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1500 Midget cam specs

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GTsRFine

Jedi Hopeful
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Hello All -
I'm trying to set the cam for a Midget 1500 (RB '76) engine. The Haynes book shows
-----------------------------------
Int opens 18d BTDC closes 58d ABDC
Exh opens 58d BBDC closes 18d ATDC
and
Rocker arm/valve clearance 7 & 8 (for valve timing only) 0.050"
-----------------------------------
I've got 2 cams that I've stuck in there and dialed out. They both show valves closing significantly earlier than the above specs. Roughly 20 degrees earlier. Total lift is 0.240" and 0.245" (intake & exhaust). I think those lifts are correct but I'm just guessing.
Does anybody know where I can find the cam specs for this engine?
The Triumph Spitfire used the same engine, but I don't know if the cam was the same.
 
The cams are not advanced on the sprockets. The lobes just aren't as long. If you set them up for opening per spec, they close too soon - & vice versa.
Thanks for the website.
 
It sounds like maybe a mix up of cam specs between durations measured at .050 & .020. It seems that the various cam manufacturers have never really standardized on the "checking" lift.

Is the total duration 20° short of specs at .050 lift? It would be considerably more at a lower checking lift.

The most important thing would be to have the overlap split equally no matter what the actual duration is.
D

[ 11-16-2003: Message edited by: Dave Russell ]</p>
 
Here's the numbers. I've got a little slop in the dial indicator contact foot. I need a mushroom (button) tip to tighten it up. Otherwise discount about 3 degrees.
___________7_____8 _____1_____2
Open @ .050 15 55 55 17
Close @ .050 15 32 32 15
start open 71 68 72 72
start close 92 38 36 92
total lift .241 .240 .240 .245

Are the four bolt holes in the cam sprocket exactly 90 degrees from each other? If they are, then there's not much you can do, adjustment-wise to the stock sprocket.
 
I think that you are actually pretty close to stock split overlap with an IO of 15° btc & EC 17° atc.

There are enough measuring errors in pushrod flex etc. to account for minor differences in numbers. This would help account for slightly shorter duration numbers, as would slightly different checking clearances.

I think that the four bolts have to be symetrical. The only way to change it is to slot the bolt holes. Not a good idea. Or to get a special adjustable or fixed offset sproket.

Although split overlap is generally the target, it depends on the engine design & intended use. Advancing or retarding the timing about five degrees by use of a modified sprocket can be used to change the torque/rpm characteristics of the engine.

Since the point of intake valve closing is the most critical event, if it closes earlier (advanced cam) there will be more compression in the cylinder thus more low rpm torque. Later closing (retarded cam) of the intake will allow more time for fuel ram (inertia) filling into the cylinder at the higher rpm.

The usual range of adjustment would be five degrees advanced to five degrees retarded for all events.
D
 
Educate me on this "split overlap" stuff.
This head is not siamesed. Individual ports. I am using headers with at least 18" of tubing run before they join (1&4, 2&3) then about 30" of collecter runs before joining at a final Y. Would a split overlap be needed in this situation?
 
Hello GTsRFine,

the Triumph 6 cylinder engine cam timing is designed to be adjusted by using one or the other pair of bolt holes (and therefore they cannot be at 90 degrees to each other) and also by reversing the camwheel giving four adjustment points. As the 4 cylinder is of the same family I'm fairly certain the same will apply to your Midget engine. If I have read my manual correctly the four adjustments span one chain link variation so your accuracy should be within a 1\4 tooth.

Alec
cheers.gif


[ 11-17-2003: Message edited by: piman ]</p>
 
Alec is correct. The "fine" adjustment is accomplished by turning the cam gear around, so the other side faces the block. It's generally a pain in the butt, but you can get well within street tolerances this way, with a bit of patience.
Jeff
 
Jeff,
If I read GT's numbers right, his cam is only one degree retarded from stock timing. (IO=15° - EC=17°) This would seem hardly worth the trouble to try to change. All of the other confusion seems to be about duration numbers being different from the specs. If he was trying to actually advance or retard the timing for a specific purpose then some adjusting is required.

How much range of adjustment is provided by the sprocket hole/frontward backward setup?
D
 
Dave, my factory book says that a total of 3/4 of a tooth adjustment is obtainable. If my memory is correct, there are 48 teeth on the cam gear, so 5.625 degrees of total adjustment.
BUT, while looking this up, I noticed that the factory book specifies a checking rocker clearance of .080" instad of the .050" that he's using. There could be the reason for the discrepancy in the numbers. The cam figures I have (18-58, 58-18) agree with the Haynes book. Little things like this are why I switched to vernier adjustable cam drives a long time ago!
Jeff
 
Interesting info there - thanks Bugeye.
I have almost convinced myself (and the wallet) to go ahead and get a mild road cam, like a Piper 270. I've opened up the head ports a bit, changed the carbs to SUs and put an exhaust header on it. I might as well take advantage of the extra breathing capability. It should improve the efficiency without sacrificing any reliability. It's another delay to completion though. I was hoping to have this done by Christmas. It doesn't look like it is going to happen.
And (oh the joy), I get to dial in the cam AGAIN.
 
David...I'm gonna ask a dumb question..where'd you get that manifold - actually both of them? I've a 3 tube header for a 1500 Midget but no intake...what am I missing? Or, should I just go get a cup of coffee & look at my car awhile?
 
Beautiful photos...but, where'd you get the manifold? What's it fit? I want one!
 
confused.gif


Uh - from the places (websites) noted above.
Intakes and carbs salvaged from UK spec 1500 Midget. Exhaust is a Pace for UK spec Midgets. Fits 1500 Midgets and 1500 Spitfires.
 
Oops, sorry, David...didn't see them....I'll call Ted at T&S
 
Has anyone used a reground cam from an outfit called DeltaCam out of Tacoma, Washington, US?
I'm about halfway into trying to turn Midget 1500, the red-headed step-child of the MG family, into a sporty little car. The stock cam exhaust closing is just too restrictive so I'm going to put in a street cam. I guess I'll call the project the Silk Purse Sow Challenge, or David's Waterloo. We'll know in a few months.
At any rate, they offer a reground cam profile:
"mild street 1500-6200 rpm" Grind# KC
262 int & exh Duration
217 at 0.050 Duration
24/59 int 51/24 exh
108 degree separation
This should be good for you Spitfire fans also.
 
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