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Old 09-01-2010, 11:25 PM
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Tunnel Ram/Vacuum Secondaries/Cam/Torque Converter

Hi guys,

Have a T Bucket with a 396 block, 454 Hi Perf rectangle port heads, tunnel ram, 4:30 rear gears backed up by a Turbo 400. Cam is a relatively mild Comp Magnum 270. Two Holley 1850 600cfm vac. secondary carbs.

Been fighting an over heating problem for years, which due to some great advice, has pretty much been resolved with throwing in some 76 jets, 37 squirters, distilled water, 180 stat and a few other changes.

Now that the heating issue is over, the larger jets and squirters have made the engine come alive a bit. When I put purple springs in the secondaries with the 66 jets and 25 squirters, the engine would take a huge bog when I nailed it, now it just goes. I'm going to keep moving up the scale with the spring selection and just see how much of the "more than I need" CFM the engine will take.

So, I know what I have isn't an ideal race engine and that isn't the intent of the car. But, I would like to take better advantage of what I have, and get the most out of it.

The two secondaries are connected so the vacuum is equal in both chambers when applied. Both diaphragms seal up nicely and hold on a leak test. From what I can tell, they are both operating correctly and I just need to find the combination that works best with the different spring rates that are available.

One of my questions is this: What is really happening with a tunnel ram in terms of vacuum vs. a lower plane manifold? I'm at 12" in gear at idle, 15" at a steady speed. Does the spring choice change because of the larger volume in a tunnel ram? In other words, do you need a lighter spring to accomplish the same secondary opening than with a shorter manifold.

Part two of this question involves the vacuum at the secondary ports on top of the diaphragm. Since I can easily tee a gauge in between the two, does anyone have an idea of what the reading should be - or when I should see a reading? Will the secondaries begin to open at 2""? 5"? I know the engine helps to decide the opening of the diaphragm along with the spring rate, but curious to know if the tunnel ram vacuum/volume is preventing the necessary amount of 4 barrel operation.

Certainly going to lighter and lighter springs may help to find this answer, but anxious to know if there are numbers to support where I should be in terms of selection.

As far as the performance of the car goes, the carbs work great. Very streetable, smokes the tires with ease, etc. No need, for me at least, to downsize the carbs, go to mechanical secondaries, etc. It works! Just playing at this point to make it better.

In terms of the cam...with my 4:30 gears, I'm out of the 5800 rpm range of the Magnum 270 in no time. As I play with the Comp Cams Camquest, it looks like I can take the hp from 450 to easily over 500 with a more aggressive cam. I'm a bit concerned about my idle quality - this is a parade car, but if you have some ideas on a cam selection, I'm game to hear it. I have roller rockers, but not a roller cam.

Finally, I have no idea what torque converter is in the car. The one time I have seen it, I know that the factory holes were not used to mount it to the flex plate. Imagining that maybe something like a Vega converter was used.

My basic problem is idle creep. I have no stall per se, and can't hold the engine against the brakes at anything off idle. Now, again, I can smoke the tires at will, so I know that I don't need a huge stall speed to help me launch, but maybe there is something more to torque converters than I understand.

Especially if I run a new cam with a lower limit of 2200 on the power band. Understand I probably need to give the specs to TCI or someone like that to help me decide what to get, but does something in the 2400 stall range, sound like a practical street application for a 1900 pound car with 4:30 gears?

Thanks for the replies in advance. Time to have some fun over the winter working on the car. Not wanting to invest a lot of money, but a cam swap is easy in this car. Let me know what you guys think!!




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Old 09-02-2010, 12:18 AM
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First: if you are smoking the tires at will, you need traction- better stickier tires.
at 1900lbs You can probabily lower the rear gear ratio. 3.55ish.

A big cam wants a high stall converter. A high stall converter in a very light car looses a lot of its undesirable traits (loose coupling at part throttle) as the vehicle weight/load is low.

