hi everyone,

quick question for you people,

Friend of mine bought a gen 2 rx7 with a v8 engine already in it (I know! Why!) sounds good tho!
He has rover sd1 engine in the car, but the car tops out at 80mph a mechanic who is doing some work on he car reckons the stock diff cannot keep up?!?!

Will a RX7 gen 2 auto diff fit and be better? The mechanic said that it will that this would be a better option

what do you guys think?
do you think he will have a better increase in top end speed with a gen2 AUTO diff ?


please help !


thank you in advance


ShaQ

 

hmmm...i don't think switching to another differential ratio is going to help you out much. there is obviously something to work out with the transmission or something else in the drivetrain.
HOW does it top out? i am assuming the ratio runs out (engine hits redline)...

 

change ur gear ratios...

if u want more top speed but yet still wanna keep the crazy acceleration get a big block if u can make it fit. If not then the new fuel injected corvette engine :P. The small block v8's only got good 0-3000rpm power with ideal tuning. So if ur looking for more top speed u can prolly get a lil more than that, sounds awfully low... but still fix those damn gear ratios see wat happens.

 

thanks for the replys guys,

my friend says the engine is really loud at that speed (80mph) says he hits redline?
you guys think its transmission problem?



shaQ

 

im curious as to which tranny he put in there... cuz my cuzin did a v8 swap as well and he had to make his own custom tranny...

i really dont think its the tranny. almost 100% sure its the gear ratios.

 

well what's inside the transmission determines the gear ratios

 

Unless he put mud inside the rear diff, it has NOTHING to do with it. There are only two available ratios - 3.90 and 4.10. They are so close that changing it would only make about a 5 mph difference at that speed (80mph).

What size tire is on the rear of the car? What type of transmission is in the car, manual or auto? By redlining, tell me how my RPM the tach reads (and tell me you know for fact the tach has been calibrated for a V8). How many gears does the trans have?

A few possibilities here. The tach could be uncalibrated - this can cause reading from zero to redline at almost any engine speed. The clutch could be slipping. The transmission could be slipping. It could be topping out at 80 mph in 3rd gear and not shifting to a higher gear. The exhaust could be leaking so badly it sounds like it's redlining - or have so much backpressure the engine can't breathe well enough to go any faster.

 

Big blocks are better for low end torque than top speed. They can fit (sort of) but it's not worth the hassle for anything other than "cool factor".

There are several small block powered RX7's running 9's in the quarter mile - isn't that enough acceleration?

Tuning has little to do with idle-3000 rpm power. That's all about proper cam, head and carb selection. Most manufacturers go for the 1800-5000 powerband with small blocks because that's what people usually want. But with the right selection of parts you can build a small block engine that will idle at 400rpm, redline at 3000rpm and tow a tractor trailer up a mountain. Small blocks have grown past the 400 plateau - how about a 454 small block (or bigger)?

As I previously posted, you can't "fix" the gear ratios in an RX7 - without changing to a different rear setup options are extremely limited. There may be a 3rd option, I can't remember - but 3.90 and 4.10 are pretty much it.

 

Thanks for the replys guys,

Crashtestjoey


What size tire is on the rear of the car?
----He has 17”
What type of transmission is in the car, manual or auto?
---It is a 5 speed manual

By redlining, tell me how my RPM the tach reads (and tell me you know for fact the tach has been calibrated for a V8). How many gears does the trans have?

----He hits about 5k when he hits redline the tach has been calibrated for a V8 also.


thanks

shaQ

 

Ok - if he's in 5th gear doing 80 MPH and hitting 5000 rpm, either his tires are flat or his clutch is slipping. Or someone put 140 wt oil in the diff maybe and the friction is straining the engine. Something is keeping the car from keeping up with the engine. Have you checked the clutch out? You can usually feel when it's slipping in lower gears than 5th - like 1st and 2nd is normally pretty obvious. Other than that, I really don't know - do you know the diameter of the tires (the actual height, not the rim diameter)? Mine are 26.2 inches tall and with my 4.10's 80 is around 3100 or so in top gear. The transmissions are totally different but most 5spd transmissions have an overdrive.

 

Ya know, I read the original post again, and I'm wondering about that engine and transmission. What type of vehicle was it in? If it was from a Land Rover the problem could be in the transmission gearing after all. I've never seen a Land Rover with short tires - most I think are in the 28-30" height range. This could make dropping to a 24-25" tire a HUGE deal and could be the problem. If this is the case, it's NOT a good thing. Because you have to swap to a Vette or Cobra IRS to change your gears to anything more driveable (like a 2.73, 3.08, 3.27).

 

hmmm...land rovers don't exactly have a very high top speed. combine that with an extremely different tire size, and bingo. i guess you have redline in 5th @ 80 mph...