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MGB-FV8
Jacques Mathieu
Alexandria, VA
(299 posts)

Registered:
09/11/2009 08:55PM

Main British Car:
1977 MGB Small Block Ford, 331 Stroker

Did I chose the wrong gear ratio for my Ford conversion????
Posted by: MGB-FV8
Date: February 14, 2010 12:36AM

Hi everyone, since receiving my 8" rear axle center section from Currie Enterprises I have had second thoughts on the 3:42 gear ratio I had them install. It can be a confusing task going through all the factors to select the perfect gear ratio that will work in harmony with your design; itíll give you heart burns worrying how some minor change will affect/work with your original gear selection.

I don't have the car on the street yet but after changing a couple of options I now have these final factors; the engine is a '92, Ford Mustang (EFI) modified 5.0 Liter H.O., aluminum flywheel for faster acceleration (and engine braking) coupled to a Summit Racing T-5. Torque will drive a set very light weight BBS wheels and P205/60VR15 tires (subject to change), the 3:42 gearing is attached to a Detroit "Truetrac" that uses helical screws instead of clutches.

The camshaft is a Ford Racing (roller type) E303 which is a switch that I've made to the engine after the gear selection. My worry is that I may have too much gear for the kind of engine torque itíll generate on a light 2,200/2,300 pound car. Someone suggested that a 3:23 gear ratio should be installed or a camshaft with a higher power band than the E303 be installed. Others have said to leave it alone that it was just fine the way it is.

Does anyone have advice on a balanced gear ratio for this set up? Thanks


kstevusa
kelly stevenson
Southern Middle Tennessee
(943 posts)

Registered:
10/25/2007 09:37AM

Main British Car:
2003 Jaguar XK8 Coupe 4.2L DOHC/ VVT / 6sp. AT

authors avatar
Re: Did I chose the wrong gear ratio for my Ford conversion????
Posted by: kstevusa
Date: February 14, 2010 08:26AM

Jacques, Sounds OK except we do not know the tire diameter. If you have a T-5Z (5th is .63) you should be ok. The e-303 cam may not be the best choice. It was ground and based on the Crane 2040. Ck Ford Racing description and then check Crane's description. Doesn't sound like the same cam?? I first chose the E-303, but was advised to ck out the differences. Glad I did.
I chose a Comp Cam 35-349-8 and am very please and the performance is great. Dyno indicates power band 1800- 5200 and great low end torque . I run a 3.55 8.8 Mustang R/E. T Ford E & B cams are 15 yr. old technology and have not been upgraded. The guys on the Mustang boards indicate a person can do better now with more modern grinds. Just my thoughts.


MGB-FV8
Jacques Mathieu
Alexandria, VA
(299 posts)

Registered:
09/11/2009 08:55PM

Main British Car:
1977 MGB Small Block Ford, 331 Stroker

Re: Did I chose the wrong gear ratio for my Ford conversion????
Posted by: MGB-FV8
Date: February 14, 2010 08:05PM

Thanks for answering Kelly, I do have the 0.63 O.D. but I had a feeling that my E303 camshaft was going to end up on Ebay.

Taking a second look at my power train design made me realize that it may be a good idea to move up the power band while taking advantage of the extremely advantageous weight to horsepower ratio. I'm using the same intake (GT-40) as you have and the heads are aluminum AFR165. The T/B is 65MM with 24 pounds injectors, this set up limits how far I can move the power band up as the intake airflow has the good velocity but will lack volumetric efficiency at higher RPM. The next move would be to force feed it with a turbo.

I maybe able to maximize acceleration by playing around with rear tire seize and if my lower end torque still excessive then I'll have to look into going to a 3:23 ratio. Dan Master's car has a 3:23 gear ratio; I wonder how he came up with his decision? Iím sure that with all of the Ford conversions out there, someone has some proven/tested advice to give us Ford fans.


BlownMGB-V8
Jim Blackwood
9406 Gunpowder Rd., Florence, KY 41042
(5935 posts)

Registered:
10/23/2007 12:59PM

Main British Car:
1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS

authors avatar
Re: Did I chose the wrong gear ratio for my Ford conversion????
Posted by: BlownMGB-V8
Date: February 15, 2010 02:07PM

You might be a little small on your injector size.

I'd say try it with the gears you have now before deciding it's not right. You may be able to go up an inch or two on the tire size, about the same as the lower gear ratio and an easy change to make. Plus you might find you like that ratio real well. Especially with that OD.

