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Old 09-28-2009, 10:58 PM
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Detonation or pinging now with 700r

Thought this was going to be sweet, but now very very very upset. 350 vortec heads, small cam (isky 262/270) 3:73 gears 31" rear tire. I installed an OD tranny over the weekend and it seems to ping at 60mph on the hwy. One other change I did was change distributors to a MSD street fire. Something needs to be adjusted inside the vaccum module. Have not done that yet.

Anyway, my biggest problem is how do I get rid of detination to enjoy the added expense of the OD tranny I just added. Also added a 1800-2200 stall with this set-up. I have always run 91.. so this is not it.

Also, I have never had a distributor where you adjust the advance through the module. Is this common and DOES it need to be adjusted. I have not touched it yet.. and instructions say to use a 3/32 allen key to adjust. First start by turning clockwise (in) then out to adjust. Has chart.

I am seeming not to be able to enjoy the OD feature because of the pinging. Even when I try and pick up speed.. it gets worse.

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Old 09-29-2009, 12:08 AM
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The adjustable vac advance is there for that very reason. You must adjust, test drive, adjust until the pinging is gone. when you added the 700R4 you lowered the cruise RPM even more due to it`s overdrive, which means vacuum is higher due to the lower RPM. Less throttle blade opening: more vacuum. I had never fooled with one either until the last engine I built for a friend, the kit we got for the HEI included it and I`m grateful it did. Since his engine has 10:1 compression it pinged like crazy, I took the time to adjust the vacuum advance and the result was outstanding power with no pinging. adjustable vacuum advances are common on aftermarket distributors.
There are several reasons why they make them adjustable, but I would think the biggest reason would be higher compression ratio`s, plus it being adjustable makes it where you can tailor in the amount of needed advance or set it where the engine runs it`s best.
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Old 09-29-2009, 02:51 AM
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You may also have 1 or more cylinders leaner than the others.

if you are looking for big economy gains with the th700r4, you are going to be disapointed overall. Especially compared to the amoun t spent to make the change.
With 31" tall tires and just 3.73's, your cruising rpm was not high to start with.
The cam in your motor is too big to see the gains you expect by running the engine at such a low cruing rpm in OD.
You need to adjust the rate of vacuum advance (diaphram spring tension) and also the vac adv limit by physicaly limiting the advance rod travel.
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Old 09-29-2009, 11:08 AM
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I kind of concur with answer #1 more so in terms of what's wrong with my problem. I think I need to adjust the vacc. on the distributor. Fbird88, I thought my cam was rather small. The rpms were not way up there but they were excessive or more than I wanted my motor to run. I was told a 3:73 is a good minimum gear for OD? that was one reason whay i went that route. Now I'm hearing no benefit or not much?

Do you agree with anything doublevision had to say.
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Old 09-29-2009, 11:15 AM
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Quote:
Originally Posted by DoubleVision
The adjustable vac advance is there for that very reason. You must adjust, test drive, adjust until the pinging is gone. when you added the 700R4 you lowered the cruise RPM even more due to it`s overdrive, which means vacuum is higher due to the lower RPM. Less throttle blade opening: more vacuum. I had never fooled with one either until the last engine I built for a friend, the kit we got for the HEI included it and I`m grateful it did. Since his engine has 10:1 compression it pinged like crazy, I took the time to adjust the vacuum advance and the result was outstanding power with no pinging. adjustable vacuum advances are common on aftermarket distributors.
There are several reasons why they make them adjustable, but I would think the biggest reason would be higher compression ratio`s, plus it being adjustable makes it where you can tailor in the amount of needed advance or set it where the engine runs it`s best.

