OEM idle gear fitment for my 1978 GTS is 27/30, which gives 0.9:1, the same as shown in the GT4 workshop Manual. 5th gear is 28/24 which gives 1.16667:1 The pinion to crown wheel ratio is 17/63 which gives 0.26984:1 If one runs the numbers for 5th gear: 30/27*24/28*63/17=3.529 or 1/0.90000*1/1.16667*1/0.26984=3.529 overall. The above calculation agrees with my owners manual for overall gear ratio in 5th. I have a chance to purchase an alternate idle gear set with a ratio of 27/28 which gives 0.96429:1. If I run the numbers with this ratio: 28/27*24/28*63/17=3.294 or 1/0.96429*1/1.16667*1/0.26984=3.294 overall. The new idle gear set lowers my overall ratio from 3.529 to 3.294, a useful 6.7% reduction in rpm for a given speed. A cruise rpm of 4000 is dropped to 3732 rpm. I'm told this gear set will fit my 308. The top gear gets replaced. I do not know which of the other 2 gears get replaced, but one of them does and I suspect it is the transmission input gear. The problem I'm having is tying to understand how this can be, using the OEM housing and the 2 new gears. It seems to me, the rotational centers of the gears need to be adjusted relative to each other, to accommodate the new gear ratios, but I'm not a gear expert. Is it possible that one can change the overall idle gear ratio without changing the center-to-center shaft spacing? Is it possible to change the pitch circles without changing the pitch centers, just by changing the pitch form on 2 of the 3 gears? Bill
Yes this is indeed possible. The gear centre remains the same. There is a gear design calculation which allows you to increase or decrease the number of teeth for a given diameter. The gear tooth angle changes but this is not uncommon. I'm not a gear expert but I am a mechanical engineer and have seen this done. In fact I have a 308 GTB with idle gear ratio of 26/31 in the standard case with std gear centre. My gears are damaged and I am replacing with std 27/30. The 26/31 was a custom set made be do engineering in the uk and they do not have any left. Difaz
Thanks Difaz. I wish I'd seen your post before my trip yesterday. I visited the owner of the multiple ratio gear sets yesterday and took a look. The gear sets are paired up and coated with what looks like Cosmoline. The gears of each set are helical (angled teeth-not straight cut) and have a splined ID suggesting they are meant to replace the engine output and transmission input gears. The splines are somewhat different from each other. My transfer gear case is assembled so I had no way of doing a visual comparison with my existing 27/30 gear set. TAV 24 shows a sketch of the 3-gears and are numbered 7, 8, and 9, top (engine) to bottom (transmission). Gear 7 was most like the alternative gear sets. Gear 9 was close, but the Parts Catalog shows an extended snout for the ID that is not present on the alternate gear set. In the end, I left without the gear set. I could not convince myself that the gears would indeed fit. I did not see you reply prior to my trip yesterday. Bill Does anyone have pictures of their transfer gears?
Could you give me the contact details for this supplier. And does he have any other ratio sets. I'm looking for a shorter set similar to my DK engineering set. What price is he asking for the gear sets and is it a set of three (top, centre and bottom) gears. Thanks Difaz
Hi bill I forgot to mention in my previous post that I believe the internal splines are the same on both top and bottom gears. My gears are out at present as I have just rebuilt box, diff and in the process of engine build also. I could have a look at mine in about a week and let you know for sure. Difaz
Hi Difaz, The supplier is Peter Sweeney of Forza Motor Sports in Gaylordsville, CT. He has an ebay action for the complete set of gears here: Ferrari 308 GTS GTB GT4 Final Ratio Changing Transfer Gear Collection RARE | eBay It is ebay Item 200840339958 A gear sets include 2 gears. I made Peter an offer of $350 for the 28/27 gear set, contingent on if they fit. He suggested $400 and we settled on $375. Please let me know what you think. I'm a retired mechanical engineer also. Let me know on the internal splines. If a shorter ratio set works for you, a taller set will likely work for me. Bill
There seems to be some confusion, the drop gears are input gear ratios that where used to match the gearbox to the 308 engine tq curve. that said the gear ratio listed as 27/30 is the two gears that matter when calculating it, the set 27/28 is not. I'll explain. when figuring the ratio of a gear set the first and third of a 3 gear set are the ones that matter, the middle idle gear has no effect on the ratio. here are the gears found in the 308 drop cases, 1st: 27 output 2nd: 28 idle 3rd: 30 input In order to change the ratio you would need 3 gears, not two. the axial placement of them is fixed so the idle gear would need to change. The only other way round this is swapping the 1st and 3rd gears but the spline, offset and bearing faces are different. The 328 USA is slightly different, 1st: 27 output 2nd: 28 idle 3rd: 29 input However that's not a drop in replacement as the cover case is updated as well as the input main shaft. As best as I know there was someone down in OZ 20yrs or so ago that made straight cut gears with different ratios. $$$$$ and noisy!
