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SBC engine


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roppa440

Club Member

Tue 05 Dec 17 14:13

Re: SBC engine


stevel98 wrote (View Post):Not good.. Sad

Hope your going to be ok and not have to swap your C5 for an SUV. Rolling Eyes


I'm just happy it's not a wooden box. Laughing

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Daytona Vette

Club Member

Tue 05 Dec 17 14:24

Re: SBC engine


roppa440 wrote (View Post):stevel98 wrote (View Post):Not good.. Sad

Hope your going to be ok and not have to swap your C5 for an SUV. Rolling Eyes


I'm just happy it's not a wooden box. Laughing



You would not like that Dave! - Every time I have seen one of those on the road, they are never doing more than 5mph.


--------------------
Barry - Daytona Vette

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Redlevel

Club Member

Tue 05 Dec 17 17:07

Re: SBC engine


Sometimes at the mortuary here, its a Saab hearse that turns up to collect the deceased.

As if dying isn't bad enough! Sad

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roppa440

Club Member

Wed 06 Dec 17 10:35

Re: SBC engine


stevel98 wrote (View Post):Sometimes at the mortuary here, its a Saab hearse that turns up to collect the deceased.

As if dying isn't bad enough! Sad


It's been like a mortuary on this forum lately I have noticed.

Back to the original post subject, I just want to add that the high initial advance will probably sort most of the problem by giving you enough rpm to get the throttle blades closed enough to pull the idle mixture circuit online. But sometimes with a very big can you will also need to drill a small hole in each throttle blade to get the RPM up enough. Starting at 0.5mm and going up in 0.5 steps until correct idle speed is achieved.

I have head Ross say "you have to run rich with a big cam to get it to idle". This is not actually true. It is down understanding how to best modify the distributor and carb to match the engine modifications.

Here are the readings from my 550hp 440 Chrysler when I had it. Idle mixture 14.2 rpm 880rpm, 9.1 inches of manifold vacuum.



Last edited by roppa440 on Wed 06 Dec 17 10:37; edited 1 time in total. [2 %]

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roppa440

Club Member

Wed 06 Dec 17 10:37

Re: SBC engine


I have noticed there has been no response from the other experts on here.

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roscobbc

Committee Member

Wed 06 Dec 17 11:05

Re: SBC engine


What I actually said was "low vacumn means stock power valves will flood engine with too rich a mixture at low rpm" i.e. HiPo cam usually means low intake vacumn. If using a carb (in this case 4160) the low vacumn will 'trick' the carb into thinking that it needs an enriched mixture and opening up the power valve (as it would under conditions of acceleration). Simply put - the OP needs to establish which power valve (valves) are installed in his carb before doing to much more.



Last edited by roscobbc on Wed 06 Dec 17 11:06; edited 1 time in total. [4 %]
--------------------
CCCUK Chairman
'68 coupe - 4 speed - 3.08 - 489 cu in - 570 bhp @ 5850 rpm - 606 ft/lbs torque (821 Newton Metres) @ 3850 rpm
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roppa440

Club Member

Wed 06 Dec 17 11:30

Re: SBC engine


roscobbc wrote (View Post):What I actually said was "low vacumn means stock power valves will flood engine with too rich a mixture at low rpm" i.e. HiPo cam usually means low intake vacumn. If using a carb (in this case 4160) the low vacumn will 'trick' the carb into thinking that it needs an enriched mixture and opening up the power valve (as it would under conditions of acceleration). Simply put - the OP needs to establish which power valve (valves) are installed in his carb before doing to much more.

Gotcha. Thumbs up

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roppa440

Club Member

Wed 06 Dec 17 11:44

Re: SBC engine


Just to add a couple of things for interest.

Main primary jets I adjust to around 13:1 mixture or just a touch leaner. I find this gives the best power and plug colour with modern fuel. In the old days we used to aim for 12.5:1 which seems too rich now.

If you have a Holley carb with metering plates instead of jets on the secondaries your only option used to be to drill the holes in the plates if they needed to me richer - making it a J-B Weld job if you want to go back the other way.

But Promax carbs came out with this neat metering plate that takes normal Holley jets. I used them on my 440 and they really take the guesswork (well some of it) out of secondary tuning.

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roppa440

Club Member

Wed 06 Dec 17 11:48

Re: SBC engine


Also bear in mind that on an auto transmission car the mixture will be leaner and the rpm higher when idling in neutral. You need to tune the idle mixture in gear on an auto.

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Dellb

Club Member

Fri 29 Dec 17 12:54

Re: SBC engine


Hi All
Many thanks for your advice, it worked. I have increased the secondaries, and decreased the primaries and she is fine, with the volume control screws now doing their job.
However another issue has raised its head that roppa440 touched on. The engine is fitted with a comp cam magnum 270. It is smoothest (vac disconnected) at an enormous amount of initial advance. 23 degrees. At this it is quite smooth and pulls 15 inches vacuum.
If I drop to the 15 degree norm, it becomes quite rough and the vacuum drops to 13 inches which is right on the limit of the 6.5 power valve. The current vacuum is taken off the carb. With the Mallory unilite distributor mechanical advance, the total advance is going to be unacceptable.

On another issue, I got a Gunson gastester as a Christmas present. My thoughts are, with a dual plane inlet manifold, how do you determine which control screw to adjust?

Any ideas?
Delb


--------------------
Derek L G

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