454 chevy tuning

[prairiehotrodder]

I need some help with the 454 in my 89 mustang. Above 4000 rpm in shakes and stumbles and doesn't pull very good. Its a low compression mismatched setup. About 8-1 CR with a big isky solid cam 240 @ .050 .530 lift 108 LSA. It has 90 psi cranking pressure on every cylinder. The timing is locked at 37. I thought that because it has such low CR i could run regular fuel. Am i wrong? Also the carb is a 750 holley vacuum jetted 82 / 86 with a 6.5 PV and the yellow secondary spring. Its got 4.10 gears and a 2800 stall turbo 350. I'm starting to also wonder if my secondaries aren't opening. In my shop i can load up the engine against the brakes then floor it and the secondaries don't open. I used the paper clip on the rod trick to check this. I thought that with the yellow spring they would open rael easy. Could this be my problem? Any idea's
thanks
brian

[Alan Roehrich]

It probably wants closer to 42 degrees timing with that low compression and 20 degrees too much duration. I can imagine it won't pull the secondaries open, it probably won't pull enough air to do it, you'd probably be better off with a 650. With 90 psi cranking compression, it's a wonder it will even start.

[Bob Hollinshead]

90psi with the throttle blades open during the compression test? 90psi sounds way low for even a mismatched combo. Do you have another carb to try just to rule it out? Maybe the secondary diaphram is leaking. You may be able to advance the cam for more compression.

[rfoll]

The 750 vacuum holley probably had 72 jets on the primaries. If your secondary side has a power valve, the jets were about 76. I'll bet the spark plugs are black.

[prairiehotrodder]

It starts and runs good and has loads of power. I wasn't a beleiver in this kind of combo untill a friend showed me the idea. I've ordered a holley 750 HP double pumper to try. I'll let you know how it works. I'm gonna try a few more things on my 750 vac carb first though. The carb was 70 front with a plate in the rear for jets and a 6.5 PV in the front only. I converted it to a 4150 with a rear metering block. it still has just the one pv in the front. I've never see a carb with a rear pv.
brian

[rfoll]

Is it stumbling on a slow roll to wide open, or when you floor it? The yellow spring could be opening too early on a mismatched combo. The rear jetting is probably a little lean, the front is about 8 sizes too rich. On my 3310-1 vacuum secondary carb, 74/88 was pretty close, but the older carbs like mine had downleg boosters power valves on both ends, and were 780 cfm. On the bigger carbs, removing the power valve required going up about 8 sizes.

[Alan Roehrich]

The statements "it starts and runs good, and has loads of power" and "above 4000 rpm in shakes and stumbles and doesn't pull very good" are diametrically opposed.

You have low compression, and too much cam for the low compression, so your solution to your current problems is to add to that a mechanical secondary carburetor?

I had stuff like that 30 years ago, when I was in my teens and did not know better.

I'd offer some advice, but it seems you're dead set on ignoring good advice. It's funny how people ask questions, but don't want honest answers, they just want someone to tell them to do what they want to do and it will work just fine, when the truth is, it will never really work well.

[rally]

paririehotrodder you need to quit complaining about you engine setup. For your info Allan Roehrich is a top notch Chevy engine builder and racer who knows engines like the back of his hand. You missed out by not listening to his expertise and advice. Allan has been great to me and when i have issues or need advice he always steps up to the plate at no cost. Guys like him are very rare. He is trying to help you, but your dead set on trying to figure it out yourself, to bad for you.

[ClassicComp]

90psi sounds like the cam is in wrong..........

[prairiehotrodder]

o,brother, give me a break. Who do you think is gonna figure it out? Me. I'm the one who owns it and i'm the one working on it. I'm asking for advice and telling you what i've got. No need to get cranky. I've posted on 2 different forums and got guys telling me its to rich, to lean, to low compression, to big carb, secondaries aren't opening, secondaries opening to soon........... lets just be friends allright? I know just as much as anyone that 90 psi is very low and i know the cam is to big. But anyways back to the facts: We got it figured out. It was way to rich. The secondaries are opening really quick since i was using the yellow spring and i may change it still but it works but will likely bog if i put sticky tires on it. But the real problem was i had it jetted 86 / 82 with the 6.5 PV. I changed the PV to a 3.5 which i had originally tried but with bigger primary jets. Now i jetted it down to 72 on the front and it made an amazing difference. I haven't changed the back jets yet but i still may bring them down a bit too. I don't have o2 sensor or a dragstrip and i live on a gravel road so thats why this is a little bit trial and error for me. Before the car would shake and labor at 4000 rpm in 3rd till about 5000 rpm then start to smooth out and pull again. I guess it was after 5000 that it couls start using up all that extra fuel i was giving it.
BTW - i weighed the car last night and its 2930 without me and i'm another 190. I also set the timing to 40, it was 37 if i remeber correctly. i'm thankful to those who offered help. BTW- when i said it had loads of power i'm talking maybe between high 11's to mid 12's in the quarter. To me thats fairly quick, I'm sure thats slug slow to some others. thats ok..... Power is maybe 400 -450. good power to some, wimpy to others, thats ok too.....
I'm trying to sell the car right now but if i get it to the track i'll know for sure and post the times. Maybe i'm dead wrong then you can laugh at me for building a mismatched piece of junk. thats ok too... I also built a 10 second malibu street/strip car that has run 9.80 on the bottle so i know a little about what i'm doing
brian

[rally]

You sure you know what you are doing? And you say you do? What are you doing on this forum with questions and issues. Gotta love this back yard perf builders. Still having issues, not surprised.

[prairiehotrodder]

Well so far i don't yet have any real evidence of this cars performance but a funny thing happened. A young guy that helped out putting the car together borrowed it for a local car show then made a youtube video without even telling me. Its kinda neat and at least shows the car blasting through the gears. Its a little soft off the line because the convertor is only a 2800 stall but actually stalls more like 2000 because of the lack of torque. Once it gets moving it pulls pretty good. If it holds together i am still hoping to make it to the track this summer.
brian
http://www.youtube.com/watch?v=bR0Nsjf-tzY

[prairiehotrodder]

So I changed the torque convertor to a trans specialties 10" rated at 3100-3600 now it has terrible tire spin. Not a surprise. I got a g-tech timer from my wife for my 35th birthday a few days ago. If you trust those timers (I think there usually pretty close) I ran 13.4 at 106 with brutal tire spin then running out of breath way before the end of the quarter. It actually discarded my first 2 passes because i let off the throttle way to early not realizing i wasn't past the quarter mile yet. I think i'm going to find a set of 3.55 or 3.27 gears so the motor can tug instead of rev like a small block. Another problem i found was that as soon as i changed the convertor i would run out of fuel at the top of every gear. My fuel system was barely adequate so when i was up in my power more with the higher stall it was no longer adequate. I've now upgraded the whole system and that problem is solved. If all goes well i should be able to get some real track times in august. This has been a fun project and a good learning experience. If I don't sell the car maybe i'll stick in some pistons this winter to get the compression up where it really should be to work with the cam and heads then it should really fly.
brian

[johnretired]

If I was to buy this car, I think I would change to thicker head gaskets or run two on each side so all the extra fuel can get a straight shot at the intake valves.. But then I see you went to smaller jets and less fuel. Never mind.

[427Z06]

If I was to buy this car, I think I would change to thicker head gaskets or run two on each side so all the extra fuel can get a straight shot at the intake valves.. But then I see you went to smaller jets and less fuel. Never mind.

I'm guessing the Flux Capacitor will be of no help here.