Valve timing

[zefarelly]

I've got 11.5 : 1 on an A3 cam in my road car . . .thats running a 1700 crossflow bottom end with flat top pistons on a precrossflow head . . .looks period, goes well !

I know I can get to 12:1 with a little courage, when I rebuild the engine over the winter I'll do it

[dtype38]

OK, thanks for that Graham. Nice to know I'm not completely misguided all of the time :-)

[graham bahr]

Originally Posted by zefarelly

I've got 11.5 : 1 on an A3 cam in my road car . . .thats running a 1700 crossflow bottom end with flat top pistons on a precrossflow head . . .looks period, goes well !

thats quite high, what are you using for carburation?

[R59]

When I got my motor over from Tennessee, it had a comp ratio of circa 13:1, and had been running on VP C12, with booster....

After some filing of the top of the pistons, and another 20thou of head gasket, I've got it down to a rizla under 12:1.

The fact that the motor is alloy, allows me to run much higher compressions than a similar iron lump due to better heat dissipation.

The other thing to take into consideration is that this is a static compression ratio, ie: what it works out to be on paper. The real compression ratio depends on what the motor can draw into it's lungs and sqeeeeeze. Hence A-series running very high 12-1's on Optimax/4*, but try running that on a 16v Vauxhall and it'll clatter like someone emptied a bag of nails in the cylinder.

One thing that you all have forgotten in your quest for answers is: Volumetric Efficiency - the ability to make use of what you have got.

The theoretical figures are: 2 valves per cylinder : 100bhp/litre, 4 valves per cylinder : 150bhp/litre.

Now those figures are ballpark, but they have been beaten by many builders. Even the old non-crossflow Ford 1500 can achieve 105-108bhp/litre without getting to exotic.

Take the modern extremes of F1, they have achieved 300+bhp/litre, with some of them pushing the limit towards 1000bhp with just 3000cc's.

Dave Vizard explains VE in many of his books, well worth a read.

Rob.

[Al Weyman]

David Vizard, I remember getting him and Roland Hayes together at a practice session down at Brands when David had prepared a Seirra to run in the ModProd championship. That was a meeting of minds I tell you.

[zefarelly]

Originally Posted by graham bahr

thats quite high, what are you using for carburation?

Graham, its running twin 40's, 32mm chokes, 125 mains, and I set timing at 14 degrees static, 34 full advance. It has a 4 into 2 into 1 old magnoletsi manifold . . . . although I have a nice 28" primary 4 into 1 I borought back from Australia as hand lugguage to go on it now!

I only run it on posh juice as its an occaisional hooning around car

[dtype38]

Is your occasional "hooning" on the Sussex coast a local custom, or an arrestable offence??

[graham bahr]

zef,

that goes to prove what i suspected your pre x chamber is burning better than x flow, as we often used to have them running on 40 degrees advance to get them working properly,

go for that 12:1 you know you want it

[graham bahr]

Originally Posted by graham bahr

how much compression as cylinder filling also plays a part,

that is volume metric efficency

[zefarelly]

we have a custom built flyover just outside Shoreham which has a fantastic roundabout for checking handling charecteristic on quiet evenings, and I'm only 5 minutes from the sticks and some nice country roads. which is nice !

just by looking I'd say precrossflow would burn better, from some of the heads I've done, run and stripped you can see where the burn is, theres usually one little bit where it doesnt between the inlet valve and the side wall by the plug, either that or its the easiest path for the mixture to get to (more likely) and its gets well cleaned and doesnt mark. I've not yet done two the same so its difficult to make any great sweeping statements, but we're generally producing more and more power so we must be doing something right !

[zefarelly]

we have used A3 cams in several engines, and A6, in both cases higher compression seems to be the biggest factor in making them work. on the 1700 (with bigger followers and an A3 cam) I was wondering about trying a set of high lift rockers as well to raise the lift a bit

[graham bahr]

more cam needs more compression to make it work and both A3 and 6 cams are quite a lot of cam especially considering your realatively small engine sizes

high lift rockers may work, but they will also increase the lift on overlap which the engine may not like, i would be inclined to close the tappets up a couple of thou and see what happens on the dyno, if it likes the tighter tappets then i would try high lift rockers but with std tappet clearances.

[zefarelly]

that sounds like trouble . . . .for a road car anyway, I'll stick with the std A3 for the time being

I have heard some of the hot rod engines runnind downdraught with soil pipes for exhaust manifolds, offset valve centres, short stroke, big bore screamers etc use A8's and high lift, they also breath in through 48's . . . .different gravy !

[graham bahr]

sorry i thought you were talking about your RACE not road car

[zefarelly]

I use the road car as an every day test bed . . .we don't have the luxury of a dyno, and until I can befreind someone at Ricardo (1/2 a mile from home) we'll be doing things the old fashioned way . . . .