grumpyvette Posted June 18, 2004 Share Posted June 18, 2004 this will be a learning exercise for the newer guys, below are two dyno sheets from another site thanks to new2novas it compares the SAME ENGINE with TWO DIFFERANT CARBS,A 600cfm and a 750 cfm, notice that the 600cfm makes more power so if you just look at the power the 600 cfm carb would seem to be the better choice .......RIGHT??? WRONG!!!! heres the 600 heres the 750 this will be a learning exercise for the newer guys, below are two dyno sheets from another site thanks to new2novas it compares the SAME ENGINE with TWO DIFFERANT CARBS,A 600cfm and a 750 cfm, notice that the 600cfm makes more power so if you just look at the power the 600 cfm carb would seem to be the better choice .......RIGHT??? WRONG!!!! heres the 600 heres the 750 yes I looked at the dyno sheets and the 600 carb has a slight advantage AS THEY ARE NOW TUNED, after BOTH CARBS are correctly adjusted I think youll find the 600 carb makes for a better street combo, is more responsive and gets slightly better mileage, BUT the 750 carb will make MORE PEAK HORSEPOWER, and its easier to fine tune correctly, myself Id go with the 750 cfm why? look at the dyno sheet closely......... the engine was running BOTH richer and cooler with the current 750 cfm carb adjusted that way and that, loss of heat and rich mix cost you power!! the BSFC should stay close to about .45 your oil temp should have been 215F minimum and if you had both carbs running the same a/f ratio the 750 SHOULD have a slight advantage you were running about 10 degrees hotter and significantly closer to the ideal a/f ratio with the 600 than with the 750 BTW you should be running about 12.8:1 a/f ratio for max power and about 14.7:1 for low emissions and mileage at 9:1- 10:1 like the 750 runs now,and the oil temp under 215f your giving away a bunch of potential hp/tq BTW you should be running about 12.8:1 a/f ratio for max power and about 14.7:1 for low emissions and mileage at 9:1- 10:1 like the 750 runs now your giving away a bunch of potential hp/tq Quote Link to comment Share on other sites More sharing options...
Kevin Shasteen Posted June 29, 2004 Share Posted June 29, 2004 I whole-heartedly agree about learning from Dyno runs. Another tool to look at is BMEP on every dyno run for each rpm, TQ, and HP. BMEP is the indicated cylinder pressure. BMEP stands for Brake Mean Effective Pressure. Take for instance the two examples Grumpy listed, where the only alteration was swapping a 600 cfm carb for a 750 cfm carb. BMEP for the 600 was: 164 BMEP for Peak Tq at 4100 rpm 139 BMEP for Peak HP at 6000 rpm BMEP for the 750 was: 165 BMEP for Peak Tq at 4200 rpm 136 BMEP for Peak HP at 5900 rpm The 165 BMEP would put this engine in the category of the high end range on a Mild Street engine; to the low end range on an H/O Street Engine. Learning to recognize patterns is the first step into understanding the engine. Airflow Velocity and Cylinder Pressures at a specific rpm is what separates one performance engine from the next. We all simply need to understand how the Compression Ratios and Port Volumes effect the Airflow Velocity and Cylinder Pressures with the spec's of a given Cam Profile. You can do this by learning how to read a dyno run. Unfotunately the high performance magazines rarely give the full data when they list their dyno runs....so this makes it a little difficult to fully interpret what you are reading. Kevin, (Yea,Still an Inliner) Quote Link to comment Share on other sites More sharing options...
pparaska Posted June 30, 2004 Share Posted June 30, 2004 Thanks for that lesson, Grumpyvette! Also, note that VE% is 2+% better over a wide rpm range with the 750. That means if it were run under the same conditions and tune as the 600, there'd be around 2% more power too. Well, I believe that's true... The air going in is also higher (A1 and BSAC) with the 750, meaning that the 600 is choking the engine under WOT. But like you said, it'd be more streetable at part throttle with the 600. Wow, 9-10:1 A/F with the 750 - that's pig rich!. The differences in oil and water temps are really significant. Quote Link to comment Share on other sites More sharing options...
Guest Marshall Posted June 30, 2004 Share Posted June 30, 2004 Confusion like that is why I like fuel injection. It's so much easier to control, and it's consistent. Program it to give the engine an optimum mixture, and it'll do it everytime. With controllability and efficiency like that, it's easy to see why 100% of new cars have switched to fuel injection. Still cool to see the art of tuning old school devices. Quote Link to comment Share on other sites More sharing options...
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