Reducing Revs at Higher Speeds

[Martin AH 3000 MK III Phase 1]

My BJ8 turns approx. 3.500 revs when driving in 4th gear with overdrive. I want to reduce the revs for long distance drives and see two technical solutions:

1) the standard rear axle for cars with overdrive has a crown wheel and pinion with 11 x 43 teeth (3.909:1 ratio). Cars without overdrive have crown wheels and pinions with 11 x 39 teeth (3.545:1 ratio). Changing the crown wheel and pinion to the 3.545:1 ratio spec would reduce revs and ‚lenghten‘ all gears.

2) the 100/6 has a different ratio in the laycock overdrive. As far as I know the support is 28% compared to 22% at the BJ8. Changing the sun wheel, planetary gear and annulus to the Laycock Overdrive A specification would lenghten only when activated the 3rd and 4rd gear.

The effect with the change in the rear axle should bring revs down 10% from 3.500 to 3.150 and the modification of the overdrive should reduce revs down to 3.231 revs if calculations are correct.

The big question is: how will I like the side effects of these modifications? Variant 1 would also make the ridiculously short first and second gear longer. Good when everything is normal. But how about starting to drive with the second gear, especially when the oil is still cold and I struggle shifting in the first gear? Today/unmodified I can easily drive off from a traffic light with the second gear, but would this also be possible with the 3.545:1 ratio? And how about a bendy road when you get out of a curve uphill with the second gear - would the unmodified 3000 engine be strong enough to still provide a sporty drive?

The second variant would keep all gears the same and only reduce revs when the overdrive is kicked in. Sounds great, but the rev-reduction is smaller than with the modified rear axle and who knows, maybe the longer first and second gear would be great and still have enough power to drive off?!

Does anybody have experience and did either the one or the other modification?

Best regards, Martin

 

[Bob_Spidell]

I put a 3.54 rearend in my BJ8 years ago mainly because, at the time, I was taking at least one long road trip of up to 5K miles every year. Since, if you're going to drive long trips you'll be spending a lot of your time on highways and freeways--although I avoided them as much as possible--it makes 'cruising' a lot more pleasurable (the 11% drop reduces vibration and noise considerably at highway speeds). Around the time I put the taller diff in I had some clutch judder when launching, which I attributed to the change, but when I finally pulled the gearbox I found one of the damping springs on the clutch disk was broken, new disk fixed the problem; I still need to slip the clutch just a smidge for a smooth start. Your quarter-mile times will drop from, what, 12s to maybe 14s--you're not going to beat a '97 Civic anyway--but if you take any trips longer than 100 miles or so you will be glad you made the change. A strong engine is (always) better, but first and second are plenty low already and IMO the taller diff makes second, in particular, more 'friendly' (I usually can't get out of first quick enough). Rolling starts in second on flats or downhill are still possible, but on uphill slopes you'll need first (lots of fun on Gough, the backside entry to the famous Lombard Street in San Francisco). My dad and I installed my diff, and I think we did it correctly, but there's some whine, but I only notice it with the hood up (not often). Your problem getting into first is universal, as Healeys don't have a synchromesh gear in first; if you're barely moving it'll go in easy, or you can 'kiss second'--just nudge it towards second with the clutch depressed--to get smoother engagement. Using synthetic multi-viscosity oil in the gearbox should help on cold starts.

I also have a BN2 with the shorter rearend and taller overdrive and it's fine for the shorter trips I take with it; you don't want to run those cars north of 4K RPM for long anyway due to their inclination to fracture crankshafts at higher rotation. The late, great Gary Anderson thought the taller (28%) O/D and the taller rearend were the best possible combination for a 'cruiser.' Note if you change the diff you'll need to modify your speedometer (I did) or fit a ratio converter to get (somewhat) accurate speed readouts. Some opt for powered, GPS-driven aftermarket kit. Since driveline speed is taken after the O/D no change is necessary for an O/D gearing change.

Edit: Corrected; the 28% O/D is, of course, the taller of the two options.

 

[Jack T]

I swapped to a 3.55 diff in my BT7 and it totally transforms the car. Every gear is more useful, and where before I might have switched up to overdrive on a back road at 45 - 50 mph, standard 4th is perfect. With the torque of the Healey 6 I have never notice Id a lack of low-end grunt. Unless you're running the Monte Carlo rally I can't imagine needing any more power when coming out of an uphill hairpin in 2nd. I still start out in 2nd a lot around town. Your car should have a base 20 +/- more hp than mine, so I can't imagine you would ever regret making the change.

I also have a 28% overdrive, and the combination drops the rpms a lot on the highway, although since my speedo is far from accurate I can't quote actual rpm/mph values. I even went to 185/15 tires to get more rolls per rev. The vague Healey steering is the same at 70 - 75 mph, though.

 

[Bob_Spidell]

...

I also have a 28% overdrive, and the combination drops the rpms a lot on the highway, although since my speedo is far from accurate I can't quote actual rpm/mph values. I even went to 185/15 tires to get more rolls per rev. The vague Healey steering is the same at 70 - 75 mph, though.

