Technical HB Blower boost gauge hookup

Believe me, when you stab it, you ain't got time to look for a boost gauge!

 

I've run a street blower on several SBC's for about 30 years. I do run a boost gauge and I like it, but I've found that the higher pressure isn't everything.

I've seen more pressure (with the same blower set-up) on small cubic inch, small valve iron heads, because of less overall swept cylinder volume and restriction through the ports.

Then I switched to big inch and big valve Brodix heads (with the same blower set-up) and made more power on 2-3 lbs. less boost. It's all about flow not necessarily pressure!

Cams work the same way. Smaller duration cam will make more boost pressure than a larger duration cam with the same blower set-up, because of flow. Increase the flow, and boost pressure goes down.

Plus, higher boost pressure increases intake charge heat (which increases chance of detonation) and on small port, small valve iron heads, port velocity gets out of control easy, causing increased chances of more detonation. Not a good thing at the expense of flow.

 

Indeed. It's the pressure you actually get in the cylinder that is useful, boost pressure in the intake can partly be a measure of the air that DOESN'T get into the cylinders because of restricted flow.

Supercharging an engine that doesn't flow well is just a band aid, the better solution would be to improve the flow first - but sure, for the engines we talk about here the look and cool factor may be a higher priority than actual performance.

 

G-Son:
You are close. But your statement needed one correction.

"Indeed. It's the DENSITY you actually get in the cylinder that is useful, boost pressure in the intake can partly be a measure of the air that DOESN'T get into the cylinders because of restricted flow."

It is all about how many oxygen atoms you can stuff into the cylinder. Density is a function of Pressure and Temperature. Roots type blowers are notoriously inefficient and do a better job of heating the air up then increasing it's density. As the temp increases the density drops. This is where the intercooler comes in. In fact, a good intercooler setup can often add more horsepower by increasing the air density than what the supercharger creates on its own.

 

Yes, you are right!
It's the age old adage of the air pump... air/fuel molecules in = power out... and it takes a pressure difference to make it flow.

 

lot of good this does for me. the f.e.d driver(my son) doesn't read gauges. shifts cross handed and by the seat of the pants. hey...it saves money not buying gauges.

 

The shift light was invented to let you see that you have reached the right rpm without looking at a gauge. Not sure when they were first used, they may or may not be traditional.

 

I am sure they are not traditional in a HAMB perspective. But that when did that stop anyone?
I have one in a convertible I do a little 1/8th mile stuff in. The digital, rev limiting, amber and red LED shift tube was not something you want to see when walking past a classic ragtop. I sectioned a sun tach, glued a piece of PVC pipe in the center that slips right over the shift light. You can see the back of the tube sticking out in this angle but other than that it is invisible...when the ignition is off.

 

Do you think some of us might want a boost gauge because it's cool, fun to look at, &might impress our friends. I agree an air/fuel ratio gauge is extremely valuable for tuning a modified engine, blown or not. Greg

 

I do have vacuum gauges in 2 of my cars.... One is factory installed....

 

I am sure there are plenty of guys out there that do that sort of thing. I am a Test Engineer by trade, I don't put a gauge / sensor / transducer onto an engine or anything else unless I expect to get valuable and useful data that will help me improve the performance of said engine. I forget that other people don't think in the same screwed up ways that I do.

 

Hi, Had the 23 T Bucket for about 6 months, I always like to know everything about all my vehicles. The bucket is pretty radical and has some history behind it. Started out in Washington State in 1985, then went to California in 2014 where all the engine work was done, then up to Texas. The engine block is a 1967 corvette 327/365 w/ HB 671 blower, E Street Edelbrock USA aluminum heads, high lift cam, MSD ignition, 2 Edelbrock 600 cfm carbs, Sanderson headers, GM 700R 4spd auto and 411 rear end. The bucket weighs in at 1800 lbs. the previous owner guesstimates at around 600 hp. It’s definitely a neck snapper


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I see.

 

^^^ said the blind carpenter as he picked up his hammer and saw.........
Sorry, couldn't resist....

 

1985 327/365 Corvette motor?

I don't think so.

 

If you want the boost gauge and be period correct I would look into a aircraft manifold pressure gauge that was used in a supercharged powerplant.
They are calibrated in inches and are very accurate. It would be a cool setup.

 

I missed the specs at first.
I bowled a 365 last night. WTF

 

Questions for all you Blower guys....this is my first full blown 327 Bucket 671 HB blower

1) Regarding boost, at idle (850 rpm) should I see any boost at idle ?
2) Should I see boost come up by blipping the engine ?
3) Do you see boost when the engine is not under load
4) I understand that the blower is generating boost all the time, but how much
Thanks
WildBill





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Thanks I rephrased that post it’s actually a 1967 block, bucket was built in 85


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The blower is generating boost all the time, in that the pressure below is higher than the pressure above it...but until you open the throttle a ways, that "boost" is still a vacuum, relative to the atmosphere. So you won't see boost on the gauge, until you actually get the engine under load and the throttle open a ways.

 

Thanks


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1) Not on the gauge.
2) No, you will only see a vacuum drop on the vacuum/boost gauge until you put a load to it.
3) No. Just a manifold pressure difference between the carb and the manifold at part throttle.
4) I had roughly about 5" of manifold pressure difference.

When I had a 144 blower on a 383 in the bus (capable of 4-5# boost and 20,000# load), I used a vacuum gauge at the carb and a vacuum/boost gauge in the manifold. It always showed about 5" manifold pressure difference between the carb and the manifold gauges.

That's 5" more vacuum at the carb than in the manifold. As I opened the throttle under load, the vacuum/boost gauge would go to zero right about the time the secondaries started to open. Then it would start to show boost on the gauge.

That would probably be much different with more boost capability of a 671 on a smaller cu. in. engine, in a much lighter vehicle. My motor was a high compression, low rpm, torque monster, capable of just under 600 lbs of torque at about 3000 rpms.

Remember, the blower is also a vacuum pump at part throttle when seen at the carb. The vacuum brakes worked much better than normally aspirated, and it also messed with the vac. adv. at the distributor, because the carb had 5" more vacuum at all times. It also gave about 1/2 mpg increase in gas mileage because of better fuel emulsification. Win-Win situation!

 

thats correct there Squirrel.................

 

If you can think back to 8th grade science class, remember that pressure is resistance to flow. The blower is a fixed displacement pump dependent on RPM for flow. The faster it is turning, the more flow it outputs. The resistance to that flow is the pressure you see on the gauge.

But, there is no need to get all sciencey about it. If you know that your head gaskets will turn to vapor at 15psi, the boost gauge is a nice way of keeping them from doing that. Don't over think it and you will be OK.

-Abone.

 

Thanks guys, I called HB blowers, they confirmed to remove a small Allen screw and connect boost gauge to this port. Gauge works great. Got another question for you blower guys. I have a 36 tooth pulley on Top and 33 tooth pulley on bottom giving me 6 pounds of boost. I believe this is considered under driven. If I turn the pulleys around with 33 Tooth on Top and 36 on bottom, giving me over driven any ideas what boost I should get with this ratio....Bill



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Maybe 11 psi?

Give it a try, see what happens

 

Thanks


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