[SkylineUSA]

I pulled this off of www.turbomustangs.com this is the best turbo site on the net, point blank. A bunch of very smart guys willing to type, and help.

Go there, you will not be dissapointed.

A/R is mearly a mathematical calculation!! It has more to do with RPM relationships for a given application than anything else.

The larger the A/R the more time it will take a given compressor wheel to create pressure.(in the case of the compressor) Or how long it will take a given volume of exhaust gasses to generate a certain amount of pressure or kenetic energy on the turbine wheel to achieve a certain RPM. (in otherwords spool-up time when putting both together)

The smaller the A/R the faster the compressor wheel can generate pressure in the compressor housing and the faster a given amount of exhaust gasses can generate a certain pressure to drive the turbine wheel to begine spinning the compressor to make possitive manifold pressure.

Now, Yes A/R ratios have an impact on HP potential. Simply by deciding where in an RPM range the engine will generate boost and reach optimal airflow.

For example, you have a given compressor wheel and A/R Compressor housing, lets use and example of a T64E. This turbo has a compressor wheel that roughly measures 2.5" Inducer, 3.28" Exducer and has a E housing with a .60 A/R. We then have a turbine side consisting of a P-trim wheel and a .69 A/R turbine housing. This turbo on a given 302 will generate say 9psi by 3,000RPM. It will them make say 450RWHP at about 4,800-5,200RPM.

We then replace the turbine side housing with a .81 A/R. It would then reach full boost at about 3,200-3,300RPM and make a HP peak in the 5,300-5,500RPm range at about 475HP.

Ok, so now lets take the same turbo, add a Q-trim wheel in the .81A/R housing. It will spool up at about 3,400-3,600RPM and generate about an additional 10-20HP at about the same RPM, maybe a tad higher.

So you can see A/R has an impact on power production but more importantly it mearly determines the RPM range in which a given set of wheels will operate on a particular application.

More important to flow is Compressor and Turbine wheel SIZE. The physical size of the wheel has a huge impact on flow capability. The larger the wheel, the more flow.

For example, we will use a T04 with a E .60A/R compressor housing. Both with a .69A/R turbine and P-trim wheel. One will have a 57trim wheel the other the aforementioned T64E which uses a T66 wheel. What would be the difference in these turbos? How would they perform comparitively on a 302?

I think pretty much everyone will agree the 57trim will have a tad better esponse (less inertia in the smaller wheel) and offer much less flow potential.

My point is, before buying a turbo of A/R this, A/R that, have who ever it is that is selling it measure the wheels. That is just about the most critical aspects of turbo sizing.