I've been doing a lot of reading about turbo vs n/a gearing and how each should be optimized for the 1/4 mile.

Should a turbocharged vehicle cross the traps near their peak power output rpm like a n/a vehicle would?

This is my first turbo build and it feels like a turbocharged engine keeps pulling and pulling unlike a n/a engine that one can feel hit a wall and the rpm's drop off.

 

Yes if you want it optimized.
Heres where goals come into play so gearing and convertor can be selected for mph based on et goal.

 

Part of the reason for my question is that I'm considering switching to 28" rear tires from my current 26" tire. If a turbo car is no different than a n/a car as far as gearing for the traps; I'd have to change rear gears.

I currently run 3.55 rear gears. I haven't been to a dyno yet but my past n/a 347's all made peak power well before 6000rpm. The 28" tires would screw the pooch evidently. I run an Astro 5 speed manual and I can use all of the traction that I can get. The 28" rears would offer a larger footprint.

 

Since you already have a combo together, just put the 28s and it see how it does. Might just mean you can leave with more rpm to keep the motor happy while the tire takes the hit.

 

Good point. I just found a friend who will loan me a set of 28's. Time for some fun!

 

3.55 gears are great, stick with the manual trans, the main point you should get 28’s for track use only (not street) and stick with your exact setup is because it will increase the engine load and make the rpm range longer so you can keep the boost controlled boost in happy land torque output wise for longer! Be safe!

 

Nevermind. I prefer the 26" tires. The 28's caused a long flat spot mid throttle before picking up steam.

 

If you run the same engine config (cam, heads etc) and fit the right turbo, it will make peak power at the same rpm. What will change is trap speed - much higher because you have a lot more power.

 

That makes sense.

 

Not from my experience. Boost carried the rpm higher every time.

 

I always feel like the engine never stops pulling. It just never hits that wall like a n/a car.

 

With PWM boost control and enough turbo you can put peak power wherever you want, beginning middle end or the entire graph can be one long straight line.

For 1/4 mile racing you would generally select the gear that put the engine into its peak power for the longest duration for an entire race which is usually matching the trap speed to the highest possible RPM which maximized the area under the curve for every shift.
But modern vehicles have more gears and some engines dont breath well at high RPM and not everybody has PWM boost control or big enough turbochargers to do all that. So the question must be answered on a vehicle per vehicle basis. E.g. dynojet the car to get an idea of the shape of the power curve when its at its best then dial the gear ratio to match the curve area for every gear to the style of racing, For example 3.55:1 rear gear on a 28" tall tire could probably 135-140mph using 1:1 final gear perhaps 6k 7k rpm window. I tuned my RB25 that does 7,800rpm so with the 3.54:1 rear and 28" tall tire it can go faster than it needs to for a 1/4 mile, but that is just one type of racing some people do 1 mile some do 1/8 some do actual turning which is matching gear ratios to specific corners and track shapes.
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won't need 165mph all it does it street car stuff , roll racing
Haltech NSP uses PWM output to control boost so if I want a little peak on the end or a bump in the middle or whatever its just a click away

 

Are you talking about torque converter slip? At the same wheel speed (turbo vs NA) rpm will be the same unless gearing or slip is different.

 

Converter slip based on torque. Boost more torque = higher rpm for the same wheel (tire) speed

You seem to be implying that turbo and NA will make the same torque. Its possible but I don't think that is traditional or expected.

 

What makes you think that?

I said "rpm will be the same unless gearing or slip is different". . . and yes - I do know that slip will be different if torque is different . . . and . . . not being from the USA, I don't assume that every car has a torque converter.

 

Don't get defensive I only interpreted what you said. You literally said 'unless slip is different' which seems to imply you thought that a turbo engine and a naturally aspirated engine would have the same slip with the same torque converter at two completely torque outputs. The discussion is about converter slip after all. In your mind, what situation would you possibly be referring to where slip is the same for both a natural aspirated and turbocharged engine using the same exact converter? It doesn't seem possible. One will produce far more torque and thus more slip. So of course slip will always be different. Your comment does not seem to reflect that obviousness. But I don't care what you say, its what you meant. You meant something and I know you are a smart guy I don't think you are not smart just because of a mis-interpretation of your meaningful thought. I'm not one of those guys on the internet who points out spelling mistakens and grammar or word play issues to try and sound smurf. I really don't care what you type its what you meant

so what did you mean because both myself and underpsi68 seem to notice the same idea

 

Put the engine on an engine dyno with and with out the turbo. The hp will peak higher with boost.

 

I guess I should have mentioned that my car is a manual. The only thing slipping is my rear tires.

 

Ive seen many engine dyno comparisons that show the peak power is always the same na or boosted.
The na induction system and cam dominate the peak power rpm.
The difference is the power will carry out farther in rpm but it is on a downward trend.
So for in the pants feel its hard to catch peak power event.
Not sure of what a chassis dyno will show.

 

I will add, a fella had an engine with a stock cam. I know the engine well and it peaks and 47-4800.
He claimed with a turbo it pulled to 6000 at the track. He later took it to a chassis dyno. Peeked around 4800ish on 20psi and slowly dropped hp on its way to 6000. The turbo is well big enough for the engine and hp level it achieved.