firechicken_3

iTrader: (4)

So I have a vortec headed 350 that is as follows

.040 over flat top 4 eye pistons
its decked down to near 0 which depending on the calculator puts me in the 10.25 to 10.5-1 range
xe274 cam
750fm vac secondary holley
vortec rpm airgap
GM vortecs from sdpc
hedman longtubes


When i mash on it there is a pretty big lag before it screams, loaded or free revving. if you roll on its fine this occurs off idle or doing it at cruise rpms.

it doesnt really stumble, so much as go silent and then snap to life.

its got 70 jets, and 65 powervalve in the front, a yellow pump cam, and 31 shooter, silver spring in the secondary.

i read sofakingdoms holley tuning sticky. and need to get going on tuning this thing up right, cuz it went to the track last year and ran 14.20 something at 101mph which is way lamer than similar combos. and a whopping .7sec and 6 mph faster than the lame smogger truck motor i had before. for the $3k i have in the front of this car im not pleased at all.

i cant say for sure its not a timing issue either i have an MSD pro billet in the car with 12deg initial and 18deg centrifugal advance all in at 3000rpm according to the provided chart.

o btw im getting roughly 14-15 mpg in highway biased mixed driving, with a t56 and 3.73 rear


and suggestions or direction would be great.

built91Z28

You may want to try upping the timing a little. I'd put it at about 17 or 18 degrees initial and have it all in by 2500 or so. I dont know much about timing a motor, but doing this on my car did help.

firechicken_3

iTrader: (4)

i do believe it can take some more timing, just not sure how much due to the highish compression ratio, i detonation and ping is bad i know, but im not certain what it really sounds like.

the thing HAS to run on 93 octane or less. preferably 91 or less because of its higher availibility.


also on the timing front, i am using the vacuum advance also it gives 18 degrees and is connected to full vacuum on the carb base plate

built91Z28

Are you setting your initial timing with the vacuum advance connected? If so, thats probably your problem. You only have that advance when there is vacuum. Under WOT, the vacuum advance does nothing.

firechicken_3

iTrader: (4)

no its set without vacuum, i know that it goes away at wot

skirkland1980

iTrader: (2)

i like to add a metering block to the secondary so i can tune it better. usually a slow rev is from exhaust back pressure. if you have an o2 sensor bung in your exhaust you hook up a back pressure gauge and check it under load. you can also try different secondary diaphragm springs to tune the secondaries. what was your 60ft?

onebad82z

iTrader: (7)

Doubt it is exhaust back pressure.

So at idle with vacuum can hooked up you have 30 degrees timing? 12 base and 18 centrifugal also gives 30 total which is low for a performance sbc. You need more total timing 1st.

skirkland1980

iTrader: (2)

tell us more about your exhaust system and the custom y pipe. maybe some pics. does it have a cat connected to the cat back? full length exhaust systems are great for street driving but can be restrictive for racing. this car should be low 13s or better.

firechicken_3

iTrader: (4)

dont recall for certain the 60 ft it was awful tho, street tires and enough sping to create a haze according to a friend that was in the stands.

as for excess exhaust back pressure not likely headmans to 2.5 in to 3 in no cats al mandrel bent, that should be plenty of flow.

truth be told the 1/4 mile time doesnt concern me i just dont feel im getting what i should from the motor, and it needs a good tune or i need a teacher to show me the way


y pipe pic

https://www.thirdgen.org/forums/atta...20-10_1255.jpg

full thread my pics start at post 30

https://www.thirdgen.org/forums/exha...longtubes.html

skirkland1980

iTrader: (2)

thats exactly what i have but 3" into 3.5" and also the flowmaster y.

skirkland1980

iTrader: (2)

i was curious about the y because i had a muffler shop make my first exhaust system and where the pipes came together they just made about a 1" diameter hole. it caused me a lot performance problems.

onebad82z

iTrader: (7)

I stand corrected. Read right over the fact you have vortecs. Your total timing sounds good in that case.

But I still ask, idle timing is 30 with vacuum can attached? If so...

