I've been modding an AUII SR 5.0 for some years. It has a crow cam, roller rockers, double row timing chain, yella terra 1.6's, edelbrock performer intake manifold, tri-y's, 3 inch system, ss inductions cold air and k&n. It has been dyno'd with a flash tuner. I recently got my hands on a 75mm pro-m maf which I have not put in yet. So I decided to finish off the intake properly and replace the stock 65mm tb. I also got my hands on a new air box lid from an xr which has the aluminium bell mouth, I took to the rest of the air box with a dremel and removed all of the moulded ribs, I think it gave it a bit more toqrue in the low end before it kicks down, perhaps just the cold climate when I drove it after that, seemed like a sensible free mod to do. I had 165 rw kw with the mods before the edelbrock intake manifold and with a 2.5 inch system before I went 3 inch.

The edelbrock manifold has a 70mm opening and the instructions say it can be machined out to a maximum of 75mm. On the stock maf the outlet inside diameter is about 75mm, so is the pro-m unit, though both of them are larger on the inlet side. the stock maf has the aluminium restriction in the centre and the pro-m restriction is narrower and only goes from one side to just past the centre.

So I was thinking that putting on a 75mm tb may be a waste because if you take into account the restrictions in the maf, outlet diameter less area of sensor restriction, it's less than 75mm for sure.

I guess the 220kw xr8's with their 70mm tb are running the same restricted stock maf as I have, so my theory may be wrong as they would not put it on if it did not give a performance increase.

Is a holley or bbk 75mm tb too big in that it would lower velocity ? They look somewhat challenging to mount because the metallic fitting which the cable connects to is at the other end compared to the stock unit. The metallic fitting and the rubber grommet hole are reversed. Has anyone fitted one of these to an AU ? I would make up my own spacer or buy one, I hear that by not using a spacer there will be a loss in low end torque, that's why the 70mm tickford tb has a built in spacer thicker than what was on the 185kw v8's, then again the au3 strokers with their 82mm tb are mounted right up against the plenum with no spacer ! I see the spacers have 2 studs at the back to hold other stuff, I guess I could make up a bracket to hold it on after the tb/before the stud nuts.

So, the 70mm tickford tb is a bolt on while aftermarket is a lot of stuffing around but perhaps worth it to get to a 75mm tb. If 70mm is the way to go and I can not get a tickford unit then I have to stuff around fitting a 70mm aftermarket anyway.

Thanks for reading this long post.

After reading the forums I see some members have noticed a drop in low end torque going from 65mm to 70mm and have gone back to 65. Does this mean the au xr8's with the 70mm can have their bottom end torque increased by going smaller to 65 ? I don't think anyone has done that or would do it. So is a 70mm going to be a step backwards for a daily driver ?

Had the 5L on an engine dyno - I went from 70 TB to 75 TB & it smoothed out a couple numbers - but really made b all difference - mind you I have scrapped the MAF & am using MAP. The MAF I had was 75mm. In fact with the 70 TB there was still vacuum in the Performer manifold so switched to 75. Missed the opportunity to stick the 65 TB on to prove / disprove TB size theories.

Richie.