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Old 06-24-2008, 12:03 PM
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Rebuilt 350, detonation problem ??

Hi,

I buy camaro 1986 305 TPI original

The motor is a 355 with flat top piston and 416 head with 1.94 and 1.60 valve with no port job, just the valve is change

I don't now the compression, but with the TPI kit and normal gaz , the motor can ping ???, or the computer can resolve the problem


thank you

Éric

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Old 06-24-2008, 12:20 PM
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with 58cc heads and flat top pistons you could see compression ratios in the 10.5-11.5 range depending on the volume of the valve reliefs, head gaskets, and deck height.

I don't know if the computer will be able to compensate that much but I know you should be using premium with that range.
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Old 06-24-2008, 12:32 PM
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I have a 10:1 compression 350 in my truck, runs on 87 octane. It has a stock
throttle body injection with knock sensor and ignition retard. On the hottest
days when fully loaded the computer cannot compensate enough to
eliminate auto-ignition.
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Old 06-24-2008, 02:20 PM
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[QUOTE=Parisienne70]Hi,

I buy camaro 1986 305 TPI original

The motor is a 355 with flat top piston and 416 head with 1.94 and 1.60 valve with no port job, just the valve is change

I don't now the compression, but with the TPI kit and normal gaz , the motor can ping ???, or the computer can resolve the problem


thank you

ɛ/QUOTE]

Computers don't know what they don't know and have no real means of learning.

To start with, if your TPI is a MAF system it is a bit more flexible than a MAP system. MAF is identifiable by a wire screen sensor ahead of the throttle body. There is a heated wire whose resistance changes proportional to the change in its temperature caused by the amount of air passing it at any given moment; this system also monitors throttle position and RPM. MAP systems measure throttle position, RPM, and manifold vacuum computing air flow from those conditions. This is a simplification of what's going on but is sufficient to explain changes that upset or go beyond the system's abilities.

The MAF system is considered to be the more flexible of the two in responding to changes, however, either system's ability to respond to changes is limited to the programing on the chip. The factory program is largely built around the expected operating range of the vehicle. The program doesn't have the ability to feed the engine forever. The program operates with a list of commands for injector switched on time and for ignition timing. The sensors are read several times a second for the conditions they see. The results are run thru an integration program to arrive at a Control Number. The Control Number is compared to a table of conditions, when a match is found, the computer switches the injectors on for the specified time period it found. The computer does a similar function with the ignition timing, using the Control Number to select an ignition timing value.

Where you're pushing the envelop is using a 305 system on a 350. The computer has no way of knowing it now has to feed 15 percent more engine. It has no way of knowing the heads have changed and the compression is now extremely high. It has no way of knowing about any other changes. It only knows what its been told, which was it's looking at a 305 with a mild cam and moderate compression. If a MAF system it can resolve some of these issues, for a MAP system it'll try to do what it can to get by.

In a mechanical sense you're also constrained by the 19 pounds of fuel per hour flow of the 305 injectors where-as 350s came with 22 pound/hr injectors which is 15% greater flow capacity, where did we see that number before, imagine that. The smaller injector will have to be on longer which is called "duty cycle". The more on time it's exposed to the hotter it gets and the less life the solenoid will have. This really isn't too much of a problem unless you have a heavy foot.

Now a new chip with a new program needs to be built for the computer that has a program which recognizes these changes. It needs to be built around the injectors being used. If you stay with the 19 pound injectors the computer needs to know that so it will hold them on longer to get the needed fuel flow. If you switch to 22 pound injectors it needs to know that so it will not hold the injector on too long running the engine rich.

Games that can be played:

The knock sensor will read detonation and will cut back the advance automatically. While saving the engine from destruction, the result is running a retarded spark which isn't good for obtaining the best power or fuel economy. It defeats the reasons for high compression. But when programming a new chip the compression can be accounted for by programming the mixture a little richer than needed so the extra fuel cools and slows the burn rate. The program can also be trimmed such that the engine can be optimized around a cooler operating temperature. Both of these will reduce the tendency to detonate with less of a demerit to power and economy than just retarding the timing.

Mechanically, detonation can also be reduced by running an oil cooler. As the crank spins oil is tossed off some landing on the bottom of the pistons where it absorbs heat before dropping back to the pan. An oil cooler will reduce the temperature of the oil inside the engine with the effect, among others, of reducing piston crown temps. In fact, before emissions became a prime concern, every OEM engine builder provided an indexed oil stream from the big end of the rod to the underside of the piston. Many of these were a groove that passed from a notch in the rod bearing where the cap and shank join. It passed to either side of the rod bolt to exit as small notch at the outside of the cap to shank interface. Others were a small hole drilled in the top of the big end aimed at the underside of the piston with a small hole thru the bearing to provide the oil feed. Ford's 429 Super Cobra Jet went so far as to provide a hole all the way thru the length of the rod to provide pressure lube to the pin which also sprayed the underside of the piston. This is a technique often found in big WW II era aircraft engines and certainly today in heavy equipment diesels. It is an effective means of reducing piston crown temps, but comes at the cost of a little increased oil consumption.

Certainly getting intake air temperature down as low as practical is also effective in reducing detonation. Sourcing air from behind the grill rather than under the hood will do much for detonation reduction and increased power thru it's greater density than that of hot under hood air. For power this is good close to a 1% increase for every 10 degree temp reduction at the throttle body.

Bogie
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Old 06-24-2008, 09:22 PM
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Thank you for your help


Éric
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