A question on camshaft measurements and carburetor choice

[S30Stig]

I am in the process of picking out new carbs for my 240Z with a 'Stage I' Datsun Spirit motor. It's an L24 based 2.7L stroker that has been (in my opinion) burdened with the Weber DGV setup that came with my Z. At the time of setting up this motor I found myself unable to replace the carbs the car came with but I'm now in the position to do it right. After some back and forth I thought I would go with some Z-Therapy SU carbs since they should be plenty for the engine and I've heard nothing but great things about them. The problem is I don't really have 4-5 months to wait for a set.

 

I also don't have experience with sidedrafts which would be my second preference. So after reaching out to a few people and dredging old forum posts I got in touch with Josel from "SidedraftSpecialties" in CA about getting a new set of 45mm OER carbs. He explained to me that the most critical choice in picking a set that will run well, particularly on a mild 2.7, is the choice of venturi size which would naturally be based on the size of the camshaft. So here's my problem:

 

I don't know my cam specs. I've reached out to Datsun Spirit who even found my old measurement and spec sheet which was able to narrow things down to it being a "DSI274 non drilled cam". Unfortunately he didn't seem to have a cam card anywhere for this particular cam or recall the lift and duration. The cam itself has "Japan" and "E30" in raised letters along with an "A" stamp near the firewall so I'm assuming this may be a re-grind cam. After deciding to measure it myself I got (I think) inconclusive results. Using a dial indicator and measuring directly on the cam in the car I found an intake and exhaust lift of .317" but a duration of 195 degrees measuring from .050" of lift from of base circle back to .050" of lift. I could see the .317 lift because the engine is pretty mild but the 195 degree duration doesn't make sense when the original "A" cam specs I find say the original duration should be about 248 degrees. I tried measuring from .006" to .006" which gave me 289 degrees which seems a little long for a mild cam. I could assume that "DSI274" refers to a 274 degree advertised duration, not too far off from 289, but we all know what happens when you assume things.

 

What am I doing wrong in this case? It's been a few years since I had to measure a cam but I feel like I'm missing something. Should I be measuring duration and lift from the moment the rocker arm moves from and back to .050"? Or does taking these measurements straight from the cam base circle suffice? Was the advertised duration of these L-series cams based on a standard before the .050" standard I remember being taught about? I appreciate any advice at this point because time is a factor for me but I'd rather do it right in the end.

 

EDIT: I'll eat some of my own words before someone else makes me.... Just found this wonderful thread about cam timing with a degree wheel and dial indicator by BRAAP and others:

Pretty much narrows my questions down to should my .050" measurement always be taken from the top of the valve retainer?

 

Edited March 5, 2019 by S30Stig

[jonbill]

IMO venturi size is driven by the amount of air you need to flow, which is dominated by displacement and max rpm you're going to use. Not cam specs so much. 

 

There are a few variations of a formula around. Frank Honsowetz's book has this:

Venturi size=sqrt(cylinder displacement * max rpm ÷ 1000) * 0.65

Which for a 2.7 with a 7k rpm used redline gives 36mm venturi size. 

Which sounds right to me. 

 

I have a 2.9 which I rev to 8k with 39mm venturi. Should prob be bigger but that's getting to the limit of 45mm body anyway. 

 

Some will say you should have smaller venturi for the street: The bigger the venturi the harder it is to get the transitions between idle and progression and main jets right but it is possible. 

[S30Stig]

Thank you jonbill. That makes good sense to me. In my use I wonder if I should go with a slightly smaller venturi. Using Frank Honsowetz's method and considering I don't really go over 6k on the street I came up with a 34mm venturi size:

 

sqrt(458.79 * 6,000 / 1,000) * 0.65 = 34.103

 

Even though this would probably limit the 6-7k range, my cam definitely doesn't have any hidden power up there and if the slightly smaller size makes for easier tuning and or more driveability then that tradeoff could work.

 

However if that 2mm reduction in size would affect things across the whole rpm range then it may be better to stick with 36mm.

 

[jonbill]

No I think you're right, if you don't go over 6 ish much, stick with the 34s. They'll work as well up to 6k as bigger ones could. 

Edited March 5, 2019 by jonbill

[S30Stig]

9 hours ago, jonbill said:

No I think you're right, if you don't go over 6 ish much, stick with the 34s. They'll work as well up to 6k as bigger ones could. 

We'll have to hope so! I just bit the bullet on a set at 34mm. Hopefully the worst case scenario now would be having to change them out for the next size up or down.

[jonbill]

I'm sure they'll be fine. I guarantee the buying has only just started though! 

[turbogrill]

Regarding measuring the cam, did you measure directly on the cam or on the valve?

 

0.050" on the cam equals  ~0.075 on the valve.

 

This is also not true because your lash will affect the real lift as well.

 

How do you find exact TDC with engine in car? If it's a cam with fast lobes a 5 degree difference will make a huge differnce. My current cam has about 0.006" lift per degree change during it's opening phase. 

[S30Stig]

9 hours ago, jonbill said:

I'm sure they'll be fine. I guarantee the buying has only just started though! 

Tell that to the 123ignition distributor sitting next to me and the fuel fittings going out today! There's suddenly a lot going on at once for my Z.

 

 

1 hour ago, turbogrill said:

Regarding measuring the cam, did you measure directly on the cam or on the valve?

 

0.050" on the cam equals  ~0.075 on the valve.

 

This is also not true because your lash will affect the real lift as well.

 

How do you find exact TDC with engine in car? If it's a cam with fast lobes a 5 degree difference will make a huge differnce. My current cam has about 0.006" lift per degree change during it's opening phase. 

I was trying to measure directly from the cam which is probably why I was getting such strange numbers then. I imagine a proper valve adjustment would be key in measuring real lift since lash has an affect on that measurement as you mentioned.

 

Also, and you'll have to excuse the ignorance of youth and lesser experience, but because I was originally only trying to find the lift and duration from on the cam I figured TDC was irrelevant to this case; only the starting and ending degrees of the measurement. I wasn't trying to degree the cam itself or change cam timing, just to get a better idea of what cam is in there since that was the original direction I was pointed to in picking a starting venturi set.

[turbogrill]

On 3/6/2019 at 11:57 AM, S30Stig said:

Tell that to the 123ignition distributor sitting next to me and the fuel fittings going out today! There's suddenly a lot going on at once for my Z.

 

 

I was trying to measure directly from the cam which is probably why I was getting such strange numbers then. I imagine a proper valve adjustment would be key in measuring real lift since lash has an affect on that measurement as you mentioned.

 

Also, and you'll have to excuse the ignorance of youth and lesser experience, but because I was originally only trying to find the lift and duration from on the cam I figured TDC was irrelevant to this case; only the starting and ending degrees of the measurement. I wasn't trying to degree the cam itself or change cam timing, just to get a better idea of what cam is in there since that was the original direction I was pointed to in picking a starting venturi set.

 

Your right TDC shouldn't matter. But it's hard to measure these thing, I am no expert and I have to remeasure several times. It's hard to get accurate numbers with the engine out and a proper timing wheel, 1 angle is really nothing but the lift can differ for just 1 angle. Also your .317 lift should be ~0.470 at the valve. If it's DSI motor I am sure it's good!

 

[S30Stig]

It definitely wasn't easy to get a decent reading. With the engine fully dressed and in the bay I didn't have anywhere good to attach a dial indicator or have a magnetic base handy so we ended up making this jerry-rigged contraption to hold the indicator in a good position. It uses two bolts from the cam cover to hold itself down and worked well for something thrown together. Figured someone might get a good chuckle out of it here...