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

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.

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.

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.

I just picked up my new A15 race motor last night and as the owner of numerous A 12's and a daily driver of an A14 this will be my best motor yet. It was built by JTS in Sydney and I'd regard Jeff as the premium A series builder in Australia. On my Weber 40's this is a 140HP motor with a 9500 rpm redline. With Jeffs max head work and 45's it will generate 160HP. When I picked up the motor I spent some time looking over a 200HP 50mm trumpet injected A15 in the best race coupe I have ever seen. So I guess that answers the race engine question A12, A14 or A15 - it's the A15 and it'll still rev enough to scare you. In standard form I'd reckon the A14 will have the edge - it's got the most HP and revs higher than the A12 or A15. In standard form the A15 is a torquey van/ute/forklift motor and the A12 is a great power plant but the lower redline and 200cc smaller displacement hurt it against the A14.

ddgonzal,
I'm in your area, and looking for info on some parts which can be sourced around us. Email me for details.
Thanks.

I thought Nismo had adjustable cam gears, but their site seems to have disappeared. Maybe AFRacer can pick up some from Pitroad.

This is a classic technique; even the auto manufacturers retard or advance the same cam from year to year to shift the torque curve or reduce emissions. There is a great picture at stylinconcepts.com, but it appears they don't have these for a-series engines. Simpler ones have several different key slots on an otherwise standard-type gear.

Okay guys, we all agree that displacement rules all. However, I think there is such a thing as "horses for courses". For road racing, the engine is always on the boil, and pulling at high revs. An A14 might benefit here, (epecially if the same amount of hp is achieved using a thousand more revs) but for drag racing, an A15 with all out torque wins. Agree?

but olboy regardless of what it revs to

we are more interested in where the max power hits and i guess at what revs you would change gear to achieve max accel

how much info can you give us on whats been done to and how much power your motor makes?

my 2 cents,
after reading all that i am going ahead with getting an a15 (which is going into a 1000 wagon mind you) i think itll go just nicely. i have to say tho, hi rev's generaly ( but not always comes down to a direct relation to the amount of money you invest into the engine.