high lift rocker.

Post by **callyspoy** » Sun Sep 07, 2008 5:49 pm

what does it do and provide??

Post by **wanderinstar** » Sun Sep 07, 2008 5:57 pm

Opens valves further so allowing more fuel into cylinders and so more power. But they are expensive and put more strain on valvegear.

Post by **callyspoy** » Sun Sep 07, 2008 6:03 pm

oh i see. interesting. i am looking at rebuilding my engine at some point, well, i'd rather buy a knackered one as mine is pretty bloody good, and having a bit of fun with it. but i am truly a novice, so its kind of interesting! i assume you have to fit a different cam to compensate for the different rockers?

Post by **wanderinstar** » Sun Sep 07, 2008 6:11 pm

No, you can keep the same cam if you want. I don't know the exact figures of standard rockers . But lets say 1.2:1. that means for sake of argument that for every 1" the pushrod goes up, the valve goes down (opens) 1.2".
With high lift rockers they can be in the region of 1.5:1.
Hope that makes sense to you.
Minisport do them.
http://www.minisport.com/acatalog/Mini_ ... ckers.html

Post by **callyspoy** » Sun Sep 07, 2008 6:15 pm

wanderinstar, thanks very much.

Post by **bmcecosse** » Sun Sep 07, 2008 6:37 pm

Complete waste of money - and sure fire way to ruin the cam/followers and valve guides. And all for ~ 1 bhp gain in top end power and some loss of power at lower revs! Don't get me wrong - higher lift is a GOOD thing on a highly modified engine, but not for most road-going engines. You will get enough air in using larger valves and gas flowing the head. The 544 cam (285 duration0 has slightly more lift than all the other cams up to that duration (I'm talking BMC cams here) - as Wanderinstar well knows - the cam he has in his engine has very lazy timing but high lift - it's been reground on a standard cam and gives similar engine characteristics to the MG Metro cam - but with more wear etc due to the steep cam ramps/small base circle and the higher lift. beware if using a cam like this (or the 544 or 649) if using a 12G940 head on a small bore engine - the valve pockets would need to be very deep to avoid collisions! Probably so deep they will be very close to or even foul the top ring. Same problem applies if using high lift rockers.

Post by **callyspoy** » Sun Sep 07, 2008 6:47 pm

is that the same case with a 1275 engine bmc?

Post by **bmcecosse** » Sun Sep 07, 2008 6:51 pm

Well - the valves/pockets problem etc doesn't apply of course! Otherwise - YES!

Post by **callyspoy** » Sun Sep 07, 2008 6:52 pm

oh...

Post by **bmcecosse** » Sun Sep 07, 2008 6:57 pm

Just get an MG Metro head (with the larger inlet valves) - and gas flow that as best you can. MG Metro cam -or 544 if you want it to REV (but not great fuel economy) - and with a decent carb (HIF 44) and exhaust it will go plenty!

Post by **PSL184** » Sun Sep 07, 2008 7:04 pm

BMC, do you know the numbers for an MG head?

Post by **callyspoy** » Sun Sep 07, 2008 7:05 pm

ha, i cant really get much worse fuel economy than with the dellorto 45! and the exhaust is already sorted, as i said, im just looking at things to do really...

Post by **bmcecosse** » Sun Sep 07, 2008 7:12 pm

They are ALL 12g940 - over many years, very confusing. Need to measure the inlet valves - they are 'obviously' larger.

Post by **PSL184** » Sun Sep 07, 2008 7:18 pm

I've got a head stamped 12G940 and 171 Valve ports measure roughly 25mm and 30mm Any idea's what it is ?

Post by **wanderinstar** » Sun Sep 07, 2008 8:05 pm

wanderinstar wrote: But they are expensive and put more strain on valvegear.

Roy, I didn't recommend them. I just answered the question.

Post by **IslipMinor** » Sun Sep 07, 2008 10:22 pm

Not a waste of time, and definitely one of the items to consider after fitting a good head with good valve sizes, flowed etc., decent exhaust and at least a 266° duration cam. The perceived benefit is they give the performance on the next cam up, without the disadvantages of more overlap, such as a more lumpy idle.

A-Series engines need all the valve lift they can get to overcome the restricted flow characteristics of siamese ports. Many of the 'modern' cams from Kent and others do provide more lift than the earlier cams from BMC, such as the 731 and 544, but in this case more is really better!

If you are going to consider high lift rockers, go for at least the roller tipped type to avoid unnecessary side load on the valve stems and early wear. The full 'roller' rockers are very expensive, and not necessarily much better in longevity.

Post by **bmcecosse** » Mon Sep 08, 2008 5:34 am

Ian - yes, I know.
PSL - sounds like standard head
Richard - I disagree - recent test in one of the Mini mags showed precisely 1 bhp gain in power for expenditure of well over £100, and the Mini forums have many many threads on subject of seized/worn guides/dropped valves and rapid cam shaft wear. I agree at least 266 duration, but if you want lift it's better done at the cam (544 eg is ideal) - although extreme lift is not required except on full blown competition engine. That's a different matter all together!

Post by **Stig** » Mon Sep 08, 2008 9:25 am

Roy & I have had a discussion in the past. I've got Minisport high-lift rockers fitted and gained a noticeable increase in torque with a standard head & cam. I'd agree that they're an expensive way to increase power (I got mine free) compared with, say, a cam change. They're a bolt-on mod rather than an engine-out mod though, but if you're rebuilding an engine anyway there are other options as already mentioned. Worth reading what Vizard has to say.

Post by **Mogwai** » Mon Sep 08, 2008 10:10 pm

Ive been running a 266 cam & high lift rolertip rockers in my engine for about 8-9 years now, runs fine

Post by **callyspoy** » Mon Sep 08, 2008 10:34 pm

mogwai, how would you know, you can't even hear your engine! i'm suprised you can tell if its on!