what do they mean by trim when talking about turbos im assumeing they are talking about the pitch of the blades but i want to make sure also what would be better for lower rpm use smaller trim or largeri would think you would want less to make it spool faster but i could be wrong
thanks
tony

 

In this case the world "trim" refers to a variety of different parts, including compressor/turbine size/pitch as well as housing size/style.

"stock trim" turbo versus "race trim" turbo, etc. Personally I think the word is being misused. "Trim" to me is dangly stuff installed on something else - like body side moulding or tinsel on a festive December tree.

Turbo's can be sized for any displacement and RPM range. It's all math. Any turbo, if attached to an undersized engine as compared to the turbo's flow rates will be less useful in lower RPMs and more useful at higher RPMs as the flow increases. The reverse is true also. Undersized turbos will spool up quickly in lower RPM ranges but not do a heck of a lot at higher RPM ranges.

One of the reasons why diesels are turbocharged rather than supercharged these days - the useful RPM range of a diesel is very narrow compared to gasoline engines.

And BTW, you're not limited to stock turbo sizing. There is nothing stopping you from combining different compressor housings (fresh air) with different turbine (exhaust) housings. T3/T4 combinations seem very popular lately, for example.

Smaller turbine housings/wheels spool faster than larger because it requires less flow.

Remember, a gasoline engine is nothing more than a big air pump in which we explode fuel.

 

Trim is the relationship between the inducer and exducer of both wheels on a turbo (both turbine and compressor), think of it as an area ratio.

The inducer diameter is defined as the diameter where the air enters the wheel, whereas the exducer diameter is defined as the diameter where the air leaves the turbine/compressor.

Check this diagram - it will explain it better:


A higher trim wheel will flow more than a smaller trim wheel - but there are lots of other things involved too (like pressure differences, lag etc.)

On edit: Frederic beat me to it

 

thanks guys i was just looking on ebay last nite and people where saying that there turbo had a 60% trim and others were a 40% trim and i wasnt sure witch was better as this spring i am looking to start my twin turbo prodject which will be very simmilar to frederics whom as been reading my mind apparnetly the last few years and research i have found is a good way to start any project

 

Yes, research is definately the way to start a project. My first twin-turbo pickup (75 Dodge with a 451 stroker) didn't have enough research ahead of time. Enough to get started, and get into trouble several times along the way, which wastes time and money later, and potentially breaks parts.

This time around I had plenty of "research" from the Dodge that I only needed to do the math over and made decisions from there. And I ended up with slightly undersized turbos anyway... I have a pair of T4/T3 hybrids, and a pair of full T4's and the more I look things over the more I realize the T4's are the choice for my particular combination.