Does DD2000 calculate VE correctly...
Thread Starter
Instructor
Joined: Mar 2002
Posts: 181
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From: Allen TX
Does DD2000 calculate VE correctly...
Guys,
I am trying to size up a carb for a potential buildup. Is the volumetric effiency calculated by DD2000 accurate, or should I just size for 100% efficiency at max rpm?
Thanks,
John
I am trying to size up a carb for a potential buildup. Is the volumetric effiency calculated by DD2000 accurate, or should I just size for 100% efficiency at max rpm?
Thanks,
John
Le Mans Master
Joined: Feb 1999
Posts: 5,877
Likes: 1
From: Park Ridge IL
Re: Does DD2000 calculate VE correctly... (John W)
Peak volumetric efficiency occurs at the peak torque rpm point, and declines gradually as rpms increase beyond that.
DD2000 is fairly accurate at predicting VE, at least based on the last 2 engines that I've built comparing DD predictions to actual Superflow dyno results.
DD2000 is fairly accurate at predicting VE, at least based on the last 2 engines that I've built comparing DD predictions to actual Superflow dyno results.
Drifting
Joined: Dec 1999
Posts: 1,462
Likes: 1
From: Reno nevada
Re: Does DD2000 calculate VE correctly... (Monty)
I like the formula someone posted on the forum.
1.6 cfm per hp. For example 400 hp X 1.6 is nearly 650 cfm. In real world driving that is the correct carb size. Drag cars that are only concered with WOT worry about pressure drop. So they use larger carbs.
1.6 cfm per hp. For example 400 hp X 1.6 is nearly 650 cfm. In real world driving that is the correct carb size. Drag cars that are only concered with WOT worry about pressure drop. So they use larger carbs.
Race Director
Joined: May 2000
Posts: 12,712
Likes: 2,264
Re: Does DD2000 calculate VE correctly... (Lt1er)
Four-barrel carbs are flow rated at 1.5" Hg. depression and you want the carb to deliver peak air flow at somewhere between 1.0 and 1.5" Hg. Bigger than this and you have a change of having low speed metering problems and poor throttle response at low revs.
As a rule of thumb for a street high performance SBC, VE will drop to 80 percent somewhere in the range of the power peak to ten percent over peak power revs, so if you compute 100 percent VE at the rev limit, that size carb should put you in the 1.0 to 1.5" Hg depression range at maximum air flow.
For a 6500 rev 327 the calculation goes:
(327/1728) x 6500/2 = 615, so a 600 CFM carb is the way to go.
At 80 percent VE the actual flow will be 492 CFM, and since flow is proportional to the square root of depression (or depression is proportional to the square of flow rate) the actual depression across the carb at peak flow will be:
1.5 (492/600)**2 = 1.01"
Using the formula from the prior post for 375 gross HP, 1.6 x 375 = 600 CFM.
Duke
As a rule of thumb for a street high performance SBC, VE will drop to 80 percent somewhere in the range of the power peak to ten percent over peak power revs, so if you compute 100 percent VE at the rev limit, that size carb should put you in the 1.0 to 1.5" Hg depression range at maximum air flow.
For a 6500 rev 327 the calculation goes:
(327/1728) x 6500/2 = 615, so a 600 CFM carb is the way to go.
At 80 percent VE the actual flow will be 492 CFM, and since flow is proportional to the square root of depression (or depression is proportional to the square of flow rate) the actual depression across the carb at peak flow will be:
1.5 (492/600)**2 = 1.01"
Using the formula from the prior post for 375 gross HP, 1.6 x 375 = 600 CFM.
Duke
