Disc Brake Hold Off & Prop Valves

Brakes for your Classic Mopar

  1. jonn6464

    jonn6464 1970 Duster

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    Hi Fabo,

    I'm building a '70 Duster and I'm running power disc brakes to all 4 wheels, with a Moser 8/34 rear end. I'm about to purchase the brake line kit and was wondering about the correct valving. I'm using Right Stuff components and the calipers are already installed on all 4 corners, just waiting for the lines.

    1) Will I require a Hold Off Valve to the rear? I'm under the impression that this valve was used with rear drum brakes to allow the fronts to grab first, and avoid the back end from getting sideways. Is that a drum brake specific thing, or does this also apply to rear disc applications?

    2) Proportioning valve? Should I be using a prop valve, or just a metering valve for a 4 wheel disc set up? I always thought the prop valve was just to regulate fluid to the rear drums (wheel cylinders) to avoid lock up.

    I'll be using a 1 1/8" master cylinder, with a 9" dual booster.

    I just want to make sure the car is safe, so any help with the brake line/valve set up is greatly appreciated.

    Thanks in advance!
     
  2. Veryfastdart

    Veryfastdart Well-Known Member

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    Wow,
    I hope someone answers this! I am doing the same thing and had the same questions!
    Thank you
     
  3. BigBlockMopar

    BigBlockMopar BigBlockMember

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    Residual pressure valves are only to keep the shoes and pads near the drum or disc.
    Drums use a 10lbs valve and discs a 2lbs valve.
    They shorten brakepedal travel by not having to take up the gap between brake shoe and drum, or discrotor and pads.

    However I also tend to believe they help in keeping air out of the system which could enter at the wheel cylinders or calipers.
     
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    • my68barracuda

      my68barracuda Well-Known Member

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      [QUOTE
      However I also tend to believe they help in keeping air out of the system which could enter at the wheel cylinders or calipers.[/QUOTE]

      ,,,don't you think that if air could leak in, brake fluid could leak out,, especially at the 700 to 1400 psi at the wheel cylinder - caliper with the brakes engaged,,

      My 68 Barracuda was a 4 wheel drum car, converted to 4 wheel disks, used the 73 and up mopar A body disks / calipers on the front, a Dr Diff disk kit on the back, using a 68 A body distribution block in the brake line system, no proportioning valve, no hold off valve,, stops fine.
       
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      • pishta

        pishta I know I'm right....

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        disk brakes provide their own "standoff" by the spring action of the piston seals. Drums needed a little pressure to seat the lip. Id think a non ABS 4 disc distribution block would be your easiest bet.
         
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        • jonn6464

          jonn6464 1970 Duster

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          ,,,don't you think that if air could leak in, brake fluid could leak out,, especially at the 700 to 1400 psi at the wheel cylinder - caliper with the brakes engaged,,

          My 68 Barracuda was a 4 wheel drum car, converted to 4 wheel disks, used the 73 and up mopar A body disks / calipers on the front, a Dr Diff disk kit on the back, using a 68 A body distribution block in the brake line system, no proportioning valve, no hold off valve,, stops fine.[/QUOTE]
          Cool, so just lines from the MC straight to the calipers? No valving at all?
          Thx for the reply
           
        • 67Dart273

          67Dart273 FABO Gold Member FABO Gold Member

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          In my case "I got lucky." Mine is 73/ 74 Demon duster factory up front, and a Lincoln Versailles disk brake rear axle. I just ran a nail through the residual valves, LOL, and "tried" the factory drum cylinder. No prop valve, and it works great

          Depending on your tire size/ weight bias, and the disk setup front to rear, you may need a prop valve. There is NO WAY TO KNOW until you try it a bit
           
        • famous bob

          famous bob mopar misfit FABO Gold Member

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          ,,,don't you think that if air could leak in, brake fluid could leak out,, especially at the 700 to 1400 psi at the wheel cylinder - caliper with the brakes engaged,,

          My 68 Barracuda was a 4 wheel drum car, converted to 4 wheel disks, used the 73 and up mopar A body disks / calipers on the front, a Dr Diff disk kit on the back, using a 68 A body distribution block in the brake line system, no proportioning valve, no hold off valve,, stops fine.[/QUOTE]
          ------THIS !! I have a diff. disc set up on all 4 wheels w/ the stock brake line block ! (68 fastback)
           
