Gear Ratio Question - December JBL HEAT Status

Machiavelli · #1 (**permalink**) 12-19-2010, 11:26 AM

Hi gang, time for an update on the build and a request for some gearing advice. The build is progressing along fine, Larry is well into completing the body and chassis and George has the motor configuration set. Motor build webpage: http://www.gessford.com/cobra/images/BAU-427.htm

Here are the changes to the build since my post mid October -

12/19/10

WHEELS -
Style: Schott Sprocket
Front: 18 x 9.5"
Rear: 18 x 11.5"

HUB CENTRIC - Outer Hub Boss Dia 2.5000" (front/rear)
These rims will also have an "even" outer dish dimension of 3.5", front & rear.

MISCELLANEOUS -
Roll Bar: Two 3-point loops (driver & passenger), now race legal.

Gauges - Stewart Warner "Heavy Duty", Smith doesn't have the full compliment I want in a specific series, oh well.
I also replaced the clock with a vacuum gauge, still 10 gauges total - but all business now.

Now, some gearing talk. (Larry & Richard, please chime in if you're reading). From the BOM on the motor build page you can see George has me setup with a TCET4617 TKO 600 (.82 5th) Road Race Transmission. Assuming my tire diameter will be ~26", and that I'm expecting ~400 RWHP, does the 3.73 rear end really make sense? I've run some numbers through this on-line gear calculator: http://www.5speedtransmissions.com/gear_calculator.html
As you can see, with a 3.73 at 80mph I'm taching 3163 rpm. With a 3.31 @ 80mph I'm at 2807rpm. I like taller gears generally and want good gas mileage for cruising/long trips. I think with the power I'm expecting along with a car weight ~2250 lbs, I don't see a problem here. I'm also the 1st to admit I'm an idiot with this stuff, there may be other ramifications I'm totally unaware of. My driving preference is I DO like extreme speed as well as handling, and the pavement in my area is real good (like glass). When I take a cruise on my BMW K1200S sportbike, I'll find a way to run it up to 160mph every time. I know a 3.31 has a better high end, but it has to all work together as designed. So there's the question, you guys with the experience let me know if a 3.31 rear end will be a problem (from any perspective), or if you think it's ok. Any advice would be greatly appreciated, thanks.

patrickt · #2 (**permalink**) 12-19-2010, 11:46 AM

I don't think you're going to like a 3.73 rear with a .82 TKO600, at least out on the highway that is. One of those gears needs to change. You don't have to go all the way up to a 3.31; I have a 3.54 along with a .64 and it is perfect for highway cruising, in my neck of the woods -- but you know what speeds are like where you drive, and you know how to use a gear calculator. "Pleasant highway cruising" is the only thing you'll ever use a .64 fifth gear for (that, and top speed runs). But, I love mine.

Dwight · #3 (**permalink**) 12-19-2010, 12:39 PM

I have a 600 with .64 5th and 3:55 rear gears. My tires are small at 25 1/4". I cruise 70 to 80 1800 to 2200 rpm. 17 mpg.

400 rear wheel hp. 2315 lb car.

Love my set up. Lots of low end torque. I can take off in third if I want to.

Cruise at 1500 with no trailer hitch jerk.

Dwight

jdean · #4 (**permalink**) 12-19-2010, 02:46 PM

Mostly track - 3.73
Mostly street - 3.27-3.50

Having a street car/track car is about compromises on everything from gear ratios to suspension/tire choices to cams etc. etc. Serious racers will change the gears depending on the track anyway.

Wbulk · #5 (**permalink**) 12-19-2010, 03:10 PM

I looked up all the information you had on your engine. The costs reminded me of the last time I went to the hospital.

Comp Cams recommends 3.73 gears for the cam #35-426-8 you have choosen. But that is assummed it is for a heavier car. That cams power range is 2200-6200 rpms with a duration @ .050 of 236-240, and has a rough idle. Not really a cruising and gas mileage cam that you said you wanted.

You said:
"I like taller gears generally and want good gas mileage for cruising/long trips."

Taller gears and a high duration cam are often times in conflict. This is all about trade offs. My .02 is that the 3.73s are going to keep the revs up for this cam but you most likely will have to start out in second gear as first will be to short. Get some ear plugs to from where the rpms are going be at. On the other hand with the 3.31s you may have some low rpm issues around town since you may not even get in the power range of that cam. If it were me I would go with the 3.55s and one step lower on the cam. I'm a firm believer in building for where you are going to be running most of the time with some left over. My opinion!

Machiavelli · #6 (**permalink**) 12-19-2010, 04:19 PM

Agree, .82 OD with a 3.73 just doesn't have the headroom I want in gears. Turns out the 8.8 differential that JBL mods to a limited-slip 3.73 starts out as a 3.27 I think, and that gives a nice 2773 rpm @ 80 mph.

