123 replies to this topic

[EunUCh]

Is there anyone who who has had the privelige of using flow bench who can tell us what is the optimum area of intake port compared to valve throat area that gives best results (for the purpose which engine is built) for cylinder filling.obviously there are varibles involved but there must be some point where air speed thru port and thru valve hit the so called sweet spot,for example,do we want air speed thru throat faster than air speed thru port or vise versa or do we aim for same speed thru both throat and port.

[Tiny]

I'll be watching this thread!

Only because it's a really good question, but i dont think that you'll get too many people give you their secret out in the public like this! (Unfortunately!!)

Cheers!

[EunUCh]

I agree that those sorts of figures would be kept close to chest by those who know or have a good idea of what the figure is,however I have managed to find on the net that a figure of 650 ft/sec is about the max,but still doesnt answer the question of the speed thru port/valve throat,my own thoughts would be to keep them the same. Anyone else got ideas to throw in,throw em in lets churn it over,after all we all want the best results from our engines and I figure a good place to start is put a bit of theory into what we are doing then go from there.The main reason for good air speed into cylinder is to obviously fill the cylinder quick and get as much as poss. in there.I have had personal exp. with a torana that was fitted with 253 ,twin 2in ex,big valve heads of well known manufactuer,mild cam,336 gears on 13's,roch.carb and the thing went worse than a stock 253 auto with wwstromy with 308 gears on 14's.only reason can come up with is that the big valves killed it.(both engines were in similar condition).a mate of mine has almost the same setup (same profile cam big valves)in a tonner and it has the same charactoristics as the torry did and his motor is fresh.hence the question where do we aim for with air speed,and once we know that what is the formulae for working out how much air/fuel do we get into the cylinder at a given air speed(keeping in mind of course wot purpose we build the engine for)

[Struggler]

Mmmm, where do I start. I'll see if I can cut and paste something later on.

[rodomo]

Does 9" between the butterfly and the valve head mean anything?

[Struggler]

NOBALLSUC, I'll give you my take on things......

Every cylinder head has a point of most restriction/smallest cross sectional area (CSA), some have 2 or 3. The best place for this to be is at the valve seat. The best heads act like a funnel, but are not shaped like one. Air needs greater CSA to turn a corner (ie in a bowl behind a valve). For best cylinder filling air speed has to be greatest at the valve seat. With your 253 fitting the bigger valves may have moved the lowest CSA to another part of the port, this is what happens with standard heads with big valves fitted and no bowl work.

1QUICKLJ, I have said this before, the fact that the valves hang over the edge of the bore is mainly irrelevant. As Toranamuk said, the majority of the air travels on the outside of the port, across the bowl and valve and heads toward the spark plug. The bore lip is not in the way. What works on a flow bench doesn't always transfer to reality. A flow bench measures air flow, an engine flows air and fuel. If the bore lip was a problem then Clevo's and BBC's would go better with the lip removed. They flow more air this way but make no more power.

EVERYONE, from my experience L34 sized valves are NOT needed for a mild 253 or 308. DON'T believe the hype, Yella Terra heads are WORSE performers than standard stock valve heads, both on an engine and on a flow bench. Any head with L34 valves NEEDS a fair bit of bowl work to perform effectively. In my experience standard sized valve HQ heads with a good valve seat job and very little clean up work on the bowl and short side radius perform remarkably well.

For computer aided engine design including CSA calculations check out....
http://www.maxraceso...ipemax36xp2.htm


This is all based on MY personal experience, please feel free to comment.

[TerrA LX]

So where is the best head job in town?

[EunUCh]

Dont know if these are in calculator but here we go;
MEAN GAS SPEED THRU PORT =PISTON SPEED/60 x D squared/d squared
MEAN GAS SPEED THRU VALVE = PISTON SPEED/60 x Dsquared/VxLxPI
D=Dia of piston,d=dia of port,V=valve throat diameter(measured under valve),
L=lift of valve.All measurements in inches and ft/sec.
As already stated CSA plays a big part in air speed as well valve lift,rpm,port shape/size and it does get a bit complex,if we crunch a few numbers with the above formulae it is easy to see that what seems a fairly minor change in port or valve throat area (CSA) we end up with major differences in air speed which obviously affects cylinder filling, hence torque/power.One would assume(i hate that word) that if we aim for max air speed thru valve throat(at some given rpm) that we should aim for the same value thru port and try and maintain the speed probably from the carby/s? and keep the air stream as straight and smooth as possible right to the valve.could be why one carb per cylinder work better(in most cases) than one single carb.Back to poor little 253 problem,yes it had 308 head gasket dont know how far the valves were sunk and the cam was around 208@50 270 lift,all would have had some affect,but man it was woefull.if i was computer savy i'd know how to add this link but i'm not so i'll have to type it out http://www.racetech.com.au/custom.htm

[Struggler]

Struggler, can this problem be compensated and or moved to a different rev range by the choice of the carburettor size?

Choking the engine with a smaller carb is a bandaid fix at best. It can make things a bit better but if the heads are poorly designed/ported then the engine will always be lacking.

[EunUCh]

As already said big valves/throats dont always make more torque/power and as also already said that smaller valves work better in some cases,would the fitting of smaller valves also involve reduction in valve throat area or is flow gain achieved by the fact that when you fit a smaller valve it is unshrouded in chamber?also if we use a smaller valve and reduce valve throat area accordingly would this not let us use a camshaft of higher lift to increase the curtain area under the valve resulting in maintaining good velocity over the valve lift operating range and at the same time better flow because of increased curtain area.I've not had any experience in building hot engines but because of the plan to build up up something abit better i figured a good place to start would be do a bit of research to find out on which way to go,so far this topic has yielded some execelent info,and the article mentioned obout the standard valve/port on the torana sounds very feasible as i have mate who is a mechanic and he did his mates 202 in similar manner,was only a daily driver but the results accordin to him were quite outstanding.as this discussion continues my personal point of view on big valves big ports big flow numbers dont mean big power,i say this with regret but i think a lot of folk suffer from what i call glossy magazine syndrome (from which i used to suffer),and bigger is probably only better in the area of air cleaner size!.as 1quiklj has said use the airspeed he aims for,flow comes from very carefull removal/adding metal in bow area and around where the valve stem guide protrudes.

[Struggler]

i have the upmost respect for struggler i am certainly not trying to challenge his knowledge. this is just my point of veiw from what i have learnt from over 20 years of playing with engines.

No problem 1QUICKLJ, I'm old enough to know there is more than one way to do this stuff, there is no real right or wrong. How I do it is to concentrate the flow across the valve to the spark plug. I only unshroud the intake in the area around the spark plug. This is what works for me (maybe I could be doing it better ?). This does NOT mean this is the only way that is effective.

Its also worth noting that HQ308 heads respond differently to other heads (Ford, Chev etc) to port changes.