joshp14

iTrader: (5)

I am running 11.4 SCR and it's detonating like crazy under heavy load. I dumped about 2.5 gallons of 110 octane fuel in my 1/8 tank of 92 and it ALL went away. Do I really have to tear the motor apart to install thicker head gaskets to try to fix this? When I was doing these mods everyone was saying to "bump compression" and "optimize quench"....it turns out that it detonates like no other under load. What gives? I have tried reducing timing, octane booster, cleaning MAF, nothing helps with the exception of $5.50/gal 110 fuel. Mods are as follows:

Stock Shortblock, FM13 112+4, AFR 205 62cc, .040 Cometic's

Any help is appreciated.

ExceSSive

iTrader: (13)

Well this happened to me with 11.3:1 compression. I must admit when the compression went from 10.8 to 11.3 I did not get a retune right away...anyway here it is.

brad8266

iTrader: (23)

Im good on 93 octane with 11.9 ish SCR

Thimble

iTrader: (2)

11.6 SCR, 8.7 DCR, no problem at all on 93.

It's the DCR that matters..High SCR means nothing if your big overlap cam is bleeding off the pressure. Have you calculated your DCR?

SS101

iTrader: (2)

+1 on the DCR

valve events may lower your compression by keeping the exhaust valve open on the compression stroke.

joshp14

iTrader: (5)

That DCR stuff is confusing....I dont know if its right but I got a DCR of 8.78 using this website: http://www.kb-silvolite.com/calc.php?action=comp So I'm guessing a 8.78 DCR is excessive for 92 pump gas? What are my options to get this thing to not detonaate w/o using 110 race fuel?

Engine Spec's:

Stock shortblock
62cc AFR 205 Heads
3.930 Gasket bore .040" Cometic
FM13 Cam 112+4


Here is the info from my cam card:

Intake & Exhaust
LOBE SEPERATION = 112.2 CAM DEG
VALVE OVERLAP = 7.4 CRANK DEG

INTAKE
VALVE OPEN = 6.9 BTDC
CENTERLINE = 108.2 ATDC
VALVE CLOSE = 43.8 ABDC
DURATION = 230.7 CRANK DEG
LOBE LIFT = .35028 IN
VALVE LIFT = .59547 IN
LOBE AREA = 28.18 IN * DEG

EXHAUST
VALVE OPEN = 51.5 BBDC
CENTERLINE = 116.2 BTDC
VALVE CLOSE = .5 ATDC
DURATION = 232 CRANK DEG
LOBE LIFT = .34551 IN
VALVE LIFT = .58737 IN
LOBE AREA = 27.99 IN * DEG

chuckyt1

iTrader: (33)

DCR near 8 and SCR 11.25 no problems at all on 93oct...

pdd

iTrader: (3)

im at 11.3:1 scr and 8.6dcr and have no problems on 93oct

JScamaro

iTrader: (6)

I have read that 8.7-8.8 dcr is the max for 93 octane. Since you only have 92 octane you might want to lower your dcr a bit.

A few ways to lower your dcr are to run a thicker head gasket or retard your cam to +2 or +0.

If you retard it to +2 it will lower your dcr to ~8.6 and +0 ~8.5.

slow trap

iTrader: (22)

something don't seem right. 8.8 is fairly high but even so i wouldn't think you should have the issues you are having but i'm no expert.i had between 11:6-11:7 scr and around 8:9 dcr on 93 and mine ran great with no knock at all unless i was in 6th gear at a very low rpm under a pretty decent load .
some combos are different though and with your setup you may have alot of cylinderfill with those high velocity afr heads.going to a thicker gasket may not help at all since you will increase the quench.is the afr alittle on the lean side or possibly to much part throttle timing ?

allngn_c5

iTrader: (3)

Not sure on my DCR but its up there and yes I do have quite a bit of overlap. Running a big cam. SCR is right at 11.5 and I have no issues at all as long as I use a good name brand 93 octane such as BP amoco or Mobil 1. Used to use Sunoco 94 and got horrible gas mileage. No clue why, but I did. Good luck.

JScamaro

iTrader: (6)

I'm not sure how much the extra point in octane helps but this guy does not have 93 octane according to his post.

joshp14

iTrader: (5)

The highest pump gas you can get around here is 92 at BP. I think I can get 93 at Walmart but it has ethanol in it.

I reduced all the timing tables by 2 degrees to see if that would do anything and it didnt. I also bought a WBO2 but havent had the time to install yet to check the AFR. It runs fairly rich (thats what people behind me in traffic say).

It doesnt knock all the time....it would only knock if I was in 6th gear going up a hill trying to maintain speed, or if I mashed the pedal in 6th gear. It would also knock under a WOT run in the upper RPM's.

98RedZone

iTrader: (1)

The four degrees of advance increases your dcr. You could get an adjustable timing chain and retard it. Have to calculate to see how much, but it would be easier than a head gasket swap. Also changes your ve's.

slow trap

iTrader: (22)

well that sorda sucks but an easy solution would be to either go down to 5th gear or if afr is close maybe pull out 4-6* or so at the timing table where it is knocking at and see if that helps.the tune may just have way to much timing at where it is knocking.
as for wot knocking that is more of an issue.is your motor using oil some and maybe sucking oil through the pvc system and getting on the plugs ?

Gannet

If your DCR is 8.78, that is borderline for 93 octane. "Borderline" means it will work if everything, including tune, is *perfect*. And you're running 92. On a new build and tune. So yeah, it knocks, as expected.

Your choices:

Retard timing
Retard cam
Lower static compression (thicker gaskets, etc)
Change to a bigger cam with less DCR
Run more octane

If it was me, I'd run octane booster for now and keep working on the tune. Don't keep going WOT if it knocks! If you go into the engine, I'd pull the DCR down to the 8.4-8.5 range, maybe even a tenth or two lower. You want to leave yourself some slack for a bad tank of gas, heat soak in traffic, etc. Yeah, ok, maybe you'll leave 3 or 4 rwhp on the table. And maybe your engine won't blow up.

The stock LS1 has a DCR in the low 7s. Getting into the low 8s should pull you up 3-4% on hp, or 12-15 rwhp on most combos. Risking a street engine over that last few tenths of DCR, and hence literally 5 rwhp or so, just isn't worth it imo.

Big Z

iTrader: (6)

Im trying to play around with the DCR calculator, but im assuming that the advertised specs for the cam are at .050, right? it also asks for .006 and .200, how do i enter those without having a cam card? The cam my engine builder recommended is 230/234 570/575 113, heads milled to 60cc and the pistons have 2cc reliefs. thanks guys

ringram

Id retard the cam. You could also flycut a bit to get an extra cc or so. But maybe thats a crazy idea. A thicker head gasket might cause more knock as the tight quench will keep turbulance up to help prevent knock. if you have a crap exhaust setup it wont help as hot unburnt charge will hang around.
Also round off your plug ground straps to break the sharp edges and set your fans to come on sooner and hold the temp lower. You should be able to run at least 15* lower than stock with the stock thermo. That will also help. Oh and run colder plugs like 6 or 7's even. There is a good popular hotrodding article about compression and how to manage it. Do a search there