Turbo Lag......engine load vs RPM's (gears)

ScreamingFast · Site Admin Joined: Aug 17, 2008 Posts: 11378

So a while back on another forum a guy told me to come down in gears in Crystals car since it has 3.73 (done before the turbo). He told me that engine load will create boost quicker than gearing. At the time I didn't give it much thought thinking that getting into higher RPM's quicker would create boost quicker......just seems to make sense but my other thought was the down fall is you are not in boost as long because you get to redline quickly which just mean you will be shifting faster during WOT. Right now her car has some lag very little but you can feel it. I think it starts building boost at 2500 rpm and full boost very shortly after that. So yesterday I was under the hood checking fluids and put on a new Air Filter. After I was finished I started it up just to make sure all was well. Well after I started it up let it idle for second and the I tapped the gas just to hear the exhaust and turbo for fun. Well I just happen to notice the boost gauge, I noticed that when you tap the gas that it never really builds full boost even if I rev it to 4000 or 5000 rpms. Why? My car hits boost....full boost if I tap the gas enough just sitting.

So today I started thinking about what the guy on the forum was trying to explain. Because the car was sitting and I gave it gas it rev freely and easily which means that it doesn't have any load......meaning the motor isn't working hard to rev to hit high rpm......which also means that it doesn't require much air coming in (intake) which inturn means less air going out (exhaust.....essential to turbos). Now put the car in gear and romp on the gas......it will not build rpms as fast because now the motor is under load of moving the weight of the vechile. This mean that the motor will require more air (intake) to get rpms up which also mean more air going out.......more exhaust. So what does that mean? Basically more engine load mean more exhaust pressure.....good for boost!! So what does this have to do with gears? Well take a car that is NA with 3.31 and run WOT for a quater mile. During that run the motor was under "more" load to move the car fast. Take that same car and install 3.73 or 4.10's and run it down the track. During that run the motor wasn't having to work as hard to move the car because of the gearing was helping move the car.......meaning the motor had less load on it which mean less exhaust pressure.

So what did I figure out.....well orginally I figured gears would get more more RPMs which lead me to think that it mean more exhaust which meant boost would come on quicker. Nope RPM's don't build exhaust pressure......load does. So if I go down to a 3.31 in Crystals car will build boost quicker because the motor will have to work harder (engine load) but will pay off because it will make boost faster. So does that mean that her car will see boost at a lower RPM (build boost quicker) and if so that also means the car will be in boost longer too.

Any thoughts.....I am sure I could have looked at other forums and had this explained to me but it just seem to hit me all at once just thinking about it and to me makes sense or am I wrong? Ryan who drove the car at the track thinks the gearing is perfect but now I am starting to thinking may be not. Wayne had 3.55 and went to 3.31 (I think....I know he went down in gears) and went faster with the new set up in his late foxbody.
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TOWGUNNER · Posted: Wed Feb 23, 2011 11:08 pm Post subject:

wow lots to read but i get what your saying. i thikn by dropping down a gear will get you what your looking for.

SuperDan · Posted: Wed Feb 23, 2011 11:35 pm Post subject:

BUT, would the power difference be worth the cost of doing a rear end gear swap? What your saying is true, im trying to explaine the same thing to my cousin, when he cranks his boat up in his yard it runs like a champ, but it falls on its face in the water.

But back on subject, wouldn't there be other things you could do for the same price that would result in the same or better gains?
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ScreamingFast · Site Admin Joined: Aug 17, 2008 Posts: 11378

It wouldn't make more power just improve lag for a better power band. It should improve track times with the right driver. I would be swapping rearends with Josh if I decide to do it.
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recordbreaker5 · Posted: Thu Feb 24, 2011 5:36 am Post subject:

well...what's your end result here? you wanna see faster times on the track or just have fun with the car on the street? what kind of turbo it is again?
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stang910 · Posted: Thu Feb 24, 2011 6:28 am Post subject:

I don't think the car is gonna gonna feel like the boost is coming on faster. I'm pretty sure that has to do with the AR housing. You'll probably build boost at a lower RPM from the gears since the car will be under greater load sooner, but I doubt it'll be noticeable. One thing you might like from a gear swap is the car will be in boost longer through whichever gear your in. Auto is mod your looking for.

The main reason turbo guys preach about lower gears is cause 1st gear becomes usable and 2nd gear also in some setups. Think of it like this, if you have 4.56's 1st gear is a very short gear so short that the turbo never has time to build boost in that gear. 2nd gear may be the same depending on the turbo setup. Now if you put 2.73's in that same car 1st gear becomes very long and there's plenty of time for the turbo to spool. When it comes down to it the only way to find the best performance for your setup is to test different gears and record your track times with each gear.

ricanstang · Smokin' Tires Joined: Oct 09, 2008 Posts: 2480

it makes sense to me ive also read up a lot on this but i dont think id go lower than 3.55 gears. 3.31 seems counterproductive.
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heavy94 · Posted: Thu Feb 24, 2011 7:01 am Post subject:

I have some 3.55s you can try....and some 3.31s. I may even have some 3.73s. To find the setup that You want your gonna have to try different things. Its Your car so set it up like you want it. If you want quickest possible et than drop some numbers on gears. If you want to row through the gears fast then leave it like it is. You can ask a million different people what they would do and you will get a million different responses.
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Agent007 · Posted: Thu Feb 24, 2011 8:27 pm Post subject:

But there is an advantage to swapping out with me...I have performed the rear end swap on S197's 5 times and can do it with an extra hand in about 3.5 hours. Plus!!!! You will only have time invested!
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SpeedJunky · Laying Rubber Joined: Jan 17, 2009 Posts: 1379

ScreamingFast · Site Admin Joined: Aug 17, 2008 Posts: 11378

ScreamingFast · Site Admin Joined: Aug 17, 2008 Posts: 11378

ScreamingFast · Site Admin Joined: Aug 17, 2008 Posts: 11378

cipher · Joined: Apr 18, 2010 Posts: 28 Location: Currie, N.C.

I know i'm new and the only real turbo experiance i have is on the 2.3 lima motors. I completly agree with the load verse spool time due to gearing theory but the biggest difference in spool time and boost pressure i have found was from opening up the exhaust. Going to large diameter pipe with straight thru muffler i was able to produce boost at about 700 rpms lower and i gained 3lbs of boost in the process. Nort sure of your current setup but this may be an angle to incress pressure at a lower rpm. Just a thought. Oh and i'm running 3.73 gears behind the little turbo4 and boost comes on at about 2500 rpms as apposed to stock of 3200rpms.

Mike J.

ScreamingFast · Site Admin Joined: Aug 17, 2008 Posts: 11378

Well I can tell you that when we first had the turbo installed it had SLP loud mouths (basically straight pipe) but it was just god awful loud. So changed it out to bullet style muffler (magnaflow) and it seem like that it ended up with more lag. When the SLP where on the car it felt like boost was more instant. But I really doubt that little bit of a change would affect lag at all.
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