smg2, Thanks for your input. I agree the center gear does not enter into the calculation. It only ties the input and output gears together and ensures they turn in the correct direction. So, the OEM transfer gear train for the 308 is: 1st: 27 engine output 2nd: 28 idle 3rd: 30 transmission input The ratio I was interested in was the 28/27. Presumably this would replace the 27/30 end gears and give a taller transfer gear ratio resulting in: 1st: 28 output 2nd: 28 idle 3rd: 27 input Part of the debate I was having with Difaz was whether changing the end gears necessitated changing the radial distances between the gears. Presumably, a 27-tooth input gear could be changed to a 28-tooth gear if the 28-tooth gear form were made a little taller so that the new mesh occurred deeper in the middle 28-tooth gear. This would mean the gear tooth profiles were different between the 28-tooth input gear and the 28-tooth middle gear. Different, but could it be compatible? A similar game might be played between the 28-tooth middle gear and the 27-tooth bottom gear, on the transmission input shaft. I think it is possible to play this game with a 1-tooth difference between original vs alternate gear, but it would likely be impossible to replace a 30-tooth gear with a 27-tooth gear without changing the radial distances between the gears. I've heard of the OZ sets you mention, but have no personal knowledge of them. Whether straight cut or helical, the radial distance issue is the same. I wonder how they addressed it? Probably some sort of floating rotational center for the middle gear would be my guess. The other possibility that comes to mind is a stacked middle gear (2-gears sharing a common rotational point) with an axial offset input or output gear. Bill
The 328 set while sharing the same 27/28 out/idle does indeed have a slightly different pitch radius and tooth profile, but as noted they aren't useable to the earlier 308's as there was a change to the drop case and input mainshaft. The other model that had a drop case and gears is the TR, the ratios there are 29/27. when I get some time I'll put a tq curve up with the different drop ratios and where it would place the curve, hopefully that made some sense.
My shorter ratio set of 26/31 uses the standard 28 tooth centre gear. Top 26 Mid 28 Bot 31 The gear tooth angle on the 26 and 31 gear are different to original to allow more and less teeth to be used for the same shaft centre distance and gear diameter. I have plotted a comparison thrust curve for each gear set using excel and the shorter set gives more thrust in every gear but runs out of thrust in top gear at 240km/h compared to std. It's also quite a bit louder due to the higher RPM. Good for track days but it's a love hate around town. Better acceleration but feels more revy and uses more fuel.
Hi Bill I had a look at the gears via the link. I think you made the right decision not to purchase. The gear diameter looks quite different in most of those sets which means the gear centre must change in this drop gear configuration with centre idle gear. In my 2 sets the diameter of the gears is the same. Hence the centre distance is the same. Difaz
Here is a correct way to provide an alternate transfer gear ratio. Note that the center gear must be changed in this example. The gears run at different levels so either the top and middle, middle and bottom, or top middle and bottom gears may be changed. The rotational centers do not change. The height of the gear set (axially) gets taller to provide a strong enough gear so a spacer must be used. Bill Image Unavailable, Please Login Image Unavailable, Please Login
The guy in OZ still does them, I think. Frank at Modena Engineering in Melbourne - they have a web page
Thanks ferraridoc. I found ME on the web and sent them an email asking if they had any gear sets for the 308. They really don't give much information on their website as most sections are under development. Bill
No worries. While you're at it, ask him to get cracking on my Lotus gearbox - he's building me a straight cut. Let me know if he doesn't get back to you in a reasonable time and I'll give him a call
Ferraridoc, Are you having him make a gear set for your S3 Elan? I went for a custom BGH gear set for a Voights T9, 5-speed conversion, for my 1966 S2 Elan. Bill
Yeah, blew 1st gear on the 1st stage of a tarmac rally last month - my first foray for 10 yrs with typical result. I have a 5 speed Toyota based box, but want a CR straight cut with a dog change for my bullet box, which has Mg casings - figure about 3-4 seconds per minute faster. The gearbox has to be (at least LOOK) standard for class restrictions. I'd love an S2, but can't find a suitable donor, which has to be basically trashed - I'm not going to destroy a nice road car to feed my OCD to build the ultimate super-dooper lightweight. (My S3 is about 100Kg too heavy) Image Unavailable, Please Login
Awesome car Patrick. What does it weigh now? Do you know Rohan out of Melbourne? I'm looking at importing a lightweight 26R body from Kelvedon in the UK to put on my new TTR 26R chassis. That would make my original S2 body and chassis surplus with a new set of 26R flairs. Too bad you're so far away. My new engine, tranny, and rear end are all aluminum. The engine will be a dry sump, 2-liter Cosworth BDP or Steve Jennings block, like Jay Leno used on his 26R restomod. Please see: https://www.youtube.com/watch?v=1cPcpMmN18M I'm expecting about 1450 lbs with roll bar and 200 bhp using a new John Stowe twin cam head. My plan is to keep the pop up head lights though. Bill
I’m going to bump this up. Does anyone currently make alternate helical style drop gears for the 308? I’d like a longer gear ratio to relax rpm on the freeway. Aaron
Hi Aaron, Modena Engineering does a helical drop gear set for the 308 gearbox. I'm in the process of fitting one to my 328 gearbox at the moment. Bill
Hi Aaron, IIRC about 3k USD, for one ratio, plus shipping. I'll be going back for additional ratios after some road time. The one thing not included in the kit are new drop gear cover studs. The new case, with outboard bearings, is about 1-inch taller, so all 10 studs need to be replaced. These studs are tough to find because of length and thread details, but I have a source. My friend Mark is building a Lancia Stratos rally car, which will be using a Ferrari 3x8 engine/transaxle and one of these quick change kits. His ratios will be different than mine. Frank is a great guy. Tell him I sent you. Bill
