Dynamic CR - how much is too much

[tencentlife]

They all come with events at .050" lift on the cam card (except in parts of Europe where 1mm (.040") seems to be the standard). The difference between hydro and std. cams is in the first several thou of lift, mainly that the std. cam has lash take-up ramps and the hydro doesn't, after that it's all the same.

All these valve events, intake closing being the most important for CR determination, are moving targets. At high rpm, the rate that gases are escaping thru a certain small lift opening may be less than the rate the charge is being compressed at a certain piston speed, so compression is effectively already building well before the valve is on the seat; at low rpm, a lot of gas is going to slip past that same opening, so you can't yet call the valve closed until it's closer to, or on the seat. As far as detonation or preignition are concerned you're generally more concerned with the lower rpm situation so you really have to judge the intake closing as effectively quite a bit later than you could at a higher rpm, but it's hard to say just how much later that should be because there are so many other variables.

So the only real usefulness of either 1mm or .050" lift as a measure is for comparative purposes: if you have an engine that you know the effective CR works with a particular valve timing, then you can make comparisons as to potential effective CR of a similar engine, or a similar cam in the same engine, but if you go too far afield you're really starting all over so you make your best guesses planning, go ahead and build it, and see how it performs.

How tight did you get the squish clearance on that? That's the whole trick with these wbx's, you can run sick high CR's if you keep the squish really tight, which you have total control over when you are cutting down the piston tops like I do.

[syncrogreg]

Hi Chris, I went .005"over deck and used gowestys rings which are .040". I annealed the rings before fitting so they may have also compressed some. I measured my dish volume at 39cc which fell in line with the table you kindly supplied on this thread. I measured the cylinder head repeatedly and got 17cc (genuine vw heads). I have read several times that others get a reading of 15cc on these heads. This may be where my figures are out and I find myself with too much static/dynamic. I am running an 82mm crank, 96mm slugs and standard length rods.
What I am unsure about is if I compare the 36 valve close on the 2280 and the 51 on the 2254 am I comparing apples for apples? On the calcs it brings my pressures down considerably. Is it possible that simply swapping cams could get me in the knock free zone. The overlap on the 2254 and change in power band worries me. How rough was the idle when you ran it? I have big wheels and tyres on my syncro and need it to pull from low rpm.

Why don't cb at least give us the inlet valve opening spec online, we are working blind ordering cams it seems.

The other cam that might work for me is the original mv cam, but at $300-500 + shipping to Aus they can jamn that.

Stripped had the great suggestion of retarding the cam, which I can easily do thanks to your cam adaptor. But even if I go 6 degrees it does not considerably bring down the numbers. I guess the only way is to try it and see.
Do you have any suggestions on a cam with a late closing inlet. Is the 2254 too much of a top end cam with all that overlap? Which way would you go if you did not have alot of time to play with? Thanks Chris. Greg

[tencentlife]

You just gotta run the numbers on whatever combo you have. At .050" lift is your "apples-to-apples" comparison, so long as you're using the same rocker ratio, if different then you should correct for that in your figures. I think 6º retarded is more than I would want to use just to alter DCR, that'll shift the torque band way high.

I thought the 2254 was a good cam if the engine will rev into its power band, at the stock wbx rev range it doesn't come into its own, you'd want a good 500 or 1k higher rev limit. It did give great off-idle torque with 1.1 rockers on intake, but more lift killed the low-end velocity.

You're not limited to only hydro-grind cams, y'know, hydro lifters work perfectly on non-hydro cams, lots of moderate-duration ones will work and I've written about how to evaluate them elsewhere here (but you have to have the cam in hand). The only cost to using hydros on a non-hydro cam is a very small drop in idle vacuum.

I annealed the rings before fitting so they may have also compressed some.

Neither copper nor laminated steel combustion seal rings will compress significantly, the steels will go down maybe .001" at most, coppers too little to measure. Neither is enough to figure into your DCR calcs.

Why don't cb at least give us the inlet valve opening spec online, we are working blind ordering cams it seems.

You might get them to read off a cam card if you call, but that's doubtful. If you know the LSA (CB's are all at 107 or 108) and the duration, you can derive the events from those so long as the grind is symmetrical, and most CB cams are. You don't have to determine the precise intake close position since, as I said, in actuality it's a moving target dependent on rpms. It's only of determinative use comparatively.

[syncrogreg]

Thanks Chris, I have worked out how to do the math and get the figures. Its actually pretty easy once you understand the way the cam grind works.
So what I have done is worked the numbers on the 2280 and 2254 that I have cards for to verify what cb are doing. They are ground on 107 centres as advertised. They both have 3 degrees of advance "ground in" and the figures on the hydro cams cards are derived from "advertised duration" and not the .050" duration that the solid cams are measured on.
So after crunching all these numbers I am thinking that yes I want the increased duration of the 2254 but with wider lobe centers to decrease my overlap. Which brings me back to the stock mv cam, 109deg inlet lobe and 238 dur at 1mm and similar lift to the 2254.
So I guess my course of action has to be to get it back out and recheck my head volume and see how much material I can remove from the chamber and hopfully do something benefical in unshrouding the valves in the process. If I can't get the cr down enough I might have to go the stock cam which should work fine with two deg retard.
Any input welcome. Tell me if have it all wrong, this is all new to me. Greg

[tencentlife]

the figures on the hydro cams cards are derived from "advertised duration" and not the .050" duration that the solid cams are measured on.

No, hydro cam cards from CB and other US grinders have lift at .050" just as plain grinds do, so cams from different grinders are comparable at that lift. It's the advertised duration that isn't always calculated the same way so it's not as useful for comparison between grinders, or even necessarily from the same grinder (witness CB's 2253 vs. 2254).

[syncrogreg]

OK, maybe i have got something wrong but I thought I would just post the two cam cards and try to explain what I am talking about. When my novice eye looked at the 2254 cam card and saw figures in the 70's and 80's it just did not compute. First you can see that the print where the duration figure is on both cards is different. The solid cam says ".050". The Hydro just says "duration". The text below the images is cut and paste straight off the CB site. It shows that the duration figure on the 2254 card does corespond with advetised duration and when you work the figures off this number and add three degrees of advance you get same numbers. The numbers in small writing are what I worked off the .050" duration figure and seem to make sense.
My intent here is not to labour the point for the sake of it but to understand why CB do this. The only thing I can come up with is that the take up ramps give a false impression of the cams characteristics when measured at .050"?



CB 2280
Adv. Duration 274°
Dur. @ .050" 222°
Lift @ cam .359"
Lift w/1.1:1 Rocker Arms .394"

CB 2254
Adv. Duration 308°
Dur. @ .050" 236°
Lift @ cam .319"
Lift w/1.1:1 Rocker Arms .350"

[Piledriver]

Looks like on the hydro cam card they have it listed both ways.
(see the tiny 42 and 48 to the left and right of the card)

@.050 you are well past the ramps.

Beats me why they did something stupid, I gave up thinking about that sort of stuff 20+ years ago.

[syncrogreg]

No the small print is my writing.

[Piledriver]

No the small print is my writing.

Ah. I'd ask CB what the checking timing is at .050" used by all cam manufacturers in the US.
What's on the card is BS, but it's all they give you, so check it at zero.

Perhaps they figure folks running a hydro will never check it anyway

preload 1/2-3/4 turn, and check your adjusting screws, if they get beat up find some that don't.
Knife edges don't stay adjusted properly.