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Ignition timing after porting

General engine tech -- Drag Racing to Circle Track

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Biteme
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Ignition timing after porting

Post by Biteme »

Is it normal for a 23* sbc head to need the ignition to be advanced after porting?
I’ve got some pro1’s on a motor that I’ve done done ssr work to, basically layed them back a couple of mm and reduced the swirl ramp a bit.
Timing was set at 36* when engine was previously dyno’ed. No other changes other than a bit off the roof of the exhaust port, but not much.
Now motor is showing some soot on the plugs and can be a bit sluggish at low revs.
Wondering about giving it some more timing?
My thinking is that it had a lot of swirl before, and I’ve traded some of that for outright high lift flow, and now the burns not as efficient.
Keep to hear others thoughts on this...
mag2555
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Re: Ignition timing after porting

Post by mag2555 »

If there's any part of a Intake port in a well developed aftermarket head like yours that you do not want to grind on without the help of a flow bench and a velocity probe, or better yet a swirl meter it's the short turn area that you ground on!

Without the aid of a flow bench and welding material back on the short turn the next 2 best thing to do will be to recut the valve job to max out the valve OD and in the process make the bottom cut taller, or install a oversized valve and then replicate the original valve job.

Also the gasket matching that you did on the Exh side likely increase reversion at low revs and in turn the higher consentration of Exh gases diluting a fresh intake charge can kill off low speed Torque and make a need for additional ignition timing to try and burn the crap again!

The area of the Exh port at the flange should always be a minimum of 1/16" less in circumference then what the port is dumping into, even if it eats up 5 or so cfm in high lift Exh flow numbers.
rebelrouser
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Re: Ignition timing after porting

Post by rebelrouser »

not an expert, but never had to adjust timing much after porting a set of heads. what I usually notice, is an improvement in idle quality, and a lot more snappy when I crack the throttle. Even when I do a basically stock engine, the idle and throttle response is noticeable.
I will also agree with the critical short side radius, what I have been doing is, try to do a decent 3 or 4 angle valve job, with back cuts on the valves, smooth out any casting flaws, usually widen the push rod pinch a little, open up the bowl under the valve, and blend the machine work when they made the seat. Then take a velocity probe and measure the air at the short turn and slowly lay it back noting the velocity, and turbulence with a string attached to a piece of welding rod. I have noticed just a couple passes with a sanding bur usually helps a bunch on the flow numbers. I have the velocity numbers I look for written down at the shop, do a search it has been talked about several times on the forum, by people smarter than me.
And I am sure I leave some on the table, but the heads seem to work very well using this method. If you play the internet game comparing numbers it can make you crazy. I also have seen a lot of CNC ported heads that while they flow pretty good CFM, the velocity in the runners is down, and they do not run as good, as my hand ported heads that flow a little less, but have good velocity in the runners.
mag2555
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Re: Ignition timing after porting

Post by mag2555 »

The bottom line is that without a Intake flowtest being done you do not know if out of the box any given head even needs it short turn layed back or rearced!

If a any point above .500" lift you see the flow numbers drop, then that is a strong indication that the short turn arc and or width is not all it could be.

On the other hand if the high lift flow numbers just level off and stop rising, then that indicates a restriction Area before the short turn like at the window area, roof, push rod pinch point or a combination of all 3.
Biteme
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Re: Ignition timing after porting

Post by Biteme »

Thanks for the replies.

I’ve had the heads about 15 years and do a bit more porting on them every few years or so, just as a hobby but I enjoy learning and seeing what works and why.
They were flowed a few years ago and yes, they were backing up above .500 lift.

My grinding is slowly getting more adventurous 😊
A friend of mine is a bit of a guru with these things and he guided me through altering the short turn. We have access to a flow bench but it takes the guy a week or two to get around to doing the test, and he doesn’t like us hassling him, so I haven’t tested them since.

The exhaust work was my idea. They were always a bit smaller than the flange to try and reduce reversion, but I decided I’d see what happens if I remove that anti reversion step at the top of the port. It’s not to difficult to enlarge the opening at the header flange if I need to.
Sounds like that may certainly be at least part of the problem.
Many thanks again.
1980RS
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Re: Ignition timing after porting

Post by 1980RS »

The only set of heads I had to set the timing lower on were on 2nd set of Vortec's as they worked the best with 29-31° timing. The stock set I used ran from 34 to 36° but the 2nd set had some chamber mods that may have helped make some more power with less timing.
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Re: Ignition timing after porting

Post by Belgian1979 »

I wonder whether there is a relationship between the intake port and soot being deposited.

