Open header crankcase evac design

[vette427sbc]

Im designing an exhaust for my corv...(ahem) boat, and would like some design advice

Its a 302 SBC, with high flow water cooled manifolds to dual 3" pipes out the back. The tips are not under water. Id like to have a crankcase evac system through the exhaust to pull a vacuum.
I cant seem to find any data on tube entry angle, depth or angle of the cut for maximum efficiency.

It seems like the kits to do this are ~45 degree entry angle, and a 45 degree cut to make the hole perpendicular to exhaust flow. The tube seems to fall in the center of the tube cross-section as well.
Would a steeper entry angle (more parallel to the exhaust flow) and some kind of small bell flare create a better low pressure area at the end of my tube?

 

[BBShark]

Just guessing here but, it seems like you would want to increase exhaust flow velocity (to increase vacuum) in the area that the end of the tube is located. However, that means a more restrictive exhaust.
There is probably information on pitot tubes for aircraft that might be useful.

 

[phantomjock]

Crankcase evac is a neat method. I was interested in it until I found out the the SCCA and other racing associations don't permit it. There was a 'vette in Nevada that ran that system on the Silver State (?) road race a few years back. With luck maybe you could find him?

Best of luck.

Cheers - Jim

 

[rtj]

You might try searching on static pressure probe misalignment.

It looks like guys are saying open headers in the collector are best, as you can’t get back pressure from the exhaust.

So the trade-off is collector diameter, vacuum desired, and exhaust back pressure. I think collector dia is pretty much set by header tube dia. You could add a Venturi to speed the flow to drop static pressure, but the engine might not like that.

We built all different kinds static pressure probes back when I did that, things like a 90deg tube with a flat plate on the end to a 90deg capped miniature tube with tiny holes around the circumference for static pressure oscillations.

My guess is people use what they use/sell now as the gains from optimizing geometry are minimal.

Looks like there are lots of forum discussions.

 

[vette427sbc]

Just guessing here but, it seems like you would want to increase exhaust flow velocity (to increase vacuum) in the area that the end of the tube is located. However, that means a more restrictive exhaust.
There is probably information on pitot tubes for aircraft that might be useful.

I thought this was going to be my answer, but it seems like its just for pressure/velocity reading, couldn't find much on low pressure/vacuum,etc... Interesting reading though!

 

[vette427sbc]

I think youre right, RTJ... I might be trying to reinvent the wheel here. tube in at a 45, angle cut at a 45, call it a day...
and because everyone loves pictures, here’s the motor in all of its prismatic metal flake and polished aluminum goodness (so far!)

 

[phantomjock]

Very Nice. Cerakote - or? What will the engine bay paint be? This is a real attention grabber.

Cheers - Jim

 

[rtj]

If you search hard enough, someone has probably written a scientific paper on the 45 deg tubes.

But, at the end of the day, if it will do what you want, it seems like it would be easy to fab.

 

[vette427sbc]

Very Nice. Cerakote - or? What will the engine bay paint be? This is a real attention grabber.

Cheers - Jim

No, just a high temp base coat/clear coat... The deck will be this same color (the inside of the hull is not nice enough to do much other than basic white for now) . The outside of the hull will be black but with that same "Caribbean blue" flake with Porsche miami blue/neon pink numbers and name. It should look right at home in a Miami marina circa 1988.

 

[vette427sbc]

Ok so now I’m curious after reading some vague forum Posts… I’d like to build a test rig to see what actually does work best. One post said 55 degree cut with a 30 degree tube entry, some are showing whistle cuts too… who knows what works in one application might not work in another. So I need someone to confirm my math here:
The exhaust is completely separate for each bank. If I have 302 cubic inches, split that in two so 151 Cubic inches which would take two revolutions to spit that volume of air out of the exhaust.
So the lower end of my rpm range is ~4000 rpm. Multiply 151 cubic inches by 2000 rpm to get 302,000 cubic inches per minute which is about 175CFM. Correct?

ive got to look at my compressor or maybe a leaf blower to see if I can get that level of airflow (or more) for testing.

 

[rtj]

The cylinder pressure will always be higher than atmospheric, but for a ballpark number it is probably okay.

Just scale your test rig to the best compressor you have to get reasonable velocity.

A cheap and easy pressure measurement device is a utube manometer if you are on a budget.

Some leaf blowers now have multiple stages I believe. Pretty sure mine does.

You could modify a car with headers for your test for actual conditions.

Edit: on the exhaust stroke I’d expect higher than atm, obviously on intake stroke not the case.

 

[phantomjock]

This guy has a 3rd Gen, and he has a straight pipe setup and shows a "test" of the Moroso setup.

Would your leaf blower get to about 350 fps? That would be about the low end of the exhaust - I think. Not sure about the CFM, but you've run those numbers.

I do like the idea of using the leaf blower as the "exhaust" with a manometer/magnahelic gauge to check the pressure (vacuum). EBay has som digital ones for about 50 bucks or less I recall.
You could "sacrifice a section of the leaf blower - or purpose-build a test section of pvc (with JB Weld) and trial several angles.
At least the speed for the tests would be relatively similar.
Then some tubing for a full scale test on the engine?

I'll be watching for your "findings." Sounds like a cool idea.

Cheers - Jim