I had an epiphany moment

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Posts: 499
Joined: Sat May 09, 2015 12:25 pm

I had an epiphany moment

Post by mooreken »

Ok. Sense no one is posting here’s another of my long winded stories.

On motor 1 and 2 I had noticed that the cylinder compression was averaging around 255lbs. As the compression ratio was only in the mid 9’s I was concerned that the cylinder pressure was too high. That is where the tidbit started nesting deep in my mind.
With the new Dart motor the comp ratio was getting into the very high 10’s so I now had another tidbit added to the file folder. Also when I took apart motor number 2 (motor has less letters to type than engine) I saw what I think was some detonation on two pistons. Very light but I’m almost sure it is there. Another tidbit.

While decreeing the cam on Dart motor 3 and with the cam straight up. Two rods were hitting the cam lobes. Now the cam, lifters, push rods, and heads were the same that I had used on motor 1 & 2 also they had no interference. The gear set was new. More tidbits.

I couldn’t find the cam card so I looked at Summit’s site and such for the lobe center info. I saw a setting for 107 degree lobe separation angle and jumped to the conclusion that that was the number. (As I am writing this we can assume that was incorrect).

After I pulled the offending piston/rod assembly’s so I could rotate the crank to do the INTAKE LOBE CENTER setting it was at 103. Another tidbit for the file.
My new gear set had 3 key ways but none got me to 107. Summit to the rescue with a 9 key way gear set. Now I have to admit that having to retard the cam around 6 degrees was leaving a nagging tidbit that this was maybe not exactly right. But as they say “can’t stop now” As I was way too deep into the build to back out now.
As the nagging thought was still there I checked and rechecked many times my procedure and readings and decided I had the 107 degrees in place.

I put the rods back in and everything thing clears. I used a long zip tie as a feeler gauge to see that the lobes and rods had at least a 1/16” clearance deep into the middle of the motor. That trick worked great.
Motor runs great I have around 500 miles and it seems to be doing great.

Now getting to the epiphany moment.
I am doing a chassis Dyno Friday if it doesn’t rain so I was trying to find my cam spec’s in case I was asked. Looking at Summit’s site again and there are 3 Thumpr cams along with the Mutha thumpr so I went and dug into the file that I had ready in case the State wanted receipts to get the summit part number and found the Cam card.
As I was reading the card there is no mention of 107 degrees. But there is a 102 degree intake center degree. Remember the 103 setting at the beginning… Oops

So I wore out the internet for retarded cam timing info and found these things.
Kills some torque at low speed (not all that bad with lite car and hi hp)
Makes more hp at higher RPM (Other than bench racing that isn’t an issue either)
These 2 are not a problem other than the bench racing jousting.
The other thing I saw was a video of Dyno testing advanced and retarded settings. They mentioned advanced timing will raise the cylinder compression. The dyno test showed around a 50lbs difference in an extreme settings test.
So I ran out today and tested a couple of cylinders and they only are showing 230lbs. Even with a higher ratio. So motors 1&2 had advanced cam timing 102 is pretty far advanced from what I saw in the examples hence the higher cyl compression numbers.
And now all the tidbits are sorted and fitting into the puzzle.

I went to my engine machine guy and quizzed him on all of this and he showed me where he has to take a small notch out of the rods on stroker motors to clear the cam. And we discussed any effects that the retarded cam might have for me and I’m leaving it alone (unless the dyno numbers won’t hold up in a bench racing session).
With the hint of detonation from the 102 advanced motors. I now feel better with the lower cylinder compression numbers in the Dart motor.

The main reason for the dyno session is I can’t load the motor on the street to get an accurate O2 reading without running around 140 MPH on the interstate. So the $75 for 3 pulls is cheaper than the ticket. I want to make sure all is good before coming off NASCAR turn 4 and running down the front stretch at Daytona.

I’ll try to video the Dyno runs and post.

Hope you enjoyed the visit into my innermost thought processes.

Posts: 266
Joined: Mon Jul 13, 2015 4:09 am

Re: I had an epiphany moment

Post by BlackSnake »

Very interesting epiphany Ken! Did your machine shop guy really say he notches the rods for clearance…gulp…😳
Posts: 499
Joined: Sat May 09, 2015 12:25 pm

Re: I had an epiphany moment

Post by mooreken »

Yep...he showed me some that he had just done. It's just a little spot on the big end near the bolt that catches up with the lobe as they are all swinging around....
Tom Wells
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Joined: Fri May 08, 2015 7:34 pm

Re: I had an epiphany moment

Post by Tom Wells »



Possibly Mike had the same reaction to the word "notch" as I did. When my 521 for the Cobra arrived in 2000, it was immediately disassembled and checked. Among the things that were discovered was a notch in the side of one of the rods about the size of half a dime; these were "hand selected truck rods" direct from Ford Racing. After much argument and stress, Ford replaced them with a set of Eagle rods - subsequent crate motors had Eagle rods as the default. Hmmm...

Anyway, a little bit near the bolt sounds a lot more benign.

Very interested to see the dyno data.

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