Machiavelli · #7 (**permalink**) 12-19-2010, 05:17 PM

Dwhite, you're at the other end of the extreme, very tall gear setup - even with 25 1/4" wheels. I don't imagine that you use all that 400 rwhp in 5th, nor ever intended. Simple math, if 80 mph = 2200 rpm, then if you can get that beast to 5500 rpm you're doing 200 mph lol.

jdean, makes sense - I'll be a "mostly street" guy so I'm thinking the 3.27 will be right in the sweet spot.

Wbulk, yeah - the cost is substantial, more than I originally planned for, but the JBL comp roadster is deserving lol. Good input on the cams, I'll check with George, need to know where the engine cleans up. As far as 1st versus 2nd goes, would rather have a useful 1st - even at 2200 with a 3.27 rear, I'm doing 18 and town is 25 so I should be ok. Didn't know the cam was that aggressive, #35-426-8 was George's selection so I'll ping him on that one.

All good comments, thanks!

Wbulk · #8 (**permalink**) 12-19-2010, 05:25 PM

If you are going with the 3.27s I really think you would be happier with the 35-424-8 cam, since you will be dropping rpms. Check it out.

WardL · #9 (**permalink**) 12-19-2010, 05:33 PM

I've got a 3.46 rear end and the 0.82 OD with 17" wheels and cruise at 75 MPH and 2,700 RPM. This seems to be a good combination for me.

CHANMADD · #10 (**permalink**) 12-19-2010, 05:45 PM

I had the 3.31 with a high revving 327......it needed longer gears ( my rires are 26")
The 3.07 was better and now I run a 2.88 with a 600 hp motor, and it gives me the best of everything..I have a 4 speed Jaguar gearbox with an electronic Laycock (Heavily modded for the power....Bronze facings on the cone clutch ).with a o/d ratio of .73.... I have'nt had anyone beat me yet!!!! bikes included, and i cruise at 80mph at 2000 rpm

Machiavelli · #11 (**permalink**) 12-19-2010, 07:45 PM

Wbulk, the email to George is on it's way explaining the situation along with your recommendation to look at CompCam 35-424-8, thanks for doing some digging there!

WardL, ok - I may be moving up to that area of ratio after all is said and done.

CHANNMADD, sounds like you found the right recipe with all the variables, and that's what makes this not such an easy to nut to crack, all the variables.

Richard Hudgins · #12 (**permalink**) 12-19-2010, 08:13 PM

Machiavelli,

With a big displacement low revving engine such as you are building I like either 3:27 or 3:31 gears. Please see below.

At 2.2k rpm (where I expect the engine will run cleanly.)

At 5k rpm

Machiavelli · #13 (**permalink**) 12-19-2010, 08:36 PM

Hi Richard,
You engineers with all the software! Thanks for the data, another solid vote for the ratios I like. We'll see what George says about the cam.

Machiavelli · #14 (**permalink**) 12-20-2010, 05:29 PM

No issue with the cam, right choice for the car for a multitude of good reasons and fully vetted with comp cam engineers.

... when you have Anthony Bourdain in your kitchen making authentic Toulouse Cassoulet, to suggest B&M baked beans in lieu of tarbais probably won't fly ...

vector1 · #15 (**permalink**) 12-20-2010, 09:32 PM

curious why the choice of hub centric over lug centric?

Machiavelli · #16 (**permalink**) 12-21-2010, 03:30 PM

Wasn't a must have, Schott said they could make the rims hub centric at no extra cost, wheels go on centered without cranking a couple of lugs to force centering, makes 1-handed pinning easier, little things like that - free, so why not.

vector1 · #17 (**permalink**) 12-21-2010, 07:46 PM

with the closer fitting hub/rim the tire has to be put on with the holes lined up pretty close, you can't spin the rim on the hub or have any wiggle room. i don't see any benefit if there is no other reason. the rim does rest on the hub protrusion when the wheel is mounted and the lugs can be inserted fairly easy with the lug-centric. just a fyi

vector1 · #18 (**permalink**) 12-21-2010, 07:53 PM

regarding the gearing, you might want to consider the mph with the car in 1st and idle if you do any parading or stop n go traffic jam stuff, could be a lot of clutch work.

i like the tko600 for gear spacing, i think a 3:55 or next lower number would be a good combo imo. i have the 600 with 3.73 and smaller engine and it works pretty well. also have a 3.55 with .64 od and highway cruising is nice. these lighter cars are more forgiving with regards to gearing imo.

CHANMADD · #19 (**permalink**) 12-21-2010, 10:39 PM

With Hubcentric wheel the shock of the road forces are transfered to the hub and then you are not relying on the strength of the lugs alone

Machiavelli · #20 (**permalink**) 12-22-2010, 01:16 PM

victor1, you probably should google 'hub centric wheels' to learn a little more about the benefits.