So I bought a giken built 350 diff off of mikellucci awhile back, I had the day off and experimented with the f diff and the 350: diff housings are different but both work just fine on the F. You can't swap the f ring gear over to the 350 giken unit, the spacing and ring gear thicknesses are different, we tried to but just reassembled both as they were and we dropped in the 350 diff to have fun for drags this month! The thing rocks, I only went on a 10 mile cruise around the area (mountainous/curvy road) turned off trac/vsc half way and the thing carriers so amazing, acceleration is what a f should be like, the 2 way lsd holds lock up so good. I'll be selling this diff at some point to get the f giken unit unless I can find a different (3.7ish) ring n pinion to fit the 350 diff.

 

Wasn't as nearly as good as I hoped (need slicks to run the giken+4.09 gearing), this was the best run of the night (ran with my gf little sis so I think it would have been a tad better without her in the car also (This is also track that is about 100 feet shy of being true 1/8th mile)

My specs are on the right:

 

Sorry to brought this old thread up.
We did the 3.7 Diff on the 3rd Gen IS350 and the speed/rev limiter come in 6th gear.

The ISF 7th gear 130mph is the same, if you remove the speed/rev limiter, your car will go again.

 

Yes I am aware, I figured it will just keep me from going that fast on open roads which I was notoriously doing when times permitted (I work early in the morning and have a wide long stretch of highway!) you sell any speed limit kit like the hks or sard?

 

Bumping this old thread. For those who did the is350 diff into their ISF...it is to my understanding that the is350 diff is NOT an LSD. Seems like an interesting give-take from a 2010+ ISF. Are you guys welding the is350 diff?

 

Its not LSD...so yea for us 2010+ guys you lose that. Maybe it doesnt matter if you have drag radials at the track or something? I can imagine it just spins like crazy on the street though and honestly with that crazy aggressive gearing I really wonder if the car is accelerating much faster or you are just spending a lot of time going through gears, shifting. Theres gotta be a point where you have too much gear when you are going through 6 gears to hit 100 mph lol

Would be interesting to see 2 similar cars race from different speed with factory gearing and one with the 350 gearing.

 

Not really. It only makes a big difference from idle to shift in 1st with an 8 speed gearbox. Once you get in the powerband, gearing is a whole lot less important unless it is so widely spaced that you fall out of the powerband.

 

I would love to see 350 diff vs ISF diff side by side. Anyways, you could always weld the diff to get an "LSD" although this would be hilarious in a nice car like an ISF

 

Yea, im not sure, on paper its going to multiply the engines torque at a higher rate in every gear so theoretically it should accelerate faster everywhere id think. Thats why id be curious to see a car with the 350 gear race one without it in the real world from a stop and a roll but as you said, may not have much if any affect after 1st gear in these cars

 

That's what is intuitive. It isn't supported by the math and physics. I got into a huge argument with a good friend over this thinking exactly what you are saying, but he proved me quite wrong. Once you get into the peak power the engine produces, gearing is unimportant. You can't get more than peak power. You can change rotational speed and the subsequent force (torque) associated with it, but you can't do work any faster than the available power. The IS F will jump off the line harder with lower differential gear ratios if you can get it to hook up, but it won't trap at a higher speed. Work is work, and distance is distance no matter how you slice it up. That's why it's not hard to take just weight and power to figure out what a car will do reasonably accurately.

 

Correct, you arent actually increasing engine output, just multiplying the torque at a higher rate in lower gears, helping the vehicle accelerate quicker, but yes, by the time you are crossing the 1/4 mile you are far up in the gears and gear multiplication will be basically the same as with stock rear end gearing.

It makes perfect sense that trap speed is basically the same as you arent increasing power and trap speed in the 1/4 is pretty much a function of power/weight.

The idea is that with the lower gears the car will accelerate much quicker up to 60- 80- 100 mph and lower the ET, even if the trap speed is the same.

I would still think the ISF with 4.09 gears would accelerate quicker on the street from any mph below like 80 personally. I wish there was an easy way to swap in the 4.09 and keep my LSD diff.

Thats been my experience with gear swaps in the past.

 

^^^^^
easy and cheap enough to swap in an open one, then get it yanked & welded for a carton of smokes unless you're high rent or somethin..

 

One factor that isn't taken into consideration here is tire height.
We've all presumed were using stock size tires.

However, if someone can stuiff a 27" tire in those fenders, this could be a win-win.



couldnt get the damn pic to resize

 

Is this a better picture?

 

Yeah, thanks I just realized i fat finger 4.06 not 4.09 but its like 1mph difference.
Was just browsing 275/40/19s and they are significantly more expensive!