Honda Car Forum

for those of you who work with older d-series motors, have you ever
wondered why the cam timing between the d16a6 cam is "half a tooth" off
from the d15b2?

here's what i think is the answer - [please check the math]:

d16a6 stroke = 90mm
d15b2 stroke = 84.5mm

difference = 5.5mm. that's 2.75mm difference in deck height between the
two blocks.

how does that affect cam timing? if the deck height is different, the
belt length between crank and cam has to be 2.75mm longer [if we're
using the same head, which we are].

now, diameter of the cam wheel is 114mm. radius is therefore 57mm. <-
[you will see this number again!]

so, if the chord of the circle is 2.75mm out, that's 2.75/57 = 0.0483
radians difference in angle. convert radians to degrees = multiply by
180/pi = 2.768 degrees. but, the cam/crank rotation ratio is 1:2, so
2.768 * 2 = 5.535 degrees. there's 38 teeth to the cam gear, so 360/38
= 9.47 degrees per tooth, so 5.5 degrees is nearly half a tooth.

BUT!!! also note, 5.5 degrees is numerically the same as 5.5mm. why?
180/pi = 57.298 degrees per radian, or approx. 57 degrees, close enough.
make the cam gear 57mm radius and the 1:1 ratio gives you an easy way
of figuring out how much to adjust the cam timing for the difference in
deck height.

honda engineers? smart dudes i'd say.

how do i know all this? this weekend i went junkyard surfing and scored
a zc cam - single cam head variety. wondering whether this was a
bolt-on for my d15b2 crx, i wanted to check facts, so i bought a stock
d15b2 cam as well [$14 each btw!!!]. you can see the difference in cam
profiles [although it's not radical], but with the naked eye, you
/can't/ see the difference in key-way angle for the cam gear. well, you
kinda can, maybe, but it's not something you'd want to bet money on.
so, you therefore have to measure. some blocks of wood, a spirit level,
a school protractor, various other bits of metal for pointers, some
messing about, and you have a measuring jig! sure enough, what is the
angle difference in the setting of the key-way between the two cams?
5.5 degrees. best i can measure at any rate.

conclusion:
the good news is that for $14 and with some good luck, you can put a hot
cam in an old banger just for fun. the bad news is that you then have
to spend $120 on a variable cam gear so you can account for the timing
difference caused by the difference in deck height.

otoh, if you work with decks and heads that have been skimmed, or
non-stock head gaskets, you now know how to time the cam - simply
calculate the difference in stacking height from stock in mm, and that's
the cam timing difference in degrees. easy! don't need no dyno to
figure this stuff out!

next step - measure the difference in cam profile durations...

jim beam wrote:
> for those of you who work with older d-series motors, have you ever
> wondered why the cam timing between the d16a6 cam is "half a tooth" off
> from the d15b2?
>
> here's what i think is the answer - [please check the math]:
>
> d16a6 stroke = 90mm
> d15b2 stroke = 84.5mm
>
> difference = 5.5mm. that's 2.75mm difference in deck height between the
> two blocks.
>
> how does that affect cam timing? if the deck height is different, the
> belt length between crank and cam has to be 2.75mm longer [if we're
> using the same head, which we are].
>
> now, diameter of the cam wheel is 114mm. radius is therefore 57mm. <-
> [you will see this number again!]
>
> so, if the chord of the circle is 2.75mm out, that's 2.75/57 = 0.0483
> radians difference in angle. convert radians to degrees = multiply by
> 180/pi = 2.768 degrees. but, the cam/crank rotation ratio is 1:2, so
> 2.768 * 2 = 5.535 degrees. there's 38 teeth to the cam gear, so 360/38
> = 9.47 degrees per tooth, so 5.5 degrees is nearly half a tooth.
>
> BUT!!! also note, 5.5 degrees is numerically the same as 5.5mm. why?
> 180/pi = 57.298 degrees per radian, or approx. 57 degrees, close enough.
> make the cam gear 57mm radius and the 1:1 ratio gives you an easy way
> of figuring out how much to adjust the cam timing for the difference in
> deck height.
>
> honda engineers? smart dudes i'd say.
>
> how do i know all this? this weekend i went junkyard surfing and scored
> a zc cam - single cam head variety. wondering whether this was a
> bolt-on for my d15b2 crx, i wanted to check facts, so i bought a stock
> d15b2 cam as well [$14 each btw!!!]. you can see the difference in cam
> profiles [although it's not radical], but with the naked eye, you
> /can't/ see the difference in key-way angle for the cam gear. well, you
> kinda can, maybe, but it's not something you'd want to bet money on. so,
> you therefore have to measure. some blocks of wood, a spirit level, a
> school protractor, various other bits of metal for pointers, some
> messing about, and you have a measuring jig! sure enough, what is the
> angle difference in the setting of the key-way between the two cams? 5.5
> degrees. best i can measure at any rate.
>
> conclusion:
> the good news is that for $14 and with some good luck, you can put a hot
> cam in an old banger just for fun. the bad news is that you then have
> to spend $120 on a variable cam gear so you can account for the timing
> difference caused by the difference in deck height.
>
> otoh, if you work with decks and heads that have been skimmed, or
> non-stock head gaskets, you now know how to time the cam - simply
> calculate the difference in stacking height from stock in mm, and that's
> the cam timing difference in degrees. easy!

duh, mistake.
make that a 2:1 ratio and stick with double the deck height difference.
2.75mm deck height difference ~= 5.5 degrees timing difference. i
think you know what i'm getting at...


> don't need no dyno to
> figure this stuff out!
>
> next step - measure the difference in cam profile durations...

jim beam <spamvortex@bad.example.net> wrote in
news:U9Odnetaku_98EHbnZ2dnUVZ_gSdnZ2d@speakeasy.ne t:

> for those of you who work with older d-series motors, have you ever
> wondered why the cam timing between the d16a6 cam is "half a tooth"
> off from the d15b2?
>
> here's what i think is the answer - [please check the math]:
>
> d16a6 stroke = 90mm
> d15b2 stroke = 84.5mm
>
> difference = 5.5mm. that's 2.75mm difference in deck height between
> the two blocks.
>


You've considered that the difference in strokes may have been achieved
simply with different crank throw offsets?

This would be much more economical for the manufacturer than making a whole
new block with a different deck height.


--
Tegger

The Unofficial Honda/Acura FAQ
www.tegger.com/hondafaq/

Tegger wrote:
> jim beam <spamvortex@bad.example.net> wrote in
> news:U9Odnetaku_98EHbnZ2dnUVZ_gSdnZ2d@speakeasy.ne t:
>
>> for those of you who work with older d-series motors, have you ever
>> wondered why the cam timing between the d16a6 cam is "half a tooth"
>> off from the d15b2?
>>
>> here's what i think is the answer - [please check the math]:
>>
>> d16a6 stroke = 90mm
>> d15b2 stroke = 84.5mm
>>
>> difference = 5.5mm. that's 2.75mm difference in deck height between
>> the two blocks.
>>
>
>
> You've considered that the difference in strokes may have been achieved
> simply with different crank throw offsets?

yes - i'm sure that then the belts would be the same. the d16 & d15 use
different belts - the d16 is one tooth longer.

>
> This would be much more economical for the manufacturer than making a whole
> new block with a different deck height.

sure, but all you're doing is machining the deck. that has to be done
anyway.