R34 Gtr Garrett -5 Turbo Lag, Need Advise

[XGTRX]

This is a good little basic article to explain the effects of diff gearing. From WALLACE RACING. http://www.wallaceracing.com/reargear.htm

Rear End Gear Selection

As most Pontiac fans realize, changing rear gear ratios can yield a tremendous performance increase. Three questions sometimes arise when considering a change in rear end ratio: How will the change affect top speed?, How will the change affect fuel economy?, and How will the change affect exhaust emissions?

A change in rear gear ratio can be one of the most satisfying modifications you can make on your Pontiac, and it can be one of the most cost effective. For instance, changing from a 2.41:1 ratio to a 3.73:1 ratio will provide a rear wheel torque increase of 55%.

Figure I below is a chart of rear wheel torque increases; to find the torque increase for a proposed gear ratio change, find your present ratio in the left column and move across to the column that is headed by the proposed ratio, the percentage shown is the torque increase (negative percentages are torque decreases).

Providing you can get it to the ground (have sufficient traction) the 55% torque increase should substantially improve your quarter mile E.T. and push you back in the seat a lot harder! On the street,the drivability and responsiveness will be greatly improved.

There are some interesting points to ponder in a rear gear change. For instance, a 1978 Firebird with -the standard 400 (non T/A 6.6) engine originally equipped with 2.41:1 gears. With this combination the torque peak occurs at about 52 mph and the horsepower peak occurs at about 118 mph. The same gear with 3.73:1 gears reaches the torque peak at about 34 mph and the horsepower peaks at about 76 mph. The 3.73 gears convert this car from a "Freeway Cruiser" into a "Surface Street Stomper" that would have tremendous response at around town speeds and would probably get the best fuel economy at city speeds of 30-40 mph.

Now, consider a 1978 Firebird with the T/A 6.6 engine. When using 2.41:1 rear gears the torque occurs at about 92 mph and the horsepower peak occurs at about 131 mph - With 3.73:1 gears, T/A 6.6 car reaches the torque peak at about 59 mph and the horsepower peaks at about 85 mph

The 3.73:1 gears convert the T/A 6.6 car from a "Freeway Flyer" into a good highway or multipurpose car.
On both examples above the cars and gears considered were identical, the only difference was the engine, yet the peaks and most practical uses for each car varied greatly. The point of these examples is that a gear ratio that is perfect for one car could be a disaster in another car aiming for a similar performance goal, so you must consider the engine combination carefully when choosing the rear gear ratio. Another point that became obvious in the first example, is that the non-T/A 6.6 400 is woefully undercammed for high performance use. As you can see from the road horsepower graph (figure 2) below, the maximum vehicle speed is influenced by the rear gear ratio. The graph shows increasing horsepower and torque on the vertical axis and increasing vehicle speed on the horizontal axis. NOTE: All curves shown in this article are arbitrary but representative of actual relationships.

You see five curves on the graph, two torque at rear wheel curves (for 2.41:1 and 3.73:1 ratios), two horsepower at rear wheel curves (for 2.41:1 and 3.73:1 ratios), and one horsepower required to maintain road speed curve. The two rear wheel torque curves show that increasing the gear ratio both causes the torque peak to occur at a lower road speed and increases the peak torque value. This is the primary explanation for the performance increase. The two rear wheel horsepower curves show that increasing the gear ratio causes the horsepower peak to occur at a lower road speed but the peak horsepower value is the same. The single "horsepower required" curve indicates the amount of horsepower required to maintain vehicle speed. The respective maximum vehicle speeds are indicated where the rear wheel horsepower curves intersect the "horsepower required" curve. Taken as a whole, the graph shows that a torque increase of 55% can be had while reducing maximum vehicle speed by only 8 mph,only a 6 % loss.

Edited March 25, 2015 by XGTRX

[XGTRX]

Spreadsheet is here

[r32-25t]

Awesome so where in that article does it show how boost comes on at a lower rpm when changing the diff ratios?

[XGTRX]

Because the torque at the wheels is higher at a particular rev point. So even though the engine characteristics have not changed directly the torque at the wheels have so therefore you will come on boost at a lower speed. As a result on a dyno you will see the torque curve plotted against wheel speed shifted to the left and even though the torque against rpm will be similar, it will be seen to ramp quicker against time because of the torque multiplication factor of the diff. But the torque against wheel speed and rate of acceleration is where it is at and that's what matters really.

