wrong but think because it is a solid roller cam it can have much steeper ramps therefore I would assume the duration would not neccessarilly need to be so great and maybe less overlap would mean more midrange power, as you can see it has a lot of valve lift at .615.

[graham bahr]

true to a point, but i'm certain the only real difference between your race cam and most others is in the meassuring, a few thou of valve clearance can alter the duration by quite a few degrees, in your case your talking 10 thou difference in the meassuring point, plus the difference between measuring the lift figs at the cam and at the valve, which is multiplied up by the rocker ratio