Try a #427 pump cam on the #2 hole on the primary side. Keep the 31 shootr in there, just try the pump cam.

firechicken_3

iTrader: (4)

yes idle timing is 30, what color is the #427 cam

i have a 105 power valve to try as suggested in sofa's sticky.

what about primary jet size is it in the ball park at least? i know altitude makes a difference as far as what the carb should be set up with. if i recall from my schooling we are around 700ft here in eastern iowa

onebad82z

iTrader: (7)

That is more than enough.

Typically blue, but I do have one that is red. That is why I gave the number, look on the back side and you will see it.

I too used that one when I had the XE274 in my car, although I had AFR heads. I like to set my PV much like sofa. Drive the car around and note where the vacuum typically is when I am cruising. I then will watch it as I drive up long hills and see just how low it goes under normal throttle, when I dip into it to keep speed I note where that is and set the PV there. I swapped heads/cam and now I tend to get into a 10.5 too much so I dropped to a 8.5. Converted to annulars this winter and since they pull much harder on the jets I now am running a 6.5. All depends on combo and driving style.

Honestly, get the tip in good with cams/shooters and then worry about jets. I would start with whatever came in it. Then once you have it transitioning smooth drop 2 jet sizes till it surges under steady cruise, then go back up 2 to get it smooth again. Try that in 4th gear. Typically Holleys are not into the main jets till 2500 or above, you will be moving in 5th. Not ure I would worry about 700ft, just tune the car to what it wants. Start with Holley supplied.

Aside from the vortecs your car is very similar to mine when I had the 274 in it. I ran a 12.47 @ 110 with a slipping clutch so you have a lot left in it.
I also had a T56 and 3.73.

firechicken_3

iTrader: (4)

ok, thanks for all the info and tips.. i have a set of pump cams on the way.. also i scrounged up a 37 shooter from my step dads old racer stuff, and put it in for kicks, and it did change things but there is still a bog. so i'll put the 31 back and try the #427 cam (blue) that seems to be suggested by alot of folks having similar stumbles and such

i guess i need to get a vacuum guage hooked up. so i can actually figure out the power valve situation..

if i understand that correctly, the 65, 105, etc relates to inches of vacuum.. it opens and provides more fuel when vacuum drops below that number. sort of a delayed accelerator pump in simple terms?

on the idle timing, should i try to remove some of the advance provided by the vacuum?

i think the pro billet is supposed to have an adjustable one, but i couldnt find an allen wrench that grabbed anything, guess i will call them and see what the deal is there

as for the time i ran with the car, that was with my old t5 in the car. i dont see it making a huge difference tho the ratios arent much different 1-4 at least.

end of the month it will likely go to the local strip for a midnight mayhem street car fun night, less than ideal conditions for traction but its cheap and you can run the thing as many times as you can get it through the staging lanes.

i still cant find my old slips for 60 ft times, i wanna say they were like 2.4x something on the best run, so i know im losing a ton of et right there but the mph isnt as high as it should be still for the power this is supposed to be capable of

a couple people have told me i should have gone mechanical double pumper since its a stick, but i went vacuum due to being more forgiving to tune. i kinda want to try one but at $400+ im willing to tinker with what i got since i just bought it last summer

onebad82z

iTrader: (7)

In those terms, yes.

If it starts fine, does not ping at part throttle I would leave the timing alone. I know my pro billet did not have an adjustable can. I had to add one that fit a points dizzy.

I prefer DP for a stick car but that does not mean a VS cannot be tuned to work. Either one tuned for the car it is in properly really leave neither with an advantage.

hamaf

This chart(attachment) should help. If you have a wideband, and can make log files its easier to see what pump cam you need. Probably the blue one(pos.2) as someone allready said.


I think that the secondary spring is way too stiff. especially with manual transmission. Maybe try the yellow one?

There has to be some major issue, because my car is faster. It's got stock suspension, 235/45/17 tires, gmpp 330hp crate motor, 600 cfm holley 4160, TCI th700, 3.42 rear, Dynoed 280hp to the wheels.

firechicken_3

iTrader: (4)

tried the blue cam, it seems somewhat better.. been to busy with work and helping others to get anything else figured out. hopefuly i can get some time to try the powervalve this weekend.