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 Posted: 07-17-2005 12:26 pm
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Ron Earp
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My motor on the JH SCCA IT car is done, in the car, and we're tidying things up for start up. I've a couple of questions about some "details":

*On the intake manifold, right above where the temperature sender screws in, there is a little brass tube - what is this and what does it connect to? Is it an air bleed for the cooling system?

*My distributor has no vacuum advance/retard, someone removed it eons ago I suppose.  I don't want a vacuum advance/retard however, but, I need to know roughly what does the stock distributor do for mechanical advance and when is it in by? I'll have to tune it and fool with it I'm sure for best power, but I'm just sort of wondering what I'm dealing with as a mechanical base - will it get 34 degrees of advance in at 2500 RPM?, etc.  I know it has weights and springs in it, I verified that, but what they'll do I have no clue. Does anyone re-curve these things in case it is needed?

*Driveshaft - related to my other question you fellows were right, my driveshaft that was sold to me as a "complete 5 speed kit for a 4 speed" does not work. It is about 1.5" too long. I'm going to have a driveshaft made for the car, in *cough, cough* ummmm, aluminum. Anyone have any sugestions, tips, or pitfalls to avoid with the driveshaft?

Thanks much for all the help folks. Jeff Young and I are pretty confident the JH and the TR8 will race as a pair at CMP in early August - barring any problems on my end with engine, tranny, or suspension setup. All the electrics (I made all the harnesses from scratch) work, it seems, brakes work (all new lines, equal length and separate for the back with bias control), and if she'll just fire up and run we'll be in good shape. Fire up in Monday or Tuesday night, I'm hoping it'll be fine.

Thanks,

Ron


Last edited on 07-17-2005 12:57 pm by Ron Earp

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 Posted: 07-17-2005 03:09 pm
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Mark Rosenbaum
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What a lovely, lovely photo you've provided. 

Brass Tube.
This is the manifold vacuum spigot.  It's pressed in, and if it fits loosely you'll have a hard-to-find vacuum leak.  In a street car this is the manifold vacuum port for the vacuum switch and the distributor.  If you won't be hooking up vacuum to either of these two devices, the spigot should be capped off or replaced with a threaded plug.

Distributor.
If you look at the General Specs in the shop manual you'll see the centrifugal advance specs.  Note that the curves for the 23D (Del'lorto) and 25D (Stromberg) distributors are the same.  The 45D distributor is intended for smog cars with greatly retarded static timing and thus gives a lot of advance just above idle -- not a good choice for performance.
     The stock distributor provides 0 crankshaft degrees of centrifugal advance at 1000 rpm and 17 crankshaft degrees at 2500 rpm.  Bear in mind that centrifugal advance is in addition to whatever you use for static advance, so you'll have to add the two together to see what the actual advance is at any particular rpm. 
    Given the odd things previous owners do, once the engine is running, it is prudent to verify by actual test that you have the correct centrifugal advance at each of the rpm's stated in the manual.  If there are doubts or problems, open up the distributor and verify that you have the correct advance plate (AKA 'action plate').  The one for a JH with a 23D/25D distributor is marked 11^ which means 11 distributor degrees or 22 crankshaft degrees of centrifugal advance.  This is the maximum possible centrifugal advance available, and (guessing) will occur around 3200-3500 rpm.  See http://www.mgcars.org.uk for further information.
     For a stock or near-stock 907 engine, the factory advance curve is generally appropriate.  However, you might be able to improve things a bit if you put the car on a dynamometer, determine what advance gives max power for full throttle at various engine rpm's, then modify the distributor to suit.  Any racing shop with a distributor testing machine should be able to do that, assuming of course that they can get or make the appropriate parts.  
     If you are not going to be using the vacuum retard function of the distributor, and can not remove the thing for some reason, a bit of epoxy in the right place will keep the capsule end of the coupling spring from moving due to vibration of the diaphragm.  This may result in more stable ignition timing under some conditions.

