gibsongod716

Any tips are welcome for tuning a Mighty Demon Blow Through 750cfm carb/Single Turbo T-70

Should I run rich at idle?

What should the fuel bowls be set at? I was told between 1/2 and 3/4 mark?

I was getting a pinging sound almost like a rattling clamp, could this be detonation due to improper carburetor tuning?

Thanks for any help

squeeezer

what kind of fuel pressure regulator do you have???

gibsongod716

I didn't run one, both thirdgen and barry grant said I could get away with a new mechanical pump and reference the port on the top of it for a 1:1 ratio of boost to fuel pressure

The fuel pump is a summit 110gph fuel pump putting out about 7 to 8 psi

gibsongod716

I won't have access to the internet for about 3-4 days as I'll be away, hopefully someone can give me some good advice because I'm gonna work on it right away when I get home, I really cant wait to get this done!

Thanks in advance,

Jim

5literTPIterror

i wouldnt know anything about using a mechanical pump with boost. im sure it can work, but i wouldnt know how to set it up, you definately do want the 1:1 increase with boost tho.

from what ive heard/learned you should be as close to 14:1 afr at idle as possible.
do you have any sort of wide band or air fuel gauge?

junkcltr

iTrader: (1)

1) Do you have a vacuum line from the intake to the fuel pump to increase pressure 1:1? If you don't have the vacuum line then you are not increasing the fuel pressure.

2) Are you using a fuel pressure regulator?

3) Test the fuel pressure with the engine at idle by hooking up air pressure from an air compressor along with a air regulator. Slowly turn up the air pressure to simulate boost and make sure your fuel pressure gauge rises 1:1.

4) Are you using a power valve or just increasing the main jets 8-10 sizes over stock for a starting point? I like using a power valve with drilled out channels over the large main jet method.

5) Rattling sound from the engine is likely to be detonation from either too lean or too much timing, or both. Don't run it like that. You will hole/melt a piston.

gibsongod716

Do you have a vacuum line from the intake to the fuel pump to increase pressure 1:1? If you don't have the vacuum line then you are not increasing the fuel pressure.

Yea, I ran a vacuum line from the bonnet to the fuel pump atmospheric reference port.

2) Are you using a fuel pressure regulator?

No, I was told I didn't have to.

3) Test the fuel pressure with the engine at idle by hooking up air pressure from an air compressor along with a air regulator. Slowly turn up the air pressure to simulate boost and make sure your fuel pressure gauge rises 1:1.

That I'm limited to because I don't have a fuel pressure gauge, just a whole bunch of air compressor psi gauges.

4) Are you using a power valve or just increasing the main jets 8-10 sizes over stock for a starting point? I like using a power valve with drilled out channels over the large main jet method.

Not increasing the jet sizes per say, just trying to tune the carburetor jets for a street car not a full out drag car lol

5) Rattling sound from the engine is likely to be detonation from either too lean or too much timing, or both. Don't run it like that. You will hole/melt a piston.

Alrighty, the timing is 8 btdc I believe, I didn't time it my dad did that, it's most likely from running lean...

gibsongod716

directed to 5liter:

No I don't have an oxygen sensor on the car, nor is there any sort of computer system at all luckily XD

I can tune it old school by ear but I'd prefer to get some good tips before hand

junkcltr

iTrader: (1)

If you tune using the main jets then to get the boost AFR correct then the idle will be rich. That is how it has to be.

Is using a distributor without a boost retard box with the base set at 8 degrees then you have way too much timing for 4 PSI and above depending on your compression ratio. Using a stock type distributor I weld up the advance holes and grind so that it can only advance to 26 degrees.

I would check you total advance. That is, check the timing with the engine above 3000 RPM or where ever your springs give total advance. Be careful.

junkcltr

iTrader: (1)

That works OK with NA engines. Turbo setups are very sensitive to detonation. It will melt a piston quickly.........much quicker than an NA engine.

gibsongod716

So your saying I should retard the timing, ok I can do that, and since unfortunately I don't have any way of using an AFR gauge than I'm just gonna have to do the best I can...

comp ratio of the motor is only 8.1 to 1, what should the timing be set at?

calebzman

iTrader: (3)

Do you still have the power valve in the carb? Like said earlier, you either need to increase the jet sizes or drill out the PVCRs, but probably both. A blowthru carb will run rich at idle and cruise without drilling the PVCRs.

How much boost are you going to run? At 8.1:1 compression, you won't have to retard timing much. A good starting point is to reduce timing ~1°/1psi boost. Also, be sure to fill up with premium gas always to help limit detonation.

junkcltr

iTrader: (1)

I would do exactly as calebzman said and also keep checking the spark plugs. Since it sounded like you already rattled it I would pull the plugs and look at the electrodes and ground straps.

