Wtf is wrong with this combo?

Discussion in 'Classic Mustang Specific Tech' started by 68stang351, Aug 28, 2011.


  1. 68stang351

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    I kind of don't really want to post this, but I'm up for some suggestions. The new dyno day thread on another forum I'm on got me thinking about my results from the last one. This is a mild setup but I thought it would make more than it did.

    My 68 Coupe:
    69 351w
    TFS TW 170 heads, 1.6 RR
    Comp XE284 cam 284/296 adv, 242/246 @.050, with .541/.543 lift
    Performer RPM intake with Holley 780 carb.
    T5 with 3.25 gears. Makes no power past 5500.

    Last time it put out 288 rwhp, 342 rwtq. I was told on the last dyno day that the vacuum secondaries were only opening a little more than halfway, and I have put a lighter secondary spring in since then. One of the guys was watching the linkage for the secondaries, and showed me how far it was opening. It runs noticeably better now, but I still feel like it should run better.

    It does have low compression pistons (not sure how many cc's the dish is, but they are huge dishes, I'm thinking mid 8.?:1), so I know that hurts it.

    I don't know, just looking for advice. I have most of a wet kit. Maybe I should spray the **** out of it? I know it needs gears too, lol!


    I'm about to try something different for my 351w fox, some 11:1 pistons, Roush 200 heads, bigger cam, see how that works. I may end up with a similar combo for my 68 depending on how it works. I would like to maximize what I have for now in this car. Should I expect much more the 300/350 to the wheels? It runs okay in my sub 3000 pound car, but I still can't hang with a lot of cars from way out here in the sticks
     
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  2. 65ShelbyClone

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    Huh, I would have expected a 302 with the same parts to put down the same, if not more than 288rwhp. The heads and intake are fairly conservative , but it sounds like maybe the carb is still not completely sorted out(and a bit large). That's partly why I like mechanical secondaries with stick trans cars. :D

    What rpm does the power/torque peak at?
     
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  3. 2+2GT

    2+2GT Well-Known Member

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    Have you ever had the distributor advance curves professionally set?
     
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  4. rbohm

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    first off the heads are a bit small for the displacement and the cam you are using. you need an intake runner more along the line of 185-200cc, the 170cc runner is choking down the engine.

    second, you need to calculate the actual compression ratio, there are a number of good compression calculators on the net. for good power you should be around 9-9.5:1. if you have the stock dish pistons and the 64cc chambers, you are going to be about 7.9-8.2:1 and that is just too low for good power production.

    third, what fuel ratios are you running? holley jets their carbs to be a 5-10% rich to give you room to tune the fuel curve.

    fourth, as asked, have you had the distributor curved? and what initial timing are you running? your initial timing should be 10 degrees, with a total timing of 36-38 degrees, and it should be all in by 2500-3000 rpm. by the way, total timing only takes mechanical advance into account.

    with proper heads, and proper tuning, you combination should be making about 350-375rwhp.
     
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  5. htwheelz67

    htwheelz67 Member

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    The heads are fine, TW heads flow about the same as an AFR 185 dont let the numbers fool you as the intake is closer to the chamber.

    I would agree with checking the advance curve and also fuel pressure.
     
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  6. Couped Up

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    (1) What size are the chambers on your heads?

    (2) How much Vacuum do you have at the intake?

    (3) What flywheel did you use for your conversion?

    :shrug:
     
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  7. rebel65

    rebel65 Member

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    I think you're biggest problem is just low compression.

    you're running on 87, make it so you can only run on premium

    A super charger will fix that ; )
     
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  8. PoppyMod

    PoppyMod Member

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    +1 for increasing your CR. I would shoot for minimum of 10.0. As well as, what was mentioned, regarding the AFR and dizzy curve. Don't throw money at it, fine tune what you have.
    Good luck!
     
