boost question
 

I hooked up a vacuum gauge to my motor this weekend on the dyo. The program for boost on the dyno was not right. When we started to make some boost the gauge held at one then would flutter back and forth between 2 and 3 lbs of boost. Why would this happen? Is the belt maybe slipping when it starts to make boost? Or is the vacuum gauge to touchy for preasure? Also what size should the pulleys be for the crank, and the blower pulley. I want to make 5 to 7lbs of boost.
Mark

Re: boost question
 

Are we supposed to be Psychic? Maybe you should call Miss Cleo.

1) You hooked a vacuum gauge to your motor?
Do you mean a combo vacuum/pressure gauge?
Exactly where did you plumb it to your motor, and are you sure the hose connections are tight and secure and that the hose is not leaking somewhere? The gauge should be taking its reading from the intake manifold under the blower/carb/intercooler/whatever.. Do not take this reading from anywhere on the carb. Do not take this reading from a pressurized carb box. Take this reading from the intake plenum itself. On the flipside, if you are using a carb box, then you must reference your fuel pressure regulator to the box pressure, not the plenum pressure. Got it?

2) you mention the gauge would jump from 1 to 2 or 3 pounds (positive boost pressure, I assume) intermittently. You also mention that it was on a dyno when this occurred. Did the power output jump around in the same manner? Seems that the motor wouldn't be able to hold a steady rpm if this was occurring. Was the pull smooth or did it jump around to match the pressure spikes you were witnessing? Was the throttle wide open while this was occurring (it should have been) or was it being opened and closed for some reason (which will always affect manifold pressure)?

3) Is the belt slipping? Maybe. Maybe not. Some answers to the above questions will help determine that.

4) Is the vacuum gauge too touchy for pressure?
I don't understand what you are asking here. Question makes no sense to me.

5) What size should the pulleys be on the blower and crank - I want to make 5 to 7 pounds of boost?

Look at my hands. The blower pulley should be this big. Look at my hands again. The crank pulley should be this big. Understood?

You haven't told us ANYTHING about what motor you are running: displacement, heads, cam, compression ratio, intake, carb, exhaust style, blower style and size. All of these affect the ability of your motor to pump air. And that affects the "restriction" the motor presents to the blower. And the "restriction" presented affects the manifold pressure that a blower will make.

And you want to make between 5 to 7 pounds of boost?
Which is it?

5 pounds will USUALLY be a safe number on most setups and will allow use of pump gas when tuned properly for the application. 7 pounds is ONLY safe on pump gas when all parameters are matched to the setup.

Since your dyno operator/engine tuner does not seem to possess the ability to diagnose a slipping blower belt, I have extreme doubts about his ability to match your motor's state of tune to your boost pressure.

Please find a knowledgeable guy to help you set up your blower motor. Money spent at THIS stage (matching parts to the setup and proper tuning) will prove to be far cheaper than money spent at the NEXT stage (picking parts up out of the bilge and having the boat towed in from who knows where)...

mc

Re: boost question
 

MC Your Killing me. :D

Re: boost question
 

Cmon Billy Thought you were Psychic. :D Ofcourse Hes right.

I thought You and Tommy had Ms. Cleo tied up in the back :D

Its Monday AM Cant ya tell.
Look at my hands its this big. :D

Re: boost question
 

She already SAW that she was gonna escape. That's why she didn't give you any trouble when you tied her up.

She also knew how big it was. That's why she left.

And she knows how big mine is.
That's why she's ringing my doorbell. (Hang on, Cleo, I'll be there in a minute - but wait, you already know that don't you..)

See you later. I've got to go let Cleo in. I'll have the blower pulley answers to you in a little while.

mc

Re: boost question
 

#1 I have a 525sc all of the components of an sc, would like to know the pulley size.

#3 yes the gauge was hooked up in the right spot. could have been a fitting problem.

#4 no the motors hp did not jump around, ran great.

#5 I stated 2 to 3 lbs of boost, I would say that is positive, if it would have been negative I would have called that vacuum.

#6 sutphen, The dyno was at dts dyno. these where just some questions I came up with at night, not at the dyno room.

#7The belt is a sepentine belt.

I realize I did not give out a ton of info, it was late and I was in a hurry. Then guy running the dyno seemed to know what he was doing, like I stated earlier, these where just some questions I came up with at night.

Mc I was just asking a few questions, Sorry if my questions were not up to your standards,

Re: boost question
 

Dayum MC! Are we getting a little grumpy in our old age?!? :eek: :D :D :D

Re: boost question
 

Wildchild 212: Don't be offended, sometimes us grumpy old bastards have heard the same question too many times. MC is full of good info, and he has a PhD in supercharging. If it was a DTS equipped dyno, post the dyno sheet with boost pressure as one of the columns. We'll have you edumacated in no time.

