My Big Block Wont Rev!
Moderator: John_Heard
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My Big Block Wont Rev!
Hi Guys, need some help with my combo. Got a 427(+30) in a Bracket racer weighs 1990 lb (with me in). 4 bolt block & Manley rods, 13:1 Aria pistons, closed chamber big port heads 2.3" In & 1.95 exhaust stainless valves, heads have been worked but haven't been flowed. Cam was 260/270 @ .050" .648" lift solid roller, rev range 3200-7200 rpm. PB was 9.1 @ 145mph.
Changed cam to 280/286 @.050" .710 lift solid roller, rev range 4500-8500. Same times, but lost 5 mph & she just wont rev!
Tunnel-rammed with 2 x 600 Holley DP's. Tried changing diff gears, tried jetting up, tried with power valves, with-out power valves! (use C10 race gas). Timing is 38 degrees total. Got new springs 230lb installed pressure. Lower diff gears dropped by 60 foot by 1/10th but still wont wind out in top gear! (Old cam would rev to 7,100 rpm O.K.)
Are the carbs too small? Is the new cam too big? What do you think I should try? Any suggestions welcome!
Changed cam to 280/286 @.050" .710 lift solid roller, rev range 4500-8500. Same times, but lost 5 mph & she just wont rev!
Tunnel-rammed with 2 x 600 Holley DP's. Tried changing diff gears, tried jetting up, tried with power valves, with-out power valves! (use C10 race gas). Timing is 38 degrees total. Got new springs 230lb installed pressure. Lower diff gears dropped by 60 foot by 1/10th but still wont wind out in top gear! (Old cam would rev to 7,100 rpm O.K.)
Are the carbs too small? Is the new cam too big? What do you think I should try? Any suggestions welcome!
Re: My Big Block Wont Rev!
I would bet that going to the bigger cam is causing it of to rev. The heads must be big if they have valves that big. Putting a bigger cam in it may not have been the right change. Your 427 can only move so much air and if the heads are too big (cam included) it will stop rev'ing. The cam can add to or take away from the head flow. I bet if you were to go to a smaller cam it would rev better.
Bigger is not always best.
Bigger is not always best.
Blackhoodmafia!!!!
1972 Nova SS
572 C.I. BBC
Best to date: 1/8
et: 5.28
mph: 134
new wt. 3340 lbs
1972 Nova SS
572 C.I. BBC
Best to date: 1/8
et: 5.28
mph: 134
new wt. 3340 lbs
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- Posts: 5
- Joined: Tue Apr 10, 2012 11:58 pm
Re: My Big Block Wont Rev!
My next plan was to change to a cam in between. You don't think the carbs are too small? I thought about changing the throttle bodies from 1"& 9/16" to 1"& 11/16" effectively turning them into 650 DP's. Either that or bolt on a couple of 750 dominators! (but that would be over-kill).
Re: My Big Block Wont Rev!
I think the 650's would be a better choice.............But the ones you have should be ok.
Blackhoodmafia!!!!
1972 Nova SS
572 C.I. BBC
Best to date: 1/8
et: 5.28
mph: 134
new wt. 3340 lbs
1972 Nova SS
572 C.I. BBC
Best to date: 1/8
et: 5.28
mph: 134
new wt. 3340 lbs
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- Posts: 5
- Joined: Tue Apr 10, 2012 11:58 pm
Re: My Big Block Wont Rev!
What about changing the cam set-up? I dialled it in as per the cam specs. What if I retarded the cam 2 or 4 degrees? Would that make it rev better? Then only thing I was worried about there, was piston to valve clearance!
Re: My Big Block Wont Rev!
I'm thinking the cam is just too big. Put som nitrous on it and see is it will rev on up. I bet it will..... I had the same problem with a cam and it would rev on nitrous.
Blackhoodmafia!!!!
1972 Nova SS
572 C.I. BBC
Best to date: 1/8
et: 5.28
mph: 134
new wt. 3340 lbs
1972 Nova SS
572 C.I. BBC
Best to date: 1/8
et: 5.28
mph: 134
new wt. 3340 lbs
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Re: My Big Block Wont Rev!
Do you have enough valve spring for it? Weak springs will make one lay over early. Certainly seems related to the cam as that was what was changed before it started doing it they way I read your post.
My 1971 X275 Nova | Facebook
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Re: My Big Block Wont Rev!
Yeah. I have tried bigger springs that were 300lb installed height but they were too much for my 3/8" pushrods & bent a few (as they required 7/16") so instead I dropped the spring pressure down to 230lb which is more than the cam makers specify. I think Supernova is correct - the cam is too big for my combo. I have heard that 396ci - 427ci naturally aspirated don't like more than 285 duration @.050" & I found out why!
Re: My Big Block Wont Rev!
Springs can be a issue. I changed the cam in my 572 over the winter. The one took out wouldn't rev past 6500 on motor. Put a smaller cam in it and now it revs to the moon. The only thing I changed was the cam.
Last edited by supernova on Thu Apr 12, 2012 10:09 pm, edited 1 time in total.
Blackhoodmafia!!!!
1972 Nova SS
572 C.I. BBC
Best to date: 1/8
et: 5.28
mph: 134
new wt. 3340 lbs
1972 Nova SS
572 C.I. BBC
Best to date: 1/8
et: 5.28
mph: 134
new wt. 3340 lbs
Re: My Big Block Wont Rev!
Not disagreeing with the cam being too big,...but. What kind of fuel delivery system are you running? Give us the whole system from tank to carb please. Regulator(s)? Float settings in the carbs?
Bending the push rods raises a few concerns. How are you doing with coil bind? What lash are you running? Does it do better by increasing lash? Have you leaked this engine since bending push rods?
Don
Bending the push rods raises a few concerns. How are you doing with coil bind? What lash are you running? Does it do better by increasing lash? Have you leaked this engine since bending push rods?
Don
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- Posts: 5
- Joined: Tue Apr 10, 2012 11:58 pm
Re: My Big Block Wont Rev!
1/2" ID fuel lines from tank to pump to 'Y' fitting to 2 x Holley fuel regulators (Mallory 140GPH Comp Pump), then 4 x 3/8" ID lines from regulators to bowls. I did a flow test once into a bucket, I can't remember the exact quantity but she was fair pumping! Holleys are east-west mounted & have centre-hung floats with bottom corners cut off for racing. Hilborn style air-scoop. Carbs also have K&N Racing Sub-stacks #85-0200 which are supposed to "reduces turbulence, improves metering accuracy. Increases the airflow by reducing restriction. Straightens and speeds up air-flow". I could always try it with them OFF?want-a-be wrote:Not disagreeing with the cam being too big,...but. What kind of fuel delivery system are you running? Give us the whole system from tank to carb please. Regulator(s)? Float settings in the carbs?
Bending the push rods raises a few concerns. How are you doing with coil bind? What lash are you running? Does it do better by increasing lash? Have you leaked this engine since bending push rods?
Don
When I changed the springs, I did a vacuum test when the heads were off & they were fine! The 3/8" push rods I had were only hardened ball type ends & they are not load-rated for big spring pressure so I purchased a set of Manley chrome moly 3/8" & reduced the spring pressure & they work fine! I learnt the hard way that hardened tipped push rods are no good for racing! Good call Don! Coil bind was checked & has plenty or clearance.
I haven't done a leak-down because there is absolutely no chuffing on the breather cap or popping the dipstick up! Besides, I only had the motor stripped down & freshened up in November. Lash is set at .024"-.025" Hot! (Haven't tried increasing lash).
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