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Username Post: Big block aluminum head question        (Topic#351481)
Docjim 
Contributor
Posts: 249
Docjim
Reg: 01-11-07
06-08-18 04:12 PM - Post#2736344    

Hey everyone! I have a question about big block chevy heads. I have the 781 heads on my 454 now. It has hundred thousanths pop up pistons in it and I get a bad ping at 2500 RPM. Tried all kinds of timing tricks to get rid of it but no good. My question is. Are there any good aluminum heads (or head, gasket combos) that would get me around 9.5:1 with these pistons so I can run pump gas? Greatly appreciate any help. Best to ask the pros questions before spending the money


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Edited by Docjim on 06-08-18 04:13 PM. Reason for edit: No reason given.

 




Shepherd 
Frequent Contributor
Posts: 1371

Loc: Lake George, NY
Reg: 11-11-15
06-08-18 05:13 PM - Post#2736349    
    In response to Docjim

What does your static compression ratio work out to.? How much cam is in the engine?



Edited by Shepherd on 06-08-18 05:14 PM. Reason for edit: No reason given.

 
Docjim 
Contributor
Posts: 249
Docjim
Reg: 01-11-07
06-08-18 05:48 PM - Post#2736352    
    In response to Shepherd

Was supposed to be 9.5 to one but It now doubt it. It has 520 lift 240 duration.

Visit my webpage at http://www.picturetrail.com/docjim


 
Shepherd 
Frequent Contributor
Posts: 1371

Loc: Lake George, NY
Reg: 11-11-15
06-08-18 05:55 PM - Post#2736353    
    In response to Docjim

Before any changes to the engine are attempted you should find out what you have for a starting point, otherwise a lot of guess work could trip you up.



 
grumpyvette 
Senior Chevytalk Moderator -- Performance Subject Matter Expert --
Posts: 17188
grumpyvette
Age: 70
Loc: FLORIDA USA
Reg: 03-16-01
06-08-18 06:42 PM - Post#2736357    
    In response to Shepherd

you need to know the piston dome volume,and deck height and the block deck height, to calculate the correct head gasket and quench


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IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!


Edited by grumpyvette on 06-12-18 06:13 AM. Reason for edit: No reason given.

 
IgnitionMan 
Valued Contributor
Posts: 3373

Reg: 04-15-05
06-08-18 06:52 PM - Post#2736358    
    In response to Docjim

What is the current quench distance in the engine, please?

If it isn't where it is supposed to be, you may never be able to tune pinging, and possibly detonation out of the engine, no matter the other settings like timing, carb.

And, simply adding a thicker head gasket, could very possibly adversely effect that quench, making things worse.

Also, how is the ignition curve situation set, for initial, and both mechanical and vacuum advance, what vacuum sourcing for the vacuum advance, full manifold or ported??

Quench is the distance from the flat edge of the top of the piston, to the flat surface of the head, at TDC, measured in thousandths of an inch.



 
Docjim 
Contributor
Posts: 249
Docjim
Reg: 01-11-07
06-10-18 10:32 AM - Post#2736515    
    In response to IgnitionMan

Thanks for the tips. This is over my head. I am not a professional engine builder. I am getting a ping at 2500 RPM that is not there at lower or higher RPM. A bit of race gas additive and it goes away. I thought by going to aluminum heads that absorb energy better than cast iron and maybe a bigger head volume or/and thicker gasket I would solve the problem. Now I am not sure where to go with this???????????????? I do appreciate the help. Doc

Visit my webpage at http://www.picturetrail.com/docjim


 
IgnitionMan 
Valued Contributor
Posts: 3373

Reg: 04-15-05
06-10-18 06:18 PM - Post#2736539    
    In response to Docjim

doc, just a bit more info for you to contemplate.

"bigger head volume or/and thicker gasket". When emissions controls came about, fuels had key detonation/ping reduction components removed, and the fuels went to being just plain bad.

Many engine builders, parts manufacturers,and "techs" erroneously figured out that a thicker head gasket, and/or pistons down into the bore at top dead center, and other measures would take care of the issues created by ongoing down tech in many areas of engine design and tech.

We got from an, at that time, a very high volume piston manufacturer, a piston line that dropped the top of the piston down into the bore another .020 from stock, erroneously called "destroked", in an effort to lower compression pressures, to stop the pinging.

Gasket makers jumped at the chance to sell the daylights out of thicker head gaskets, to0 accomplish the same thing.

Neither really helped. When combined, engines pinged worse than before, because the quench was adversely altered. That distance between piston deck and head flat was altered from what was needed to control and manage the air/fuel mixture positioning around the edges of the piston tops. Tis created lean spots, over heating of top rings, and a vast array of other issues, because the quench would no longer control the edge mixture capture and placement.

Larger volume of the chamber didn't help the incorrect quench distance problems induced by the piston/gasket "fixes".

That is why we asked you what you had in mind, and what you were seeking. To do the thicker head gaskets would more than likely make things a lot worse, and that would not be a good thing to do.

