• The Roadster Factory Recovery Fund - Friends, as you may have heard, The Roadster Factory, a respected British Car Parts business in PA, suffered a total loss in a fire on Christmas Day. Read about it, discuss or ask questions >> HERE. The Triumph Register of America is sponsoring a fund raiser to help TRF get back on their feet. If you can help, vist >> their GoFundMe page.
  • Hey there Guest!
    If you enjoy BCF and find our forum a useful resource, if you appreciate not having ads pop up all over the place and you want to ensure we can stay online - Please consider supporting with an "optional" low-cost annual subscription.
    **Upgrade Now**
    (PS: Subscribers don't see this UGLY banner)
Tips
Tips

best diff

bodyshell2

Freshman Member
Offline
Hi all i am thinking of fitting a 649 cam in my 1340cc S fitted with twin 1 1/2" H4 carbs 1.5 roller rockers , could i get away with a 3.7 diff ( for limited road use only ) i know the works cars used 4.1 but this would make it a screamer especially with straight cut box and drop gears . Your thoughts please .

Best Regards
 

dklawson

Yoda
Offline
I can only offer conjecture since my car still has the standard gearbox and a 3.44 final drive.

With straight cut gears and a 3.7 final drive your car will certainly be driveable around town and it should accelerate quite well. It is also going to be very loud if you ever take it on the highway. Consider that a Mini on 10"s with a 3.44 FD turns about 4000 RPM at 65 MPH. You'll be closer to 4300 with the 3.7 FD and the straight cut gears are just going to be that much "louder".
 

dklawson

Yoda
Offline
You asked "could I get away with a 3.7 diff" if you fit the 649 high-lift cam. That all depends on what you are looking for and asking about. You mentioned limited road use. Are you concerned about how driveable the car will be on the street or how it will perform on the track?

The higher the FD number, the more revs it takes to go a mile. That means the available torque will allow faster acceleration. Your 649 cam is supposed to have a power band between 3k and 7k RPM. Kent considers it a race cam. When driving at lower speeds and around town it will benefit from higher final drives... but as I said earlier, you will have those higher revs at a lower road speed. Starting from a stop and driving at low speed (RPM) may be less than smooth. As I mentioned earlier, it will be loud.

I suggest you contact the folks at Kent regarding using high-lift rockers with this cam. Long ratio rockers have their own problems, not the least of which is/are higher contact forces. Make sure your valve springs don't collapse to solid using the high-lift cam and long ratio rocker arms.
 

Jerry

Darth Vader
Gold
Country flag
Online
My mini is a 1380 with 2 1.5 inch SU, a 288 cam and dyno at 5800rpm to be 95HP. My rear final drive is 2.9.
things I would do differently if I took the engine out:
1. The cam has moved the power band to far for street driving. Not much power below 2500rpm. I would use the next cam down, (around 275)
2. The final drive is great on the freeway, At 3800rpm, I am going 70mph. But, loss of acceleration around town due to the final drive ratio and cam.
3. I have tried about 10 different needles in the carbs using the MInty program. Those 2 1.5 inch carbs are a little too much at lower RPM. Perfect on the freeway.
Still, it works pretty well. Once you hit 3000rpm, it takes off like a rocket. Fun to drive.
Jerry
 
Similar threads
Thread starter Title Forum Replies Date
D best oil to use in the diff? Triumph 7
mctriumph TR2/3/3A the best kinda folks Triumph 2
K TR2/3/3A What is the best way to prevent the crank from turning ........ Triumph 27
tr6nitjulius For Sale "SOLD" Winter projects: Upgrades 4Sale or Reasonable Best Offer Triumph Classifieds 0
AngliaGT MGB-GT The Best Laid Plans....... MG 3
69MGC T-Series Best Technique to Clean a Canvas Top MG 3
Michael Oritt Best Seat Trim Adhesive Austin Healey 8
J Best Aluminum Fenders? Austin Healey 3
1 TR2/3/3A Best Rustproofing for inside of body. Triumph 7
G Best fuel option for a 72 Midget Spridgets 5
Celtic 77 MGB Looking for best info regarding throttle cable replacement. MG 16
JPSmit some days your best tool is a credit card Spridgets 2
dripdripdrip TR2/3/3A Best place for tonneau & hood stick covers? Triumph 2
D The best place to get shocks rebuilt. Spridgets 8
Joel Lester TR2/3/3A Advice for which head gasket material is best Triumph 8
BoyRacer Best-Sounding Muffler - Stainless Steel or Mild Steel? Austin Healey 9
W Best source for Bugeye front shocks Spridgets 3
J For Sale For Sale - 2x TR6 seats in red vinyl - Best Offer! Triumph Classifieds 1
Bob Claffie Best cruise in of the year Spotted 8
longbridgehealey Best way in Austin Healey 2
B TR4/4A What really is the best clutch/bearing combo Triumph 4
I Best glue for rubber door panels? Spridgets 3
N Best cam choice??? Spridgets 5
R TR2/3/3A Best day ever [so far] Triumph 1
C How best to restore red script on valve cover? Austin Healey 7
F Best $25 I've ever spent on my Healey - 2019 Austin Healey Concours Guidelines Austin Healey 0
FlyingCat Spitfire Which carburetor is the best? Triumph 9
T TR2/3/3A Where is best spot to check temp with infrared thermometer? Triumph 17
D Best little tube bender Restoration & Tools 8
N Best lube for a Datsun gearbox?? Spridgets 6
Kleykamp For Sale 1978 MG Midget Best Offer Spridgets Classified 0
DornTRoriginal TR2/3/3A Electrical Wiring Best Practices Triumph 13
I General TR Best car cover Triumph 8
M Best Interiors Austin Healey 20
Jer Confirming the best choice for clutch / release bearing for my 67 Midget Spridgets 2
N Best carpet glue???? Spridgets 5
S Best Ethylene Glycol/Water Ratio to avoid overheating? Why? Informal Survey. Austin Healey 27
Boink Pre-War MG-TB wins best in class at Forest Grove Concours MG 7
T TR2/3/3A What is best way to flush engine after antifreeze got in oil?? Triumph 2
GTP1960 Not the "best car ever made" by a long shot Other Cars 14
glemon Mercedes 450SL Best Car Ever Made? Other Cars 7
cpbol Best alternative option to sleeved thermostat? Austin Healey 9
MisterB Spitfire Best year for a Spitfire? Triumph 12
D TR4/4A Best SU overhaul service to use? Triumph 10
K TR2/3/3A What is best way to remove old nylon/rubber bush 525-020 from steering box shaft? Triumph 5
Gerald_Gordon TR4/4A Best carpet for TR4 Triumph 2
C Best Replacement BN1 Leaf Springs Austin Healey 5
Jim_Gruber Headlight Trim Rings on my BE - Removal - Best Methods Spridgets 5
A Best fibreglass/GRP Bugeye bonnet Spridgets 21
D TR2/3/3A Best TR3B seats - repair or replacement? Triumph 11

Similar threads

Top