        • my68barracuda

          my68barracuda Well-Known Member

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          once again, I have the Mopar 73 and up disks / calipers on the front and a Dr Diff disk kit on the back. I spoke with Cass / Dr Diff, he recommended staying with the OE distribution block.
          I used new brake lines and an OE 68 drum brake distribution block from Inline tube. The distribution block is just that, no front to rear bias, it works fine on my car.
          Inline Tube - The Professional Restorer No1 Choice

          the brake lines from Inline tube fit great, the distribution block was an exact copy of the OE unit

          for your specific installation, you should contact the tech group at the company the put together your disk kit, they should have a recommendation.
           
        • my68barracuda

          my68barracuda Well-Known Member

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          "However I also tend to believe they help in keeping air out of the system which could enter at the wheel cylinders or calipers"

          BigBlockMopar made that comment and I disputed that,,
          well,,, I ran that by an old time mechanic that has much more experience than I do on these early brake systems, and he agrees with BigBlockMopar
          said it is due to the design of the wheel cylinder seal, and they would suck air if not under pressure,,he said that in the mid 60's the wheel cylinder seals were changed to eliminate that issue.
           
        • Murray

          Murray FABO Gold Member FABO Gold Member

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          My experience says that disc brakes need no proportioning valve/ hold off valve, drum brakes do.
           
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          • Phily

            Phily 3XL, loud pipes, loud music, loud weed

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            I would use a prop valve. Modern cars even with all around discs still brake harder in the front. That's why you may only do you rear brakes every couple years or longer while your fronts always last about 4x longer.
             
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            • Killer6

              Killer6 Well-Known Member

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              Residual check valves came in master cylinders to provide a small amount of pressure(~10psi) to the wheel cylinders while they were not applied, this kept them from seeping
              and prevented them from taking in air & moisture past the flared cup seals. This would occur esp. when they were heated up good, then parked to cool...the colder the worse.
              At some point, the spring & spreader discs arrangement was added to the wheel cylinders, these contact the inner part of the tapered seals and kept them flared snug so that
              the MC's no longer needed a residual valve. Only the large trucks w/the tandem MC's still contained a RCV for the rear drum brakes.
              Metering valves are provided in a Disc/Drum setup to maintain ~3-30psi to the discs until the gap between the rear shoes is taken up, & the pressure spikes up.The valve, or
              "Hold-Off" per Mopar terminology, set-point..or"Cut-In", is 117psi for std & tk., and 165psi for PoPo/Taxi apps.
              Proportioning valves are there to balance front & rear braking forces by incrementally restricting flow/pressure to the rear as pedal/MC pressure increases, for a given amount
              of frt. psi, the rears are reduced a certain amt., say 1000psi/540-650psi for a 3.2slope, & 1000psi/430-550psi for a 3.7slope.
              4-wheel drums were basically proportioned by the wheel cylinder sizes, large in the frt. & smaller in the rear. Since they have identical operating properties, they functioned
              OK in that manner. 4-wheel discs are also the same, caliper bore/total areas are proportioned the same to provide a correct balance, but both systems work better with a
              proportioning valve because of the dynamics of hard braking. The weight distribution and available traction frt./rear changes dramatically under hard braking, so a system
              has to adapt to provide optimum performance under all conditions. An adjustable proportioning valve is always a good idea, because You can fine-tune Your particular car.
               
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              • jonn6464

                jonn6464 1970 Duster

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                Thanks for the knowledge. I ordered my set today, with the prop valve, but no hold off valve.
                 
              • AJ/FormS

                AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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                I rundisc fronts/ drum rears ,big rear tires and no proportioning at all, AND I upsized my rear wheel cylinders.
                Point is, with the big and littles I run; 235/60-14s, and 295/50-15s, The car stops excellent, in any situation.
                I had trouble with the factory dual-diaphragm when I installed a 292/292/.508 cam. I swapped to a newer Single unit off a 73 Swinger. Problem solved.
                 