Re: boost question
 

also that is a roots type blower you may have been seeing so blower surge we see it on the dyno when we run these type of blowers

Re: boost question
 

Pulley ratio on a stock 525SC is 2:1 overdriven. That makes 5.5lbs of boost.

Re: boost question
 

Understood. That makes it much simpler. The 525sc is a "known quantity". Good motor. Stock pulleys should give you right around 5 pounds of boost. If your motor is "stock" and makes less than this, you either have a blower in need of freshening OR you have a slipping belt OR you have a manifold leak.

Are you using a "damped" gauge or at least a liquid filled gauge? Cheapo gauges can not only be inaccurate, but can do everything you describe. Have you checked the gauge against a known calibration, and have you checked it for smooth movement? Internal burrs on the little brass gear rack can cause stick-slip and jumpiness.

This would seem to indicate that your boost pressure was NOT fluctuating as the gauge seemed to show. Erratic boost pressure will show up as erratic output.

Well you sort of called the gauge a vacuum gauge in the first part of your post. Then you said it was showing 2 to 3 pounds of boost. That doesn't exactly jive as I have an Auto Meter vacuum gauge in my middle drawer right now and it don't show no boost. That's why I asked if you were running a combo gauge (I actually like combo gauges). Apparently you are.

Thank goodness. Those guys should be able to provide some good info.

Those can slip, for sure. But if your pulleys are clean and straight, and your belt tensioner is not binding up somewhere, you shouldn't be experiencing much slip.

No need to be sorry. If you were only allowed one post, then you could be sorry, but you get to keep posting all you want.
And there was nothing wrong with your questions, you simply asked questions with no supporting info or basis from which to answer. If I just wanted to rain on your world, I would have done so. But since I want to try to help answer your (good but poorly supported) questions, I told you what info you had omitted. I also threw in some daggers cause I am probably not as tolerant as I could be.

Grumpy? I prefer to call it well-seasoned. Possibly even stinky.

mc

Re: boost question
 

well seasoned,,,, that's great!

Re: boost question
 

[QUOTE=Wildchild212]#1 I have a 525sc all of the components of an sc, would like to know the pulley size.
QUOTE]

I did a search and came up with 5.5" as stock? Those that were looking for more boost were running 2.5"-3.0", but recommended an intercooler.

Bruce

Re: boost question
 

5.5 lbs of boost is stock.

Top pulley is 3.5" and Bottom pulley is 7.0" stock on 525sc

A 3.0" top pulley will give about 7 lbs of boost.

Re: boost question
 

That is great news. The pullies we tested with where a 3.5 on the blower and a 4.75 on the crank. I now have the 7.0 on the crank but cannot afford more dyno time.

Griff, Do you know off hand what the belt size is? Obviously my belt is to small for the 7.0 pulley.

Mark.

Re: boost question
 

They should give you a free pull for the mix-up on the pulley size :D

Re: boost question
 

My dyno info is on a floppy disk and my computers floppy drive does not work. :mad: However here are the specs of the motor from top to bottom.

stock 525sc flame arrestor. 1050 dominator carb running 93 jets square . a 177 weiand blower w? a 3.5 pulley on top and a 7.0 pulley on crank.( was running wrong pulley @dyno time. pulley was a 4.75) rectangle port heads with 1.900 standard length inconel exhaust valves. 2.190 severe duty intake valves. Crane cam, lift intake @ cam 312 @valve 530 exhaust @cam 324 @ valve 551. cam timing @.005 intake open 5* btdc closes 43* abdc max lift 109 duration 228* exhaust opens 57* bbdc closes 1* btdc duration 236* Ground w? a 112 center lobe. piston are 8-1 compression w/ 4.28 bore. felpro gaskets all the way around and a high volume oil pump.

Re: boost question
 

That would would be great. :D probably not going to happen. :mad:

Re: boost question
 

That would definately expalin the low boost reading. They were probably correct.
The specs you posted were almost identical to a 525sc. Is your cam a custom grind on the 112 lobe sep???? All the #'s are the same as the 525sc cam except for that. Stock crane cam is 114 lob sep???
Carb jetting is also on the lean side. Stock jetting is 98's squared. I would go bigger if I were you. Nothing will kill a blower engine faster than being lean.
Other than that, the only differences I see are the .030 overbore and the CR you listed is 8:1. Stock is 7.5:1. Both will only add power. If thre Cr really is 8:1, then you mightwant to keep timing down to 32* Stock is 35*

You should have a solid 525-535hp engine.

Re: boost question
 

Griff. Yes, the cam was ground to a 112. The dyno was telling us that the motor was running rich, that is why I re jetted. That however was with the smaller pulley. I will put the larger jets in now. I set the timing at 32* total for the higher compression. Thank you for all of your help.

Mark