I have been seeing the stylinconcepts site and I think that it is not so dificult to do some adjustable cam gears for the A series engine. If you get one of this adjustable cam gears (for other model, but with the same diameter and teeths of a A series) you can always machine it for fiting the end of the A series Camshaft. Other way to do it is getting a stock cam gear, machine it and develop a similar system.

What would be the pros and cons of such a modification? Can you pull up the power/torque in a significative way?

I think that it will be much more efective on dual camshaft engines, since you can change valve crossing (I think this is the name).

Thanks

Ricardo.

Quote:

Can you pull up the power/torque in a significative way?

That's part of how Honda vTEC does it -- advances and retards the intake camshaft.

1200rallycar is right, I could (and have on occassions) taken my car to 8000rpm. There is no real power up there, but it will eventually get there if it has too - like when you get jammed in 3rd. Just out of interest, the A15 has a much longer stroke than my L18, and the same length rod as I use.

tertonga, any details on the piston and rod combo in that 9500rpm A15?

This is what I have learnt from various books and talking to engine builders. Any time you increase an engine's displacement you decrease it's responsiveness to cam timing. The biggest desensitiser is increasing stroke, because it reduces the rod to stroke ratio which determines some engine dynamics which are very significant in the 4 stroke cycle.

Increasing stroke (and decreasing rod to stroke ratio) will:
*increase rod angularity. This has a few impacts but I can only think of 2 at the moment; 1) decreases mechanical efficiency through increased friction; 2) reduces safe operating speeds due to increased stress to rod beams and piston skirts
*increase piston speeds across top and bottom centres. As these are where all the crucial valve events happen, the increased speed decreases the engines sensitivity to effective valve timing. This means the A15 will not respond as well as the A14 to the big maximum effort cams you need to get good power out of these little motors. This is a good thing for a street motor, as it is more forgiving to cam timing errors and where your cam gear limits the adjustability, but bad news for chasing peak bhp in a race engine.

The rule of thumb is that higher rod to stroke ratios are good if you are intending to run at sustained high rpm. Lower rod to stroke ratios work best at lower rpm and will tend to produce a broader operating range. best suited to road or rally applications. It's not always that simple, but a generally accepted guide.

DDgonzal

Hello there

Ok, you are right the Honda VTEC system increases valve crossing with the increase in the RPM. However, I think (maybe I am wrong) that the VTEC system doesn't advances or retard the camshaft gears. The VTEC system is DOHC and instead of having one came for each valve it has 2 (or 3 on the S2000) came for each valve, with increasingly higher and sharp dimensions. The came movement is transmited to the valve by 2 (or 3) special rockers (I think this is the name for it). When the oil pressure is low only the first rocker is activated. If the oil pressure raises a litle more (higher RPM) the second rocker will get conected by a moving pin (activated by oil pressure) to the first rocker, so the higher and sharper came is controling the valve. If the system has a 3rd came then its rocker is going to begin to work at even higher RPM.

Some other cars have fase variators (is this correct?) and those advance and retard the camshaft gears.

However I was asking about the A12 engine, since if you have only one camshaft you can only retard or avance all the valve cycle and you can not change the valve crossing (because this is determined by the came position on the came shaft).

Thanks

Ricardo

You may be right. I thought the VTEC changed the camshaft phase. It seems like all the auto makers have a variable valve timing system now. Some vary lift, some timing, and some both.

For a single-cam cylinder the cam can't be advanced/retarded as much -- just a few degress. I heard that VTEC moved the intake something like 50 degrees!

its a bit off subject but has anyone heard about variable compression like moving the whole crankshaft up or down on a engine running boost so you still have down low power

Can't say that I have heard about it!
Have you or are you just asking if it is around?

Matdat

An year ago Saab (I think) presented a engine with variable compression. However, it wasn't the crankshaft that moved. Instead the engine head rotate (pivots) (!!!!strange system!!!!) for about 4 degrees.

Ricardo

yeah i heard about it being on a saab to. I thought it would be a good idea wonder if it worked. By thery it would be excellent

SAAB has made the prototype, it will go in production in a couple of yrs, its supercharged which engages when boost is needed only, its block moves few degrees away or closer from crank to increase or decrease compression. compression ranges from 8.x:1 - 13:1(approx.) it basically combines power of a supercharged engine and economy of NA engine.
barpk