Originally Posted by oldchevy1956
My problem is that my engine easily dies when in gear and stumbles / misses from time to time at cruising speed. They two issues may be related but I am not sure why. I am not new at engines and tuning as I have done this for several years but I am somewhat at a loss for my current problem. I have a newly built 383 with slightly worked over heads with a roller and somewhat aggressive cam with a 2800 rpm stall converter in a 700r4 trans. All is fresh with this engine and I am running headers with dual exhaust, a Mallory dual point distributor and Edlebrock 600cfm carb. My dwell is set at 31. My engine vacuum is low (9) at idle (due to the cam I am sure) and 15 at 2500 rpm. I have set the base timing (with distributor vacuum unplugged) at between 5 and 7 at 750rpm and the mechanical advance will move it to 25 at 3000 rpm and then the vacuum advance will take it to a total of 35 at 3000 rpm. I have been running full vacuum off the carb to the distributor. It has good and loopy idle at around 750 rpm but with low vacuum at 9. When I put in gear the engine rpm drops slightly and then I loose engine vacuum (goes to 5) which in turn causes the distributor vacuum to drop below what will advance the engine (takes about 15sec to loose distributor vacuum) and then the idle becomes very rough and slow (advance is back to 5) and then will die within about 10 secs. To fix this I have to have the rpms up real high around 1500 (to keep vacuum up to 12) and now we have both vacuum and some mechanical advance on and when I put in gear I am now running to much advance causing around 1350 rpm which is way to fast. There seems to be no middle ground between idle and in gear in terms of advance and rpm. Too low of rpm I get no advance when in gear and it dies and to keep the advance from losing vacuum I am running to much rpm for idle and in gear. I also mentioned that the engine seems to miss from time to time at cruising speed and I don’t have a good explanation for this. I do need to add that the engine at WOT will run crazy fast with no hesitation, engine detonation or spark knock. So if I could just run at WOT all the time and not have to stop at stop lights and such I would be fine!! Any ideas how to deal with my idle – advance issue, as well as, my cruising speed issue???
Having the specs of the cam and compression ratio would be good data. But from what I can glean; 5 inches of idle vacuum sure says there's a pretty big stick in there or a large vacuum leak.
For a big cam your timing is too little too late, you need a lot more idle advance with the mechanical also coming in much sooner than 3000 RPM. Why the 600 cfm carb and a dual point ignition the carb is way small for a 383 and point ignition is technologically obsolete, it just can't solve the problems at hand adequately. This is like trying to build a fire in your gas barbeque by rubbing two sticks together.
The timing probably needs to be more like a base of 12 to 15 degrees at idle, be aware that this will greatly change the idle characteristics of the engine requiring a redo of the carb settings. Depending on the cam, compression, head chamber shape, and piston crown shape the total advance will need to be somewhere in the range of 34 to 40 degrees and all in by 2000 RPM. What's going on is all related to the molecular density of the mixture, that is the number (thus weight) of fuel and air (oxygen) molecules in a given space. The higher the density the faster the burn proceeds, at idle and into the middish RPM range where there is little throttle opening the mixture density is low therefore the burn speed is low thus more advance is required to get the burn to complete at the point of piston stroke where it can apply the greatest pressure onto the crankshaft. If the burn is too slow the pressure peak is too late and the engine feels gutless while consuming lots of gas doing it. A second problem in this zone is miss and late fires; it is harder to set a low density mixture on fire on top of its slow burn problem. An electronic multi-strike, high energy ignition is of immense assistance in getting the burn started in this situation.
The reason your engine is doing well on the top end is because you've got all the right stuff happening up there, you need to bring this stuff down to a lower RPM range. So more base advance, this may require that you not only speed up the centrifugal but that you may have to subtract some total out to accommodate how much can be added to the base and still stay under the detonation limit, For instance if the engine will tolerate 36 degrees and 15 are in the base setting then the centrifugal will have to not put more than 21 degrees in. This is managed by obstructing the amount of travel the centrifugal cam can move usually by inserting a screw, bushing, or welding of the advance slots in the cam plate to shorten the travel. Obviously this is work easier done on a SUN distributor machine but you can hack a solution without one, but it's some cut and try repetition. Given that the engine vacuum of a hot cam features very little at idle and a moderate amount in the mid RPM range only to go away at WOT, it is well to set the engine up at least initially with no vacuum advance because if it (vacuum) peaks at 15 inches at 2500 RPM and the centrifugal is in by 2000 RPM the engine will be over-advance at this RPM and may detonate. You can use vacuum advance, but dialing it in will be a way advanced procedure that will require changing the centrifugal and perhaps the base setting so rather than introduce this large tuning complication, it's just easier to delete it for now.
I mentioned combustion chamber and piston crown shape as well as compression also having an effect on ignition timing and also mixture ratio.
