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change cams worth it ?
givemethebighammer - 23/10/05 at 08:09 PM

Would fitting 275 or 285 cams to my Zetec just mean more top end and less bottom end. as I understand it differnet cams just change when the engine makes power (rpm) rather than release more of it.

Anyone want to put me right ?

thanks

Strolls - 23/10/05 at 10:50 PM

I don't know the Zetec but my understanding is that high-lift cams generally open the valves wider & put more air through the engine.

Cams come in different "profiles" - the 285 will have a bigger "lump" on it to open the valves wider than the 275, and another manufacturer's cams will be a slightly different shape altogether. So it may be the case that some cams "mean more top end and less bottom end" but I don't think this is generally the case.

What is important when changing the cams is - because you're putting more air through the engine - that you also tune the carbs to put more fuel through, too. The exhaust is also a consideration, as is the black box / timing.

The cam is certainly one way to get more power out of your engine, but I don't think it's worth doing alone. I've read of ad-hoc performance "upgrades" actually reducing the output from standard, whereas an experienced tuner will take a look at the whole picture.

I'm interested in the Rover K-series engine, so I'll refer you to Dave Andrews' webpage where he discusses that engine in detail. It may not be specifically relevant to your Zetec but it may give you an overview of the kinds of things involved in tuning a modern engine. I haven't started building a kit car yet, but I'd be inclined to get it on the road first & then take it to a tuning specialist once it's SVA'd & I'm happy with it.

Stroller.

NS Dev - 24/10/05 at 09:57 AM

quote:
Originally posted by givemethebighammer
Would fitting 275 or 285 cams to my Zetec just mean more top end and less bottom end. as I understand it differnet cams just change when the engine makes power (rpm) rather than release more of it.

Anyone want to put me right ?

thanks


It isn't possible to generalise to that extent really, as it depends on what the standard cams are like.

The three key things with a camshaft are lift, duration and overlap (which on a single cam engine is obviously rather different to a twin cam)

To generalise, any changes which increase valve overlap period will do what you say and increase breathing at higher revs at the expense of charge contamination at lower revs (i.e. typical "race cam idle"

Lift and duration changes are much less easy to identify the results of. They will not necessarily lead to less torque lower down the rev range, particularly as on your engine when fitted with vernier cam pulleys (or just swung away from std setting as there are no keyways on zetec cams anyway) you have good control of valve overlap.

There obviously comes a point where to gain any more duration (and lift within engineering constraints of not having vertical camlobes!) you have to start extending overlap period, and this is when you run out of "space" within the lift cycles on the inlet and exhaust and start to shift power up the rev range due to the increasing overlap.

What all this means is that (with one very significant proviso which I will mention in a mo) in the real world, you can fit "fast road" or "road rally" spec cams to your engine and you will not notice any real downside, though they will need timing in properly (i.e. on a rolling road so you can see the results) and the ign and fuelling will need sorting too.

The proviso that I mentioned may be the fly in the ointment though. To make any real (i.e. worth spending money on!!) gains, you need to enlarge the valve cutouts in the pistons (just like you do on my Vauxhall XE engine). If you don't you can't time the cams in right, and won't see any big advantages over the std cams.

rusty nuts - 24/10/05 at 11:41 AM

The Zetec lump came with 2 different cam profiles at least . Believe the difference between the 115 and the 130 was just the cams . Sure someone will confirm it. The 130 lump is more "driveable" without any low down loss

jon_haggerty - 24/10/05 at 05:13 PM

The two lumps you talk of were the 1.8, a difference between xr3i mk 5 with the zetec and the 1.8gti mk 6 also with a zetec. The cam and the ecu were however different. were diff making one a 115 and one 130 spec. Try www.feoc.co.uk for some really good zetec tuning advice as a lot of them have 2Ls in the escorts. Jon

britishtrident - 24/10/05 at 06:12 PM

Overlap has a really major effect by reducing the over lap on an old Lotus Twink engine hit hot cams you could reclaim a lot of medium speed torque at the expense of some top end.

givemethebighammer - 24/10/05 at 09:31 PM

interesting, so I could just fit vernier pulleys to the standard cams (130 ones in a 2.0L) to fine tune the engine ? I really don't want to get into head mods at this stage. Just looking for a few winter mods to improve things a little (and hopefully not break the bank !)

thanks

NS Dev - 24/10/05 at 11:48 PM

If you do it carefully then you don't even need to buy the verniers!

The zetec cams don't have keyways (as far as I can remember?), so you can just move the sprockets relative to the cams anyway.

There may or may not be gains to be had by moving the cam timing from stock with stock cams, I don't know the engine well enough to know.

Somebody on here has Dave Walker's (as in Emerald, not Newark engines!) article in their archive, from CCC mag on timing cams using the lift on overlap method, which is very simple and he explains it well

NS Dev - 24/10/05 at 11:59 PM

just had a search, it's in Rob Lane's archive - thanks Rob!