So I need to ADD more vacuum by turning it counterclockwise. I'm not sure where it is now, but instructions say turn ALL THE WAY IN FIRST, then out. I hope this is what's wrong.
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Old 09-29-2009, 12:27 PM
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You`ll find you need less advance, not more. Too much is why it pings now. What F Bird says has some merit. 31 inch tires are pretty tall, so we need to know what the cruise RPM is at 55 mph in OD. The taller the tire, the higher it makes the gear. a 31 inch tire turns a 3.73 into a 3.08 as a example while a smaller tire does the opposite. What F Bird is saying is that you could have gotten the cruise RPM too low, if it`s below the power band of the cam it`s going to ping and detonate no matter where you set the timing because it`s placing the engine in a lug. The power band of your cam doesn`t come in until 2000 RPM, if your cruise RPM is lower than this at 55 mph then you`ll need less cam, lower gears or smaller tires. There are other ways you can try and compensate for it, like zero lashing the valves, or installing a smaller carb or a carb with smaller primaries like a Quadrajet. Or even possibly advancing the cam 4 degrees but overall, if the cruise RPM is below 2000 RPM at 55 mph then you`ll have to lower the power band to get the detonation out.
Have you tried it on the interstate at 70 MPH? if the ping is there at this speed then it`s over advanced no doubt, but that doesn`t mean it`ll come out at lower speeds as discribed.
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Old 09-29-2009, 12:39 PM
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Have a quadrajet and as far as the timing advance goes, I was talking about the advance adjustment inside the distributor.. not initial as I believe you are indicating here. I was with you on the first post.. lost me on this one. Now, maybe I need more cam. The cam I have is pretty small. I thought.
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Old 09-29-2009, 01:24 PM
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You need less cam, more raises the power band higher. I don`t think you understand entirely what timing is. At idle, you need vacuum advance, it should be connected to a full time, manifold vacuum port or a port that pulls vacuum even at idle. Next with the vac advance disconnected and plugged, set the timing at 12 degrees before top dead center, lock the distributor and reconnect the vacuum advance, check the spark timing, it should be around 24 degrees before top dead center, as a ball park estimate I`m thinking you`ll need around 18. At cruise speed, the mechanical and vacuum advance will work together. at cruise speed mixtures are leaner and leaner mixtures burn slower than do richer ones so more timing advance is needed. As the throttle blades open more the vacuum drops off giving it less vacuum advance so the mechanical advance has taken over entirely. This is why vac advance is not important at mid to wide open throttle. You can adjust the vac advance with the engine at idle, you`ll notice as you lower it, the idle will drop. This is why you`ll need to adjust, test drive and adjust. This is so you can get it where the engine likes it. I can`t give you a base line on vac advance because every engine is different. With everything set, check the spark timing, For example if it has 24 total degress, disconnect the vac advance, adjust it down, reconnect and check the timing again. Bring it down 2 degrees at a time until the pinging stops.
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Old 09-29-2009, 02:06 PM
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your cam is small, its definitely not the problem- unless its too small. If you're runing flat tops it may be too small. The problem is you're overloading the engine, high cylinder pressure and low RPM do not mix. GOing up in cam size would actaully help you mroe than going down. Or just retarding it 4 degrees (cheap fix). Another temporary fix would be installing a small 2bbl carb. Pulling some timing out will get you there too. you need to lower the cylinder pressure or reduce the timing advance. But each has its limitations.
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Old 09-29-2009, 02:46 PM
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The whole routine for setting the distributor up w/the correct timing curve for the engine needs to be done- not just poke and hope w/the allen wrench to set the vacuum advance.

The initial needs to be set along w/the total. When that's dialed in, add in the vacuum advance- as much as the engine will stand w/o pinging.

Once the curve's dialed in, you can tell whether or not the cam is gonna be an issue. IMO, the cam should not be an issue- at least not one that can't be tuned for.
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Old 09-29-2009, 02:49 PM
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I don't agree with any of this:

At 60mph with 3.73's, 31" tires and a th700r4 in OD the engine is reving 1700 rpm.

1. The cam is too big to see a fuel mileage benefit when operated at such a low rpm while crusing in OD.
The OD combined with 31" tires and 3.73's is causing the engine to run below its sweet spot rpm.
If you want to see a fuel mileage benefit for this combo in OD you need a cam with around 190-195 @.050" duration, cause the cruising rpm (1700rpm) is so low.

With the cam you have etc the engine will be much happier running in "D".
Will not be lugging. Will not ping, will tolerate a healthy bit of vacuum advance
(optimized spark timing) and will in all likelyhood deliver better fuel mileage than it will in OD @60mph.

