good post Ken

I guess it all depends on what we want from our setups. Barbz did recommend to me a CR of 8.8 to make the most out of the engine build, and I guess nobody knows better than him on building a 20VT. J has gone a bit higher, but seems happy with this. I think I'm going to stick with 9:1 and see how it goes. Hopefully not boom

In terms of CR, the general trend whether it be factory or modified is to increase. That said, a factory product is built from the ground up with the latest technology therefore they are designed to run this CR out of the box. In many instances a higher CR is due to the prevelance of increased octane fuel, an adaptive ecu and light pressure turbos. Most VAG modders will drop their CR from 9+ to middle-high 8s, old school but nothing like as bad as the low 8s and 7s that Cossies and older Porsches run.

I think there are to many people focusing on the static C/R

would some one be able to give me the details on the C&B cams that everyone is using??

there are many other factors at play that will effect your decision on C/R

the turbo that is being used will make more of a difference

High C/R seems to suit larger turbo's running relitively low boost, remember that whats more important than boost is flow

a smaller turbo will have to run a higher boost level than a larger turbo to produce the same flow, the problem with higher boost pressures is that you will have a hotter charge entering the cylinder and as a result you will have to retard the ignition

there is a lot more at play than i have explained above but it may make some points clearer for some people

all of this is theory and some times does not playout exactly as you expect.

But what ever you do make sure you pay attention to the turbo you plan to use before you finalise your decision on your C/R

Just for the record i have driven cosworth with C/R's of 6.8-1 and 8.9-1 plus the standard setup, most people would say that 6.8-1 is really low and off boost drivability would be horrible when in fact it was hard to tell the differnce as he speced turbo that suited the low C/R

Jay, Running the GT2871 will be quite a good compramise for your setup, you wont have to run such a high Boost pressure to make the BHP you want

Also looking at the flow maps you have uploaded the GT2871 seems to cover most of the bases

I don't pretent either i am learning too that is why i choose a motorsport course. I'm still only a first year




Well in all honestly I planned to run 8.8-9:1 CR as Flea/Barbz etc runs 8.8:1 and JohnS runs slightly higher than 9:1 (he won’t say exactly what that is) so I am not the only one who has increased the static CR and I still feel there is nothing wrong with running 9:1.

The only possible mistake I have is that I skimmed my head and block too much and ended up with 9.4:1 (long story). I had the option to use a decomp plate to bring it back down to 8.8:1 whilst the engine was apart but I thought lets experiment, however it’s not the end of the world as changing the turbo to a bigger compressor could benefit to this factor. I am studying motorsports and it is all about testing your ideas and tweaking the results and I feel I have been very open in what I have been doing unlike some people who keep data away from the forum.

After another set of results if I wanna change the CR then off pops the head and on goes a decom plate. It’s no big deal for me as I am able to do this at no real extra cost (just my time and parts). I choose to experiment with my own ideas rather than follow the flock as if I don’t then I haven’t really learnt anything. My car is my toy, project and experiment and I have learnt so much from it.

So can we please now drop this and have some ideas on a turbo choice thank you

Also I’m am still compiling a graph to compare all the turbos but using this excel 2007 is a bit of a pain

You know what TurboJ, I have absolutlely no idea what you are talking about here (well I do see the larger scheme of things) but, I tell you what, I would love to know more. Is there any decent books that you recommend to get your head around this aspect of engineering?

WOW talk about rasing the dead

So what was your choice in the end j?

GT2871 48 Trim

This would be a good book to start off with

Thanks Begbie and thanks TurboJ for the info! Already ordered