A 10" "3500 stall" converter will give about 3800 max stall behind a 396 at WOT, but you will only see that under full load without tire spin on launch (slicks)

A "stock converter" will seem very tight in a low weight car becasue the coupling at idle is tight and will tend to "creep" on ya. A higher stall smaller diameter converter like a 10" wil not tend to "creep" at idle in a low weight car casue the idle coupling is much less.

A really nice high performance street cam was and is the origional GM L-78 396-375 LS-6 454-450 factory street mechanical cam. 244-244-.520-.520-114lsa "Adv duration" is about 284. nice easy high prf idle, broad power band to 6000+ rpm very easy on the valvetrain.

A more radical cam that will make more power and top end that you can bolt in is the Crane "Fireball 326" F326-2 #134261. This one has a rough idle, needs a 10" high stall converter and likes the rpm. it has modest street friendly valve lift.

Nice to see you got the jetting on the 1850's more dialed in. What is the secondary jetting?
Nothing stopping you from getting and using a AFR gauge/02 sensor to help fine tune the afr/jetting. Even a simple inexpensive narrow band type is very informative.

You make no mention on the distributor timing curve etc. This is critical and often overlooked. With cams that are bigger than stock the stock timing curve is not going to work very good.
Generally such a cam like the 270H needs increased idle base timing, a shorter advance curve. I suggest 18 to 24deg at idle 38deg at max mehcanical and 10ish deg of vacuum advance commin in at hiway cruising speed.
The more radical the cam the more the motor will want lots of timing at idle, meaning you have to shorten/limit the curve travel stop.
There is no way to evaluate whether you need a lighter sec vac spring or... unless you can load the motor down without wheel spin when testing.
(slicks...or) The "butt dyno" can be deceiving.
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Old 09-02-2010, 12:49 AM
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Wow! Thank you so much for the thorough reply!

The photo is a bit dated on the car - which now has the new MT Sportsman Street Radials (flame design). Much softer than the bias ply tires which were about 10 years old - low miles though. In either case, the tires just light up if I nail it, so it takes a gentle launch for a quick 0-60 sprint.

Wish I could take the gears down a notch - this is an Old/Pontiac rear axle and plenty of performance ratios are available, but not higher gears. Pain on the highway, but it does make the car launch pretty quickly even with this very mild setup.

Timing is handled by a MSD mechanical distributor - sorry but this was a looks thing, and wanted to keep the engine as devoid of hoses as possible. Might have been a mistake, but it works well.

Base timing is 12 degrees - blue bushing and blue springs are installed. Max advance is around 34. Previously it was at 39 which was causing detonation on the local chassis dyno so they cut the test short. It's gone now, so looking forward to another run on the rollers before a new cam goes in to see what the difference is.

The jet plate if that is the correct term on the 1850's is the equivalent of a 70 on the secondary side. The local speed shop was thinking about throwing a metering plate back there, but I'm not sure it is necessary. Would also need to create a longer transfer tube since I only have one fuel inlet.

At this point, if I could get to the point where I really think that the secondaries are doing what they should (will see tomorrow with the lighter springs), and the lack of the idle creep, I could really be happy with how quick the car is. But...if there is another 50 hp to be uncovered at minimal driveability issues, I'm all for it.

Certainly one of the concerns would be if I change to the cam that you recommend, what does that do to vacuum?

One of the threads (the one with the yelling match going on so I didn't finish reading it) had a post talking about the the manifold vacuum signal was essentially split in half due to the two carbs. That raised my brows a bit wondering if that meant that in a two carb situation would that mean a lighter spring rate would be required to make up for the loss of vacuum. Now, all I have to do is just keep changing springs, but was looking more for a conceptual rationale as to what really goes on in a two carb, tunnel ram set up.

Will know soon enough if I went too far!!!

On the torque converter, wouldn't have guessed to go to a 3500. Appreciate the advice there.

Thanks again for the help! Love this forum!!!
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Old 09-02-2010, 01:15 AM
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34 is not enough and 39 is too much. 12 is not enough at idle.

