random turbo question

[sleepinsol]

I just had a thought and wanted to ask and see if maybe someone could shed some light on this. I have seen a few dyno videos and dyno charts of 2.3 turbos, and I am noticing a pattern. It seems that the larger the turbo, the closer the hp number and tq numbers are. For example, a stock svo t3 turbo may make 220 hp, and 300+ lb/ft of tq at the wheels with various mods etc... However, a holset turbo or a .50 trim hybrid may make the same tq as the smaller turbo, but higher hp numbers (of course depending on the mods and tuning). To me, it seems that the larger or more wild the turbo and motor setup is, the less likely it will be that the tq number will be higher than the hp number. I could be mistaken and this could be completely irrelevant to turbo size and engine setup, this is just something I have picked up on and am wondering why this is the case?

[Stinger]

If you look closely at the graphs, torque peak is always the instant the engine reaches max boost. By definition, the earlier in the RPM range the engine can reach this point, the higher the number will be. A larger turbo reaches full boost at a higher rpm which 1) lowers the tq number while allowing it to make more hp (again at a higher rpm).

For a given torque and speed, the power may be calculated. The standard equation relating torque in foot-pounds, rotational speed in RPM and horsepower is:

HP = {(T [{ft * lbf}]) (ω [{RPM}]) /5252}

Where P is power, Τ is torque, and ω is rotations per minute. The constant 5252 comes from (33,000 ft·lbf/min)/(2π rad./rev.).

Because TQ is directly related to RPM and HP is derived from TQ, the HP and TQ lines always cross at 5252 rpm (when both are shown on the same scale). Therefore the higher in the RPM range that the engine is designed to make power, the lower the tq number and the higher the hp number. Notice how a Honda will make 400hp@8500 rpm and only 280 tq while a turboford might make 400hp at 6000rpm and make 400 tq as well.

Cliff Notes: Larger Turbo = Slower Spool = Higher RPM Before Max Boost = Less Torque

[sleepinsol]

now it makes sense. its all clear now. esp since you put it in terms of hondas lol. i understand those real well. since they are designed to rev higher they will sacrifice some tq for a gain in hp. got it. slower turbo spool also means higher engine rpm at the big end since the turbo will continue to make power up there, which will again mean less tq. right on. so i guess my next question is, what is better for a daily driven street car? should i try and shoot for a motor that will make high 200 hp #s and mid 300 tq #s, or shoot for a motor that is going to have hp numbers and tq numbers that are almost similar? i am thinking of upgrading my ihi to either the larger svo t3, or a small hybrid turbo once i get a spare head to open up.

[Stinger]

The "stockish" setups that make tons of torque are very fun to drive because the initial "hit" when the turbo spools up makes it "feel" much faster than it really is. On the other hand, the ability of a well build 2.3 with proper cam, porting, etc. is nice because you don't have to short shift at 5k rpm like you do with a stock setup. Just a matter of preference. If you are using a stock cam, a small turbo will be more fun.

[sleepinsol]

I am not sure what sort of cam I am going to use. I definitely want to do some head porting and bowl work on a spare head though. I have a ranger roller, and I hear they make good power when they are adjusted with a cam gear and tuned. I am also considering the A237 cam or the bo 1.5. It will probably be whatever fits my budget best. You can bet I will be tuning the cam adjustments with a gear on the dyno though. If I go with the aftermarket cam, would upgrading to a bigger turbo than the .60/.63 be better for daily driving? It is also going to be a while before I can afford a good engine management system, so I will be on a blow thru vam and an adjusted fpr for a while as well.

[Stinger]

For a daily driver, the smallest turbo option that will support the hp your supporting mods can make will be the most fun. Under 300rwhp use a T3 for a driver.

[sleepinsol]

understood, thank you. this is re-assuring because I have always wanted the svo turbo. its the perfect turbo for a mild street car with a useable power band.