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                • jonn6464

                  jonn6464 1970 Duster

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                  Hey AJ! What's up man, been a while...
                  I realize now that my previous post said I ordered the brake line kit with a prop valve, but it's actually just the distribution block. I feel okay with that set up. My tires dont have the size difference yours do though... 245 front, 255 rear.
                  I was wondering though, roughly what the largest cam I could run and still pull enough vacuum for the dual diaphragm booster? I'm not building a big horsepower motor, I was thinking the cam would be somewhere between .475 - .495 lift. I really want the booster to function properly.
                   
                • AJ/FormS

                  AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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                  Good question.
                  I took my Dual Diaphragm apart Cuz I really wanted to run it. But there was nothing wrong with it. I played with the pushrod length, but to no good result. I was running a 292/508 cam, with a pretty good tune in it, but the boost was just a bit out of sync for low-rpm use in a stick car with 3.55s.
                  The larger Swinger booster while not 100%, was huge improvement.
                  A 475/495 cam could have a buncha different durations and LSAs so the lift spec is not gonna help you make this decision.
                  Since I never tried that DD booster with any other cam, I can only speculate as to it's suitability.
                  But I can say this; that 292 barely made 11 inches at 750.If I blipped the throttle a couple of times before I backed out of my carport, it was fine for take-off. Once driving it was pretty good, the manual trans kept the rpm up until right slowing down. But somewhere down at about 7mph, the engine vacuum would become insufficient to maintain assist. So I always had to keep that in mind on long slow slow-downs. At stoplights it was no biggie cuz in's a stick. Some automatic guys have mentioned that it could not maintain assist at stoplights, and the car would begin to creep forwards.
                  And I can say that the factory put DDs on 340s,lol with 268/276/114 cams . But this cam only has 44* of overlap, compared to 76 on that 292/508
                  Soooo, somewhere in between, the happy part goes away. I'm gonna GUESS halfway of course,60* , cuz what else can I do? Of Course a cam with 60* of overlap could have a buncha different specs.,too.
                  But the ever-popular 268/276/110 has just 52*, and this is street-friendly when synced with adequate cylinder pressure.This is especially important with a 318 and manual trans. An automatic with a hi-stall can get away with all kinds of inefficiencies. And a 360 is also more tolerant of low pressure.
                  Can you go bigger? Sure; there's still 8* left in the speculation.
                  But I can't see the point with those 255s. A 360 with a 268* and 160psi/iron is just gonna roast those skinny rollers,probably all the way to 60mph with 3.23s, if you're brave enough to keep your foot in it. But if you're gonna do that often, then you should probably look to some oiling mods to keep your rod-bearings happy.And just as importantly, adequate spring pressure.
                  The 268 is about a 222*@.050. It can make a chitload of low-rpm torque. I had one once. I remember it very fondly.I ran it with Eddies at about 185psi, Daymn! that thing had take-off. And the 106mph in the qtr says it was cooking pretty good.
                  In a 318 at 8.0 compression, with a factory TC and even 3.55s,it will be IMO a huge disappointment. She want's at least 9.7Scr.
                  So bottom line, if you're NOT gonna pump it up, I, me personally, AJ, wouldn't put more than a 360-2bbl cam in it, and it will want a 2800 TC and minimum 3.23s with that 2800. With a 2500, it's gotta have 3.55s. Those are my opinions based on my experiences.
                  When I say a 360-2bbl cam I just mean like-sized in advertised duration. You can put any lift on there that your heads can take and any .050 you can find,the more the better.
                  For example
                  the typical ramps for various types of popularly sized street cams are;
                  Hydraulic roller 50* or more........ hydraulic flat-tappet 46*or a bit less.........solid flat-tappet 40* or less
                  So a 2bbl 360 cam was listed at 252/2xx/112 I don't recall the exhaust.
                  Lets estimate it at 252/260/112.....OK the .050s are 202,206, and 212*....... That doesn't look like much difference, now does it? In fact tho, in the engine, each of these is about one size bigger from the previous, and each size will move the rpm of peak power up some 150 or more rpm, and by the formula,more rpm makes more power. But they all three will make about the same cylinder pressure, which on an 8.0 teener is VERY important. In a teener at 8.0 the pressure will fall to about 130psi with any of these, in at 54* ICA. So each will be equally weak at low rpm; and less powerful than the stock teener cam. That's why you need the hi-stall TC, to get the rpm past that doughy part. Then you need a bit of gear, to let her climb up to where the power is, in a reasonable period of time, in accordance with the rate of acceleration that you may be expecting. I mean if a guy sinks 3 or 4weeks wages into an engine, he sorta expects something in return, right?
                  Anyway,the factory 360, suffers the same problems, but can use less cam for the same expectation.
                  And the Magnums start from a higher Scr, so they are less sensitive to that crap. You can put a little more cam into them,before the bottom falls away.
                  Stick streetcars spend most of their lives below 4000, even below 3500; which being 30 to 35 mph, in first gear with 3.55s. So a stout bottom-end is paramount. You don't want to be either slipping the clutch out all the time, or spinning the tires all the time......right? lol.
                  Anyways way off topic. At your power level, and since you already own it, slam that DD booster in there, and go for a spin. Give it a couple of hours in traffic and see what you think. If you're unhappy,it only takes a few minutes to swap it out.
                   