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  9. Hack

    Hack Active Member

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    I disagree with the above post that mechanical secondaries are preferred over vacuum. The great thing about vacuum secondaries is they open the correct amount to supply what the engine needs. If you think the carb is restricting the engine you can check vacuum at the intake during a dyno pull. If the carb is a significant restriction the vacuum reading will tell you.

    Just because the secondaries are only opening a certain amount doesn't tell you anything. You want to know A/F ratio and vacuum across the carb. Probably it makes sense to have an expert tune the car on the dyno. Adjustments should be based on measurements, not guesses. You aren't going to get better power just blindly changing parts. Figure out what is going on now through measurements and adjust to get the results you want.

    I would also want to know CR as the others posted above, but it's not necessarily the problem. With the right quench height and cam you should be able to run well over 10:1 on pump premium. Your problem might also be partially that the hydraulic lifter cam just might not pull much over 5500. Usually hydraulic lifters start having a little trouble around there and don't make a lot of additional power at higher RPMs.

    You want to figure out dynamic compression to determine what CR is safe to run on pump gas with your cam of choice.
     
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  10. PoppyMod

    PoppyMod Member

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    +1 Hack. Some good basic advice. Especially, when it comes to your AFR an CR comments.
     
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  11. bikefreak600

    bikefreak600 Member

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    way too low compression, the low rpm might be combo of dual plane intake and small headers ? ( you never mention what headers your running) ?
    hyd lifters are good to 67-6800 as long as there is not too much spring pressure.
    check your compression and have it tuned on the dyno

    the carb is just fine. actually spinning at 5500 its a little big. general rule to measure carb cfm size, take c.i. (351) x rpm (5500) / 3456 = 558 cfm . on your motor a 780 cfm carb is sufficient to 7500 +

    what headers/manifolds do you have?
     
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  12. 65ShelbyClone

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    I didn't say mechanicals were preferred over vacuum; I said I prefer mechanical over vacuum. ;)
     
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  13. Hack

    Hack Active Member

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    Gotcha! :) Yeah mechanical secondaries can be great if you really know what you're doing. I'm not much of a carb guy. I think Holley continues to decrease the number of applications they recommend mechanical secondaries for - possibly due to customer feedback.

    You have to have a really light, race only, manual trans, big cubes and 4.11+ geared car before Holley will recommend mechanical secondaries, IIRC. The resulting performance isn't necessarily going to be better either - unless the tuning is really spot on.

    Edit: Remember mechanical secondaries are opened by the throttle linkage, so the carb secondaries will open as much as the linkage is set for at full throttle. This will overcarb the engine at low rpm or undercarb the engine at high rpm, since the amount of air and fuel that the engine needs changes based on engine rpm. There's no way to tune a mech secondary carb to work correctly at a range of engine rpm. It can only be tuned for one particular engine rpm. That's the good thing about vacuum secondaries, the carb actually opens as much as the engine needs, rather than being incorrect most of the time.
     
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  14. Slobin3d

    Slobin3d Active Member

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    That is good to know, how would you adjust this for altitude?
     
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  15. Westmus

    Westmus Member

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    According to CompCams spec that cam should pull to 6500 rpm. Something goes wrong above 5500, afr or maybe floating valves? Do the springs match to the cam?

    I would also think a cam that size need a minimum of 9,5-10:1 in static compression to work correct.

    Trickflow says those heads make max power with 34-36 degrees. The tw170 are good heads, switching to Roush 200 would be big a step down. In some old article dyno compairing a lot of heads on a 331" with a XE274 roller cam, the Roush 200 made about 50hp lesser than the tw170! :)
     
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  16. 65ShelbyClone

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    You don't because the equation doesn't address air mass. The engine is treated as a positive displacement pump so the cylinder swept volume remains constant regardless of altitude. The only thing that changes then is the density.
     
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  17. bikefreak600

    bikefreak600 Member

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    +1 on the heads, DO NOT get roush heads ! that cam needs more compression, a carb spacer would help ($25)

    what exhaust are you using? your motor could be choked
     
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