I would go with 18 to 24base at idle and 37-38 total. It pinged on the dyno at 39 cause the jetting is not right. I suggest 75-76 pri and a #21 metering plate for sec.
( you can rob these from a 3310 or drill you own.) 70 seems a bit lean sec on this one. 73-75-76 sec jetting seems better to me. A place to start.

recurve the distributor as I outlined and it will idle cleaner better and at a lower rpm without so much throttle opening and won't tend to creep at idle so much in gear.
You need to remove the distributor and modify the curve beyond what is able with springs and bushing from a tuning kit. 18-24 at idle 38 total a 20deg advance curve will not allow this. Shorten the advance curve by limiting the travel stop.
Tunnel rams like lots of initial base timing at idle.

change to this tune up and the sec spring tension will be a minor issue that you can only truly evaluate with full traction.
You need to search further for mid 3 gearing for that rear axle. its a "factory ratio". Your t bucket is a prime candidate for a power glide trans and 3.23- to 3.73 gearing now and with a bigger cam. a 3000-3500-4000 stall would be "just right". Bolt on some slicks or ET streets and see the real definition of LAUNCH!
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Old 09-02-2010, 02:04 PM
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Yegster, the combination you have is great. The mistake you did was lowering the size of the squirter's. It should be 31. The operation of the vacuum secondary is not determine by manifold vacuum, but the amount of air traveling through the venturis in a carbs on the primary side. The more you open the primaries, the more air will travel causing a build of low pressure in narrow part of the carbs, that low pressure build up what causes the secondaries to open. The purple spring is too light. It's causing the secondaries to open up too fast. I hope you reinstall the check ball in the diaphragm housing. The check ball is used to prevent the secondaries to open quick causing it to gulp too much air and not enough fuel at the start. It will lean out and cause it to bog. Connecting the two diaphragm housing with balance vacuum line is a very good approach.

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Old 09-02-2010, 08:13 PM
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Hi guys,
Appreciate the updates from my original post and FBird's reply.

LG - the squirters are 37's. Pretty sure 31 was stock, and for whatever reason 5 years ago, we put 25's in there. So the jump from 25 to 37 was pretty huge, and it virtually eliminated the bog when the secondaries opened. The 37's plus the 76 jets also corrected a running hot situation down the freeway, so excited about that also.

With the purple springs, the secondaries were barely opening, so I did try the short yellows today - two jumps lighter. If I'm in 3rd gear at 50, it stumbles a bit, so for grins, I'm going to try the tall yellows (in between the purple and short yellows. The plain silver - heavier than the purple didn't do anything in terms of opening up at high rpm. I know purple works without stumbles or hesitation, but do want to try the tall yellows just so I can have a record of what does what.

Thanks for the explanation of the secondary operation -I was thinking that the port from the carb was feeding it vacuum - something I could measure.

I do have a check ball installed, but my local speed shop advised that I should have one with a smaller diameter. As a result of doing so, what was no secondary operation, became something I could play with. Thoughts on the small vs. large ball on a tunnel ram?



Certainly understand that I could be fooled by the purple springs in that it feels like it is faster, but may not be because I'm loosing something as they initially open.

F Bird - You need to come over. No clue how to handle the distributor curve set up on my own - outside of using the kit assortment available from MSD.

You do bring up a point in that the dyno test that revealed the detonation was with 64 jets and 25 squirters, not the 69's that were recently installed, nor the 76's with the 37's. The bushing that limited the overall timing to 32 with the 12 degree base, resolved the detonation with the lean condition. I should at least try to pump up the total to where you recommend to see what happens now that is running much richer.

In regards to the base timing at 18-24, if we go much higher than 12 at normal temps, it won't want to start because of - not sure of the term - inadequate starter torque? To describe it, that as the starter tries to turn the engine, it slows and then starts. When it is hot, then it might not have enough oomph to start the engine. Hope that description makes sense.

I don't think we can pump the base timing up that far without running into that condition. But I can replace the bushing and add another 5 degrees. It won't accomplish what you want, but I don't have the knowledge of how to do what you are asking.

I might have to compromise there.

In terms of the axle ratio, a friend that passed away knew of someone with a 3:55, but I never got his name - he was the connection. I scoured the gear suppliers at SEMA and they only offer performance gears. Not sure why I need them to go away, except for the high rpm on the freeway - and resulting poor mpg.

