17 replies to this topic

[HNSGEQ]

Hi All,

I am looking to buy a gearbox for my Torana and have a couple of questions a calculator can't answer so hoping someone might have gone through this before me and can help shed some light.

 

I've got a 355 VN headed stroker engine that is good for almost 530HP at the flywheel at 7,300 RPM.

My LX Torana will be running a 295/50/R15 tyre - 26.6 inches tall.

Diff ratio is 4.11:1

 

The box I'm looking at comes in 2 overdrive ratios and not sure which to buy. They are 0.82 and 0.64.

 

The car won't see a lot of highway driving - only a few times a year.

 

Using the calculator, the engine at 110 km/h will rev at approx:

1. 3,000 RPM with the 0.82 ratio OR

2. 2,300 RPM with the 0.64 ratio

 

Is 2,300 RPM too low to drive the car for a period of say 2.5 hours, eg from Sydney to the South Coast where the road is hilly?

On the other hand, is 3,000 RPM too much to be revving the engine for 2.5 hours?

 

Thoughts?

Thanks all...

[yel327]

The way I look at it to compare to a 1:1 4th gear.

4.11 with a 0.82 overdrive is 3.36.
4.11 with 0.64 overdrive is 2.63.

The tyres are about 5% larger diameter compared to an original D7014 or ER7014.

Does it have a lockup converter?

[Bigfella237]

My calculator roughly confirms what you've said above:

 

Tyre Size = 295/50R15
Tyre Revolutions = 470 per km
Gear Ratio = 0.64 : 1
Diff Ratio = 4.11 : 1
40 km/h = 824 RPM
60 km/h = 1236 RPM
80 km/h = 1648 RPM
100 km/h = 2060 RPM
110 km/h = 2267 RPM
120 km/h = 2473 RPM
140 km/h = 2885 RPM
160 km/h = 3297 RPM

Tyre Size = 295/50R15
Tyre Revolutions = 470 per km
Gear Ratio = 0.82 : 1
Diff Ratio = 4.11 : 1
40 km/h = 1056 RPM
60 km/h = 1584 RPM
80 km/h = 2112 RPM
100 km/h = 2640 RPM
110 km/h = 2904 RPM
120 km/h = 3168 RPM
140 km/h = 3696 RPM
160 km/h = 4224 RPM

 

2,300 RPM is kinda on the high side if you look at OEM cars, definitely not "too low", but it all depends on the how the engine has been built. If it's all revs and no low-down torque then you may be better off running at a slightly higher RPM?

 

Can I assume it's been on an engine dyno if you have those HP figures? How are the power and torque curves around those RPMs?
 

I hope you're running a big tail shaft. Have you or whoever designed your driveshaft done all the critical speed calculations?

 

With those rear tyres and that final drive ratio the ½ critical speed of a standard V8 Torana driveshaft would be bang-on 100km/h (which is not good)!

 

[axistr]

I have 4.11 gears with a T56 Tremic with 275x17" tyres. Not sure how accurate the tacko is but indicated 2,300 rpm at 108kph. Box has 6th gear .64 OD. If I had my time again I would probably go 3.9 or 3.7 gear set. I hate sitting listening to high RPM on the highway. Even 2,300 sounds like its starting to get a bit on the high side for long periods. But this is only my opinion, some people love to hear their engines screaming all day long.   

[neglectedtorana]

Not exactly the same but for comparison I have a 3.7 with 225/50-16 which is 24.9 inches and I have a .75 5th gear and .5 6th gear, I regularly use 5th on the freeway at about 2500rpm and don't mind the sound or more user friendly aceleration

If I could afford it I would change my ratios to .86 5th and .75 6th and be happy with the higher rpm

 

Presuming the box you are looking at has 5 gears is there an option for you to choose a different box that has 6 gears?

[myss427]

I have 3.9's with .75 T56 Magnum with 26.5 tyres, around 2000 cruising at 110kph, feels really good, not buzzy at all.

[HNSGEQ]

Hi All,

Thank you for all your responses, much appreciated!!

 

I've decided to go the 0.64 overdrive ratio and if I need to change gears more to go uphills rather than a lazy drive, I'll do that rather than revving the engine harder for no gain on a long(ish) drive.

 

Cheers and thanks again,

George

[76lxhatch]

I think you've made the right decision. Assuming you're talking about a TKO, the closer ratio is really for road racing rather than normal street use, that overdrive ratio is better suited to low torque small-displacement engines than V8s which would normally have at least somewhere around 0.7:1 overdrive.

