Well, in the continuing saga of reassembling my engine, this has come up.

The block has been bored .030 over and has been decked .050 for a deck height of .137. The head is from a '81 and has been shaved .100 for 43cc chambers, 1.7 /1.44 values have been installed with a home porting job done.

This works out to 10.04 static compression with a 9.5 dynamic compression with the Erson TQ 20M cam. The dynamic is high for a street engine and some of the compression calculator sites say the anything over 8.5 dynamic is not streetable and/or will require heroic efforts to make it work. Such as colder plugs, a lower thermostat setting, water injection, etc... (Hmmm, maybe I need to break out that Edelbrock water injection kit I bought 30 years ago...)

While porting the head I broke thru to water on number 3 intake. I brazed up the hole and it held brake cleaner just fine. After machining/hot tanking, it appears that the braze job was not as good as I thought. I pulled the intake tonight and it appears that some of the brazing flux was sealing the hole and this has broken out, leading to a large water leak. Which is why the head was pulled in the first place.

The question is, should I rebraze the crack? It would take less then a hour to do, a day to cool off and another day to reassemble the engine. I also risk the potential of warping and remachining. Or, should I just do another head?

I would be extremely angry with myself for ruining a head I put that much into but I would start over with another head. Which is easy to say since I have a spare on a shelf in the garage. My 2 cents.

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You're gonna be doing another head, or finding someone that has lots of experience brazing (20+ years, just to be safe).

9.5 DCR is really high... Currently I notice that the 7.X range is ok for 87 regular...up to the mid-8's is ok for plus 89, tending toward needing super... 8.8+ starts into the 92/super range...I haven't had a 9 DCR engine yet, but I would suspect that after yarding the timing down to almost nothing and pulling out the bag of tricks (water injections, etc...), you'll be visiting the race track for a few gallons of high octane to boost whatever is being fed at the pump...

-D.Idiot

I recall there being a few people on this site who used a product called Belzona on some of their heads.

I think Joshie225 was one, might try asking him how it worked out.

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Thanks for the advice, I guess I need to pull a head off the self and start again. I am more concerned about the DCR then the brazing. Another approach would be to use a cam with more overlap.

I wonder, the DCR specs seem to be a "rule of thumb" formula and I wonder if they were developed with a short stroke, large bore engine and if they apply to a long stroke, small bore engine.

I believe we can go a bit higher in DCR due to long stroke and small bore as mentioned.

I would use a cam with more overlap. Slants can handle more cam than a comparable std V8.

If intake port, then you can epoxy it and it will last for a while. I hear Indy Cyl Head can weld anything and I may get my "mistake" head fixed by them sometime. Otherwise, start over...

Lou

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Home of Slant6-powered fun machines since 1988

I haven't done the epoxy and sheet metal repair to my intake ports yet. It's been low priority. BTW, the plan is to make sheet metal patches (a new port wall really) and use the epoxy to keep the patches in place. If I'd ground into water on the exhaust ports I would have thrown out the head.

For a maximum effort head I'd like to sonic check the port walls so I don't hit water again and can choose the best head to start with.

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Joshua

I think it is Moroso that sells any epoxy for this sort of thing. It is pretty good stuff. Ive seen it on a few heads that have come throught the shop.

The DCR is kind of "subjective" scale to see how the engine is going to be compared to the valve closing event vs. just the swept volume. It's a little more accurate in determining what may happen (when ping may occur).

Our engines can use a bit more cam and be streetable, but you want to look at your LSA and overlap. More overlap lets you bleed a bit more pressure out and thus lowers the DCR, and makes a larger cam high compression engine be able to run on the street with lower octane.

My build I have the erson 280/270...has a little lump (not 340 lump...barely a .5" bump on the vacc. gauge once it all dialled in).
I chose the LSA at 111 to kind of build a bit more pressure and smooth out the idle a bit...the overlap is in the 50's which the hyperpak intake likes... This in turn allows enough bleed off that my 10.3:1 SCR is in the 8's for the DCR...runs fine on plus (vacc. adv. dialed back a bit...14 inital-16 mech-17 vacc), runs great on super....but can run on 87 *IF* I drive conservatively...
Looking back I think I could have run it more toward the 109 range and still been OK and probably would have been more in the 'regular' range for daily driving. With a short cam the overlap is so short you are going to build lots of pressure really fast and it won't have time to bleed down.
Currently beater valiant has a compcams CCX256 in it...I bumped the static compression to 9.66 last weekend...DCR says I'll be at about 8.2...so if all goes well I can still use 87...but it may like plus if I give it more advance...(current curve is 14 initial-18 mech-20 vacc...)

I would guess that if you were shooting for the 10:1 and dialed the cam in for something in the 107-108 range the DCR would come down a bit...(and that lobe separation is known to make pretty nice torque...)

2 cents

-D.Idiot

I pulled a stock, late '70's head off the shelf and cc'd the chambers. They come out to be between 58 and 61 cc's. My guess is this is a '77 or '78 head. It only has one emission sensor hole in the head, not two, like the '79 and later heads have. It may be of interest to some that these holes are the same size as the GM CTS sensor.

Running the numbers thru the kb-silvolite calculator, shows that with my combination of parts, I need a 56cc chamber. This measures out to 8.243 DCR which is close enough to the ideal 8.25 DCR for a street engine. This will give me a SCR of 8.716, which is a far cry from the 10.04 that the old head has now. I should be able to run standard pump gas without any heroics.

What does the calc say about 9.5 SCR = ? DCR for your build?

-D.Idiot

A 9.562 SCR would have a chamber volume of 48cc. This gives a 9.037 DCR.

Now what is interesting, with the 8.25 DCR combination, if I ran 10psi of boost thru the engine, I would have a compression ratio of 13.81:1 and would most likely double the output over a NA combination. Food for thought...