Help calculating dynamic compression

[Richardab1967]

I'm going round in circles trying to understand what ivc figure to input into various dcr calculators.

My static works out at 8.86
My cam is a cb2280 and says ivc=39 on the cam card
Or if i do the calc i have seen on the forums duration/2+lsa-180 i get 224/2+106-180=38
Ok maybe not a big enough difference to worry about.
I assume these figures are @0.05

But one calculator i have looked at (uempistons)
https://www.uempistons.com/index.php?ma ... 5505410b4f
asks to add 15 degrees to the [email protected] figure giving a dcr of 7.6 which i am happy with.

Where as the not2fast calculator
http://www.not2fast.com/turbo/compressi ... sion.shtml
just wants a plain ivc figure. When i input 39 i get a dcr of nearly 8.2 which feels a bit close to the edge

Is the ivc figure meant to be @zero, how do i work that out? Whats the 15degrees about?

Any help for a first time engine builder gratefully accepted
Cheers

[Piledriver]

Any calculation that does not include the ramp from valve lash to .050" is not following the standard method, so will produce different results.

The notsofast calc is nicely done otherwise. "losing" half a point in the calc might be their idea of a safety measure?
Aircooled heads run hotter than watercooled as a rule, but some watercooled vws seem to defy that.

The 15 degrees is only a reasonable approximation that is accepted.
Some online calculators have the 15 degrees built in.
A few cams have longer than 15 degree ramps from zero lash to .050"...

It's more of a quick sanity check anyway to see if your cam/CR combination is reasonable.

A DCR much above 7.0 is getting into premium fuel only territory on an aircooled engine, depending on a tight deck, chamber design, even plug location... you might be able to go a little higher or need less depending on application.

A DCR of 7.6 would need good squish, premium fuel or E85, or at least premium and a proven knock detect/retard subsystem.

[Richardab1967]

Thanks for that.

So the plus 15 degree figure is the one i should use. It is clearly stated in the uempistons calculator and i have seen that calculator recommended here often. Maybe youre meant to add it on the not2fast one, but it would seem like an important bit of info if you were.

So it seems the dcr is 7.6, thats a relief, I know 7.6 dcr is at the top end (i have been reading that 7-8 is the limit), so i am reasonably comfortable to give it a try, as uk fuel is available in 95 and 98 (approx 91 and 93 us fuel), i also have my deck down to 1mm. I can always take heads off and increase chambers a bit more, but if the dcr had been 8.2 then i would need to do alot more work.

[Dougy Dee]

Some non-VW world cams dont come with .050 numbers.. I believe the add 15* number was to take the .000 lift, or advertised duration numbers, to an off the seat duration figure. A number similar to the .050 numbers. Not that it matters but I use the Wallace Racing calculator.

Most T1 cams come with .050 numbers published. I find that keeping track of the Static and DCR numbers for each engine I build helps me to keep the compression ratio near but not over the ragged detonation edge.

I am a fan of the 2280 cam and will set it at 8.25 to 8.5:1CR with 93 octane Canadian gas.

[Richardab1967]

Thanks.

Is 93 octane Canadian fuel based on US AKI system or euro RON

I tried the Wallace calculator, same question, do you add the 15 degrees or not to your cams IVC number. It does say at the top "Use seat to seat specs for Intake spec for best results" does that mean its based on zero lift number?

[Piledriver]

Seat to seat ~means use advertised duration, so it includes the ramps and then some.
That isn't always published but is a little easier to measure than .050"

[Marc]

"Advertised duration" is not a precisely defined term and different manufacturers measure it in different ways - therefore it's not good for much besides comparing two cams from the same grinder. You can infer some about the clearance ramp profile by comparing the adv dur numbers of cams from different grinders which have the same duration at .050".

"Effective" duration at or above .050" is published by nearly all grinders and is more representative of how the cam's going to act. It's generally accepted that there's so little flow below that point that it's not significant, at least not on a running engine - but it can affect the dynamic C.R. calculation and the number attained on a cranking compression test. Final cylinder pressure depends upon both the dynamic C.R. and the volumetric efficiency, which varies with engine speed. Because of the variables involved, some "fudge factor" may be applied to the calculation to make the resultant number realistic...I wouldn't go so far as to speculate about which ONE it's correcting for. And again, the valve lash setting used can affect the duration and therefore the dynamic C.R.
In Europe you may find the "effective" duration specified for valve lift at or above 1mm (~.040") rather than .050".

In the US, the convention used by most manufacturers is for the adv dur number to represent the degrees of rotation during which the valve is off the seat by .020" for solid lifters and .006" for hydraulics. The valve lash specified on the cam card is used when making the plot, so the looser the lash the lower the advertised duration will be for a given cam (for hydraulics lash is assumed to be zero). But some grinders don't follow convention and (for example) advertise the duration where the valve is at or above some smaller amount off the seat - perhaps .012". That will inflate the number compared to using .020" lift, assuming the same lash. Another grinder might measure duration directly at the lifter (before the effect of rocker ratio) which gives the possibility of a whole 'nuther set of numbers (or two, depending upon how they roll the clearance number in). Those who do take all measurements at the valve must specify the rocker ratio used too...When you see specs listed "at the valve" along with some notation like "with 1.1 or 1.25 rockers" there's going to be some error, since a larger ratio will cause higher valve lift at all points on the same lobe. It's no surprise that two cams with virtually identical numbers "on paper" can act noticeably different and why there remains some art to cam selection - until every grinder starts using the same methodology you can only tell so much from the published specs, if you really want to know you've got to plot each cam yourself.