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SpitfireBGT

Is my new OD making me sluggish?

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Maybe I'm over analyzing this but my spitfire seems less zippy after adding an overdrive. 

The specs are

1500 running 9 to 1 CR

Head ported to Kastners comp prep manual

Rimmers stainless 4-2-1 header

Twin Weber DCOE 40

MegaJolt Ignition

AFR gauge to help get everything in tune

3.63/1 Diff

185/60 13 tires

So winter let up for a few hours and I was able to take the car for a spin after installing my J type gearbox that I rebuilt over the last few months. I don't have times to back it up but the car seems slower off the mark than before. So I think why? Did I add about 8 lbs. of rotating weight behind my engine? The anulus and planetary gears aren't light. Would a lightened or alloy flywheel get me back to where I was? Should I go back to the 3.89 diff? Am I imagining this or has someone else had this experience as well?

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So I'm thinking you started with a USA Federal spec car?  Which has a pretty frantic gearing combination with 3.89 and rev-averse 1500.

You've obviously addressed the engine situation.  You don't say what cam is in there but you ought to be at least 85/90 bhp with the spec mentioned, with strong torque.  Did you drive it like this before the gearbox and diff swap?  I'm assuming you didn't have OD before?  Would have felt pretty rapid off the line but shouting for mercy by 55 - 60 mph.....

The overdrive adds dead weight - maybe 15 - 20kgs, but really not much rotating inertia - I don't think a flywheel swap will help.  Also small parasitic losses but more so when engaged than not with the J-type.

What will definitely blunt perceived response is the diff change. 3.89 -> 3.63 is a 7% increase.  If you've not altered you speedo or it's gearing then it will also be reading low by the same amount, adding to the effect.....

3.89:1 + overdrive might be an acceptable compromise - probably depends on what you plan to use the car for.  Lots of open road use means the 3.63 in my opinion.  Probably worth sticking with it for a while and seeing if you can get to appreciate it.

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Thank you oh wise ones

It has a Kent fast road cam. I don't remember the specs off the top of my head. I drained the tank when I laid it up in Nov. so fresh fuel. I was hoping to do more highway miles and the occasional Auto X. Maybe it's going to be either or but not both.

The OD was the only thing to change. I put the 3.63 in it about 10 years ago. The anulus and planetary gear assembly rotate when you are moving and they weigh at least 5 lbs. So I figure that I've effectively added 5 Lbs to my flywheel. Or maybe gearing the car up one step at a time has caught up to me. 

Mike

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48 minutes ago, SpitfireBGT said:

weigh at least 5 lbs

Yes, but the rotating diameter is small, so the inertia effects are also small.

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