Engine vacuum

[rayfinseats]

Sorry to be such a pain in the ass but I have an 83 4x4 k10 with stock (1968) 350, edelbrock 600 carb and ramhorn exhaust. TH350 trans,
np208 and 2.73 gears. Open 14" air cleaner, short exhaust with duals and glasspacks. Weight 4200 LB.
The vacuum gauge I installed reads about 20-25 at idle. In second gear around town the vacuum is pretty high cruising, 20-30.
On the highway it always seems like it's about 10 and it always seems like I have to keep my foot in it to keep it going. Do you think it's the 2.73 gears are causing the low vacuum? (hence poor mileage)

[RUSHNBOBO]

Originally Posted by rayfinseats

Sorry to be such a pain in the ass but I have an 83 4x4 k10 with stock (1968) 350, edelbrock 600 carb and ramhorn exhaust. TH350 trans,
np208 and 2.73 gears. Open 14" air cleaner, short exhaust with duals and glasspacks. Weight 4200 LB.
The vacuum gauge I installed reads about 20-25 at idle. In second gear around town the vacuum is pretty high cruising, 20-30.
On the highway it always seems like it's about 10 and it always seems like I have to keep my foot in it to keep it going. Do you think it's the 2.73 gears are causing the low vacuum? (hence poor mileage)

Whenever the throttle plates are open wider vacuum drops.....10 in HG is pretty low for something that pulls over 20 in HG a idle. but those freeway gears are definitely going to drop it off, because your foot is going to be in it more to maintain cruising speed, especially since your running a 350 w/4200 lbs. With those gears the 350 is actually working harder than if you had 3.23 or 3.73 gears because your rpms are most likely not in the peak torque range.

Example: if your running a big block with a low rpm high torque cam, it will make well over 400ft. lbs. of torque at a low rpm. In this scenario your throttle position will be almost closed at criusing speed and the motor will show high vacuum, and good fuel mileage with the same gears.

Ideally you want your vacuum as high as possible, at cruising speed, this will show the best fuel mileage. So to answer your question yes it's the gears, and the wider throttle position causing the low vacuum reading. If you swap in 3.23 gears your engine will be running more in it's powerband for the best vacuum and fuel mileage.

[rayfinseats]

That's kinda what I was thinking. Thanks. I guess I'm on the right track.

[Pyrotechnic]

What size tires are you running? What is the MPH and RPM when you get the low vacuum?

[rayfinseats]

11x31x15 tires. About 60-65 MPH.

Around town about 40 MPH in second gear I get really high vacuum readings.

[RUSHNBOBO]

Originally Posted by rayfinseats

11x31x15 tires. About 60-65 MPH.

Around town about 40 MPH in second gear I get really high vacuum readings.

Here's a chart I whipped up for your combo scroll down to 2nd gear at 40 mph and you get 1799rpm .....high gear at 65 your at 1923rpm which is probably a little low for optimum vacuum with your truck. What's interesting is the 2nd gear number at 40 mph "1799"...here you say your getting higher vacuum readings. It could be due to your high profile 4x4 at 65mph; the wind drag increases dramatically at 65 vs. 40 mph ....so to compensate you are probably pressing the accelerator down more to keep this speed, and vacuum drops off, even though your in a slightly more desirable rpm range. If your vacuum drops off enough you may also be starting to open the secondaries...which will amplify the problem.

Take note of the throttle position next time you drive in 2nd gear at 40 vs. 3rd at 65, make sure the conditions are the same, such as wind speed and how level the road is. If you have a larger throttle opening in high gear, a numerically higher gear may help. A easy way to see before you drop the bucks in gears is borrow a set of 29 or 30 inch tires and wheels and see if it makes any difference (it has the same effect as a gear swap). It may very well be that with that power to weight ratio and high profile there may be little or no effect. It's like having an overdrive and going up a hill in 4th vs 3rd; in OD most vehicles will get worse mileage than if they kick down to 3rd because the engine is working so much harder (less vacuum).....just making the rpm's lower does not always equate to better mileage.

*******************************
Final Gear Ratio : 2.73
Tire Size : --
Max Power RPM : 300
Limit RPM : 5000
*******************************

*********************
Gear Ratio Compound
*********************
1 2.520 6.880
2 1.520 4.150
3 1.000 2.730

***************************************
Gear MPH/1000 MPH @300 MPH @5000
***************************************
1 13.41 4 67
2 22.23 7 111
3 33.80 10 169

*********************************
MPH RPM (in Gears)
*********************************
1 2 3
*********************************
5 373 225 148
10 746 450 296
15 1118 675 444
20 1491 900 592
25 1864 1124 740
30 2237 1349 888
35 2610 1574 1036
40 2983 1799 1184
45 3355 2024 1332
50 3728 2249 1479
55 4101 2474 1627
60 4474 2699 1775
65 4847 2923 1923
70 3148 2071
75 3373 2219
80 3598 2367
85 3823 2515
90 4048 2663
95 4273 2811
100 4498 2959
105 4723 3107
110 4947 3255
115 3403
120 3551
125 3699
130 3847
135 3995
140 4143
145 4291
150 4438
155 4586
160 4734
165 4882
170 5030
*********************************

Gear Change RPM drop (change @300) RPM drop (change @5000)
*************************************************************
1->2 -119 (To 181) -1984 (To 3016)
2->3 -103 (To 197) -1711 (To 3289)
*************************************************************