any tips?

No, not really. You have to start with the biggest restrictions first. The engine has 6 39mm inlet valves and the exhaust ports are roughly the same size, but the manifolds go directly from this size into a pipe the equivalent size of one valve.

Anything before the manifold is not causing a restriction.

Hi David, Are you saying the exhaust manifold is the restricting factor?

Yes, combined with the turbine housing.

[Alpineandy]

Hi David, Are you saying the exhaust manifold is the restricting factor?

Yes, combined with the turbine housing.

And what's the restricting factor on a non turbo (please don't say money ) ?

Exhaust manifold (lol)....it does rev better with the proper kit, camshaft duration and lift - intake, but more the design of the inlet manifold, rather that the size of the inlet

A sorted atmo like above will rev cleanly to around 7k. Intake size isn't as much as a problem as people may think (the attraction with the big carbs), as currently, people are fitting efi V6's still using the standard single 55mm throttle body, and still able to rev to 7k and make 200bhp, but this is due to the better design of the Efi manifold.

On the atmo, with the standard inlet manifold, none of the cylinders breathe equally, hence the attraction for the equal length triple carb setup.

For all the money you could spend tuning the atmo lump, sticking in a 2975 Efi PRV would reap so much more benifits.

You have bigger capacity, even fire, bigger inlet and exhaust valves, bigger ports, better crank bearings, stronger conrods, better cam profiles and lift, better inlet manifold and EFI...

No, not really. You have to start with the biggest restrictions first. The engine has 6 39mm inlet valves and the exhaust ports are roughly the same size, but the manifolds go directly from this size into a pipe the equivalent size of one valve.

Anything before the manifold is not causing a restriction.

Hi David, Are you saying the exhaust manifold is the restricting factor?

Andy, I don't know if you know, but the turbo manifolds are far far smaller than the atmo manifolds (and these are known to be small anyway! )

[Alpineandy]

the turbo manifolds are far far smaller

So the company/person that does a large bore manifold will be worshiped by GTA turbo owners?
Or is there more to it after the exhaust manifold?

the turbo manifolds are far far smaller

So the company/person that does a large bore manifold will be worshiped by GTA turbo owners?
Or is there more to it after the exhaust manifold?

lol, nope, because it will be laggy as hell

Ive designed a system like this but it works out very expensive, as it consists of two proper flowed manifolds bringing the single turbo in front of the engine instead of on top. It has a whole new turbo backplate which holds the turbo too. It also consists of a completly different exhaust system and new oil feed/returns and intercooler plumbing. Problem is it would work out about £1500 and thats not including the larger turbo you'd need to make the most of it..

So then ,the best/value for money solution would seem to be ,Atmo with turbo inlet running big carb/blower and exhaust manifolds .About 1k in parts Half the price of a new twin triple on its own .

So then ,the best/value for money solution would seem to be ,Atmo with turbo inlet running big carb/blower and exhaust manifolds .About 1k in parts Half the price of a new twin triple on its own .

You forgot cams....The standard carbs don't really run out of puff on the standard setup. You need the cams and exhaust to make the engine suck harder and want to rev higher thus the need for better carburation.

Also, due to the longer duration cams, the compression would need to be raised as long duration cams lower the dynamic compression ratio.

Also the V6 Turbo inlet manifold needs to be the A610 variant, the GTA one won't fit.

I knew you'd say that

Trust me, get the ignition timing advanced first.....you will notice a difference

It doen't matter how much air and fuel you get into an engine, if you ignite to late, it's all a waste of energy!

Ok ,I need the carbs refurbed first Looks like a long term project on the atmo Looks like an uprated Turbo is calling ,much easier

thanks all for the information - i now just have to try driving one! also wondering what other engines could be placed with the minimum of fuss - does a latter renault v6 go in (ie connect to the 25 gearbox).

As soon as i get the cash together i will be getting manifolds done and i will be putting a cossie exhaust housing on the turbo im a man on a mission looking for 300bhp+ and will get there eventually

thanks all for the information - i now just have to try driving one! also wondering what other engines could be placed with the minimum of fuss - does a latter renault v6 go in (ie connect to the 25 gearbox).

No, the later V6 can not be connected to the GTA/25 gearbox.

The other option is the R21 Turbo 4 cylinder engine which I am doing soon to a new GTA I have purchased.

285bhp has been acheived on a standard engine with just larger turbo and remapped standard ecu. 300+ with a decent cam and proper management.

You list those figures for the 21t, but in Retrocars on a RR session earlier this year, even the most heavily modified 21 failed to make 250bhp and he had everything you list and more.