Can't help with the driveshaft issue but would be interested in hearing about whatever you discover.  Do you expect an improvement in acceleration with an aluminum driveshaft, or would this be done only to reduce overall car weight?  And as it sounds like you've got all the other issues under control, I'll wish you good luck in your endeavors.





Update: the link above is incomplete.  The correct one is:
http://www.mgcars.org.uk/electrical/25ddistributor.html

Last edited on 07-17-2005 08:27 pm by Mark Rosenbaum

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 Posted: 07-19-2005 06:56 pm
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Ron Earp
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Thanks for the info Mark. I am going to use a MSD Timing Advance/Retard unit and I might just fix the dizzy at a static of 10 BTDC then use a MSD electronic advance to handle the advance duties.  The MSD Timing unit will let me advance or retard 15 degrees which will make dyno tuning easy and it easy for me to change timing based on temperature - I can run more advance when cooler and extract that couple of hp.

I capped that tube - the manifold must have some odd internal castings to do what it does!

Driveshaft - it might help acceleration. I can say this though - RTP Racing has two spec Miatas we rent and one of ours, #39, was one of the fastest spec Miatas around about 1 year ago as it got off corners faster than anything else. We used a lightweight 3 puck clutch with lightweight pressure plate (all legal).  Weight savings is about 8 pounds.  I experienced it first hand - off corners it would get half a car length and that would translate into a car length or more over a decent straight. With cars with only about 100 rear wheel hp anything helps!!!!!

So, I think a lightweight driveshaft could be of benefit. Definitely a lightweight clutch and pressure plate will, and I am getting ready to investigate that. Obviously an aluminum flywheel would be great, but it is illegal.

I will have one made next week and let you know. Also will be dynoing the motor around the 1st of August so I'll post that as well - providing it doesn't let go on rev up for the first time, at which I'll call Judson and fly him up for emercency services!

R

Ron

 

 

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 Posted: 07-20-2005 12:57 pm
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Tim Knowlton
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Ron,

What kind of an alternator are you running and how big is it? It looks different than the others I have seen

Tim Knowlton

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 Posted: 07-20-2005 11:36 pm
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Ron Earp
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Tim,

That is a Powermaster one wire alternator and it is quite small. It is only 60amp and is rated for severe duty at high RPM. They aren't cheap, typically around $175, but they last forever and are really nice on hookup. Since I made all of my own wiring harnesses and connectors (you can disconnect the entire motor harness with one connector and that includes oil, water, pertronix, transponder, starter, ignition, etc). I like the fact the one wire alternator only has one output wire that goes to the positive lug on the starter. You can see my one connector on the back wall near the coil. Makes a nice and simple installation and ease of service. There are no field wires etc. to enable the alternator to work and the regulator is internal of course.

I had to make the brackets to hold it, but I had some old alternator brackets of various types lying around and it only took a little bit of time.  In the end I think it is better since rebuilds are easy on it, and cheap, and I don't have to deal with the crazy OEM unit that nobody seems to know what it is.

Ron

Last edited on 07-20-2005 11:38 pm by Ron Earp

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 Posted: 11-11-2015 06:35 pm
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thatmak1
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Which Powermaster alternator did you buy? I don't have an alternator problem but interested in newer technologies and lighter.

Thanks,

Tommy

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 Posted: 11-12-2015 05:43 am
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Frank Schwartz
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Hi Ron...I see you are back into racing again...ae you still using that JH chassis I sold you????
I thought you had given up racing...
Frank Schwartz

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 Posted: 11-13-2015 03:15 am
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thatmak1
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Ron, which connector are you using? I see it beside the coil on the back wall but not the brand or model no.

Can you also provide the model number of the alternator?

These will go well with my Jensen.

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 Posted: 11-13-2015 11:34 am
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jcdean
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Not sure if Ron monitors this site any longer. If you need to get in touch with him, he is always on the GT40s forum.

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