Mix HT/HS

[GrampsintheSand]

Us old guys are having a "debate", then nap time of course.
So I want a tad thicker HT/HS of around 3.6-3.7
I have cases of Shaeffers 10-30 and a couple of cases of Shaeffers 20-50. Both of the Series 7000 variety.
The 10-30 has a HTHS of 3.16
The 20-50 has a HTHS of 5.31
I have a 8 quart sump. So why couldn't I just use 6 quarts of the 10-30 and two quarts of the 20-50 and get 3.7 HTHS?
I arrive at this by 3.16+3.16+3.16+3.16+3.16+3.16+5.31+5.31=29.58/8
=3.69 does it not?

 

[GrampsintheSand]

I left some off. The debate is the higher HTHS from the 20-50 is more diluted and it is not linear. They are saying I would end up with than less than the 3.7 the math tells me I would get.

 

[hate2work]

Sounds right to me.

Keep in mind that you would then have an oil with a 12.5W-35 weight.

What are you going to use this in?

 

[GrampsintheSand]

600+ horsepower Iron Block stroker motor.

 

[JAG]

Originally Posted By: GrampsintheSand
I arrive at this by 3.16+3.16+3.16+3.16+3.16+3.16+5.31+5.31=29.58/8
=3.69 does it not?
That is a weighted average. When different viscosities mix, the viscosity of the mix is more biased toward the thinner oil than the weighted average predicts. I'd assume that the mixing laws for HTHS viscosity are the same as they are for kinematic viscosity. So, find one of the online mixing calculators and put the HTHS viscosities in and find out what percentage of each is needed to reach your desired HTHS viscosity.

Using this: Calc , it predicts 70%/30% of the thin/thick oils gives 3.7 cP viscosity.

That's close to the 75%/25% that the weighted avg. calculation predicted. As the difference between the viscosities of the two oils increases, the weighted average method gets less accurate. here, it did pretty well.

 

[[RT] ProjUltraZ]

maybe just go 50:50 and get a 40 wt oil

 

[FrankN4]

Using the 75%/25% on the Rhomax calculator comes up with a HTHS of ABOUT 3.52 and a 100C cSt of ABOUT 12.4. I did not know the 100C cSt of your two oils so I plugged in a mid weight 30 and a mid weight 50.