Edited March 25, 2015 by XGTRX

[GTR_JOEY]

Shit, based off that I'll go find some 6:1 diff gears and put it in the GTR. I forgot that it doesn't matter at what point in the rev range it comes on boost.

Apparently boost vs road speed is all that matters. Can't wait to shift at 25km/h after 0.3 seconds of boost

[XKLABA]

That's a nice theory, to bad it is incorrect as far as GTRs are concerned

I have seen this first hand from PRACTICAL experience with the race cars

So it really depends what you want from your car, a dyno plot or a fast car

[XKLABA]

Shit, based off that I'll go find some 6:1 diff gears and put it in the GTR. I forgot that it doesn't matter at what point in the rev range it comes on boost.

Apparently boost vs road speed is all that matters. Can't wait to shift at 25km/h after 0.3 seconds of boost

I think 5.2:1 is availble for the GTR diffs

[XGTRX]

Nah sorry mate the theory is correct and I have also seen it first hand. And on the road they are a lot different. Nothing to do with dyno plot either it's just used to illustrate torque to wheel speed difference. Rate of acceleration is night and day when comparing to standard gears. Obviously your experience is exactly the opposite to what many others have experienced, don't know how though.

Edited March 25, 2015 by XGTRX

[r32-25t]

Because the torque at the wheels is higher at a particular rev point. So even though the engine characteristics have not changed directly the torque at the wheels have so therefore you will come on boost at a lower speed. As a result on a dyno you will see the torque curve plotted against wheel speed shifted to the left and even though the torque against rpm will be similar, it will be seen to ramp quicker against time because of the torque multiplication factor of the diff. But the torque against wheel speed and rate of acceleration is where it is at and that's what matters really.

so what you're saying is boost doesn't come on at an earlier RPM and therefore lag has not been reduced?

Road speed yes it happens earlier including gear changes but engine speed (rpm) all happens exactly the same, hence it being an illusion of response but not a gain in actual response

[XKLABA]

It might be correct for some 70s POS but a highly tuned 350kw+ manual GTR it's incorrect but hey what do i know im just some guy on the internet, so do it and report back

http://www.sau.com.au/forums/topic/334990-diff-ratios/

[GTR_JOEY]

Hey mick, what ratios did have on your 32 when you ran a 11.3 with standard turbos ?

[XGTRX]

Nah I won't put the gears in or my -5 26 setup will be as responsive as a -9 or 26/30 setup, way too responsive. I need lag to keep me happy.

[XGTRX]

Shit, based off that I'll go find some 6:1 diff gears and put it in the GTR. I forgot that it doesn't matter at what point in the rev range it comes on boost.

Apparently boost vs road speed is all that matters. Can't wait to shift at 25km/h after 0.3 seconds of boost

Put some GT12s on it while your at it.

[XKLABA]

Other then twin plate driveline is stock standard as it was when it came from the factory in 93

And it's run 11.1 still on stock turbs

[GTR_JOEY]

I stand corrected

[Stixbnr32]

I have seen multiple videos and talked to some people and even the the extra shift (r34 getrag) and R32 ran a 10 over old 11 second passes.

So I think that equals real world speed and response. I understand that the tops speed is slightly down (not buy much) and engine RPM at 110km is about 300rpm (odd) more. Also talked to someone that said even with his big HKS 2530's (505kw atw's) he said the 6spd conversion with 4.11 ratios has been one of the best modifications to real world speed.

I know boost doesn't come on earlier in RPM but have heard good things. Enough for me to take it into serious consideration later in the year.

Let me run a 1/4 in the coming summer season and then when 3rd goes run it again with the getrag haha.

[SimonR32]

I have seen multiple videos and talked to some people and even the the extra shift (r34 getrag) and R32 ran a 10 over old 11 second passes.

Watch the video again, the Motive DVD GTR car actually traps at a slower speed (so acceleration is actually worse with the 6 speed). The only reason it runs a 10 is because it gets off the line better...

Seat of pants feel vs actual performance usually is significantly different

[XGTRX]

Performance is delivered differently. Much better response. As described in the rest of the vid. So yes, shortening diff ratios will be a good mod to do if your goal is a car centred around response.

[Stixbnr32]

so his trap speed was less but time was better with the extra shift. Wonder how much softening the suspension helped.

im pretty interested in this convo. I can see both points been a fine line and valid.

[fatz]

so you can go faster but change gears more which almost makes it a waste

mick

anyone bought your sex speed box?