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..........................

[greenamex2]

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

[greenamex2]

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

[dtype38]

Originally Posted by graham bahr

d type, what are you running carb wise?

Originally I had three Weber 45 dcoe's with 36 chokes, but when I put those cams in I upped the chokes to 39s. I've since been told that they're still too small so I'm currently trying to get hold of three Weber 50 dco's with 42 or 44 chokes.

I'm surprised at the surprise at my 285 cams. The standard cams for it are only 252 deg duration so relatively speaking they are quite long! (its a 3.8 litre straight six btw)

[graham bahr]

i would certainly go along with your 45's being too small for anything other than a road engine.

do bear in mind that carbs and cams are somewhat linked, 50's could be a bit big for your current cams, so you may loose more at the bottom end than you gain at the topend, so if you cant find any 50's, 48's could be a good compromise unless your going to later on change cams for a race profile, looking in the kent cams catalog they list the following

282 fast road
308 rally
310 race

which might go some way to explaining why we are all suprised at your using a fast road cam in a race car

[R59]

Originally Posted by 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.

Al, I use this one:
Special ground for my builder by Comp Cams.

Gross Valve Lift: 0.630
Duration at .020 Tappet lift - Intake 298, Exhaust 306
Valve timing @ .050:
Inlet opens @ 29 BTDC Closes @ 53 ABDC
Exhaust opens @ 65 BBDC Closes @ 25 ATDC
Lobe lift .420 (inlet and exhaust)
Lobe separation 106.0deg

Using it with Jesel 1.7:1 rockers.

Graham, for comparison - it's in a 6.6L (403ci) V8.

I'll concur with your observations about duration vs engine capacity:

Example:

Kent AST9 (313deg) in a 1300 Vauxhall has a 4500-8500 band, with peak around 7900, though it doesn't work very well at all what so ever!

Put it in a 1600, with the same timing, it gives a band of 3800 to 8000, and a peak at 7000. Bags of grunt. And was the choice of the rallycross boys in the old Nova Challenge. I still use one, slightly retarded for more whizz, in my old Nova. Revs to 8800, peaks at 7600.

[dtype38]

Originally Posted by graham bahr

do bear in mind that carbs and cams are somewhat linked, 50's could be a bit big for your current cams, so you may loose more at the bottom end than you gain at the topend, so if you cant find any 50's, 48's could be a good compromise unless your going to later on change cams for a race profile......

Its a sort of staged process (although not necessarily planned). You are right about the 48 probably being a good match at the moment, but I hope to carry on moving up the power ladder so I'm going to jump straight to the 50s. I'm also hoping to shortly to aquire four of them (3 and a spare) within my budget

Point noted about them possibly being a bit big for the cams, but its also related to theoretical compression ratio. Mine is quite low to match the cams on my engine and may not be high enough to cope with longer ones. That's something I have to measure properly (easier said than done with crowned pistons and a squish zone that is thinner than the head gasket!) before contemplating new cams.

[Al Weyman]

Why don't you go straight to throttle body injection as triple 50's are going to cost may be cheaper in the long run and more tunable.

[greenamex2]

The answers probably going to be cost but I totally agree with Al.

On my touchy little 1600 we spent a day trying, 42, 45 and 48mm throttle bodies. The 48's, predictably, produced the most horsepower.

Suprisingly they didn't lose out ANYWHERE to the 42's and 45's on the power curve.

OK my engine has inlet ports the size of a small planet which means it's less sensitive to the inlet size being large but it did prove a point.

[Al Weyman]

It was because with the injection you do not need a head of vaccuum to pull the fuel through like you do with carbs which is why you can run much bigger. If you did the same with carbs it wiuld probably bog down at lower revs and be undrivable. Back to chevys and Holleys (sorry) but it is the same if you put a big 850cfm carb on a single plane manifold, every cylinder is seeing 850 cfm (not quite true actually as one is on a half stroke) so if you can be better off with a 750 or even a 650 cfm carb. I had an 850 on my black car and it bogged bad at low end it is much better with a 750.

[dtype38]

Interesting stuff. I have indeed seriously considered throttle bodies and a package including controller I've priced in the region of £1500. It would also seem silly to do that and not to manage the spark, which pushes the price over £2k. OK, that's not the end of the world, because it would give me better control, more flexibility, and much reduced ongoing tuning costs......

But...... its a D-type Jaguar..... and D-type Jaguars use tripple Webers. It just wouldn't feel right not to have them. It's the same argument that keeps me using a 1965 engine that weighs about the same as the moon with an output less than a decent hot hatch. I could bung in a modern 2.0ltr engine, all of which seem to weigh about a bag of sugar and produce a gizillion horsepower, but it just wouldn't be the same. What can I say? That's not to say I might not sneek a bit of spark management in (~£500 for what I want )

So back to carbs Interesting what you say about bogging down Al, and your findings too Graham. I have been told that pushing a 45mm carb up to 39-40mm choke gives more probems with low speed running than going up to a 50mm carb with a 42mm choke. The reason given what if the choke size gets too close to the carb bore, there can but insufficient increase in velocity, and hence drop in static pressure to feed the fuel properly. The bigger carb may have lower overall air velocity, but, suitably jetted, the bigger % increase in speed through the choke will give a better overall balance. Does that make sense to you?

..... Errr actually this thread is s supposed to be about heat wrap isn't it? I'll see if it dies a natural death soon, but if not I might split this part of it off into one about cams and carbs

[Al Weyman]

Thottle body fuel injections systems have been out for a long while D-Type. Admittedly not the electronics though. I sort of see where you are going but it could then be argured that you should really run a triple SU setup!

[greenamex2]

Aaaggghhh, SU's, no, don't do it!!!!

The time spent balancing them alone would more than pay for fuel injection!

Ex BL B series owner.