16 replies to this topic

[Sputnik]

Hi,

I've tried to find this answer on the forum, but with no success.

This questions has two parts both is for full race use only:

1. I have a 1330 with 296 kent cam, 40 weber, modified head. The head was so far skimmed, that the pistons had to be pocketed to prevent the valves from bending at high revs. I am running the standard rockers as is! Is this correct or should I be using the standard rockers on 2mm rocker post spacers, or use the 1.5 high lift rockers instead ??

2. The same question, but on my newly build engine, a 1425 (1275 crank with Cooper S con rods) 45 weber, swiftune 300 deg plus cam and a possible race spec head from either calverST or slark.

Your advice and recommendations would be appreciated.

Thanks

Ps: i am are not racing in an appendix K class in south Africa !

[Sam Walters]

You need to work out how deep the pockets are in the piston. Then you need the head off without any valve gear attached put it on a flat surface such as a bit of glass then put a dti on the valve and see how much clearence you have before it hits the table.

You need 13.5mm clearence on the inlet and 14.2mm clearence on the exhaust for a 296 with 1.5:1 ratio rockers. This takes into account of a 1.6mm safety clearence.

Adding shims to the rocker posts just correct the geometry but wont affect valve lift really.

However i see no major benefit of running 1.5:1 rockers on that cam.

Stick to 1.3:1 roller rockers. You need 12mm clearence on the inlet and 12.5mm clearence on the inlet

As for the latter, id speak to swift tune. I have no experience with that cam but i cant see it being much diffrent.

Edited by Sam Walters, 30 April 2012 - 03:17 PM.

[ACDodd]

Are you looking for maximum top end output and minimal low end output?


AC

[racingbob]

on my appendix j ( historic ) in AUS

i ran a spv4 vizard cam with 1.5 roller rockers

also ran a kent 310 with 1.5 roller rockers

running a slark head

speak to neil slark for his opinion

[Sputnik]

Thanks for the feedback so far,

@ACDodd, it seems that to build a high bhp NA engine in the region of 125 is not that difficult, but in order for me to competitive, I need more torque, or so I think? My idea is to get a 125bhp engine with the maximum amount of torque. Our track does not have such long straights, and with my 3.4 diff, 10inch wheel, close ratio gearbox, I only get to about 6500/7000 rpm at 3/4 of the end of the two straights, although I do fall off cam when in 3rd (Kent 296 cam with power only starting at 3200) on two of the corner exits due to understeer, but working on the suspension setup and tyre pressures.

My friend's mark one golf is dyno'ed at 125bhp and 150lbs and is doing 1min31sec around the track. With his car weighing in at nearly a ton and mine at 570kg, I am thinking that with a favourable power to weight ratio, and maximum torque, I should be able to get close to the same times.

I just bought a set of Cooper s con rods (second hand) for that purpose, but will sell them again, if the need to get higher torque between 3200 and 6500 rpm is not possible by using them.

I am not to keen to run much over 7000rpm .... Might sound stupid for a race car, but as a privateer, I rather want to have a reliable engine that does not cost an arm and a leg to maintain.

Again, thanks for showing interest and awaiting your reply.

[Sputnik]

Racing bob,

What was the engine size on the vizard and Kent cam respectively ?

[racingbob]

one was 1330 with the vizard cam - excellent for what you want

the other was 1275 and a 1380 with the kent

i realy liked the vizard cam spv4 scatter cam perfect for you

my engine lasted all year not rveing more than 7200

with this 1330, powermax pistons, s rods, fully balanced

slark head

[bmcecosse]

Stick with standard ratio rockers - roller if you wish - but not 1.5 ratio.......... If the rockers are sitting at an unusual angle - then you may well need to put spacers under the pillars - if you do, be sure to maintain the oil feed to the rocker shaft! The 40 Weber is far too small on the first engine spec you mention. 45 will be fine - with nice large chokes fitted I assume........

[Sputnik]

what is meant by the 'unusaul angle' i am not sure how to check this ?

althought I know a bit about engines i have not yet studied the rockers to see how they should fit/look like

[racingbob]

i nkow i had to put spacers under my rockers

as they were sitting at an angle

[Cooperman]

Each rocker should sit so that the centre line from shaft centre to roller centre is horizontal at 50% of maximum lift. Hope that makes sense. Use pillar shims/spacers to get it as close as possible. it's not totally critical so withing few degrees either way at 50% lift is normally OK. As Roy says, make absolutely sure the oil flow to the shaft is confirmed as good.

[ACDodd]

The 1.5 rockers will move the power band up another 500rpm though you will only be getting 110bhp with stock rockers and more like the 125bhp with the 1.5:1 in this spec. You should be making 112 to 115lbft of torque woith the 1.5 rockers at about 4800 to 5000rpm. I think the diff is a bit low, you should be using a little more RPM.

AC

Edited by ACDodd, 01 May 2012 - 07:05 PM.

[Cooperman]

A 3.9:1 final drive would seem more appropriate for your application and engine spec.

[ACDodd]

Sorry to mislead, I meant you need a lower diff 3.65 or lower still.

AC

[Ethel]

You should also consider swapping your valve springs if you are upping the rocker ratio. 20% more leverage means 20% more force pushing the followers against the lobes - power lost and more wear.