I see star came out with a better vacuum pump. Now wondering on my NA engine what would be lowest vacuum, and lose hp. Or what would be too high ?
I don’t know, but 25” of vac seems to be the top standard, and in my opinion you start to lose a little for every inch of vac under that. But I ain’t actually sure. I just know we ran better with 20” then with 15” but it ain’t a 50 hp deal. On a SBC with no pump, we we’re down between 25 and 27 hp when compared with a star pump at 20”I see star came out with a better vacuum pump. Now wondering on my NA engine what would be lowest vacuum, and lose hp. Or what would be too high ?
Here has been my rule of thumb based off building several various engines. Pulling 0 to 12 inches of vacuum is worth a lot. 12 to 16 inches is another good amount. 16 to 18 is probably a couple more. Once you start getting in the 20 range, I feel it becomes tit-for-tat. Ya, there is still more power in vacuum but the resistance it creates on the oil pump is basically a draw after 20 inches.I see star came out with a better vacuum pump. Now wondering on my NA engine what would be lowest vacuum, and lose hp. Or what would be too high ?
Woudlnt it be a draw on the under side of the pistons on the upstroke too.Once you start getting in the 20 range, I feel it becomes tit-for-tat. Ya, there is still more power in vacuum but the resistance it creates on the oil pump is basically a draw after 20 inches.
An argument could be made for diminished returns when using an inefficient vac pump. Driving any component off the crank is like your paycheck, there is a gross and a net gain (consider the Star electric vac pump that provides all the vacuum benefits with zero parasitic loss). The most efficient mechanical vac pump will require a slower pulley ratio to obtain the desired results, equating to more net. And we all want more net!Other pistons in the crankcase are descending, unless you have paired cylinders sealed off very well from other pairs. I think Nick is saying is that the HP it takes to drive the pump for 20+ inches offsets any gains achieved. Approaching or crossing over the point of diminishing returns.
Assuming no regulator present or a regulator that is maxed out, you would typically increase the speed of the pump to increase vacuum. However, all mechanical vacuum pumps have a RPM "ceiling". Once you have reached the limit of the pump, increasing the pump speed will not increase vacuum. In this case you could say the vac pump (not the engine) has reached the point of no return. If you look at any of the vacuum pump flow graphs you will see that flow results are not infinite. If you have reached this point, and are sure the engine is sealed adequately to achieve the vacuum level you want, you probably need a pump capable of more CFM.Slower pulley ratio not faster ? Or you mean speed up the vac pump?
The sweet spot would be a 50% pulley ratio for 3700 pump RPM. For more vacuum you could go to the next larger drive pulley increasing the ratio to 56 or 58% depending on the current size of the pump pulley. Increasing the drive pulley by two teeth would only increase the belt length by .375" so you could likely utilize your current belt. I would not recommend a ratio beyond 58% on an engine turning 7400Star I run a star large pump. What rpm would be max on the pump? Engine runs 7400 rpm.
The short answer to your first question is "it varies". Typically an external oil pump has far more capability than an internal one. Yet generally, even an internal oil pump can tolerate 8-10 Hg without affecting the gauge reading. We have Comp Eliminator customers running as much as 20Hg with an external oil pump with no affect on the oil pressure gauge although in theory it should show about .5 PSI less for every 1Hg. Several factors are involved here. The most obvious being the particular oil pump involved and the type and temperature of the oil. Personally, I think the main reason for such wide variations between engines is due to internal clearances. We all know that increased bearing clearances will result in less oil pressure. An engine with clearances right on the edge of being excessive would be more susceptible to oil pressure changes when applying vacuum. Even .0001" here and there could add up to a sizable difference. The following starts to address your second question because in the scheme of things they are both related as one thing leads to another.When running a wet sump external oil pump using a vacuum pump how much is the pressure gauge affected if your pulling 10” of vacuum?
Secondly does the suction affect the output gpm the oil pump puts out?
This was an argument I had been having with a friend.
The parasitic loss would be more governed by the amount of vacuum load the pump is under more so than the RPM.Very good information. I will look at my pulley to see about my ratio. I believe i am close to max. Ratio. If you were at max ratio roughly what hp would that be dragging out of the engine. If it’s noticeable I would consider electric.
Lots of variables for sure. We refer to gains/results from vacuum as a jig saw puzzle: many little reasons that result in gains but when added all together you get the bigger picture. Low tension rings are the largest piece of the puzzle for sure. Consider how much ring area is contacting the bore on a typical V-8. It's especially true these days with the trend toward larger CID that have bigger bores. You could expect very little gains with standard tension rings unless the vacuum was masking some other issue such as a serious windage problem that may be present because reduced windage is the second largest piece of the puzzle. You would be amazed at what windage looks like at 400 fps in a crankcase under positive pressure when compared to the windage on that same engine with 20Hg. Not only is your rotating assembly ploughing through less windage the oil becomes less aerated and stays cooler resulting in better lubrication.If standard tension rings are used how much power gain can be seen with a Vac pump ?
I’m sure there would be 100 variables in that question.