Jonathan Smith Posted January 26, 2020 Report Share Posted January 26, 2020 Hi I'm after some advice / opinions on the the merit of increasing overdrive from standard 22% to 28% At the moment I have standard TR4 with 3.7 diff and J type saloon overdrive gearbox I am in the process of replacing the J type saloon overdrive with a rebuilt TR4A overdrive unit and have the option of increasing the overdrive from 22% to 28% Question is - Does anyone have an experience of running a standard 4 cylinder engine with the increased overdrive reduction and does it make much difference? Does the 4 cylinder have sufficient torque to cope with the increase to 28% What is the driving experience like? eg am I likely to be constantly changing gear and flicking between overdrive in the lower gears particularly when accelerating and passing other cars Is the car going to run out of steam in 4th with the overdrive engaged? I cover about 7,000 to 8,000 mile a year mostly on UK roads typically A and B roads, there are inevitable occasions when motorways are used, there is at least one long distance european holiday each year on a variety of roads and car doesnt go over 70mph very often usually cruising at around 60 to 65 mph. Any feedback / comments would be greatly appreciated as I've a decision to make and at the moment I'm torn between staying standard with 22% and the potential benefit of reduced engine revs when touring Jonathan Quote Link to post Share on other sites
Ian Vincent Posted January 26, 2020 Report Share Posted January 26, 2020 (edited) I have a 3a with a 3.45 axle and a 22% overdrive. It works well. My engine is very similar to a 4a it has 87mm liners, a Siamesed 4a manifold and twin HS6s and the head has chamfered combustion chambers similar to a 4/4a. I don’t know how much heavier the 4 is compared to a 3a but if you only use the car relatively gently, based on my experience with a longer set of ratios, I don’t think you will have any problems. Rgds Ian PS the difference in final drive ratios between a 22% O/D and a 3.45 axle and a 28% O/D with a 3.7 axle is negligible. The only real issue might be the gap between the top gear and overdrive. Edited January 26, 2020 by Ian Vincent Quote Link to post Share on other sites
rcreweread Posted January 26, 2020 Report Share Posted January 26, 2020 Jonathan I have a TR4 with a stag box and 28% ratio OD and its great - I would have no qualms recommending the conversion . My engine is lightly tuned and has a 89mm piston and liner set, high torque cam and phoenix exhaust but with a standard 4A head and HS6s - I wouldn't call it a highly tuned engine, and I've never noticed any issue with the engine struggling with the OD in either torque or revs terms. I've never dared tried to see whether it would pull max revs in OD top because I believe that would be about 130 mph at 5000rpm - I'm running a 3.7 diff by the way. I've never felt it was running out of steam at high speed in OD top - I chickened out before I got to the point of proving the point or otherwise! If anything I seem to change gear less, particularly in urban driving - I can easily start off in second and then just flick it in and out of OD second so I dont usually have to change gear at all. As for general driving and cruising, I much prefer it to the previous 22% and if you want to drive spiritedly, which I do, it doesn't seem to affect that, so I would go for it! If you were closer, you could come and try my car! cheers Rich Quote Link to post Share on other sites
rcreweread Posted January 28, 2020 Report Share Posted January 28, 2020 Jonathan Just as an afterthought, although it sounds like you are catered for with your new bits, but just in case you are not, I have a Stag gearbox, fully converted to TR spec, with a 28% ratio overdrive ( have both A and J type available) all ready to go, which I will be taking to Stoneleigh on 9th Feb . If of interest, get in touch via PM and I can give you all the details Cheers Rich Quote Link to post Share on other sites
tim hunt Posted February 1, 2020 Report Share Posted February 1, 2020 I agree fully with Rich. When my 4A overdrive was overhauled by ORS two years ago I took the opportunity of having it uprated to 28%. Like Rich's my engine is a bit better than standard but only lightly tuned and I also have the 3.7:1 diff. Under light load on a level road I can still change from direct to o/d top at 40mph and the car will pull happily. I appreciate the additional 150rpm reduction in engine speed when cruising at 70mph in o/d top. The new gearing does bring the o/d 3rd ratio very close to direct top making the change from o/d 3rd to direct top redundant, not that I used to do this very often anyway, preferring to change from o/d 3rd to o/d top. The higher road speed available at peak revs with the 28% o/d is sometimes helpful in an overtaking manoeuvre when the instant change from direct 2nd or 3rd to the corresponding o/d ratio can be made with both hands remaining on the steering wheel. I would say go for it. Tim Quote Link to post Share on other sites
8iggles Posted February 21, 2020 Report Share Posted February 21, 2020 I have been running 28% in my TR4A for over 20 years. It's a simple swap of planet and matching sun gears (if I can remember that long). I obtained mine from a jaguar in a local scrap yard, when you could do that sort of thing. The reduction was stamped on the Laycock serial plate. On continental tours it makes high speed cruising so much more comfortable. John Quote Link to post Share on other sites
ianc Posted February 21, 2020 Report Share Posted February 21, 2020 Unless one spends a lot of time on UK motorways or (unlimited) German autobahnen, then I think that the standard 22% is better because it puts OD2nd midway between 2nd and 3rd, and doesn't push OD3rd to the level of top(4th). The result is that the car is somewhat nippier - for example, on a 3.7 axle, OD2nd comfortably allows 60mph and OD3rd allows 90mph. Also, the sums are easier because, apart from 1st, the direct gears give speeds in multiples of 5mph - and I have always driven on the tachometer. Ian Cornish Quote Link to post Share on other sites
ChrisR-4A Posted February 21, 2020 Report Share Posted February 21, 2020 Depends a lot on your local roads, I have std set up, 3.7 and 22% od and I find on our local roads which seem to be busier each year, 3rd is a bit low for 30mph limit where one is often only doing 20-25 due to traffic and 4th is too high but od 3rd is just right, allowing mini roundabouts to be taken by dropping for od 3rd back to 3rd. On motorways od top is effortless at 70/75. Chris Quote Link to post Share on other sites
Jonathan Smith Posted February 22, 2020 Author Report Share Posted February 22, 2020 Thanks for everyone’s help on this. All your comments have been a great help. After much though the final discussion has been to stick with the standard 22.5% overdrive based on roads and we tend to most frequently use, these being mostly B and A roads which after readings everyone’s comments the standard set up sounds like it will suit us best. Off to the garage now to start pulling out the old gearbox Any recommendations for best supplier / manufacturer for new TR4 clutch plate as I’ll have to replace the smaller diameter saloon clutch plate to suit the new A type gearbox. I’ve read a few posts / reviews re quality of clutch plates and ideally don’t want to be pulling gearbox out again for a few years at least Will report on the outcome next month once new gear box is in Many thanks Jonathan Quote Link to post Share on other sites
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