Turbo on a 235 inline. ?'s what does close tolerance mean?

Bill, do you have specific knowledge of how to tune carburetors for blow through supercharged applications?

These guys do, http://www.theturboforums.com/smf/index.php?board=13.0

The reason I ask is, it seems you may be disseminating false information based on prejudice or misinformation. What you seem to be stating is a carburetor only works at a specific atmospheric pressure and to change that pressure, the fuel delivery would not change. If this is what you are stating, then you are incorrect. Most carburetors work on pressure differential by means of the venturi effect to regulate fuel mixing and this happens on airspeed over the venturi creating a low pressure area and pressure on the fuel supply (float bowl) relative high pressure.

Tlowe described in basic terms what I understand from reading and talking with people that have actually done this. It works extremely well, so much so that I really don't understand why the negative connotations about blow through. But if you have specifics on how and why your statement is true, please continue to educate and inform.

 

I do not have any knowledge of tuning a standard carb at pressure. I did mess around a lot with the Fish.

Yes, I am disseminating information that while it may be incorrect, has been held to be absolutely correct for many, many years. Among other things, it explains why a carb must be retuned for higher elevation. It has also received much ink from the turbo press from some very knowledgeable folks, including the likes of David Vizzard in "How to Modify Ford S.O.H.C. Engines" and Hugh Macinnes in "Turbochargers".

I have always read that the pressure differential you speak of is due to the change in velocity. Double the mass of the air, but send it through the venturi at the same speed, you get the same pressure drop and fuel delivery. Is that not the truth?

I do have an important question, do the guys that are getting a thousand hp out of a blow through setup have streetable engines? If these dudes have figured out how to get drivable performance out of a high pressure blow through setup, my hats off to them. It stymied a lot of smart guys for many years. I do know that tuning an engine for power is pretty easy. Tuning it for power and drivability is tough.

Bill

 

so bassiacly all i need is a regular two barrel from holly and special filled floats and im set? i dont need to mod anything else?

 

I am not a carburetor guy, I don't claim to be and I don't want to go down as someone who has experience with blow through carburetion. Even so, I can read and I have practical experience with turbocharging.

Yes, sort of. Air under pressure is also denser and therefore capable of creating a stronger vacuum signal in the venturi.

Emphatically YES they have done it through very simple tuning and modifications with multiple makes of existing carburetors.

 

http://www.theturboforums.com/smf/index.php?topic=51153.0

 

ELpolacko, just read a tad bit of the driveablity stuff. In a nutshell, they have "issues" and try to pick a tune that varies from pig rich at low power to lean under boost, or use fuel enrichment to solve the problem. They mention using a Hobbs switch to squirt fuel into the carb. That is an all or nothing enrichment I mentioned. But there is a problem. Significant boost can occur as low as mid 2000 rpms with the engine reaching maximum power at 6000 or so. That is a huge fuel requirement range and cannot be addressed with a simple "squirt" arrangement. So nothing has changed. In order to attain drivability, a standard carb needs help when used in blow through configuration.

Since posting on this issue, I've been wondering if a person could modify a carb to raise the metering rods under boost as well as partial vacuum. A pressurized piston arrangement should work. It would require a lot of ass-scratching design work, but could result in a carb that would properly carburate a turboed engine across its full operating range. Another possibility, rig a 4 barrel so the secondaries open only under boost. Once again, operated by a piston operating off of boost. Of course they would have to properly jetted, I'm guessing wildly rich. Should be fun to work out!

Bill

 

Just about every single blow through I have seen uses a CSU or C&S pre-prepared carburetor without any external switches or apparatus to make them work.


http://www.csucarbs.com/750.html
http://www.candsspecialties.com/store/html/blowthru.html

The only guys I have even heard of useing Hobbs switches are to control ethanol spray for intercooling and a bit of punch to their fuel requirements. And that is usually the guys that are making north of 1500 HP.

To be on point, this is much ado about nothing related to the original post. Fact is for the power levels needed a bit of smart carb tuning will suffice.

