cam choice and rockers

[Dean.s]

1700cc W code engine running twin 40 idfs

As i have to replace the barrells and pistons I'm also going to change the camshft.

Not sure on the b & ps yet but i have found a cam on Newman cams, the details are below.

duration
inlet. exh
270. 270
valve lift

2500 revs. inlet. exh
0.392. 0.392
6000 revs. 9.95. 9.95


cam lift

2500 revs inlet. exh
.0.350. 0.350
6000 revs. 8.88. 8.88

timing

inlet open 25-65
exhaust close 65-25

Full lift. 110

From that info can someone advise on the compatible rockers? standard being 1.3 i think.

also only pistons i find in 1700 90mm are for ca cd engines. any help with info on these would be appreciated.

Failing that it looks like I'm buying 1911 pistons with a little head cutting.

[Nine One One]

I am sure KS will be able to help you on this?

[Bruce M]

Domed “W” piston set are hard to find but do appear occasionally. There’s a NOS KS set (or 2) on eBay currently but for a chunky price.

Other than the flat top 1.7 or 96mm set, you could pick up a 2.0 crank & rods set. A “AA” 2.0 B&P set is £400 (£320 in black Friday sale but i think you’ve missed that).

That Newman cam is a beetle spec but they can grind a type4. Best to phone them and discuss. Rockers are notionally 1:1.3 ratio but i’ve read claims the intakes are slightly more (never checked that however).

[Dean.s]

I have an update from newman cams. I've also decided to go with the 1911 p & b set (so long as the strokes match)

This is their recommended cam.

Power band 2000 - 5000

duration. in. ex
264. 264

valve lift. in. ex
0.358 0.358
9.10 9.10

cam lift. in. ex
0.320. 0.320
8.12. 8.12

timing. in. open. ex. close
18. 58. 58. 18

full lift. in atdc
110

valve. in. ex
clearance. 0.008. 0.010
0.203. 0.0254

[Bruce M]

What lift is the duration (open / close angle) measured at? Industry standard is 1mm or 0.05” but I suspect that is less, maybe 0.1mm? Finding out the duration at 0.05” (or even 1mm) would be useful to make a comparison with other options- although one measure of duration doesn’t tell the whole story)

The valve lift has been calculated with 1:1.12 rocker ratio. So either the valve lift will be more or the lift at cam will be less. Would be good to clarify.

[Bruce M]

For comparison, the stock camshaft specs:

cam lift IN: 7.71mm EX: 7.26mm.

IN: 12 - 42 @1mm (234 degrees)
EX: 43 - 4 @1mm (227 degrees)

[Dean.s]

Thanks for the help Bruce.

I've put your question back to them, however I'm not expecting a reply this year.

[Dean.s]

I was wrong about the time frame.

Here is their reply to the above question

Hello Dean,



On our camshafts we go by seat to seat duration. Which is usually a 0.25mm/0.010”.

[Dean.s]

ok, here i go again

scat c25 camshaft. 8mm swivel foot tappets. 0.060" shims. standard rockers. Standard pushrods.

0 clearance.

Are these the options?

Thicker shims, if so are there standard sizes and what is likely to work?

Cut to size pushrods, if so how does one sort that correctly

[Bruce M]

Firstly, with swivel foot adjusters, you need to grind a little off the underside of the rocker arm, to make clearance for the adjuster foot. A mm or so I think but do a bit of research on that.

Then you need to assess the rocker geometry. At half lift, the rocker adjuster should be directly in line with the valve stem. At full lift and zero lift the adjuster should be at an equal angle from that central position. If the angles are offset then you probably need a shim under the rocker. The pushrod is cut to suit, after the geometry is corrected. You can buy an adjustable pushrod to measure the correct length or cut a stock pushrod, tap the bore and fit a short length of thread with an acorn nut on top.

There is a good video on you tube by “type e motorsport” on rocker geometry. The example is a type1 engine but the theory is the same.

The C25 camshaft isn’t very high lift, so you might find the geometry is close to stock. Maybe try with a stock adjuster first & see if it is in the right ballpark. Then swap to the swivel foot version.

[Dean.s]

Thanks Bruce.

Great info for me to look into and progress with .