Turbo timing and fuel suppy - again

I could use a refresher and maybe some links to earlier posts on Turbo timing and fuel supply minimums, turbo setup is running and want to dial in the fuel and ignition timing now that basic running condition is established.

It's a 250 with DS II Distributor/Coil and HEI trigger. Ran well NA carbed and after adding turbo it started OK - initial running with the Buick 4Bbl and draw-thru turbo setup. I have Vacuum advance disconnected and static timing @ 14 degrees, centrifugal advance looks like about doubles that ( by eye's on timing light 8) ) .

Welding in an A/F meter bung for carb tuning, special carb has external Power Enrichment Valve vac-port but need to work out Vac/Boost signal. The gravity feed gas line seems fine for run-stand turbo/carb demand but fuel demand under load will probably need more than mechanical pump with regulator ?.

thanks

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I run 5psi FP at idle and my boost referenced regulator adds 1psi per psi boost. As for timing, mine is locked around 32 or 34 degrees if I remember correct. Im about to get an MSD box that allows you to mess with timing curves with a locked dizzy but it runs good as it is. Should just refine the around town out of boost driveability some.

If you have a wideband, then you can take notes on what your A/F is doing at different stages while driving around. Idle, cruise, tip in and WOT etc.

Matt

Crap...noticed your runnin a draw through setup. Not sure if my info will change between what your running and the blowthrough but it shouldnt change much I wouldnt think!!!

Matt

correct me if im wrong but im thinking a draw through setup would mainly be in the jetting of the carb more than anything else as its still under vacuum always. along with making sure you have a non restricting fuel supply from the tank pickup to the fuel pump, and up to the carb. it would be like having a large cubed engine under the carb when under boost.

boost referencing a fuel regulator or mechanical fp may and or will cause fuel bowl level issues under boost in a draw through setup. the added fuel pressure would force the seats open uncontrollably (where as in a blow through setup the bowls are also pressurized equally with the regulator or the referenced fuel pump. the charge pressure and added fuel pressure would counteract each other, allowing the needles and seats to close off properly when needed) once again someone can pipe in if im off in a loony rocker.

might be able to change the jetting of the carb as needed as 66Sprint6 said with a wideband.

once again someone can pipe in if im off in a loony rocker. ...
... only from your response I can't argue'

Help needed: It seems simple enough to modify the DSII to limit the centrifugal advance . I am looking for a description or how-to post and/or info on having it "recurved" for the intended turbo use.

Until red elec' retard box becomes available ($ ), the vacuum adv. is disconnected and static timing @ 14 degrees. I wound up the 250 on the test stand to @ 5 lbs boost and it backfired (detonated?) and blew off the muffler pipe, now that consistent idle , run and hot run cycles work on static stand TIMING needs to be straightened right out.



as sthorvictor' relates, the carbureted draw-thru setup trades some complications for others. With the blow-thru carb'd turbo, the fuel supply has to match the boost pressure in the carb + plus the fuel supply pressure to work but the carb sizing only needs to be about the same as the NA engine .. With the draw-thru, the carb needs to be scalable huge from small without boost to larger full boost fuel needs. BUT the fuel supply isn't pressurized so it only needs adequate NA supply like a virtual bigger engine. A fuel regulator with adequate elec' or mechanical pump could work. External referenced PV turbo-specific Quadrajet A/F ratios are within reasonable levels on run stand but turbo needs loading to really test.

have fun

. . .

the biggest problem i could see with the PV, is that the turbo can pull higher vac when it is running which would tend to pull the PV closed when it really needs to be open. If you went to a stiffer spring (to make it open under lower vac) it wouldn't have the right signal at low boost.

q-jet could have some tuneability with metering rods but you won't be able to 'stomp on it' without running pig rich until it comes on boost.


as for modifying the DS2 distributor, its pretty much the same as any other mechanical dist. you can weld the slots or posts up to have less 'throw'

the emissions DS2's have a second vacuum port that, i think, could be used with a check valve to actually retard when boost pressure increases. you'd need a small check on the vac line, so it could pull it advanced when the manifold was under vac, and you'd need a check valve facing the other way, running to the emissions port to use boost pressure to push the diaphragm retarded under boost. You'd probably also need some tuned orifices to bleed pressure in either direction... just from looking at it, in theory it would be possible, but i haven't tried it. In my estimation its much easier to just go EFI at that point.

motzingg :

Thanks for the reply,

... currently focused on timing for the turbo 250 DSII and simply "locked out" dist. for now. Someone with a SUN type distrb machine to set up the advance ballistics for the turbo application might be better idea. Locking out centrifugal advance and limiting vacuum advance someone said ...? . I am interested in electronic solutions but typical RED box elec' retard is big investment.

Neat thing about the turbo 70's Buick Regal derived draw-thru Quadrajet is it already has the basic jetting for turbo and external referenced PV port to valve for high-vacuum/boost. ONLY early GM Turbo application Quadrajet has the external PV port and BIG-DX secondary rods (AFAIK). In 1981 GM went to electronic feedback system turbo Q'jets.

The Quadrajet/turbo 7.5:1CR 250 runs great on the static stand but I can't really wind up or build boost with no engine load . I have time to work out some details before before better weather and installing in a car for load. Also looking at Blow-Thru version possibilities now that full oiling, cooling, exhaust and instrumentation are about complete and easily could swap.

have fun

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You may be able to retard the timing with boost using the Dual Vacuum advance unit that was available for the DSII Distributor.
You also need the mechanical advance operating.

Take a look at CB Performance's Black Box. Lock down the distributor advance and let a computer do all that work. You can start with one of their maps or develop one of your own.

The DSII pickup will provide a timing and rpm signal for the computer. The MAP sensor is built into it. There's a coil driver in the unit that will send the calculated spark signal to the coil. You can start with one of their built in maps or make one of your own.

This will allow you to know exactly how much advance you have for every rpm, boost, and load level. Much better than guessing what a bunch of weights, springs, and vacuum cans might be attempting to do.

http://www.cbperformance.com/ProductDet ... tCode=2013

MustangSix":1x2ae9yf said:

Take a look at CB Performance's Black Box. Lock down the distributor advance and let a computer do all that work. You can start with one of their maps or develop one of your own.

The DSII pickup will provide a timing and rpm signal for the computer. The MAP sensor is built into it. There's a coil driver in the unit that will send the calculated spark signal to the coil. You can start with one of their built in maps or make one of your own.

This will allow you to know exactly how much advance you have for every rpm, boost, and load level. Much better than guessing what a bunch of weights, springs, and vacuum cans might be attempting to do.

http://www.cbperformance.com/ProductDet ... tCode=2013

Click to expand...

I was planning on using the MSD 6530 to do the same thing but ponying up the dough was looking like it would hurt...especially since I ponied up for the MSD 6BTM back in the day. The CB performance box does the same job for MUCH cheaper and Im pretty sure the MSD box requires you buy the Map sensor seperate. Thanks for the link...its added to my list of crap to buy!

Matt