                  Last edited: Jan 27, 2018
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                  • jonn6464

                    jonn6464 1970 Duster

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                    Well, you're right about the duration playing a big factor into the cam choice. An issue I'm having is what engine to go with, which will also require a cam decision. Here's the only things that are certain about my drive train... I'm running a 4 speed (not OD), with 3.55 sure grip rear end. I say this is certain because I've already bought them. That being said, I'm able to really put some thought into my engine, and I want to make a good decision. My first thought was 408 stroker, but I'm not opposed to a well built 360. I like the extra torque of the 408 but it's not a deal breaker for me. Also, I've seen a few 340 options that I like. Point being, I have not yet decided on an engine, and I want to make sure I put the money into something that will provide 3 things.
                    1)Decent power with "street mannners"
                    2)Enough vacuum to run accessories
                    3)Reliabilty to run well without constant adjustments
                    Therefore, there are a bunch of cam options that can/will work with different engine displacement, I'm just trying to decide which is best for my set up... 4 speed, 3.55 rear diff.
                    I have nothing against a Magnum, but will most likely wind up with an LA block. So, I guess that means flat tappet cam. Also, I can't (in good conscience) put a 2bbl carb in my car. I don't mean to offend anyone, but I just can't. There are, literally, hundreds of combos to run, so I'm just hoping to get close to a good one
                     
                  • AJ/FormS

                    AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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                    Hah the 2bbl cam remark pertained to the teener, and all examples were given in relation to the small cam you cited. Had you given a larger lift number, I would have tried to aim my comments in that direction.
                    I'm glad you have a regular box 4-speed and 3.55s, cuz that is the combo that I ended up with after about 5 years of swapping chit in and out.
                    I have never owned a stroker
                    But I can tell you that I also don't need nor want one. A long time ago I did;1998.
                    I've had several, some might say many 340s .
                    But a 360 is IMO a very capable street engine. And I am a streeter. 100%.
                    I've been to a track thrice in my life since I got my license in 1969. Twice with my 68 Barracuda; for a total of 2 successful runs.
                    It didn't take long to figure out that traction was going to be a major issue. It turns out that 325/50-15s on 10s just fit into the factory tubs, with a bit of easy work. And it turns out on an airport drag-strip, or at least the one I went to, they still spun most of the way to 93 mph.
                    So what I'm saying is; if a 360 can do that, do I really need more?
                    So I gave up trying to be quick right away. I was happy with the trap-speed and called it done. I just went there to get the itty-bitty piece of paper to shut up all the guys who kept ragging on me saying "you should have built a 340." I drove there, got one successful run out of four tries, and drove home. That was 2004.
                    So if you're a streeter, IMO, a stout 360 is what you need. Anything more than that is gravy.
                    And you might as well tub it now, or otherwise get rid of the factory rear suspension. If you don't, some guy with a well-sorted teener is gonna blow you away.
                     