There are gearing calculators online that will give pretty accurate RPM/speed numbers; some even include actual wheel/tire diameters. I used to have a link to a good one, but can't find it or the site. When I was having speedo issues I carried a chart of gear/speed/RPM values and, in California at least, the gov't stations radar speed readouts on some roads from which to compare with your speedo (and scold you).

 

[BoyRacer]

The vague Healey steering is the same at 70 - 75 mph, though.

At higher speeds, any tire will have less contact with the road. Tires grow in size with speed, the sidewalls get stiffer, the contact patch gets smaller and has less grip. If you are frequently driving at those speeds you might want to lower your tire pressure.

 

[HealeyRick]

Bob Spidell wrote: " Your quarter-mile times will drop from, what, 12s to maybe 14s-"

Yikes, Bob, what do you have under your bonnet? I'd be happy to get my Nasty Boy into the 12s. I think a stock BJ8 is in the 16-17 second range.

 

[Jack T]

Bob Spidell wrote: " Your quarter-mile times will drop from, what, 12s to maybe 14s-"

Yikes, Bob, what do you have under your bonnet? I'd be happy to get my Nasty Boy into the 12s. I think a stock BJ8 is in the 16-17 second range.

Good one, Rick. Maybe Bob was thinking 0-60, typed quarter mile.

 

[Bob_Spidell]

Good one, Rick. Maybe Bob was thinking 0-60, typed quarter mile.

Yeah, I was thinking 0-60MPH. I think I saw 11-something seconds in contemporary BMC literature way back when, and that was probably optimistic.

 

[Healey Nut]

Owned three Healeys now two BJ8s now my BT7 . Switched all cars to 3.54 gears with OD . RPM drops anywhere from 1000 to 1200 RPM with 3.54 , gears become way more useful 1st becomes useable , third becomes a really great pulling gear and you can have a conversation with a passenger at 70mph even with a side exhaust .

 

[bdcvg]

Best mod ever 3.54 is fantastic

 

[Editor_Reid]

I bought one of the original batch of the limited production run of the 3.54 gears (wasn't that Mike Lempert, the steering wheel guy, who got those made 20+ years ago?). Got them installed in my BN2 by Tom Monaco here in Portland. I'm completely happy with the new ratios, but when Steve Pike drove the car one time a few years afterwards he immediately remarked, "Where's the power?" He was clearly surprised at the reduced acceleration.

They do reduce acceleration, but I drive my BN2 pretty sedately most of the time, and never more than slightly aggressively, so I don't miss a little off-the-line acceleration.

 

[Martin AH 3000 MK III Phase 1]

Thank you for sharing your experiences. The ratios with the 3.54 rear axle should be:

2nd gear: 7,32
3rd gear: 4,62
3rd gear OD: 3,79
4th gear: 3,54
4th gear OD: 2,90

The question that drives me the most is whether I can drive off from the stand with 2nd gear without dragging the clutch. I always shift the second gear first before shifting the reverse or 1st gear to synchronize the gears as Bob recommend. However, especially when the oil is still cold it happens regularly that I can’t shift the first gear though. I maybe could with force, but as the second is good enough to drive off I go easy using this one. If the 3.54 ratio would demand dragging the clutch when driving off in 2nd gear I would probably hesitate doing the modification.

According to Jack it should be possible, especially with the BJ8 and its 150 hp. As the feel of the drive is somewhat individual, are there others with comparable experience?

Regarding the Overdrive I have a question. The parts numbers for the annulus and the carrier assembly plant vary between BN6 and BJ8. Parts numbers are:

BJ8 annulus: 17H5867
BJ8 carrier assembly: 17H5865

BN6 annulus: 17H5837
BN6 carrier assembly: 17H5836

So these are definitely different but I am not sure which abo the different ratios. I know that one is 0,78 and the other 0,82, but which ratio does my BJ8 have as a standard?

 

[Bob_Spidell]

BJ8s have 22% rev reduction in O/D (or 22% more speed for same revs.) I have a stock, freshly-rebuilt--with 'slightly better' cam--engine and don't miss the slightly better acceleration of the shorter rear-end (second is more useful in the twisties). If I want off-the-line speed I drive my Mustang GT, with 475HP--480 if I could get 93-octane gas--and nominal under 5s 0-60 times (I if was good enough). Funnily enough, I still occasionally stall the Mustang, but never the Healeys; I can pretty much dump the clutch and they'll lug a little then crawl away from a stop (just like my tractors).

If getting 'off the line' is your main concern, stick with the shorter diff; if you plan on driving 200-300 mile leg trips, get a 3.54. The 28% O/D gearing with the 3.9 diff might be a good compromise. 4-cyls have 28% O/D gearing and 4.11 diffs.

 

[Jack T]

Thank you for sharing your experiences.


