Ok folks, I've been driving this combo for a while, and I haven't had an issue. I am about to the point of changing some things on my setup, but I wanted to know why am I not knocking with only 91 octane.

12.3 Static Compression Ratio
10.93 Dynamic Compression Ratio
On AFR Eliminator Comp Port 195s...

And I'm tuning 91 octane just fine. And TR55 plugs.

Is it safe to say my timing is a bit on the weak side? I haven't had a hint of knock and drove 750 miles today. (Haven't looked at my timing yet)

Specs
6" rods
(.025) Undecked block
.028 thick head gasket
4.125 head gasket bore
58cc chambers
-5cc pistons
4.030 bore

Cam card in picture
Intake closes 42* ABC

 

Cam card

I'm using 1.7:1 rockers

 

Stock stroke?

The short block build looks a lot like my 355 but we decked the block and have the pistons almost zero deck height .005 in the hole and .027 cometics. Our cam cards are different though! I went with a tighter LSA.

 

I know this isnt what your point is, but Afr heads dont like a lot of timing.

A couple degrees less timing (vs. stock heads) makes more power on afr heads.

Esp in the 2800 to 3200 rpm region. Ive done experiments on the dyno (with my forced induction car) and picked up power in that range by dropping a few degrees of timing. (And no, i wasnt getting any knock retard before/after).

There is a misbelief that an engine will keep making power with more and more timing until the engine detonates and destroys itself. This is not true. Esp in that 2800 to 3200 rpm region. There is a plateau.

Ok back to your question:
Maybe your engine is not detonating because of the ethanol content of todays gasoline.

 

Wow, when you find your secret, let me know. Many LT1's like their timing at 36-38. Lower numbers can be achieved with an efficient combustion chamber which your heads have. Which heads have you got....the 1039's?
I'm betting your timing is down more than a little. Are you running the LT4 knock module. With no knock I'm assuming that your pistons have a nice quench pad, but .053" quench is at the high side of optimal. There's no reason that your engine shouldn't be knocking like a woodpecker with Parkinsons disease. 10.93 is a bunch.......all of it even.

 

Stock stroke, 3.48"

Yeah, 1039s

LT4 car, I've never checked, but I assume LT4 knock module.

I want to get this data logged ASAP. I'm wondering if the knock isn't audible and the car is just pulling timing before I know it.

Strange. It pulls up to 7k no problem, and sounds beastly. Doesn't feel like it's a super high compression engine.

I've also been informed, AFRs use different style plugs than what I have. Looks like they need to change!

 

Who did the tuning? Mail order or dyno. Some tuners are a bit more conservative than others. Too little timing wont have a super impact on horsepower but it will affect torque more. So the 7000 RPM sounds about right. Need to get a data logger on it. Keep us posted

 

I'm not sure what programs your using to calculate this stuff. Its going to take me a hot min to figure out what's going with your numbers... I know I helped you spec some of this engine and I would never turned something like these numbers loose.

OK so using your engine info, I'm getting

10.78 SCR
8.48 DCR

On the SCR calculation - Fooling around with the numbers I found that you are accidently switching the positive 5cc volume for the valve reliefs as a negative volume (like a dome piston would have). Valve reliefs are a positive volume so it's +5cc.

Still not sure what your doing on the DCR.

If I use the @.050 closing point of 42* ABDC and the 12.3 SCR I get 11.22 DCR. BUT you don't use the @.050 closing point to calculate DCR....

You should use the advertised duration to calculate the Intake closing point correctly for DCR - which in your case is 66* ABDC.

On the tuning deal- You never said what your total timing is so there is no way to know if its not enough or too much.... monitoring knock counts wont tell you anything about what the engine wants for best hp. Too many things cause knock counts in actual engine operation for it to be useful.... it's a tool for the computer to keep the engine from damage not something you monitor for an indication of max performance.

I can tell you from experience you engine will want about 30-32* total timing at WOT. Problem with an LT1 and it's optispark and no timing tab or degreed balancer.....is there is no way to know what it actually is with a timing light - So tuners just add and subtract timing via the tables until it makes best power....

On the plugs... AFRs are cut for 14mm .750 reach gasket seat or taper seat plugs.... Your fine using the TR55 which is a taper seat.

Will

 

That's a lot of lift with the 1.7 rockers. Hope your valve springs are up to par.