                    Last edited: Jan 27, 2018
                  • AJ/FormS

                    AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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                    One of the problems, I have fought since day one, was the traction issue. I have had 3 cams in mine and always kept the cylinder pressure up between 170 and 190 (aluminum heads). This gives a 360 a preponderance of low speed torque, and I quickly worked my way down thru the rear gears that I had, landing on 4.30s (with the 2.66low trans). this makes a real nice starter gear, and you can really buzz thru the gears. But tirespin was crazy, and so was the the cruising Rs. So enter the GVOD. Then I picked up a 3.09 low gear set,swapped that in, and back in went the 3.55s. Now I finally had some decent gears and splits. I sold that 4.30 chunk right quick. The new combo had a starter equivalent to a 4.10 and the overdrive is equivalent to 2.77, so I've got both ends covered.
                    And so traction continues to be a problem. I took off the 750DP and swapped on a 600vacuum secondary; that sucked. I tried a 750vac-secondary, but once you've had a 750DP, you're kindof spoiled. I decided I'd just learn to not stomp it all the time.Well I was never real good in school, but I did learn to buy tires in bulk.
                    Point of story is; traction will be a problem,
                    And you might as well tub it now, or otherwise get rid of the factory rear suspension. If you don't, some guy with a well-sorted teener is gonna blow you away.lol.

                    Here's my combo, just FWIW
                    367LA , KB 107s,at 10.9 Scr,with Q set to .034,and stock bushed teener rods circa 1969? IDK, the late ones same casting numbers I think as 340s,and new plasma moly rings. Plus OOTB closed-chamber Eddies. The cam is an old Hughes HE3037AL FTH with lifts of .549/.571 with 1.6 roller-tipped-only Mopar adjustable arms.This cam is 230/237/110 and 276/286 advertised. I also used the Hughes anti-pump-ups in at .020 preload,and their 1111 springs,shimmed up til she revved well over 7200 and called it done. I run a Milodon roadrace pan with their tray and pick-up, a high volume pump,some oiling mods, the Airgap, TTIs with twin 3"full-length pipes, and an ancient 750DP with fresh cold air. And a rock-solid cooling system that runs a minimum 205*F
                    >I decked the block for .005 pop-up. Up from about .011 down. With a Magnum block you might not have to do this,IDK.
                    I run the .039 FelPros,now on their third install, those are some good gaskets. (The .028s popped out about the same time I popped that 292/508 outta there).
                    >Oh yeah, This engine has never run anything but 87E10, except the very first time I tracked it. I thought that it might be prudent to put some better gas in it for that day. That was a waste in hindsight,lol.
                    >If the cam ever dies,I'm gonna try a solid 225 to 228/108-106 and bump the Scr back up to 11.2 and no other changes.; I had good results with thosePlasma Molys. I'm itching to get high 20s mpg again.............. Yeah I know, everybody says that.
                    I think I covered it.
                    Done I think
                     
                    Last edited: Jan 28, 2018
                  • jonn6464

                    jonn6464 1970 Duster

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                    Good Lord, you're an encyclopedia of Mopar knowledge. I know you said you've come to this set up after years of trial and error but, man alive, you've got it down to every detail. I do get it though, a guy should know his car, I think it's amazing that you have that kind of attention to detail.
                    So, .549/.571 lift with your duration gives you enough vacuum to run the single D brake booster... good to know. I gotta ask though... 87 gas? Where is your timing set? You feel like you're getting enough detonation from that? I don't, for a minute, doubt what you're saying. I'm just trying to learn what set ups are successful. Thanks again for your time and insight.
                     
                  • jonn6464

                    jonn6464 1970 Duster

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                    P.S. Funny how we got to this point from a "Hold Off Valve" question...
                     