As this is more of a show car than a drag car, I need to keep it happy under those conditions - too be honest - taking this car past 80mph isn't a really assuring thing. I need to check, but 90 is probably 5000 rpm, so I would run out of rpm in a quarter mile easily with the gear/cam set up I have.

No big deal to me though. 0-70 is a perfect range for me to happy with.

But I have to agree with you, it would be a blast to put on some serious meats on the rear and see what it does with a few mechanical updates - for a 1/8 mile run of course...

Now, let me ask another question to the two of you. The real issue is that I have never been happy with nailing the throttle at a steady 50 and having the car take off. It is better now with the 37 squirters - very responsive, but it seems like I am just sitting along for the ride.

But let me add something.

If I launch from 15 to save some tire in the process, the car scrambles quickly through 1st gear before the know it, then into second with a full head of steam, but the shift into 3rd takes all the fun out of it. That is what has been sending me in the direction of the secondaries, the thought of a higher range cam, etc.

As far as I know, it could be the drag from the windshield, the open body, or even the restriction in the muffler extensions, but as the car gets going - again at 50 plus, it just seems doggy.

I am curious if bumping the total timing 5 degrees with the richer fuel mixture will help to improve this condition. It's a combination I haven't tried yet.

Thoughts??? And thanks!!!
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Old 09-02-2010, 08:28 PM
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F Bird -

As a follow up on the distributor curve, these are the bushings that limit the total advance over base timing.

I believe what you were wanting was 18-24 base with 38 max. If it will reliably start on a hot day, then I could do the following:

Base Bushing Total (hope these columns hold)

18 Black 36
20 Black 38

Diameter Degree
Red 0.282 28
Silver 0.312 25
Blue 0.343 21
Black 0.374 18

If 18 is too aggressive to start the engine, then maybe 16 with a blue (21).

On top of that, there is a large assortment of springs to determine the rate of advance. Too difficult to post here, but if you have a recommendation of a fast or slow rate, I could choose from the charts.

Thanks,
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Old 09-02-2010, 08:49 PM
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You are correct on your bushing ideas, I was going to post the combo but you have the two (black or blue)that will work figured out.

You would like to use a spring combination to get the advance all in as soon as the engine will tolerate it without detonation, shoot for the 2500-3000 rpm area. Springs don't have to be in matched pairs, you can mix and match(I believe MSD tells you this too) to get a curve that fits your needs.

If it likes a lot of initial advance but gets hard to start, wire a toggle switch into the coil or ignition box feed wire to interupt the spark so you can spin the engine without load and then trip the ignition in(thats how the Nascar boys, racers, and hotrodders handle it). Works like a charm.

Do you have a way to monitor fuel pressure, to see if it runs out of fuel when put into 3rd gear?? This is the highest load situation and when fuel deficiencies show their head. What does the fuel system consist of?? (pump, line size, filter, ect.)
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Old 09-02-2010, 09:22 PM
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Eric,

Funny you should mention fuel....my speed shop was thinking I should add higher flow needle and seat combinations as he thought I may be starving the bowls. He was also the one that brought up doing something about the metering plate in the secondaries - or better put, the lack of one.

The pump I have is the Holley red. Preset at 7psi. Have an Earl's fuel filter after the pump which is pretty new - maybe 2,000 miles on it. The gauge I have is mounted on the fuel delivery manifold (Summit) if that is the correct term. In other words at the carb, but I can only see it at idle since it is parallel to the floor. It does read 7 psi right on the money, but no clue what it does under load.

I guess your thinking may be that with one small pump having to feed to carbs, it may not be enough. Actually hard to read any of my gauges during a fast run as they are so low and blocked by the steering wheel.

From a line diameter standpoint, I would say it is -8 front to back although there is some plain rubber line in between where is doesn't show. 3/8" maybe?

Back to the distributor for a second, yes, the charts show a variety of spring combos to play with. Easy to try whatever you guys suggest.