Can someone explain please?
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Re: Ignition timing after porting

Post by CGT »

mag2555 wrote: Thu Oct 15, 2020 12:19 pm The bottom line is that without a Intake flowtest being done you do not know if out of the box any given head even needs it short turn layed back or rearced!
No. The bottom line is without doing a dyno or track test you don't know. Your test would have a basis in assumption...which may or may not be correct.
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Re: Ignition timing after porting

Post by 1980RS »

I can tell you one thing for sure. After I took the Strip Dominator intake off my 461 that I used the burr finish on inc. the heads it had the cleanest runners and plenum of any intake and head ports I have ever used for running a couple of races. No fuel drop out anyplace even with the carb being a little on the fat side.
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Re: Ignition timing after porting

Post by Biteme »

Belgian1979 wrote: Fri Oct 16, 2020 8:51 am I wonder whether there is a relationship between the intake port and soot being deposited.

Can someone explain please?
My current line of thinking is that because Ive layed the ssr back there is now less swirl, particularly at low rpms.
And possibly some fuel getting flung into the side of the chamber where the spark plug is.
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Re: Ignition timing after porting

Post by BLSTIC »

It also sounds like you made the exhaust port larger when you already likely had a port and header that was too large in that area. Port energy/velocity fights reversion even in the absence of steps/chambers/what-have-you.

If you think that port matching you did hurt things you may be able to make some kind of insert that fills the exhaust port and/or part of the header to see if you get some low end back (I've seen things called 'port plates' that are specifically designed to do that). It won't be a real solution and would likely hurt up top unless you get the shape perfect, but it could help diagnose.
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Re: Ignition timing after porting

Post by steve cowan »

Biteme wrote: Thu Oct 15, 2020 2:08 am Is it normal for a 23* sbc head to need the ignition to be advanced after porting?
I’ve got some pro1’s on a motor that I’ve done done ssr work to, basically layed them back a couple of mm and reduced the swirl ramp a bit.
Timing was set at 36* when engine was previously dyno’ed. No other changes other than a bit off the roof of the exhaust port, but not much.
Now motor is showing some soot on the plugs and can be a bit sluggish at low revs.
Wondering about giving it some more timing?
My thinking is that it had a lot of swirl before, and I’ve traded some of that for outright high lift flow, and now the burns not as efficient.
Keep to hear others thoughts on this...
Wondering if you have measured port runner sizing,PRP,THROAT,HEIGHT OF SSR OFF DECK etc.
Taking 0.080" (2mm) off SSR is actually a bit of material.
I have several sets of Dart heads in cast iron and aluminium ranging from 165cc to 220cc as cast they all suffer from turbulence issues as cast on the intake and exhaust.
The throat area on dart heads are already big as cast usually 89% .
I ruined a sportsman 2 head on the exhaust port by making it to big,some exhaust ports respond to bias on the cylinder wall side but not all dart heads,cfm is easy to get on an exhaust port by just making it bigger but that's the wrong approach in my opinion.
The last couple of years I focus more on making the air sound smooth and more stable and not worry about cfm,its been QUOTED before by experienced head porters that CFM will fall into place if CSA is where it needs to be.I understand that not everyone has a flowbench and I understand that experienced head porters can port cylinder heads without a flow bench but that just comes down to experience.
For me as a novice, testing at different depressions up to 40 -45 inches has shown me some different things.
If possible some engine specs,application cam timing specs. EG - intake closing.
Might help with more information :D
steve c
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Re: Ignition timing after porting

Post by Biteme »

Hi Steve yes I measured pretty much everything.
Prp is 2.25
Top of ssr is 2.7
Floor was 1.115”
Now 1.065

Exhaust ports I don’t pay too much attention to cfm numbers, because what actually happens in the engine is completely different.
These dart heads were originally 180cc and it’s pretty obvious looking at them that dart has designed them to fit up to stock exhaust manifolds, or 1”5/8 headers. I use 1”3/4 so I’ve opened them up a couple of mm, but didn’t touch the floor of the port.
I could easily just make them larger but that’d require a larger header and that ain’t happening (it’s a Torana)
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steve cowan
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Re: Ignition timing after porting

Post by steve cowan »

I took my cast iron darts down to 1.050" at the apex as cast deck.
I will measure my dart alloys 180cc out to 203cc.
On the cast iron heads there is still 0.100" material on the apex when i sonic tested.
On my dart 180 alloys I reworked the SSR on the exhaust to correct turbulence, flange size is standard size as I am running 1 5/8"
Tri-y pipes.
I find it interesting how a 1.6" exhaust valve is used in a mild SBC all the way up to a 1100hp 400 ci pro stock engine.
The correlation between exhaust flange,exhaust throat,header flange size and engine application is interesting to me,I enjoy reading Calvin Elstons posts and threads.
steve c
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