With 8:1 compression, 93 pump gas, no aftercooler or meth injection and 8 PSI of boost I would start at 26* BTDC (advance) and tune the fuel. Once fuel is tuned, I would bump the timing a little and repeat fuel checks/tune.

5literTPIterror

i do not know about that boost refrenced mechanical pump, but as far as tuning you kind of need a wideband to get it right, you can read plugs and tune jets by the feel of the car but mamn one mistake with boost and you are starting over from scratch.
ask me how i know......

i just the other day was driving my car with only a small amount of fuel in the cell and while under boost the carb was starved of fuel and blew my motor, i havent "dissected it yet but im guessing it detonated badly and bnlew the ringland of the piston of and or bent the rod, anyways the motor came to terifying stop and i have a beat up frost plug laying on the ground and the blocks cracked.....

i have a simple innovate lm1 wideband on my blowthru setp that worked very well.... until my dumb *** didnt have enough fuel in the car.

i would start off with like 18*timing under boost and slowly work your way up as you go. my motor ended up liking about 24-25* under boost.

keep us posted

zemon1

5liter, you ran out of gas and that blew your motor??

gibsongod716

Alrighty I'm back from vaca, I'll take out the plugs and check em out over the weekend, I'm also gonna tune the carb rich and yes I still have the one way power valve installed

5literTPIterror

i cant say for sure, but i checked the cell after the damage(fuel gauge doesnt work, even tho i have a gm sender in the cell.... )
and there was hardly any gas in it. what i thought may have happened was the motor may have been partially starved for fuel under boost creating a super lean major detonation. i dont know.

maybe it was just because i was making 540ish hp on rods rated for like 400hp.
who knows

85camaro350v8

i know this probably doesnt help you but i heard that alot of people have trouble with those demon blow-thru's. i went ahead and saved up 400 more dollars and had kevin build me one at csu. he asked me quesions from what kind of ignition system to what rear gears. whether he needed all that info or not i really dont know, but it sounds like they're totally custom. hes a nice guy to deal with, or atleast he was with me, i get my carb on the 30th(ups says), and ill post some pics and get some feedback on other peoples here and at thetuboforums. i hope you get it tuned and running good luck

gibsongod716

Car makes boost, feels great, however it gets real laggy when I floor it, it almost feels like its got a rev limiter... So I actually closed the jets, upped the fuel bowls, and set the timing to 14 degrees atdc, barry grant said set it to 16 but I think thats a little too much, and it seems to respond alot better, its just pretty laggy and I can't narrow down the problem

Actually I really dont know what the heck to call the timing all I know is its timed at the 14 mark after 0 on the timing tab, is that considered 16 btdc or 14 atdc? Thanks for any clarification

Also, should I set the secondaries to open the same time as the primaries?

gibsongod716

bumppp

junkcltr

iTrader: (1)

A locked time distributor will make an engine lazy. I prefer EFI for boost setups because of precise fuel metering and spark timing. Not to say carb & mech. dizzy is bad, but to get the same performance a timing retard box is needed.

The next best thing is to use the mechanical advance and limit the max spark timing using offset dowels or welding & grinding the advance slots. Or you could use an HEI module that had the knock control input in the early carb computer control days. I think they retard 10 degrees. You could run one of them with a hobbs switch. That way you could run a normal HEI dizzy and activate the 10 degrees of timing retard with a hobbs switch turning on around 2-4 PSI of boost.

gibsongod716

my car was orignally a computer controlled carburetor, do you think my distributor would have that knock control thinG?


also any tips on how the heck to hook it up with the boost act. switch?

junkcltr

iTrader: (1)

http://www.cadillacforums.com/forums...m/t-50062.html
http://www.cadillacpower.com/forum/i...;topic=10003.0

You would basically set up you distributor as normal. The hobbs switch (pressure switch) would have one side wired to ground and the other side wired to the "R" terminal. Adjust the hobbs switch to 2 PSI to turn on. Once you make 2 PSI the switch turns on and retards timing by 10 degrees.

So if you had a normal advance of 34* BTDC and you get into boost it would retard to 34-10 = 24 degrees BTDC.

Test the timing retard with the hobbs switch using air pressure and a timing light.

http://www.megamanual.com/ms2/GM_7pinHEI.htm
You want the 5 pin module


Part Number: DR128
Duralast / Control Module - Ignition
For your 1985 Chevrolet Truck C10 1/2 ton P/U 2WD 5.0L 4BL OHV 8cyl

According to Autozone, your 1985 trans am carb controlled would have a 4 pin. A truck 5-pin is $30. Could junkyard it cheaper. Ask around if you know anyone with 1980-85 chevy carb 305ci trucks. A lot of the truck guys would remove the 5 pin HEI and install a 4 pin HEI because something went wrong with the dizzy (using ESC controller or wiring).

gibsongod716

In other words I gotta toss my vacuum advanced distributor thats in the car cause thats basically useless?

