Spark Plug gap

[Tczz]

Hello I have a max wedge motor that is sometimes a prick to start ... Dirty fuel an dirty plugs have been replaced so I'm ready to throw the switch again but ...

The engine is a 63 dated MW rb block with SR440 Indy heads which were rated with alot of work to them as a bolt on 100 hp. Reputed hp for the engine was about 700hp but I don't have the paperwork to say... to me the motor does not look that busy so time on a dyno will be useful

Crane cam is # F-268/3868-2-8 which is a moderate spec cam suited to work with 3500 stall converter

Manifold is Indy 4150 with 850 Holley DP ... Fuel is Av Gas so up over 105 depending on age

MSD Dizz without the 6al box do have a one to go in for another day of fettling around ...

Question.... thats been wearing out the brain is the plug gap for these plugs NGK 5671-10 factory set is .021 has been suggested to bump them out to .040 ... compression is 225-230 psi cylinder pressure ... 021 I think is abit small and have had varying numbers suggested to use ...I have opened them to 040 ... the manual sets them at a factory 035... so yeah its a trail and error thing but don't want to risk damage.
thanks for the help
cheers

 

[5.7 hemi]

And what is your timing set at?

 

[Outsider]

MSD box, should be able to run .040" plug gap. Of course the plug heat range selection, the ignition as mentioned above, the carb tune, and the condition condition of the motor (ring seal and valve guides) all have an impact.

 

[Tczz]

And what is your timing set at?

to be honest I'm not exactly sure ..have put a light on it but it wasn't so steady

 

[5.7 hemi]

You need to figure out what your timing is at idle, if it’s erratic, it could be timing chain stretch.

 

[RemCharger]

Sounds like about 13 to 1 by the compression.
A -7 ,-8 is what I would try, and .035 gap
Set it and forget it.
Fuel quality will be the determining factor.

 

[Tczz]

You need to figure out what your timing is at idle, if it’s erratic, it could be timing chain stretch.

ok well that is a possibility ... never thought of that one, had considered that the harmonic balancer might be the cause for that ...

 

[Tczz]

Sounds like about 13 to 1 by the compression.
A -7 ,-8 is what I would try, and .035 gap
Set it and forget it.
Fuel quality will be the determining factor.

Thanks ..is there any sort of formula to use to get an accurate measure of the comp ratio ?

 

[RemCharger]

Thanks ..is there any sort of formula to use to get an accurate measure of the comp ratio ?

You would need all the paper work , or take a head off and measure.

 

[Tczz]

You would need all the paper work , or take a head off and measure.

ok ... I have some of it head work cam specs etc ... Do you have a motor that has the same or simalair compression....
might just nip the plugs back to 30 use the 10's for now and go from there just looking for a safe starting point cheers for the advice ..
bought the car a few years back, have been kicking it around since for abit of fun but time to make it better ... all help is appreciated ...
thanks

 

[RemCharger]

I had a couple of pretty grumpy max wedge head 383s. I was getting the same pressure #s. VP C12 race gas. Wouldn't come close to running on pump gas. Race motors.
Your spark plug specs sound like someone was going to run it on a heavy shot of juice
I ran -7s at .035. It was quite happy with that.