And what a great idea about a spark cutout!! Never thought of that. The car lites easily with just a touch of throttle, but if getting the starter going first and then flipping a switch is the ticket, that is pretty easy to pull off - need to hide it though...I guess I could do something that is on, unless pushed in or a toggle that is spring loaded toward the center. Then if it is a bad day, I could push or move off center a switch and then let it return to normal.

That settled that area! Will try 16 degrees to start and leave the blue bushing in for a total of 37.

Let me know what you think about the pump selection and the needle and seat switching to something that is higher flow - don't recall the part numbers.

Thanks,
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Old 09-02-2010, 11:21 PM
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Pump should be enough, just wanted to be sure you weren't trying to do this with a stock mechanical pump.

I wouldn't do anything with the needle and seats without first verifying that the pressure stays up in third gear, otherwise changing them might be like putting the cart before the horse. Maybe hook a test guage up temporarily from the spot the Summit gauge is?? Maybe put it on an elbow temporarily?? If the pressure stays up but it still feels like it is running out of fuel, then you could investigate needle and seats and metering block conversions and jet extensions.

Quick Fuel sells a billet metering plate conversion that takes regular jets and extensions so that the carb length doesn't change, might be better for you than a metering block conversion , to fit your fuel line and space requirements.

Your three position spring loaded toggle idea sounds fine if you don't want to have to use it all the time, I can't see why it wouldn't work. Could be used as an antitheft device too when switched to the off position, if hidden.
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Old 09-03-2010, 07:01 AM
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Regarding the hard start when the idle timing is high:

did you check the dizzy rotor phasing? If it's too far off you sometimes get the spark jumping to the wrong terminal especially when starting. Some people cut holes in an old dizzy cap to check the phase but if you switch to a see throughdizzy cap you're all set. They come in pretty red, blue, clear. The position of the vac advance arm at startup has an effect on the rotor phasing.

For timing curves and dizzy weights / limiting adjustments you might appreciate electronic timing. They're not cheap. Msd's ecurve dizzy is the most common but there's also a timing box called the 8981 (I think) that I use. With either item you can set idle timing, total, advance in about 30 seconds. In 5 minutes you could try tons of idle timings without affecting total or advance.
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Old 09-03-2010, 10:54 AM
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Eric,

No reason why I couldn't just put a 90 degree fitting on the manifold and aim it right at me. Simple and easy to see since it will be right in my eyesight.

Thanks for the tip on Quick Fuel. If this comes down to making a change, I'll look into it. For now I think I'm good since the secondaries aren't coming into play too much and I can avoid any flat spots with a spring change.

Yes, I will be hiding the switch, so best to have it two position, rather than spring loaded to always on.

Bubba,

The timing issue at start up is just a normal one - when you turn the distributor to advance it too much, and it doesn't want to turn over. I don't think it has anything to do with phasing. I think Eric's solution to not kick the spark until the engine is already turning will be a great solution - if 16 or 18 degrees is too much on a hot day.

Thanks for the ideal of the electronic timing box. May not be something I need for my casual driving mode, but sounds like a great tool!!

Thanks to both!!!!
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Old 09-03-2010, 03:33 PM
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You are missing the point or just too lazy to do it right.. You need 20+++/- deg base at idle. It will never idle correctly without the right timing.
This will allow proper carb throttle position opening/idle transfer slot exposure at idle and a low idle speed so it does not creep in gear.
Take both carbs off flip over and check the throttle opening at idle on all 8 barrels.
readjust pri and sec as nessessary for correct idle fuel transfer slot exposure at idle.
reinstall, fire it up. warm it up and determine how much timing it needs to idle at the desired rpm in gear without creep.
The thottles must be within this narrow sweet spot at idle.

You need to strip down the distributor and limit the adv travel stop.
Beyond what the biggest kit bushing allows.
There is no kit you can buy. First Determine how much timing it needs at idle and Modify the distributor to allow the correct curve with the required idle timing and correct max advance.. There are many other ways to limit the advance other than a bushing. Get creative. Get your hands dirty.