 

I have similar tyre diameter with 3.91:1 diff and the 0.64:1 overdrive and a reasonable overlap cam, it drives fine down to around 80km/h on flat-ish roads and you only need to shift down for hills at 100km/h if particularly steep - do those even exist in Australia? :-D

[HNSGEQ]

Thanks 76lxhatch.

Yep, a TKO is what I'm looking at so to know you have a similar set up makes me more confident with my decision seeing as I'm spending a fair amount of $$$$.

 

Didn't want to get this decision wrong!

[Heath]

I've gone for the same combo (0.64 ratio overdrive TKO) with only 3.9:1 diff gears. I think with 4.11's it's a no brainer.
 

I've done the sums. It should work sweet. Big motor can get away with a pretty big overdrive even if the cam is half reasonable. Which surely... yours is.

 

Even if the motor doesn't have loads of grunt at that rpm... it may not deliver the economy you'd hope for, but it's at least a bit more pleasant to listen to.

 

EDIT: Ahh just noticed who I'm talking to, haha. Welcome.

Edited by Heath, 20 January 2021 - 02:23 PM.

[yel327]

Kinda makes you wonder what the Aussie Muscle Cars were like at high cruising speeds. There are magazine articles from the late 60's and 70's where the 3.36 and 3.08 rear axle 81837's doing hundreds of miles at 120mph average, and also the Mel Nichols drive of the XY GT-HO down the Hume in 1971 having it sitting at just over 140mph keeping it off the 6150rpm rev limiter. I've never got to experience no speed limit roads in my driving life, all I can relate to is what they are like at 110kmh for hours on end.

[Heath]

I think you'd put up with the discomfort for that driving experience.

Crawling along at legal highway speeds in Australia is however a different story.

[76lxhatch]

If you're going to do some spirited driving its mainly second gear

[HNSGEQ]

Thanks Heath!!! Cheers.

 

Yel327 - I think the difference from back in the late 60's and early 70's is that engines made much less power than they do today and so less likely to hurt a very expensive engine.

 

I absolutely agree with Heath that it'd be a very uncomfortable (nice way to put it) ride.

 

My HK GTS isn't great to drive at 70mph for any longish distance and was even worse when it had the cross ply tyres on it. LOL!!!

 

Thanks for all your help guys! 

[yel327]

True, the GTS327 and 350 engines were just production engines and you could rebuild them quite cheaply or buy an over the counter replacement short motor from GMH, but a XY GT-HO had handbuilt engines that in comparative terms are a lot in today's money. Let's say in 1971 a GT-HO engine would cost you $2000. Run that through an inflation calculator, gives you over $23000 in 2020 GT-HO had about 330-340hp, as you say anyone can build a 350hp 350ci SBC or Holden V8 quite easily and cheaply today. I'm pretty sure you are dead right about hurting the engines though, Ford fitted a 6150rpm rev limiter to the XY GT-HO so people wouldn't leave Clevelands all over the street!

 

If you read the December 1970 Wheels road test by Mel Nichols of a Tonawanda 350 engined, 3.08:1 rear axle HG GTS350 and also Peter Robinson in another HG GTS350 in July 1971 (this one a Canadian 350 engine but still 3.08), both ran them for hundreds on miles on unrestricted speed limit roads and averaged 120mph, running for long periods in excess of 130mph. It must be said though that both had radial tyres (Aquajets). Even the few high speed road tests of the HK GTS327's (one by Rob Luck) had radials (Michelin XAS in this case, as per what was on the Bathurst 1-2-3 finishing cars in 1968 and 1969). You'd never be able to do this in any other GTS, it would have to have the suspension developed for Series Production that only the GTS327 and the manual GTS350's had. My point was though, imagine the noise and the howl of the engines at that speed with 14" tyres, 3.08 rear axles and 1:1 4th gear!

[claysummers]

We have all gotten soft thanks to the refined behaviour of modern cars. I used to think nothing of driving everywhere at 80mph in HK and even EK (186 and yella terra) back in early 80s. Now I find my FB ute scary at over 60mph, but maybe it's just the 175 85 13 tyres which are designed for trailers I would say.

Sent from my SM-G973F using Tapatalk

[Bigfella237]

Bunch of pussies... I remember crackin' the shits when they started speed-limiting road bikes to 300km/h (186 mph)!

[HNSGEQ]

Definitely agree with all of your above.

One thing I'm glad about is that I experienced the era where cars were produced in limited numbers but if you loved Aussie Muscle Cars, they were within reach.

Driving my HK GTS or LC GRT XU-1 as my daily drive is something I never forget!

 

Have a good day guys.