JB


MGB-FV8
Jacques Mathieu
Alexandria, VA
(299 posts)

Registered:
09/11/2009 08:55PM

Main British Car:
1977 MGB Small Block Ford, 331 Stroker

Re: Did I chose the wrong gear ratio for my Ford conversion????
Posted by: MGB-FV8
Date: February 15, 2010 06:45PM

Thanks Jim. The injectors may be ok considering all other given engine components, but, if I proceed with the single rear mounted turbo then you're right to say they maybe a little small. The original Mustang (5.0L, H.O.) uses 19 pounds injectors, the '93 Mustang Cobra have 24 pounds injectors, a better flowing (Cobra) intake also a matched ECM and mass-air-flow sensor. The Cobra engine also uses a 1:72 roller rocker ratio, it improves the camshaft duration.

Contrary to what someone may think, I don't want to build a tire frying machine; as I mentioned before, it's amazing to watch the acceleration that comes out of my 160 H.P. supercharged (added-on) Miata. There's a message to learn from the Japanese approach, 0 to 60 in 5.9 seconds is making good use of thermal energy out of a four banger. I attribute it to the fact that absolutely everything works together with no waste going to heavy masses. I also like the 30 M.P.G. (LOL), although at 6 foot tall the MGB has a much more comfortable seating arrangement.

I've been reading this forum for awhile now and I've taken notice to some real smart guys with swift engineering skills and Jim you happen to be one of them. I must say that you're not afraid to design and try out new ideas and that is how we all can learn to fine tune these conversions. In my mind, what I find to be mostly appreciative of this forum is when a member advises/cautions you with the plus and minuses of an approach they have experienced with; that's real camaraderie. This is a very impressive group/forum.


HealeyRick
Rick Neville

(474 posts)

Registered:
12/19/2007 05:01PM

Main British Car:
1963 Austin-Healey 3000 Ford 5.0L

authors avatar
Re: Did I chose the wrong gear ratio for my Ford conversion????
Posted by: HealeyRick
Date: February 16, 2010 06:59PM

Jacques,

I spend a lot of time on the Factory Five forum www.ffcobra.com as there's a lot of info I find useful in swapping a SBF into an Austin-Healey 3000. Seems most of the guys there are running 3.55s with the T5Z. I think you're close enough in the ballpark to wait until you get on the road awhile before contemplating any changes One of the big complaints with the T5Z with the .63 5th gear is low rpm at cruising speed, requiring a downshift for a pass. Switching to a higher rear end, as you're contemplating, might exacerbate the problem.

Rick
My Healey build: [forum.britishv8.org]



Edited 1 time(s). Last edit at 02/16/2010 07:04PM by HealeyRick.


MGB-FV8
Jacques Mathieu
Alexandria, VA
(299 posts)

Registered:
09/11/2009 08:55PM

Main British Car:
1977 MGB Small Block Ford, 331 Stroker

Re: Did I chose the wrong gear ratio for my Ford conversion????
Posted by: MGB-FV8
Date: February 16, 2010 08:55PM

Hi Rick, I've checked out your Austin-Healy conversion, it's pretty impressive. I'm going to closely follow its development. As far as my 3:42 gearing goes, I'm going to wait it out for the big test drive as it may very well had been the right decision.

The 3:42 is a vantage Mustang ratio, I originally chosen this ratio after talking it over with one of the Currie Enterprises brother. As Jim Blackwood stated earlier, tire seize selection can help cut to the chase.


tr8todd
Todd Kishbach

(372 posts)

Registered:
12/04/2009 07:42AM

Main British Car:


Re: Did I chose the wrong gear ratio for my Ford conversion????
Posted by: tr8todd
Date: February 18, 2010 10:03AM

With small lightweight cars like ours, you have to be aware of the sweet spot in the cam. If you are trying to cruise at a comfortable speed, let's say 70MPH, and the sweet spot is more like 80 or 85, you will find that if you are not watchful of your speed, the car will continue to increase it's speed up to the sweet spot. This is a real common with the TR8s using the 3.08 gears. Most aftermarket performance cams used in the TR8 really like to cruise at 3K rpm. That engine speed translates to 80MPH. You will find that whenever you are mindlessly driving along, you will look down and see that you are doing 80. Switching to 3.45s brings that down to 70MPH. Changing the overall tire height by just an inch can effectively change your net rear end ratio by .10 to .15. My TR8 that has a ford 8.8. It has a 3.55 ratio, and stock tire height was 23 inches. By going to a 24 inch tall tire, It now feels like I changed my rear end to something closer to a 3.45. You need to factor in your 5th gear ratio, your rear end ratio, your tire size, and your cam choice to arrive at the right decision. I'm assuming your building a street car and not a race car. If your building a drag car forget everything I just wrote.


