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Chamber mod intended to improve combustion

General engine tech -- Drag Racing to Circle Track

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BradH
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Chamber mod intended to improve combustion

Post by BradH » Fri Sep 13, 2019 10:57 am

I didn't/don't like some aspects of the chambers on my BBM Victors and carved on them a little bit.
- Exhaust transition into quench area rather abrupt
- The standard 15* valve angle is a plus, but the standard location of the valves vs the exhaust-side cylinder wall is too close
- The chamber's location with respect to the bore results in a pretty large quench area -- all on the cam-side of the chamber -- and at some point I know the quench ratio can be too big
- Most current 2-valve inline chambers I've looked at have a more gradual transition from the exhaust valve into the chamber, unless they're very small for high compression
- Stuff I've read regarding "reverse flow" into quench area indicates improving this characteristic helps overall combustion and can reduce amount of ignition lead required, too

So, I decided to lay back the transition area similar to many LS-type heads; here are the before & after shots. FWIW, I don't do much porting these days anyways, and never felt comfortable messing with the chambers. I had an old head to practice on before I touched the Victors to get a better feel for how to not botch it up too badly.

I wasn't worried about the loss in compression, since it was close to 11.5 before. A quick chamber volume check showed the mod added about 2 cc to the total, bringing the CR down to about 11.25. It's primarily a pump-gas engine anyway, so the less $$$ fuel I need to blend to keep things happy the better.

"Before" pics of chambers and piston tops after dyno time to show burn patterns:
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Chambers burn pattern 1_3.jpg
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Pistons burn pattern 1_3.jpg
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"Before" pic of chamber scribed to lay back exhaust transition to quench pad:
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Before 1.jpg
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"After" pic of chamber with work completed:
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After 1.jpg
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Two more "after" pics showing different chambers / view:
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After 2.jpg
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After 3.jpg
.

:|
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ClassAct
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Re: Chamber mod intended to improve combustion

Post by ClassAct » Fri Sep 13, 2019 12:14 pm

Two things.

Very nice work.

From the looks of the piston tops, and this is just looking at the pictures, it looks like you have more swirl than I like to use.

That's the mod (one of them) I used on W5 heads.

mag2555
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Re: Chamber mod intended to improve combustion

Post by mag2555 » Fri Sep 13, 2019 12:21 pm

You could have not made the rework width the same all the way around to the chamber side of the valve because no matter what you do in a wedge type chamber head the side of the Exh valve that the side of the chamber raps around only flows may be 35% of what the rest of the circumference of the valve flows.
Compression effects Exh flow ratio also in other words.

BradH
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Re: Chamber mod intended to improve combustion

Post by BradH » Fri Sep 13, 2019 1:06 pm

mag2555 wrote:
Fri Sep 13, 2019 12:21 pm
You could have not made the rework width the same all the way around to the chamber side of the valve because no matter what you do in a wedge type chamber head the side of the Exh valve that the side of the chamber raps around only flows may be 35% of what the rest of the circumference of the valve flows.
Compression effects Exh flow ratio also in other words.
Maybe my afternoon coffee hasn't kicked in, but my initial reaction to your reply is... "???"

I can say that a 2.14" intake valve made about the perfect template for scribing the new exhaust quench line, once I'd put a reference line across the head at the bottom of each intake transition.

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ClassAct:

Re swirl... it's a dual-purpose (street/strip) engine, so I don't know that this is a bad thing. It'll idle at 1200, cruise 65 MPH at 3500, and cross the 1/4 stripe at 7K+, so it's going to have a fairly wide operating range.

Oh, you don't think I'd post pics of the ones that turned out ugly, would you? :lol:

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This is one of those "Well, it sounds good on paper" things that I'll never know exactly what difference it does (or doesn't) make, since the engine won't be going back on the dyno again. If the timing that makes the most MPH at the track and the amount of race fuel I need to blend to keep the engine outta Knock City are less that what it looked like it needed on the dyno, then I suppose I'll have something of an answer.

RevTheory
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Re: Chamber mod intended to improve combustion

Post by RevTheory » Fri Sep 13, 2019 3:32 pm

Vizard has been making that cut on BBC heads for awhile now and I want to say he says it's good for something like 10-15 hp. He lays them back quite a ways, too. Probably about the extent that you did.

wjnielsen
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Re: Chamber mod intended to improve combustion

Post by wjnielsen » Fri Sep 13, 2019 3:43 pm

The look of those cuts made on the heads remind me of relieving adjacent to the valves on a flathead Ford V8. This was a modification that was pretty universally thought to benefit breathing on a lot of engines back in the day.

Stands to reason that it might allow a little better flow out of that side of the valve area, probably more so at low-intermediate lift.

Bet it helps.
seduce the attractive... charm the rest

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MadBill
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Re: Chamber mod intended to improve combustion

Post by MadBill » Fri Sep 13, 2019 10:49 pm

RevTheory wrote:
Fri Sep 13, 2019 3:32 pm
Vizard has been making that cut on BBC heads for awhile now and I want to say he says it's good for something like 10-15 hp. He lays them back quite a ways, too. Probably about the extent that you did.
Yes, he said it eliminates the reverse flow at the bottom of the exhaust port. (which apparently drove the factory 'C' and 'D' port designs) The D port inserts I fabbed for my round port ZL1 headed BBC picked up almost 20 CFM and made it a lot quieter.
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