The question that drives me the most is whether I can drive off from the stand with 2nd gear without dragging the clutch. I always shift the second gear first before shifting the reverse or 1st gear to synchronize the gears as Bob recommend. However, especially when the oil is still cold it happens regularly that I can’t shift the first gear though. I maybe could with force, but as the second is good enough to drive off I go easy using this one. If the 3.54 ratio would demand dragging the clutch when driving off in 2nd gear I would probably hesitate doing the modification.

My experience after making the change is that I don't treat starting out from a near-stop much differently with the 3.54 than I did with the 3.91. I may engage the clutch a bit more slowly than I did before, but overall if I'm on near-level ground I don't bother with first gear unless I had to come to a complete stop. The benefits of the 3.54 for overall driving so far outweigh any downside that to me it's a non-factor.

 

[Michael Oritt]

Thank you for sharing your experiences. The ratios with the 3.54 rear axle should be:

2nd gear: 7,32
3rd gear: 4,62
3rd gear OD: 3,79
4th gear: 3,54
4th gear OD: 2,90

The question that drives me the most is whether I can drive off from the stand with 2nd gear without dragging the clutch. I always shift the second gear first before shifting the reverse or 1st gear to synchronize the gears as Bob recommend. However, especially when the oil is still cold it happens regularly that I can’t shift the first gear though. I maybe could with force, but as the second is good enough to drive off I go easy using this one. If the 3.54 ratio would demand dragging the clutch when driving off in 2nd gear I would probably hesitate doing the modification.

According to Jack it should be possible, especially with the BJ8 and its 150 hp. As the feel of the drive is somewhat individual, are there others with comparable experience?

Regarding the Overdrive I have a question. The parts numbers for the annulus and the carrier assembly plant vary between BN6 and BJ8. Parts numbers are:

BJ8 annulus: 17H5867
BJ8 carrier assembly: 17H5865

BN6 annulus: 17H5837
BN6 carrier assembly: 17H5836

So these are definitely different but I am not sure which abo the different ratios. I know that one is 0,78 and the other 0,82, but which ratio does my BJ8 have as a standard?

Martin--

Without getting into what is wrong with your transmission that is making it so difficult to get into first/reverse when the car is "cold", the solution is certainly not starting off in second and I'd suggest that you address the issue properly.

As to getting off the line with a 3.5 diff--once you have a useable first and assuming your clutch is operating properly it should simply not be a problem and, with a taller diff, first will become less of a granny gear and driving in stop-and-go traffic will be less of a chore.

 

[BoyRacer]

Ditto what Michael said. Transmission oil is only 30w. Oil is not the problem.

 

[twas_brillig]

The optimum gearing combination would probably be a 5 speed (synchromesh first) with the 3.54. The 3.54 diff was used in Healeys sans overdrive, so folks were successfully motoring about with that combination. Doug

 

[Martin AH 3000 MK III Phase 1]

Thank you all guys for sharing your experiences and your precious advice. I do own HBJ8L25.321 since 1998 and drive the car since a nut and bolt restoration for 20 years now. I will definitely take it to the next level and modify both, rear axle with 3.545 ratio and conversion of the overdrive to 28% after all the information I received. I have attached a file that I found in another forum that I found very useful.

i will take the opportunity to get into the thing with the first/reverse gear. Michael is right, this is not normal and as far as I remember this misbehavior sneaked in over the years, I just somehow got used to it and as driving off with the second gear works so easy I somehow jibbed a repair and got used to it. This is not how it should be and I will take care of that as well when doing the modifications now.

Now it is time finding the parts needed. I will first do the research for the parts numbers and also check Westminster and Wolseley parts numbers as it seems that the differential could be the same. Did anyone of you do this work already and can provide the numbers?

 

[BoyRacer]

Nowhere yet in this discussion has anyone mentioned tire size. The chart in Martin's attachment is based on a tire size of 165/15. Those are small tires. Bigger tires will lower the RPMs.
Many of us are using Michelin 180HR/15s. While more expensive than the smaller tires, they fill up the wheel wells, look better (in my opinion), allow more ground clearance and lower the RPMs.
I have a 3.54 rear end, 180/15 Michelins, a 5 speed, and at 2500 RPM I cruise down the highway at 70 MPH. This choice was based on my plan to go on many long distance trips and in the last 15 months I have covered over 5,000 miles. I believe it was the right combination for my needs. Highway driving is a bitch because of all of those fast driver's out there. It is so easy to accelerate from 70 to 80 MPH (or more if necessary) when you're only doing 2500 RPM at 70.
If I was only doing city driving I would have stayed with the 3.90.

 

[Bob McElwee]

I put 3.54 gears in a 67 BJ8 with a 4 speed w/OD and a 62 BT7 with a Toyota 5 speed (synchro 1st gear is heaven).
Initially on the BJ8 I ran 195/70 X 15 until I could no longer get them. They look much, much better IMHO. I went to 185/70 X 15 on both cars eventually.
we did a lot of long distance trips to regional meets and Conclaves. Needless to say my speedo was just for approximate MPH. GPS have me close to 75 MPH at 2500 RPM.
the 3.54 also made both cars so much more bearable around town.