Effects of adjustable cam gear

[ivymike]

hello all

I was hoping that some of you with plenty o' experience could toss a bone in the direction of the green guy.

I've been told, by sources that I consider somewhat authoritative, that adjustable cam gears serve only a slight purpose in tuning an engine for performance - they can allow you to move the peak HP up or down by a couple hundred horsepower (by retarding or advancing cam timing). This comment is specific to NA, SOHC gas engines.

I've also read at least two magazine articles where it was claimed that simply by advancing or retarding the timing of the valve events (with stock cams), dramatic improvements in horsepower could be achieved. Here's one example:

http://integra.vtec.net/carmag/aemsprockets.html

Anyone care to comment on the efficacy of cam timing changes, especially on naturally aspirated engines with (one or two) stock cams?

 

[sbc]

The article that your link points to is pretty
good. When you wrote: "they can allow you to move the
peak HP up or down by a couple hundred horsepower"
didn't you mean r.p.m. instead of horsepower?
Variable cam timing as mentioned in the article
is a great way to balance performance & economy.
Changing your cam timing will help an engine at
one end of the powerband, but can really hurt
performance at the other, also mentioned in the
article.
If you intend to try changing your cam timing, you
may want to post here with your vehicle info.
Someone may have done something similar which could
save you time & money. Only a few degrees can mean
a world of difference.

 

[ivymike]

right, I meant rpm (oops).. "by a couple hundred rpm"

I don't intend to change the cam timing on my vehicle, but I am regularly asked by friends of mine whether or not an adjustable cam gear is a worthwhile investment, and I'd like to have a broader base of knowledge from which to form a judgement. I have in the past answered "no," as most of the people asking me have done little to their vehicles aside from adding an aftermarket air intake & filter(upstream of the throttle), and perhaps a header and/or cat-back exhaust system. Most of these people are driving Honda Civics, although they are not all from similar years.

I wasn't intending to ask about variable valve timing, either, as not many people will be converting a fixed-timing system to a variable-timing system. I meant to ask about changing valve timing by a particular amount, to arrive at a new (fixed) valve timing.

 

[GregLocock]

If you mean just jumping the existing camshaft by one tooth on the belt or chain (in effect) then you are likely to get some gain for some pain. I'd guess most stock engines would be fairly insensitive to cam timing, in the absence of any other significant changes (ie just fuel and timing), something of the order of 5-10 kW in 150 kW would be the maximum I'd expect (read measured, nudge wink).

The trade-off for a manufacturer is performance at idle - emissions, fuel consumption and stability, against performance at high rpms, WOT. Since we (the posters on this board) don't really care overmuch about the first constraint, we could realise the performance advantages happily enough.

I'm surprised Larry hasn't commented, this is his meat and potatoes. Cheers

Greg Locock

 

[evelrod]

Isaac, I don't think that varrying timeing of the intake came is much more than 4 wheel steering on a truck. It's a great way to be noticed by corporate (as an engineer) if it increases sales to the performance segment of the market. It does work, obviously, but to what degree on your average 'grocery getter'?

On a race car engine, now that's another story---

My DOHC engine had different profiles , in. and ex. Lobe centers were different, lift was different , acceleration rates and ramp timeing were different and setting the timing of them was VERY difficult and time consuming and ABLOULUTELY CRITICAL to performance. All the specs were designed to mesh with carbs and headers etc. and if you altered the setup specs , even as little as one or two degrees, you could FEEL it in the seat of you pants as well as lap times and drivability. As an example, normal setup was In @ 106 ATDC & Ex @ 109 BTDC (I use this only because I remember the change on the dyno) ---changed to 105 and 110 lost hp, from 205 to 195 and 500 rpm, going to 107 and 108 was even worse. Tried 107 and 110, worked OK but took us out of our optimum rpm range etc. etc. etc.

We did a Ford 2.3 Turbo with an adjustable timing sprocket. It still worked best straight up (jumping a tooth is 10 or 15 degrees, BAD, BAD) This was with an SVO grind, so I suppose the stocker was even less sensitive.

I have set my Mini at 4 degrees advance to start, I'll let you know how it works, if I EVER get this thing finished.


Rod


PS Keep in mind that cam timing is just ONE of the things in an engine that you must optomize if you wish to get the most ---performance---mileage---best this ---lowest that, whatever. Ultimately, street cars are a compilation of some of the WORST compromises you can think of. There is not much in a 'grocery getter' that can't be improved(?) upon, albeit to the detriment of something(everything?) else. Compromise, unless you wan't to get to the grocery stoor in RECORD time???