I have always been told that with LPG you need a 10% larger cam to get the same ammount of energy from the engine, due to the known fact that the LPG/Air mixture displaces about 10% more volume compared to gasoline vapour/Air mixture per unit of equal potential energy.

SO if 100hp of gasoline/air mix takes up 100L, just for example, then 100hp of lpg/air mix takes up 110L. That is always what I have been led to believe.

My problem is that I've also been told that while running LPG you shouldn't get too big of a port volume as it slows the velocity down too much, but that kinda contadicts the volume thing.

Yet in a forced induction situation you normally size the heads much bigger as you have the velocity due to the charger(be it super or turbo), and the better flowing the heads the less boost you have to run, and the easier the power is created.

now how do I make a decision on what cc/flow heads to use???

I am building a 351 clevor with a GT42R(74mm 1100HP) and twin GRA mixers with four B2 converters, powerglide and 3500stall, would the car still behave like the heads are too big and the velocity is crap, until it reaches the very high rpms, which I really don't want, or would it be unnoticeable once on boost? thats if I used the 258cc head, I should have absolutely no lag with the roller turbo, it will prob start to spool at about 2200rpm. I guess if the heads are too big it may kill the low down TQ figures?? I hoping to get as much HP as the turbo can do, but it is still s street car.

2V-190cc(600hp) 3V-185cc(590hp) 3V-218cc(680Hp) 3V-258cc(750hp)

Based on the 10% more volume theory my take on the situation is that the 258cc would behave and flow like a 232cc head. Making it about the right size for my combo, whereas if it was n/a you would only use it on 400+cubers.

ANY advice or input greatly appreciated, Cheers.

based on your %10 theory and wanting as much power as the turbo is capable of (1100) if you went with a %10 smaller port would you really miss that 100hp?? theoreticly speaking if you did reach the point of max power of the turbo.

the 3V heads are not the only aftermarket heads out there.

see if you can find a head that flows the peak power you want with a smaller port. that way you get the flow and the increase in air speed

CHI have the highest velocity ports for their size, AFD's website is down so I can't see their figures, but from memory AFD's 2V's flow more on the exhaust side over the CHI's. But some rumours have been floating around that the AFD's aren't quite meeting expectations regarding flow numbers.

I'm not sure if all that LPG chit is actually true, was hoiping for others opinions.

The thing that started this was looking at an sr20 intake and thought those runners are huge considering its only 2L.

Yes but look at 5.0 engines pulling 800 h.p..I wouldn't worry too much about velocity etc ..
My TFS heads will flow 800 easy[with boost] there ports are alittle [lots] smaller..I'd say it would be more the turbo efficiancy more than anything...Having bigger ports is not an issue the turbo will fix that!!!
Thought of alcahol injection?? Just for performance events???

I spoke to a few head places, I'm waiting on something new that should be cast within a few months, and I ordered a 2V funnelweb and terry's gonna make it to fit onto the 9.5 block, and when I get the alloys it will also bolt onto them with some port plates, which is good cause I don't have to buy another intake when I fit the alloys.

Any pics ????

The basic rule of head sizing is that you need to keep the gas velocity as high as possible without restricting flow for the powerband of the engine.

So, let's say you've got REALLY little ports. Oviously, if you whack a cam in it with a powerband of 6000rpm-8000rpm capable of producing say, 500HP N/A, the little ports are going to restrict flow and you won't notice the cam's full potential. That's the easy bit to understand. Basically, bigger port = bigger power.

However, the opposite is true for a slow revving engine designed to develop bucket loads of torque low down in the rev range. If you put massive heads on that sort of engine you run into problems because the gas is traveling so slow. You need a fast moving gas to help mix the fuel with the air and also to build up inertia to help push the air into the cylinder.

I haven't explained it very elegantly but that's the cux of is. Big bottem end torque = little ports. Big top end power = big ports.

One adversly affects the other so it's a balancing act, just like any other mod you do to the engine, unless you've got a fancy engine with variable port size.... which you don't.

Yes in theory thats right with N/A engines, but turbocharged engines are totally different animal..Its quite easy to have twice the exhaust pressure to what inlet pressure is in higher rpm on big engines..This just doesn't happen on N/A engines..For example..
With correct manifolds, long- sh but big inlet runners torque will NOT be an issue..Well atleast 700 nm @ 2500 and 220 rwkw @ same rpm..With boost I can't see any issues.I seroiusly doubt an N/A engine this size could achievethis?. Big thing is keep camshaft small in duration with little or no overlap & around 114 lobe seperation...
More the point I'd say with gas the larger- sh ports would help..
Bwarned has bloody large ports on his Cortina...Though he running alcahol..

Thats all true for n/a but not for turboed, it still holds true when under vac but as soon as boost starts it all goes out the window to some extent.

With n/a you might have the cross sectinal area of the runner/port at about 75% the area of the intake valve, when forced you have it the same as the valve.

So I guess that the bigger heads may cause the turbo to spool slower, but it would make more power with the bigger heads.

With the flow ratings of the CHI's I am going to get, my turbo wont keep up so the irons will have to do until I can get something bigger like a PT88 or GT47. I am going to get the alloys just not fit them yet.

More the point I'd say with gas the larger- sh ports would help..
Bwarned has bloody large ports on his Cortina...Though he running alcahol..[/quote]

yeah but his is 408. I think I know what heads he is getting for it.

The whole idea that lpg takes up more volume I'm still not sure about.

Either way I'll be making more power than the LTD's chassis will ever handle.