15 replies to this topic

[robert.hedges]

I have a 1098 with an sw5-07 cam and 295 cylinder head.

The block has been decked and the head has been skimmed to get the compression up.

I thought I'd check the lift at the intake and exhaust valve on the build before fitting the engine.

With standard 998 pushrods, A+ rockers and a valve clearance of 15tho I was getting about 1.18:1. Seemed really low so I thought I'd purchase some better rockers.

Got a second hand roller rocker set at an apparent ratio of 1.3:1.

Retesting the valve lift after fitting the rollers I was still only getting around 1.22:1.

Searching around this topic I realised that I needed to sort out the rocker geometry to compensate for the decking and skimming. After a lot of trial and error with loads of bits I have laying around the garage I found what I believe was a winning combination. Ditched the roller rockers in favour of 3.35mm shims under press steel 1275 rocker pillars. This gave around 1.26:1. I was struggling with the adjustment on the tappet screw and had a brain wave about using the longer 1275 push rods. Put them in and retested the set up and I'm now getting a magic 1.285:1. Which I'm really pleased with.

I just feel that the length of the rod and the general combination is a little extreme.

Wondered if anyone had any thoughts or experience in this that they could throw in.....

All comments welcome.

[robert.hedges]

Bump?

[Ethel]

Seems odd that longer pushrods would help when the engine's got shorter. I'd aim to get the centre of the rocker shaft as close as possible to the same height as the end of the valves (or slightly under). Differences in valve lift have most effect when the valves are just open and most restrictive to flow. Things are more complex on the cam side of the rocker, but the main factor is still the vertical lift on the cam follower acting at a fixed distance perpendicular to the rocker.

 

I can't see an issue if your setup works, but have a look at where the rocker pads are sitting on the valves to assess the contact. 

[whistler]

From my reading of this topic I was under the impression that you shim under the rocker pillars only what has been skimmed from the head. If I'm correct then 3.35mm is far too much and you needed the extra length of the 1275 rods to compensate for the additional rocker pillar height. I think if it was me I'd use the correct thickness shims for the head skim and go back to the correct rods.

[robert.hedges]

Thanks for your thoughts guys.

What was most surprising to me was how poorly the rollers came out over the steel pressed rockers!

I was thinking at the time that it was strange that by increasing the simming past the 80tho (2mm) that I've lost from the head / block, increased lift. I sort of convinced myself that the distance from the rocker head to the valve had now changed (after shimming) and that to compensate for this the angle of the rocker was changed by the pushrod adjusting screw lengthening the push rod, which again changed the operating angle of the rocker and potentially the final lift.

All sort of makes sense to me but does seem a non-standard solution.

Wondered if anyone else had a simmilar set up for comparison....

[OzOAP]

Centerline of rocker shaft level with top of valve at 1/2 lift.

[Ethel]

That'll give you maximum lift, but that won't necessarily give you the biggest improvement in breathing.

 

The rocker length is effectively from the the centre of the rocker shaft to the centre of the ball on the adjuster - but, of course, winding the ball further away from the rocker tends to worsen the angle it makes with the push rod too.

[Spider]

If the Cam itself has a High Lobe lift, you won't get the full ratio of the Rocker as it moves through an Arc. You'll find that most Rocker will only give their claimed ratio with Valve Lifts in the 0.275 - 0.325" range.

 

And, if the Cam has a high Lobe Lift, you don't usually need High Ratio Rockers.

 

Depending on the Cam Lobe Profile, the highest Valve Opening will occur when the centre line of the Rocker Shaft is at 50% Valve Opening, however small dividends can be found in different parts of the rev range by moving from this.

[dotmatrix]

this is interesting. I did not know this.

[robert.hedges]

Ah ok Moke Spider, so what I think your saying is that that say a 1.3:1 rocker only gives this ratio at 50% valve opening. At all other points in its arch it is less and that the final pisition of the valve will not be 1.3:1 the height of the cam lobe. Dont think I've come across this before. Very interesting. Thanks. I'll do my measurements and see what I'm getting.

Edited by robert.hedges, 26 February 2020 - 09:45 AM.

[Ethel]

If you think about it,

 

The position of the valve is fixed. The position of the cam lobe & follower is fixed. The only way to really alter the rocker ratio is to move the location of the rocker shaft.

[Spider]

Ah ok Moke Spider, so what I think your saying is that that say a 1.3:1 rocker only gives this ratio at 50% valve opening. At all other points in its arch it is less and that the final pisition of the valve will not be 1.3:1 the height of the cam lobe. Dont think I've come across this before. Very interesting. Thanks. I'll do my measurements and see what I'm getting.

 

It depends on the design and the claims around that. Yes, some will only give their claimed ratio at 50% of lift and others will give their claimed ratio as an average over a certain lift range.

[ACDodd]

Ethel, that's not true. My '0' tolerance a+ rockers use the stock post and increase lift by moving the ball falcrum point. Result is they increase lift from 1.2:1 ish to 1.28:1 and reduce the varience.

The ratio is correct at the point around mid lift is also untrue, as that depends on how fast the contact patch is moved from the rocker first contact with the valve tip Point, to.when it moves to the furthest piont away from the rocker shaft. My recently developed 1.28:1 rockers give more than that in the first 0.050" than the rest of the range.

Ac

Edited by ACDodd, 26 February 2020 - 07:37 PM.

[Ethel]

The lever length is from ball centre to shaft centre, but how do we compare lever ratios if there's also variance in the angle between both sides of the lever at the fulcrum?

 

It's still a basic A Series truth that the valve and the follower move on parallel paths at a fixed perpendicular distance from one another.

 

The ratio of the rocker levers is only one factor in the ratio of the cam lift compared to valve lift, but to sell/buy rockers we still like to differentiate between them in simple terms.

[dotmatrix]

this is still a very interesting read. I never thought about this.

 

In the other rocker thread that is on here right now they reference keith calvers write up about rockers, which basically concludes to use 1.5:1 roller tip rockers on a large bore engines with any cam and that much more than that is overkill.

 

what would you use on a 1275 race engine?