                  • AJ/FormS

                    AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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                    comments in the quote
                    Aluminum heads don't work like iron. At Least that's the opinion I have formed. It goes like this;
                    Some 85% of a streeter's life is spent at 4000rpm or less, about where the combo torque-peaks. And it's rarely at WOT. You only need detonation-protection when the chamber temperature exceeds the anti-detonation capability of the fuel . So why is your heavily throttled engine tooling around at an effective 5 or 6 or 4 to 1 compression ratio? That was my thinking; why am I buying expensive top grade gas, and just burning it up?
                    So back to alloys. Aluminum heads extract a ferocious amount of heat outta the chamber, converting it to heat.......instead of pushing you down the road. So now your heavily throttled 5/6/4 effective compression engine is operating at even less efficiency, and so it's no wonder it sits in the carport all week.
                    So my thinking was, what if, knowing this now, I cranked the compression way up, and promised not to floor it with a flipping huge air valve? Hmmmmmm.
                    Back to alloys. The thing is this; it turns out, that there doesn't seem to be enough time during a typical street blast of 3 to 6 seconds, for the chambers to get too hot for 87E10. There's just no time! I can't say for a road racer. I can say that my engine has survived thousands of blasts to 100/110mph,since 1999, when I built it. Eight to 12 seconds evidently is safe. But I have a rock-solid cooling system which is able to shed all that heat, which shortly comes around.
                    That's my theory. And I'm gonna push more pressure next time, cuz more pressure means more part-throttle efficiency. And more pressure means more power. Now, I don't need more power, so I'd be taking cam out. Perhaps a double whammy of torque increase at PT and lower rpms. Perhaps a smaller rear gear, with no loss in performance. Perhaps near30 MPGs with the GVOD.
                    I'll tell ya; I'm never going back to factory iron heads. Probably never to iron at all. If there was available a lower-priced, lower-octane grade of gas, I would be experimenting with that.
                    I'm not saying alloys are molasses on pancakes. I'm saying I've had a great experience with mine. And I highly recommend experimentation. And I don't hold back on what I've learned; you get it for free from me.
                    Oh one more thing; on the hiway, after the head temp has stabilized, you can, I mean I could, lean thechit out of her, and point-to-point, get phenomenal fuel mileage ,so much so,that a few people have immediately called me a liar. Well they didn't actually say that ; what is usually said is "that's impossible" or "I don't believe that", or some equivalent statement. So I don't quote those results any more. I can tell you this, with a stout 360 there is no reason to not drive a low-12 second car as a DD. Your EFI Honda will get better economy around town, by virtue of it's big-dollar design, and small displacement, but point-to-point, It ain't molasses on pancakes either. Well, compared to a 9.5 iron job maybe,lol.
                    I tell you what, over 100,000 miles of aluminum headed operation, I think I saved enough money just in the fuel savings of 87 versus 93, to enjoy a free freshening up,at least once every 5 years. So I'd be on the third one now. Never mind the thousands of miles further it mighta drove. The cost difference here,in Manitoba,is about 20/25cents a liter; or about 91cents per gallonUS. That's an annual fuel cost savings of from $600 to $900. That's savings. What's that come to in say 5 years,lol?
                    Never mind if the 185psi drives 2 or 4 or 10 percent further on a dollar of gas, than the 155/160 that most iron headed SBMers run.
                    So what's not to like about alloys?
                    Well, they do convert a lot of energy to heat in city traffic. You have to have a great cooling system. And You have to keep the inlet air temps down. This is waay cheaper than running extra gas for cooling.
                    And if your heads are hot, your exhaust will be too.....So it's gonna be harder to get rid off through those wiener 2.25 pipes and Wally-Mart mufflers. Lol. I run a minimum water temp of 205*F. Min-i-mum. This to help capture some of that energy to the crank. And I run twin 3inch pipes right out the back to get those hot expanding gasses outta the chambers, and to help the headers do their thing,especially during overlap.And I try to run extra duration during the power cycle to try to extract a lil more energy into the crank. This runs contra to most guys thinking who will gladly sacrifice extraction for exhaust duration. And that's fine for racing. Terrible for a a broke-azz streeter, trying to keep his fuel tank from being empty all the time.Yeah so, next time it's gonna be a solid FTH to give a lil more time to that consideration.
                    Boy you wanna talk about a cam that sux gas, look at the 292/508/108 Mopar grind. In at 104, the power cycle is just 102*. Compare that to a 268/276/108 solid in at 108, that has 114* Yeah so that doesn't look like much, but it's 114/102= an extra 11.8%..... that's about an extra eighth part . You have 8 fingers; which of those would you call extra.lol.
                    Yeah I know, the 292 is on a nuther level from the 268. the 292 is about 251@.050IIRC. A solid 268 could be a 230@.050...so that's about 3 cam sizes. I'll tell ya, on the street, I'll take the 230 at 185psi anytime over the 251 at 155/160 in iron. I've run both, and 251 didn't make 12 months. Actually,driving, it didn't make 3 months.
                    Anyways, enough rambling.
                     
                    Last edited: Jan 28, 2018
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