3.9 vs Bigger engines

Allan Robinson · Post by **Allan Robinson** » Fri May 05, 2017 7:18 am

I am running a 7" Sierra cosworth LSD. Gear Ratio is 3.62:1 with 120 Nm viscous LSD.

Post by **Rossco** » Fri May 05, 2017 8:20 am

I've got a 3.140 7" Sierra pinion and crown wheel sat on the shelf. I purchased it from Quaife 2 years ago for a project that never hit the road.

Receipt says I paid £380 for it but I'd be happy to let you have it for £200 if you're interested.

Allan Robinson · Post by **Allan Robinson** » Fri May 05, 2017 10:01 am

Thanks Rossco. This offer is very tempting and may take you up on it. Taking into consideration all the options that need assessing when upgrading to a larger engine I have concluded that I do need a better gear box as I'm not happy with my present LT77 ponderous gear changing.

I have written to V8tuner to cost a T5 gear box and conversion kit and also require details of the 5th gear ratio. Once I have this info I can fully assess my Diff ratio requirements. I want a lively performance but for economy I assume I'm looking for circa 2500 rpm at 70 mph?

I'll come back to you soon.
Many thanks
Allan

DaveEFI · Post by **DaveEFI** » Fri May 05, 2017 10:15 am

All depends on whether you want relaxed cruising or good 5th gear acceleration. Personally, I'd prefer nearer 2000 rpm at 70. So 35 mph per thousand rpm, and bit more work with the box on country roads.

sidecar · Post by **sidecar** » Fri May 05, 2017 11:21 am

You should run all of this through an online gear calculator, you need to be realistic over the power output of the engine that you will eventually be using, it's no good gearing the car for 170 MPH in top gear at 6K revs if the engine will not make anywhere near enough power. Of course you could gear the car like this as long as you don't mind having no 5th gear performance. From memory my car is geared for 26 MPH per 1K revs, it does feel quite 'revvy' in top gear but my 0-120 MPH is very good! Horses for courses and all that!

I still think that you would get very good MPG improvements if you could run a vac advance system.

My diff is a 3.36:1 Ford LSD BTW.

Allan Robinson · Post by **Allan Robinson** » Fri May 05, 2017 11:28 am

Ok thanks. Ideally I would like a tall 5th gear for cruising only (like an overdrive) and use the others for lively performance. So it looks like nearer 2000 RPM in 5th could be a realistic target for 5th. I'll see what V8tuner come up with and go from there.

DaveEFI · Post by **DaveEFI** » Fri May 05, 2017 11:35 am

Not that uncommon for a production car to achieve its top speed in a gear lower than top. If you want relaxed cruising and decent MPG on the motorway, you need as high gearing as possible. However, with such a large engine in such a light car, top gear performance should be pretty good regardless.

It really is your choice as to what suits you best.

Allan Robinson · Post by **Allan Robinson** » Mon May 08, 2017 4:36 pm

The World Class T5 gear box I am considering has taller gears than my SD1 apart from 4th which both share a 1:1 ratio. The 5th gear is quite high at 0.63 which together with my existing 3.62 diff would give me 70 mph @ 2172 rpm. 4th would remain at 122 mph @ 6000 rpm. This I feel would suit my needs perfectly giving lively performance while providing a high 5th gear for economic cruising.

Rossco, your kind offer of your 3.14 diff gears was very tempting and with the T5 box would result in 70 mph @ 1884 rpm. For the reasons above I have decided to stick with my existing 3.62 diff and purchase a T5 WC gearbox. I do however really appreciate you coming to the rescue with what could have been a very realistic option for me.

Thanks to all
Allan

DEVONMAN · Post by **DEVONMAN** » Mon May 08, 2017 7:33 pm

From what you have said , the T5 you are considering is the type used in Mustang upgrades. If so there are some aspects of the box that you need to be aware of if swapping for an LT77.

The input shaft has 10 spline so a different clutch plate will be required.
The output shaft has 28 splines so a matching sliding yoke and custom prop will be needed.
Speedo drive needs a Mustang cable.
Rear mount will need to be fabricated. (It's at an angle)
TVR or custom bellhousing and clutch release arrangement will be needed.

If it comes as a kit then all the items within the bell housing need to be compatible with each other and your clutch.

Allan Robinson · Post by **Allan Robinson** » Mon May 08, 2017 7:48 pm

OOOO! interesting.
The one I am looking at is a V8 Tuner kit called 'New Tremec FORD T5 World Class gearbox kit (competition)' to fit the RV8.

I did intend reusing my existing AP Racing triple Green clutch and Ultra light fly wheel to save a bit of cash but from what you are saying that might not be possible. The plot thickens!

unstable load · Post by **unstable load** » Tue May 09, 2017 12:16 am

You may be able to use your clutch. You can either swap the centre of the plate to the new spline or simply get a new plate that fits the splines and the clutch size.

Allan Robinson · Post by **Allan Robinson** » Tue May 09, 2017 4:51 pm

I now find that the particular gear box is out of stock. So Rossco, could I please change my mind and buy your 3.1 sierra diff gears. I have sent a PM with my email address.

Many Thanks
Allan

Allan Robinson · Post by **Allan Robinson** » Wed May 10, 2017 6:12 pm

Just to conclude this post I have now managed to purchase a new Tremec Ford T5 WC gear box with the 0.63 5th gear. Rossco has kindly sold me his 3.14 diff gears so I should be well future proofed for a larger engine.

My task now will be to source a TVR bell housing, understand what clutch master and slave cylinder I need and get to grips with altering my existing AP Racing Triple Green clutch to accept the 10 spline gear box shaft.

For my future gear box queries I think I should start a new post.

I know I've said it before but everyones input has been very much appreciated and has guided me through the mine field of options.

Speak soon
Allan

DEVONMAN · Post by **DEVONMAN** » Wed May 10, 2017 8:40 pm

Before you close this thread I will tell you what I know about the Ford T5 conversion. My Mate Devonrod Has done it.

The TVR bell housing will have a gearbox location hole too small for the Ford T5 gear box front bearing retainer. You will need to get a TVR T5 bearing retainer and clutch release bearing carrier which will accept the LT77 release arm and bearing. The preload on your new gearbox will need to be reset with a correct shim because you have fitted a different retainer.

A new prop with a 28 spline sliding yoke will be needed.

The rear mount will need to be fabricated.

If you have a cable driven speedo then you will need a Mustang cable with the correct cog and fitting into the box. The cable comes out the box on the left side.

The spigot bush in the end of the crank will need to be reamed out to suit the Ford T5 shaft.

Hopefully the gearshift will suit your tunnel opening into the car.

Good luck.

Allan Robinson · Post by **Allan Robinson** » Fri May 12, 2017 6:56 am

I have continued this thread under Drive Chain & Transmission. The heading is "Fitting Tremec FORD T5 to RV8