And now I'm no expert on distributors or anything, but if I happen to find my old dist, how would i know what the "R" thing is or where I wire the non-ground wire too.

Sorry its so many questions me and my dad have never done anything like this b4

According to the picture, is that the R terminal? It seems to be a 5 pin also

junkcltr

iTrader: (1)

No, you keep your existing distributor. The truck distributors have both vacuum and mechanical advance just like your distributor. The black box inside (module) is changed and you would need to add another wire for the extra pin coming out. Easier yet would be to get a 1980-1985 distributor from a chevy 305ci truck with the 5 wire module and just drop it in the hole and wire the knock/retard wire to the new hobbs switch you install on the intake.

The quick way out, but expesive is to run an MSD, crane, etc timing box that has a built in boost sensor.

There are all kinds of tricks that can be used when running boost. I learned a lot when I started messing with the boost stuff.


EDIT: Yes, the center of the 3 pin side is the "R" terminal. Is that your distributor from your car?

gibsongod716

It was back when I had the stock gm rochester quadrajet carb in the car but I took it out and put in a really old school 30+years old distributor

Apparently the distributor is a 7 pin not a 5 pin, would it still work the same? And you say I can leave my vacum advance distributor in the car and all i gotta do is swap the control unit inside to the one I previously shown? Woudl that require me to plug the vacuum advance or would I still leave that hooked up

gibsongod716

bump, sorry for that i just dont want to blow up my car without more information

junkcltr

iTrader: (1)

The 7-pin is different. You need the computer (ECM) to control the 7-pin. You will need a 5-pin for the simple retard mechanism.

Instead of the Hobbs switch you could run one of the vacuum switches that the trucks ran. When the vacuum gets low from heavy throttle and now boost then it will retard the timing using the 5-pin module.

Yes, you would leave the vacuum advance and mechanical advance connected. That way it drives as normal with good timing advance. Then when you get into boost the vacuum switch closes and activates the 5-pin module to retard timing.

gibsongod716

In the pictue for the 5 pin module on autozone, there doesn't appear to be a "R" plug, only H, L, & D on the left and B & C on the right... Does this mean anything different?

junkcltr

iTrader: (1)

This is the one you would want for your setup. It is the ACDELCO 1941, or DR137 at Autozone. It will have the R pin.

Read this link: http://www.cadillacpower.com/forum/i...c,7447.40.html

You ground the R pin to retard 10 degrees. A simple vacuum switch will do it or you can go with a Hobbs switch.


http://www.autozone.com/autozone/cat...ule+-+Ignition

Part Number:
DR137

Price: $34.99
OEM Brand: AC DELCO
OEM Part Number: D1941

Applications:
http://198.208.187.182/internet/Vehi...l=A&part=D1941

Keep in mind you are going to need to get the 5th wire out of the distributor to attach to the switch you are putting on an intake manifold vacuum port.

gibsongod716

By fifth wire im assuming you mean the "R" wire? And also does it matter which port on the hobbs switch i attach it to? The hobbs switch has 2 terminals and you said one gets grounded(-) and the other terminal goes to the R port right? There's no positive(+) connections anywhere?

junkcltr

iTrader: (1)

Yes, I was referring to the "R" wire as the fifth wire.

Hobbs switch has two connections. One to the "R" module pin, and the other to ground. No positive connection

gibsongod716

Lastly this isn't about the distributor, but should I set the secondaries to open the same time the primaries open?

I tried doing it at a normal setting with primaries open up to about 3500 rpms or half pedal than secondaries kick in but the car just bogs the hell out until about 5500 rpms(i guess thats when everything gets corrected)

junkcltr

iTrader: (1)

I would wait on adjusting the carb. A locked distributor will make it lazy. Once you have the timing set to a reasonable level, then adjust the carb.

Reasonable timing is 4-12 degrees engine cranking, 10-20 degrees at idle, 30-38 degrees at light throttle cruise, 28-36 degrees at WOT without boost, 16-26 degrees under boost. All of this depends on your compression ratio, heads,cam,exhaust,intake. Those are ball park figures.
With a locked dizzy, it will have a hard time gaining RPM because there isn't enough timing advance. Once it gets to the higher RPM, the timing is where it should be. That is, you are locking it to keep it in check at WOT under boost, but every where else the timing is to late.