I read in a Brodix ad that their combustion chamber was designed to deter detonation on a particular BBC head that I was reading about (not mine, I have rectangular Race Rites on a 496). Talking to others I have heard opinions that flat tops are easier to avoid lean spots versus dome pistons that may tend to create them. In my case I have 60 over SRP dome forged pistons that called for 112 cc heads....... unfortunately my heads were 119 cc to start with and the first build had very low compression and way below my minimum power requirements so I rejected the motor.

The "cure" was a different cam and to angle mill the heads all the way down to 108 cc......... the angle mill is where some say I have a problem and that it was the cause of 4 pistons detonating and I now have a combination of heads and pistons that actually increase the possibility of detonation...... and for me not to be surprised if it breaks again because the same situation still exist.

So what I am looking for is comments from those who know on whether or not the radical angle mill is encouraging a lean condition and I might add by the exhaust valve as this is where the damage was at. BTW when I measured for clearance for cam with more lift there was plenty of intake clearance but several hundred thousands less of exhaust clearance that is down to about 1/100 clearance.

Thanks in advance

Doug

 

So is something wrong with it now? Is it hurt? Or is this preventive thinking?

Angle milling removess more material from the exhaust side of the head. The chamber shape is the same overall. You might have less dome clearance around the spark plug side....but if they aren't hitting anything...you're OK.

Angle milling is a very common procedure...mine were whacked from 129cc after porting down to 112 cc. A pretty good bunch of cutting. No issues.

Tight quench helps somewhat....but isn't a cure all. I'm a little concerned with those valve measurements. Are you SURE that cam isn't retarded a lot? I know you were working it hard to get it dialed in and it sounded like you got it....but tight ex clearance usually comes from a retarded cam.

What's cranking compression?

Other than a basic assembly issue......the rest is just tuning. You have to give it what it needs. If it needs more fuel...you give it. What do plugs look like? You can get an idea of cylinder temps from reading ground strap coloring.




JIM

 

Hi Jim

Thanks for the reply and that is good news to me

More like paranoia and listening to others give their 02 then any red flag being the cause of my concern........ although I was surprised at the big difference in valve clearance from intake to exhaust.

I am not back on the road yet but that is just due to a bunch of small issues like wiring in some new stuff (with no surprises) but a couple of free days will take care of that

Cranking compression was about 180 the last time and although I ran it on a test stand I was in a hurry to get the motor back to the house and I neglected to do a compression test......... However nothing was hitting and I will say it sounded really good and revved instantly. I did have the heads flowed by Velocity Heads in Knoxville while I was down and I now have sufficient spring pressure (260) along with titanium retainers........ the old ones were down to 220 or so and the valve float was causing the shims to get beat to death (that was the source of the steel 'threads" I was finding in the oil)

The only disappointment was I was hoping to switch cams for more lift then the 668/678 I have now but the exhaust clearance being that close means the cam will have to stay.

The cam went back in as it was and I believe that it was 6 degrees advanced but I am going to check on the "SURE" part and get back to you.

Thanks again

Doug

 

Chamber shape has nothing to do with rich or lean. Angle milling is very common to get the CC down.

Piston to valve clearance should be kept up to .100 or 1/10th of an inch.

I had big domes on my 508 BBC. I just polished the chambers and the pistons so they did not have sharp edges.

 

Thank you for your reply

Maybe I should have stressed detonation rather than leanness as the "possible result" of some combustion chamber design versus the design of others........ detonation was the word used by Brodix that I interpreted as caused by leanness

Do you think that the claim that "their head was designed to reduce the possibility of detonation to be a bogus claim?

I guess you guys can see how that I figured that if you lower the head unevenly through angle milling in my mind I would think that a uneven volume distribution would be the result. Then spark would then have to travel around the dome thus creating an uneven burn and temperatures versus a flat top that would not be affected.

This is one of those times I would have rather been wrong than right and I will go by opinions here based on real time knowledge versus my imaginative guesswork ....... Fortunately I was wrong and was creating unnecessary worries for myself.

Doug

 

I don't have a picture of your chamber in front of me, but most if not all modern heads have altered the shape of the chamber to promote high swirl fastburn technology. so they are detonation resistant compared to older open chamber designs.

The biggests % of power gain is going from 9..5 -10.5 compression something like 4% across the whole rpm band. 10.5 - 11.5 is under 3% 11.5 - 13 is under 2%.

Big domed pistons impede the flame travel from the spark plug to the far side of the cylinder. Where flat or dished work fine. The old trick was to hog out a small saddle in the middle of the dome to get the flame across faster.

I have a 17-1 compression piston here on my desk at work. It has a the crown bent down and a hole in the top from lean detonation

 

Sorry to take so long to get back on retarded cam timing possibility.

As far as cam timing goes ........ Comp called for a 107 center line but that was not possible as things were at that time and it was set at either 106 or 108.....I was there for that and I believe it was set at 108.

However I have made a decimal point mistake on the clearance issue of the intake side that we checked during the rebuild and after the personal change at the shop..... it is a 140 thousands clearance on the intake side (not 400 as I had said ) and 100 on the exhaust so the difference is not as radical as I said in my OP......... my bad....... sorry about that :o

I can't complain about not getting more things done things over the weekend....... the weather was gorgeous and I took a long ride on the bike

Doug

 

Newer design heads are incorporating a lot of what's been learned over the years to kill detonation and encourage good flame travel with no *dead spots*. There is a ton of things that affect this.

The older heads...like mine...are not known for being that *efficient* as compared to new stuff. The chambers are just big and open and were designed for huge domes. With *normal* domes or flat tops...you have a lot more open area to deal with. One of the biggest things they are doing is pulling in the *quench* side of the head (intake side) to force more mixture into center of chamber. The are also moving plugs toward center and closing up the area around the plug too. Then you will often see a larger scalloped out area around valves that improves breathing and gets cc's back to normal. All good stuff.

They are also designing them to eliminate fuel wash and keep fuel in suspension. When you pull heads and you see clean/washed areas around the side of the chamber....you are seeing it. Fuel is coming out of airstream. Take a look at Dart's website or Reher/Morrison and look for wet flow testing on ports and chambers. Neat stuff.....and is exactly why the new Dart Platinum heads make 20 more HP that the previous ones and look nearly identical until the difference are pointed out. Notice the vanes in the intake ports etc.

All of this flow stuff helps detonation too....if the mixture is better, there is less chance of random events occuring.

So yes, the newer heads are designed to help detonation issues. This usually makes good power with less timing.

Running tight quench is always a help too.

Lift has little to do with valve clearance. It's the rate of lift and the opening/closing points. Think about where the piston is at max valve lift......on the intake side it's going away from the valve and is a long way away. On the exhaust.....the valve is closing and piston is chasing it....but max lift has piston way down at bottom of cylinder. The tight part comes about 10* on either side of TDC...not at max lift.

.100 is VERY safe clearance.....you can run a lot less....especially on intake side.


JIM