Large open combustion chambers are lazy fuel and air mixers, a lot of mayhem occurs in an intake manifold to separate fuel from the air, this is the reason why everybody developing heads is so keen on wet flow characteristics of the chamber as this stuff has to be quickly remixed once it gets past the intake valve. This is where the Ricardo chamber which is named Vortec at GM and shows on the L31 engine starting in 1996 has become the chamber of choice for all OEMs and the modern aftermarket. This is why the NASCAR Cup engines run very tight chambers (40-50 ccs). These chambers do marvelous things in remixing the fuel and air while providing a high volumetric filling. For a 383 this is still a small bore like a 350 and should use a tight chamber. A 400 with its bigger bore can go with small or large chambers with a matching thought to the piston shape, but the 383 stroker works better with a 64 cc dual quench Vortec style chambered head. The problem this gets to is compression can quickly becomes too much, the solution is the D dish crowned piston. The D dish offers the best of flat top performance with the compression control using a small chamber head by placing the dish under the valve pocket instead of all around the diameter. What's going on is the relationship of swirl that remixes the fuel and air, squish/quench, and maximum mixture molecular density being presented to the spark plug.
Swirl occurs throughout the open intake valve cycle. It spins the incoming mixture streams of fuel and air performing a remix of them. The more thoroughly mixed fuel and air is and the smaller the fuel droplets are the faster and more completely the mixture burns. To show you how effective this is simply look at modern EFI engines where the fuel is puddled on the backside of the intake valve compared to all the supposed mixing of fuel and air going on at a carburetor. Squish/Quench occurs in about the same space, at almost the same time. To get this the piston's flat surface needs to come very close to the flat surface of the head we're talking .035 to .060 inch on a street engine (keep in mind short of the piston hitting the head less is more) for race engines often this is less than .035 inch as winning becomes a greater concern over the possibility of hitting the head with the piston which ain't pretty, but engineers and mechanics are in race motors a lot where your street motor is expected to go tens of thousands of trouble free miles before some peeks under the head, so we give 'em more clearance at this spot.
Squish happens as the piston closes on the compression and ignition event. The tight clearance on the far side of the chamber forces the mixture toward the spark plug. This action tears apart any remaining fuel globules and violently mixes the fuel droplets with the air, then forces this mix in front of the spark plug which increases the molecular density sitting before the plug so it is easier to set it on fire and it burns quickly once ignited, thus, these type of modem heads are often called Fast Burn heads. This reduces the need for so much spark advance and puts the burn occurring where maximum pressure occurs when the rod angle can apply maximum force on the crank. Excess advance causes the pressure peak to happen too early, it actually tries to drive the piston backward, while the early pressure peak causes detonation.
Quench is often called "mechanical octane" it is the second function of the squish/quench steps of the chamber and piston the difference of function occurs in time where squish is before the fire is lit and quench is after. As the flame front proceeds across the chamber the temperature and pressure goes way up, this can be to a point where the mixture ahead of the flame front spontaneously explodes. The closeness of the piston to the head at the squish/quench steps provides an area of very little volume with lots of surface area. So it acts as a heat sink to the mixture ahead of the flame front keeping its temperature under the point of spontaneous ignition.
Compression ratio is associated with this in that higher compression ratio increases molecular density which increases the burn speed. Radical cams require a lot of compression because the late closing of the intake valve allows the rising piston to reverse the intake flow and pump it back into the manifold at idle and in the lower RPM ranges. This happens until the engine is turning fast enough that the mixture speed within the intake becomes sufficient to overcome the reverse pumping from the piston movement. This is why big cams "come on" in the high RPM ranges. But the reverse pumping causes a loss of molecular density of the trapped mixture at lower RPMs; to overcome the loss of power because of this this requires more ignition advance and more compression. There are two compression calculations that need to be done especially with a long and high lifting cam these are the Static Compression Ratio (SCR) and the Dynamic Compression Ratio (DCR). The static (SCR) is simply the volumes of the piston movement (swept volume) plus all the volumes above the piston including the crown shape divided by the volumes above the piston including it shape. The DCR calculation reduces the apparent stroke in this equation to the position along the bore when the intake valve closes in crankshaft degrees. There is a good calculator over at the tech section of Keith Black pistons United Engine & Machine Co. Incorporated
The DCR needs to hit about from 8 through 9 to 1 depending upon the fuel octane you wish to burn and the material of the head, aluminum will tolerate more compression than iron before the octane level of the fuel is exceeded. The DCR calculating always drives you back to re-computing the SCR.
Considerations about the combustion chamber also involves spark plug location, many open chambers push the plug way off to one side, this increases the distance the flame front has to cross making the chamber detonation/preignition prone while requiring excessive ignition timing to get the burn done in time for the exhaust valve opening event. So another feature of Ricardo/Vortec/Fast-Burn chambers is that the spark plug is moved as close to the center of the chamber that the valve position will allow. This is an advantage of hemi and pent chambers as the plug can be located on center. So with a 4 inch bore engine the resulting burn is 2 inches in any direction from the plug to the cylinder wall instead of 4 inches as seen with 1970 era wedge chamber smog heads.
Anyway this is a lot to digest so I'll put an end to it for now and go back to my work.