RPM and Boost levels

[yelojkt]

I have a APH with a K&N drop-in and a 2.25 turbo back Borla exhaust. Cruising at 70 in fifth gear (its a five speed) I am sitting at 3100 rpm. At 80 mph in fifth I am at 3500 rpm. If you will post your 70 and 80 mph rpm's and what gear you are running (5th or 6th)along with what mods you have, induction, chip and exhaust. If you are running a boost guage record that also.

[Scarab_Beetle]

kind of important to ask for wheel/tire size also... if the tires have an outer diameter bigger than stock this effectively lowers the final drive ratio.

Mine on stock wheels/tires/suspension is about 3800 or so at 80 in 5th... 3.94 ring and pinion suck for highway but kick ass at the track

[NBTurboFreak]

Also the tranny code is important also. I did a swap with a GIT 180 gear box and 4th at the rev limiter and my speed changed from about 115ish to about 133ish.

[Scarab_Beetle]

thats cuz the 180hp models have the 3.64:1 ring and pinion, making it a better trans for launching and highway cruising... just kinda lags a bit in the mid-top end compared to the 3.94... but anyway, yellow, i fail to see what cruising has to do with boost levels... if you're cruising you should see any boost except if you're tipping into the throttle...

[noR]

I'll try to get some #'s at lunch.

[yelojkt]

Originally Posted by Scarab_Beetle

if you're cruising you should see any boost except if you're tipping into the throttle...

Hey Scarab_Beetle did you mean to say "you shouldn't see any boost"

[noR]

yeah. Cruising at 70 I know for a fact I'll be showing negative pressure.

[noR]

@ 70 in 5th:
3.2K rpm
-12 "Hg

....
@ 80 in 5th:
3.5K rpm
-12 "Hg

Although I saw -20 also. It was hard to decide when it was balanced and I was level.

[noR]

Really man... unless you change tire size or gear ratio... everyone's RPM should match up.

btw, mine was 225/50R16's. My 17's are off due to a technical difficulty.

[yelojkt]

So if you have a chipped car than you can get to 70 or 80 faster because you have more boost pressure to get you there but what about when you get to 120 or 130 I guess your rpms will stay up just the same as they would on a stock engine you just have more hp or torque although torque starts dieing out after 5500.

[yelojkt]

Originally Posted by noR

It was hard to decide when it was balanced and I was level.

Nor what do you mean by this statement?

[Scarab_Beetle]

speed vs rpm is a function of gearing and tire size ONLY. Boost, HP, TQ, none of these have any effect whatever on what rpm you are at when you're at a given MPH. More power will get you to that rpm and speed sooner, but assuming the same tires size and gear ratios, rpm will be the same at 70 mph in 5th whether you have a 500 hp car or a 12 hp car.

[v-dubed]

boost is gay!!!! oh did i say that out loud.. my bad

[noR]

Originally Posted by yelojkt

So if you have a chipped car than you can get to 70 or 80 faster because you have more boost pressure to get you there but what about when you get to 120 or 130 I guess your rpms will stay up just the same as they would on a stock engine you just have more hp or torque although torque starts dieing out after 5500.

You lose torque alot sooner than 5500, check it out:
http://jameswebb.ca/~ron/phrog_streetdyno_01282006.xls [sometimes the server's down; try during the day]

Yea, rpms don't change when you change the software, they're factored in by metal gears and connects; there is no way to fluctuate that except by changing the actual mechanics involved.

More boost simply means more torque. But our turbo's can't sustain high boost levels up into the rpm band.

[noR]

Originally Posted by yelojkt

Nor what do you mean by this statement?

What I meant was, if I was going uphill, there'd be more boost It would take more force to sustain my speed going uphill, less going downhill. So I was trying to find a place that was level, where my boost wasn't fluctuating much. You REALLY gotta get a boost gauge tapped in to watch it sometimes, it really helps solidify ones understanding with a turbocharged system.

It's air on demand, and when you don't need it; it doesn't demand it. (Making for most excellent fuel milage... when you're not demanding more torque. )

[Scarab_Beetle]

yeah the easy wasy of what Nor is saying is this: rpm doesn't make boost. Load makes boost. The more loaded up the motor is, the more boost it will make sooner. This is why turbo drag cars use two-step launch control so that they can generate load on the line and spool the turbo before they even move. Also why automatics tend to be able to spool a bigger turbo better than manuals.