 

SU carbs lend themselves to working on draw through applications. They are easy to mount, not fussy about position as long as the float is down, and the main jet is externally adjustable. Some where on this forum, I posted a chart which shows what size to use per dispalcement both atmospheric and presurized applications. Probably a search for SU might tur it up. The ones from Jaguars, Rover V 8's, and the like are 1 1/2 or 1 3/4 throttle bore and should be more than sufficient for the chevy 6. Remember the compressing of the intake charge will heat it up, so keep you runs between intake, compressor and manifold as short as practical.

Don't forget to look at SAABs and Volvos for doners for turbos, plumbing, linkage and air to air intercoolers. Lotsa wrecked Huffed Swedish stuff in wrecking yards. Stuff is compact. My 2.4 L turbo Volvo 4 put out a reliable 175 Hp from the factory, a bit of tuning on when the waste gate opened, pulled about 10 to 15 more all on regular gas.

Non turbo motor was 120 HP. this was with a trottle body ijector but the SU should be in the same zip code.

Don't know what this photo is of but looks like a Continental or Hercules truck motor.

A home aquarium air bleed valve in the line to the wastegate can fine tune when it opens giving you a bit of boost adjustablity, Yep 10 to 15 HP for less than a buck.

 

When, years ago, I built my Turbo GMC 270 I used a suck through. At the time it seemed to me to be a no brainer. Why is it that you have choosen to go with a blow through?

 

because i would like to be able to utilize an intercooler and a bov.
and i just think its a much more interesting way of doing things haha

 

Rich, draw through still works on low boost applications but marginally so.

From what I do know about it, intercooling is out of the question and there is no chance of creating a high boost enrichment program like what is being done with blow through.

 

I ran between 17 and 21 PSI with mine. 17 worked best. Water injection. 270 GMC with a single 780 Holly. Went 158.819 MPH in my stock roof '32 five window. Stepped up to a 302 GMC and an 800 Holley and a Vega body and went 177. Good enough for records in the distant past.

 

Yup, water injection kept that sucker from blowing up!

Can't imagine that car was all that fantastic for getting the groceries and cruising the strip on a Saturday night.

The blow through carb guys tell me they get a bit of chemical intercooling with the venturi carb. And why not, the carb is designed to be a low pressure point and we do know what happens to a body of air as it transition from a high pressure to a low pressure area, at least I hope we do

I'm just seeing way to many successfully tuned applications to believe it doesn't work.

 

I've got a couple O/T suck through cars.

One runs 12 PSI with no trouble, 15 spits out headgaskets.

For enrichment under boost, you need a boost-referenced enrichment circuit. It's easy to modify a Holley 4-barrel for it, they even sell them that way now.

You can do it with a Quadrajet as well, I've never tried it with other carbs but I bet a 2GC would be easy to convert.

Shawn

 

Actually you could punch it and then tune the radio while waiting for anything good to happen. But when it did it was fun.

 

okay 87 grand national turbo ...on a 57 chevy pick up inline 6 .... wat tranny upgrade from the originaL 3 SPEED manual tranny .... heard a t5 from a 85 s10 or blazer are good upgrade. also with the stock s10 differentIAL . ....... WHAT brake up upgrades also ? was thinkin of chopping the frame and welding on the front suspension and subframe from a 79 camaro z28 or a 70 caprice or a 79 trans am(maybe buying a car whole and swapping engine ,tranny , all disc, differential, most electronics. and if possible a/c / heater /power steering .). or just keeping it stock . any easy bolt on rotors with disc breaks , ............. anyone have any great ideas ... ? i plan on having this truck for a long time to come ...

 

Hi guys...been a while...great post...I drive Leo Santuccis stude with the turbo six.....we now have the duugan block in the car...for the 2nd round...couple things about turbo on these engines....1...we put 25 lbs to a stock block with a cast cam ...for many 9 second runs til the cam broke....so...the boost you guys have been kicking around sounds like no problem....2...dont go more than 70 over on the block....we have split cyl. everytime we went bigger....and..3....I can get Leo to answer any qeustions you might have....pm me and I will give you my or his phone numbers.......

 

OK, never did this before,SO, I've got a 56 235 chevy inline bored .060 over, rebuilt the head with bigger valves and hardened exhaust valves, with 3/4 cam, was gonna run 4 bbl carb, but would like to attempt a turbo, my question is: what exactly do I need to get/look for to do this setup on my motor? any help would be appreciated...thanks