Problem with the .7 was no matter what we did with fuel or timing it just couldnt make the boost down low of the stock cover, as is apparant in the graphs that Ben has put up. We tried to give it more down low but just couldnt. Put the stock cover back on and its back.

How would the .7 cover go with 4.11 diff gears? I'd imagine you would see the low down punch the .5 had again?

How would the .7 cover go with 4.11 diff gears?  I'd imagine you would see the low down punch the .5 had again?

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MM, not sure.

The 4.11 might effect how the driveline loads up the engine and hence boost, ie delayed boost buildup??

My mate has a TD07 25G on his sr20, just looking at the size of this sucker spells lag, and it was laggy with the stock 3.4 diff. We put a 4.2 in it and wow, huge improvment, alot less lag and very drivable.

Its easilly on boost 500 to 750 rpm earlier now, so maybe the same happy results would work on the xr6... we also re tuned it as the maps were right out with the dif change

My mate has a TD07 25G on his sr20, just looking at the size of this sucker spells lag, and it was laggy with the stock 3.4 diff. We put a 4.2 in it and wow, huge improvment, alot less lag and very drivable.

Its easilly on boost 500 to 750 rpm earlier now, so maybe the same happy results would work on the xr6... we also re tuned it as the maps were right out with the dif change

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Unless you made a typo, that is the EXACT opposite of what will happen when you fit shorter gears. Taller gears load the engine up more, and allow more time for the turbo to spool up. This is easily to demonstrate in any turbo car. At what RPM can you make 15psi in 1st gear from a low RPM pull? And at what RPM can you make 15psi it 4th gear from a low RPM pull? You will reach any given boost level a LOT quicker in the higher gears. No question.

Same thing happens when fitting shorter gears. You will need to rev the engine slightly higher in each gear to reach the same boost level as before. However you will have greater torque multiplication so it may actually accelerate faster overall anyway. This is why choosing the correct final drive for a turbo car is so important. Its quite common for smaller capacity engines with short gearing and large turbos to not even make any boost in first gear.

explained well Dennis.... hows your conversion going?

Edited February 6, 200719 yr by hiddeous

:rl:

explained well Dennis....   hows your conversion going?

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Its still all pulled apart. I'm half way through porting the wastegate. Should be going a few weeks. I'm still waiting for some parts to arrive from the US like my 550cc US spec supra injectors and cams....

Great thread BTW. I thought my 0.5 cover would be loosing me power so I wanted to change if for a 0.7 (everyone else uses 0.7 covers on 2JZ's). But I think I am on a winner now.

My mate has a TD07 25G on his sr20, just looking at the size of this sucker spells lag, and it was laggy with the stock 3.4 diff. We put a 4.2 in it and wow, huge improvment, alot less lag and very drivable.

Its easilly on boost 500 to 750 rpm earlier now, so maybe the same happy results would work on the xr6... we also re tuned it as the maps were right out with the dif change

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Unless you made a typo, that is the EXACT opposite of what will happen when you fit shorter gears. Taller gears load the engine up more, and allow more time for the turbo to spool up. This is easily to demonstrate in any turbo car. At what RPM can you make 15psi in 1st gear from a low RPM pull? And at what RPM can you make 15psi it 4th gear from a low RPM pull? You will reach any given boost level a LOT quicker in the higher gears. No question.

Same thing happens when fitting shorter gears. You will need to rev the engine slightly higher in each gear to reach the same boost level as before. However you will have greater torque multiplication so it may actually accelerate faster overall anyway. This is why choosing the correct final drive for a turbo car is so important. Its quite common for smaller capacity engines with short gearing and large turbos to not even make any boost in first gear.

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Yeah we kind of thought that too... but theories aside, it worked and solved the lag issue.

Thing is, with so many different turbo size/motor size/ gear ratio combos around these days anything can result. No one can sit there and say what will and what wont work, I wont argue about this kind of thing, there's way to many of contributing factors to take into account

I wont argue about this kind of thing, there's way to many of contributing factors to take into account

Absolutely. Whatever caused the reduction in lag certainly wasn't the shorter gears. Too many other factors.

What about a compromise; make a 0.6 comp cover.