I have worked on a newly rebuilt engine that had both the lowered pistons and thicker head gaskets in it, and everything else right as rain, and it was dismal. Literally setting the quench distance correctly fixed the problem engine, it was right there, just one of the most important factors was way wrong. Fixing it, the engine woke up, didn't have the problems it had before fixing the quench.

None of us here want to see you get into even further problems from doing something that would make it a lot worse.



 
Docjim 
Contributor
Posts: 249
Docjim
Reg: 01-11-07
06-10-18 07:33 PM - Post#2736544    
    In response to IgnitionMan

Thinks ignition man. Didnt know it was that complicated.

Visit my webpage at http://www.picturetrail.com/docjim


 
IgnitionMan 
Valued Contributor
Posts: 3373

Reg: 04-15-05
06-11-18 09:01 AM - Post#2736587    
    In response to Docjim

It really is simple, but gets technical along the way. Nothing that can't be done fairly easily once the factors to fix are known and worked with.

Ask, we'll help.



 
Rick_L 
Honored Member
Posts: 27415
Rick_L
Loc: Katy, Tx, USA
Reg: 07-06-00
06-11-18 04:47 PM - Post#2736627    
    In response to Docjim

  • Quote:
I am getting a ping at 2500 RPM that is not there at lower or higher RPM. A bit of race gas additive and it goes away.




Now that you know some of the background on the quench distance and compression ratio, now it seems to be time to discuss how to fix it without new heads or new pistons or some combination of those.


Seems to me that an ignition advance combination that has either less initial timing, or delaying the mechanical advance vs. rpm with some stiffer springs, or a combination of both might reduce the pinging at 2500 rpm. Does this only occur at WOT? If not, limiting the vacuum advance with a stop, or using an adjustable vacuum advance unit and raising the amount of vacuum required vs. the amount of vacuum advance might be warranted too.


If it's pinging hard, you want to avoid whatever situation makes it do that because it may eventually wreck your engine. But that may in turn limit how you can drive (and enjoy) the car and that's no good, so trying to tune it out is worthwhile.



 
grumpyvette 
Senior Chevytalk Moderator -- Performance Subject Matter Expert --
Posts: 17188
grumpyvette
Age: 70
Loc: FLORIDA USA
Reg: 03-16-01
06-12-18 06:11 AM - Post#2736688    
    In response to Rick_L

thats potentially very good advice hes giving you

IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!


 
Docjim 
Contributor
Posts: 249
Docjim
Reg: 01-11-07
06-12-18 08:58 AM - Post#2736709    
    In response to Rick_L

Rick L, It has an MSD distributor in it so it is very easy to change springs and such. I have done everything possible with it. Initial advance is 13 degrees and full advance is 32. The timing is all in at 4000 RPM as it is now and its the best combo I could come up with. Maybe it would be much easier to get a programmable distributor and take out timing at the 2500 mark?

Visit my webpage at http://www.picturetrail.com/docjim


 
Rick_L 
Honored Member
Posts: 27415
Rick_L
Loc: Katy, Tx, USA
Reg: 07-06-00
06-12-18 12:21 PM - Post#2736724    
    In response to Docjim

What kind of "programmable" distributor would do that? I'm not familiar with one that would unless you went to EFI with fully programmable timing tables.

That's not much timing. Seems to me that if it's pinging, it's not the "best combination". What happened when you tried other combinations?


Maybe IgnitionMan has an idea.




 
Docjim 
Contributor
Posts: 249
Docjim
Reg: 01-11-07
06-12-18 02:19 PM - Post#2736737    
    In response to Rick_L

RIck L when I tried other combinations it was better or worse. This is the best I could get with what I have. I did not try heavier springs because I thought 4000 RPM before it all came in is about all I want to go. I could go to the two heavy springs but then the all in timing would be near 5000.



Visit my webpage at http://www.picturetrail.com/docjim


Edited by Docjim on 06-12-18 02:21 PM. Reason for edit: No reason given.

 
IgnitionMan 
Valued Contributor
Posts: 3373

Reg: 04-15-05
06-12-18 06:01 PM - Post#2736761    
    In response to Rick_L

Does this MSD distributor have a vacuum advance on it? Is it degree stopped down to limit the amount of vacuum advance degrees? What vacuum pull is being used?

Most engines like between 20 and 24 degrees of IDLE timing, but the engine won't usually start with that much initial timing. So, we keep initial timing on the conservative side, and add the remainder with a device that brings in the rest of the idle timing after the engine starts, by vacuum.

Example, you are running 13 deg/BTDC @ initial and idle. Adding the rest of the idle degrees by vacuum advance, say, 10 more, we get 23 degrees timing @ idle, with using full manifold vacuum for the vacuum advance.

It also sounds like you don't have enough timing in the total, 32 degrees. This would give 19 degrees of mechanical advance. The engine would probably like a touch more, say, 35 degrees, by adding 3 more degrees to the mechanical advance.

I would also take a closer look at just where the throttle is holding the primary throttle plates in the carb, it could be that at the rpm it is pinging, it could be right at the transition overlap from the end of the idle circuits, into the bottom end of the main jet circuits, and might just be too popcorn fart lean.

Sounds very complicated, but, we can go over it all slowly, so we can help more. Stay focused, use common sense, and ask when you need help with all of it.





 




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