The limitation of many dumbbell (DB) isolation exercises is the fact that the resistance profile of the exercises does not match up to the muscle’s strength curve. This is especially the case in flye patterns.
At the bottom of a DB flye your arm is a long way from the body. This long lever means a large degree of torque and muscular tension is required to move the DBs. With every degree that you travel upwards through the lift, the lever arm is getting shorter and the exercise is getting easier. At the top of the lift, little (if any) tension is being placed through the pecs. So, at the bottom, the weight feels heavier and at the top, it feels lighter.
The strength curve of the pecs during a flye movement does not match the resistance profile provided by the DB flye. The pecs strongest in the mid-range and weaker at the two extremes (top and bottom) during a flye. Using a DB for pec flyes means it’s incredibly hard at the bottom of the lift. Throughout the rest of the range, however, you get very little stimulus.
One answer I have seen people use is to only work in the bottom third of the range. While this will train the pecs hard in this range it is not ideal for maximal development of the chest. A better solution is to call your training partner into action.
Ask him to put his hand on the inside of your arms as you do the exercise. As you lift get him to place some manual resistance against you as you go through the rep. His resistance should kick-in at about the 1/3 mark and increase gradually all the way up to the top of the lift.
Utilizing this technique means you get a full range challenge on every rep. This is far more effective than doing regular DB flyes. It is also much more efficient. In just one exercise you can fully stimulate the pecs in a flye pattern.
The alternative is supplementing regular DB flyes with cable flyes or the pec deck to fully challenge your chest. Now you can do it in just one exercise and you don’t need any fancy equipment.