kstevusa
kelly stevenson
Southern Middle Tennessee
(943 posts)

Registered:
10/25/2007 09:37AM

Main British Car:
2003 Jaguar XK8 Coupe 4.2L DOHC/ VVT / 6sp. AT

authors avatar
Re: Did I chose the wrong gear ratio for my Ford conversion????
Posted by: kstevusa
Date: February 18, 2010 02:24PM

Todd has it right! i cruise around 75-80 MPH on interstate and keep revs at 2600-2800. That is sweet spot for my combo. Mileage is great also. my 205-50-16 tires are 24.1" dia and do great. the compete drive train should be designed and then the cam chosen to compliment the goals. my speedo was only about 2% error and that seems w/i tolerance.
SAFETY FASTER!



Edited 1 time(s). Last edit at 02/18/2010 05:49PM by kstevusa.


tr8todd
Todd Kishbach

(372 posts)

Registered:
12/04/2009 07:42AM

Main British Car:


Re: Did I chose the wrong gear ratio for my Ford conversion????
Posted by: tr8todd
Date: February 18, 2010 04:20PM

Here is a link to a tire size calculator. It is a big help when you are figuring rear end ratios and plus tire sizing. I think you made a typo Kelly. 205/50/16s are 24.1". That is the same size tire I put on the front of my TR8. I put 225/45/16 on the rear. Now that it's done, I wish I had the 225s on the front and went 245 on the rear.
[www.miata.net]


kstevusa
kelly stevenson
Southern Middle Tennessee
(943 posts)

Registered:
10/25/2007 09:37AM

Main British Car:
2003 Jaguar XK8 Coupe 4.2L DOHC/ VVT / 6sp. AT

authors avatar
Re: Did I chose the wrong gear ratio for my Ford conversion????
Posted by: kstevusa
Date: February 18, 2010 05:52PM

Thanks Todd, I could have been a REAL Low Rider! I have considered the next size up, but the sizing was not what I designed the combo for. Only reason i need a larger rear tire is maybe a set of slicks to get some traction. the existing tires barely rub my 02 sensors on the headers at full lock in turns in parking lots.


MGBV8
Carl Floyd
Kingsport, TN
(3947 posts)

Registered:
10/23/2007 11:32PM

Main British Car:
1979 MGB, Buick 215

authors avatar
Re: Did I chose the wrong gear ratio for my Ford conversion????
Posted by: MGBV8
Date: February 18, 2010 08:18PM

You'll be fine, Jacques. Something in the 3.4-3.5 range is perfect for the sports car feel. Gears like 3.08 are for people that drive like grandmas!


302GT
Larry Shimp

(214 posts)

Registered:
11/17/2007 01:13PM

Main British Car:
1968 MGB GT Ford 302 crate engine

authors avatar
Re: Did I chose the wrong gear ratio for my Ford conversion????
Posted by: 302GT
Date: June 07, 2010 04:47PM

I have a 3:43 ratio rear in my MGB GT with a Ford 302 and the T-5 with the .63 overdrive and 2.95 first gear. Tires are 215 - 45- -16. Cruise is 2000 rpm at 70 mph. (With your tires this might be more like a 3:30 rear). Like Kelly, I have the XE 266 HR cam, with a power peak at about 5900 rpm, and relatively flat torque curve from 2000 to 6000+ rpm.

In a straight line, traction is only limiting in first gear. But my car is relatively heavy (about 2500 pounds).


302GT
Larry Shimp

(214 posts)

Registered:
11/17/2007 01:13PM

Main British Car:
1968 MGB GT Ford 302 crate engine

authors avatar
Re: Did I chose the wrong gear ratio for my Ford conversion????
Posted by: 302GT
Date: June 09, 2010 09:53AM

Originally, I had 205/60/15 tires. They also worked fine with the 3:43 axle ratio; the only difference was that 2000 rpm equaled 65 mph, not 70 mph.


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