Use 76 pri jets and #21 sec metering plates. 4.5" power valves.
This is your baseline starting point , Fine tune from there.
You can drill and tap your sec metering plates to accept holley jets.
When you get the jetting right the motor will want about 38deg max advance.

The 3.36-3.55 3.73 gears for your rear end are widely available.
These are OEM parts. new old stock, good used restoration supply and replacement gears are still offered.
A sales man at a show will only sell you what he wants to sell you.
the right parts are available thru many sources. You just have to look.
Starting with a internet search. A contacting a car club that matches the car that your rear axle came from is a good source for new/used restoration parts suppliers.

When you employ a spark cut off switch to aid hot starting be sure to install the required $10 GM OEM starter motor rear bracket support brace. If yours is missing get one.

Last edited by F-BIRD'88; 09-03-2010 at 03:39 PM.
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Old 09-03-2010, 04:20 PM
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F Bird -

Well, the challenge isn't being too lazy, but a bit over my head in terms of doing all of this. But fortunately not for the guys that like to help me locally.

Okay, let me answer a couple of these if I can:

My car idles perfectly at 650 rpm. Not an issue there, but it seems like you are saying that the throttle position is open more than it should be to compensate for the low (12 degree) base timing. In other words, if I'm at 20 plus degrees base, then the throttle opening will be less.

I won't say that I'm completely new to all of this, but I've never heard of a car running at 20 degrees plus that wouldn't chug like crazy trying to start it. I guess the switch would counter it by allowing the engine to start first, but as I mentioned to Eric, that was news to me in terms of doing it - much less on a mild street car like mine. Have to say, that as I recall watching NASCAR driver's start their cars from the in car camera, they are cranking the car and flipping the switch. Just never put two and two together.

As far as the distributor goes, since what I need is a larger bushing to limit the amount of timing over base, it would be relatively simple to make one larger than the black one (18 degrees over base). Still, the black bushing allows for 38 total with a 20 degree base. Not quite at the upper level that you wanted, but very close. Again, fabricating a larger bushing would do the trick.

Not sure why you don't seem to think that this is an acceptable way to limit the total advance, but you certainly have more knowledge in this area than I do.

Also not sure why 650 rpm with low or high timing would affect idle creep, but again, you have the experience at playing with these conditions much more than I have.

Appreciate the idea about the rear mount for the starter strap. I'm running one of the Hitachi (if that is correct) style starters that are very light weight. The existing bracket for the original beast doesn't fit and I never worked out a replacement. It has been on for about 6 months and starts the car much easier than the original did. Will definitely work on fabricating something that will fit before hitting the spark toggle at 20 plus degrees and shocking the heck out of it.

Curious as to why the focus on the rear axle ratio? I like that is flies off the line with it, even with the side affects of having it. Doesn't it help to get me into the sweet spot of the cam/tunnel ram quicker? From all of the talk I have heard from people complaining that TR's suck at low speed, with that ratio, it counters all of it. Plus mine starts well cold - another usual negative. I know it doesn't qualify as cold in SoCal, but I don't drive it in 40 degree weather either.

Getting excited about trying a lot of this out before installing the cam. I know that may mean double work, but I may find out that I really don't need a stronger cam, if some of this works at even 80% of what it sounds like it will.

Keep on with the education. Don't get frustated with me. I'm listening!!!
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Old 09-03-2010, 05:27 PM
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You're right, you will have to make a bigger bushing if you need/want to limit it more than 18.

A heads up - a bigger bushing than the diameter of the 18 bushing won't just slip into place, the slot it rides in isn't big enough for a larger Od bushing, but there is a way around this. You can compute the difference in size needed to get the degree limit by comparing the difference in bushing sizes you already have, then make the correct size bushing. Then you will have to file the sides of the bushing to create an oval shape, with the flat size of the oval filed to no wider than the black bushing's OD, so that it will fit over the pin and into the slot, but still be longer(oval-wise) to create more limit.

Hope I made that understandable

Doing it this way is acceptable, F-Bird is just saying you can't do it with the out-of-the-box parts that MSD provides
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