 

......... .... This is what I came up with on static compression ratio approx. 10.8:1 ................. 0.612 lift is about as far as the AFR springs will go ... always check valve spring coil bind with the retainer that you'll be using and always measure retainer to valve seal as assembled .............

 

I used www.wallaceracing.com/dynamic-cr.php to get the dynamic. And their static calculator.

I didn't know that you didn't use the .050 closing point... Good to know!

I feel like a fool, but glad I know now.

 

Just a FYI.

Cam cards were informally standardized back 50yrs ago to show timing events from the @.050 numbers.... I don't know why they choose that particular duration number but they did. At any rate it's not a very useful number for determining much about a camshaft but at least everybody does it the same. They also usually have the Intake Center Line, Lobe Seperation and Advertised Duration. If you know how you can calculate every thing about a camshaft from those numbers.

For DCR calculations you are trying to figure out where the intake valve actually closes on the seat because thats where the engine starts to make compression/cyl pressure. The closest number you can get is the seat timing number or "advertised duration". If you look at the top of Wallaces calculator is says " SEAT TO SEAT SPEC FOR....." Thats what that means.

So now everybody wants to know how to figure out where you intake valve opens and closes when it is not printed on the cam card....

To find the timing events for the intake:

Divide the Advertised Duration in half and subtract the Intake Center Line from the remainder.

That will give you the opening event. Intake Opens Before Top Dead Center (BTDC)

For the closing event subtract 180 from the duration and then subtract the opening number from that sum.

That will give you the closing event. Intake Closes After Bottom dead center (ABDC)

So in your case

Intake Open -

276/2 = 138

138-108 = 30 (30* BTDC)

Intake Close -

276-180 = 96

96-30 = 66 (66* ABDC)

Will

 

Will,

Absolutely perfect. You should teach!

Thank you very much. I had no clue. Why even have the .050" numbers if they are irrelevant?

 

@.050 timing event numbers are not totally irrelevent..... it just dosen't tell you much about the cam.

Now if a guy wanted to know how much duration (out of 360*) a camshaft had - when the lifter happened to be .050 off the base circle - Why you'd have it right there on the cam card.....

In reality it's a data point.... And it's a consistant data point... One that is the same across all camshaft grinders cam cards. Some folks "think" they can judge how aggressive a lobe is or generalize how well a cam might run compared to another by looking at the @.050 duration numbers but they couldn't be further from the truth.....

What you can do is put a degree wheel on an engine and verify every one of those numbers.... and thats about it.

I do like to teach people.... I see so much mis-information, so much BS some nobody dreamed up to make themselves sound smarter about engines perpetuated on the internet it drives me crazy sometimes.... however I understand that not everybody can be an "engine builder".... I'll leave my assessment of what an "engine builder" is for another time.

So since I am in a teaching mood today. I'll give out a little more info about camshafts....

There are other informal cam industry "standards" people should know about. These could be quite confusing if you were trying to compare 2 cams - Say one mechanical lifter and one hyd lifter.

Hyd cams "Advertised Duration" numbers are measured @ .006

Mechanical cams "Advertised Duration" numbers are measured @ .020 (to get the lifter past the lash ramp).

Before the 1960's it wasn't like this.... cam grinders used whatever refrence number they wanted... Eventually it was used a marketing tool to make their cam seem bigger or better than the other guys..... They knew most engine builders of the time didn't have a clue what they were looking at. If one cam card said 316* duration and another one said 340* duration... people would assume the 340* cam was bigger. In reality it could have been the same or a very similar cam - Just one mfgr measued the duration from .020 and the other from .006.

The last standard is a pet peeve of mine. It's on every cam card printed in the last 50yrs.... The Lobe Seperation Angle (LSA).

LSA is a very misunderstood measurement. It's given all the credit and magical powers as to how well a certain cam idles or runs ect.... in reality LSA is nothing but a made up number that only exist because in a single cam engine the intake and exhaust lobes are physically located on the same stick. LSA is calculated by adding the Intake Center Line to the Exhaust Center Line, then divide the sum by 2..... Think about it - On a DOHC engine there is no LSA.... the lobes are seperated on (2) different sticks.

Drives me fricken bonkers when somebody tells somebody else they just need to make sure whatever cam they get its a 112 or 114 LSA so it'll be easy to tune, EFI or NO2 friendly ect....

Will