If you want to run such agressive vac adv timing while lugging the motor at low rpm, in OD, you would need functional EGR and perfect even AFR ratio in all 8 cylinders to avoid detonation.
You don't have EGR..so....

Get a timing light and a vacuum gauge and see how much vac adv timing is created while cruising.
Likely will not tolerate more than 46deg. Most likely a lot lot less.
Check for uneven cyliner to cylinder spark plug colouring. Nail down which of the 8 cylinders are pinging. Which ones are lean(er).
Check what ever vacuum line is connected to the manifold vac port on top of #8 cylinder. If there is a leak there, that cylinder will be too lean and will ping.
PCV does not go there. If the brake booster is connected there and its leaking vac, that cylinder will run lean and knock.

Do not allow the engine to ping. It can/will break a piston sooner rather than later.

Don't under stand why people want to "cam up" a motor, then slow the motor rpm down. More cam (larger than stock) wants more rpm, (even at cruise) not less.
More cam timing shifts the most efficient engine operation rpm higher, not lower.

Last edited by F-BIRD'88; 09-29-2009 at 03:16 PM.
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Old 09-29-2009, 02:58 PM
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its not that the cam needs to be bigger to match the gearing, but the compression. his cylinder pressure is too high for that low RPM load. a bigger cam will run worse but it won't ping. not shifting in 4th would probably be the best fix, and one that i think we all would agree on, but since he now has incredibley low gearing and big tires, and too high of cylinder pressure, pulling out timing and retarding the cam 4 seem to me to be the cheapest way out. this engine/gearing combo would only work in a very light and aerodynamic car, like a vega or a Opel GT.
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Old 09-29-2009, 03:14 PM
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Quote:
Originally Posted by ap72
its not that the cam needs to be bigger to match the gearing, but the compression. his cylinder pressure is too high for that low RPM load. a bigger cam will run worse but it won't ping. not shifting in 4th would probably be the best fix, and one that i think we all would agree on, but since he now has incredibley low gearing and big tires, and too high of cylinder pressure, pulling out timing and retarding the cam 4 seem to me to be the cheapest way out. this engine/gearing combo would only work in a very light and aerodynamic car, like a vega or a Opel GT.
The cheapest way is to run the engine at its most efficent rpm, and optimize the spark timing and carb jetting for best mileage at that cruise rpm wher ehte motor is happy. ( this motor with this cam is not happy running at 1700 rpm. Leave the cam alone. Run the thing in "D" or re-intall the th350 trans. You didn't/ and don't need overdrive at all. It will not save you money.
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Old 09-29-2009, 03:25 PM
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Or give it a set of =/> 4.88's. First will be useless unless you were pulling w/it. But you may be able to coax 1 or 2 MPG more out of it. But even if, it will take forever to see it pay for itself.
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Old 09-29-2009, 05:45 PM
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Quote:
Originally Posted by ap72
your cam is small, its definitely not the problem- unless its too small. If you're runing flat tops it may be too small. The problem is you're overloading the engine, high cylinder pressure and low RPM do not mix. GOing up in cam size would actaully help you mroe than going down. Or just retarding it 4 degrees (cheap fix). Another temporary fix would be installing a small 2bbl carb. Pulling some timing out will get you there too. you need to lower the cylinder pressure or reduce the timing advance. But each has its limitations.
i agree with you ap, on the cam. basically it seems to me that all the OP has to do is adjust the vacuum on his distributor. i have 1 from crane in my car and untill i backed out the vacuum advance, it pinged. it took about 2-2 1/2 turns out to rectify the ping, besides going up 3 jets sizes. but i also run a holley 600cfm. a bit different than a quadrajet. back out the vacuum screw, and see how it does. if this is the stock carb that was on the engine before the performance adders (cam or anything else that was done), i guarantee its going to be lean now. bigger cam, gasket matching or porting, higher compression, headers or anything that is going to add more power is going to require more fuel. simple math, if it flows more air, its going to take more fuel. id check the a/f ratio if possible, and correct , then back the vacuum screw in the modulator out till the ping is gone. worked for me.
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