Highest DCR for 92 octane?

Is there any rule of thumb for figuring out what DCR will detonate for a given octane rating? I'm building a 381 stroker and my DCR will be 9.17:1 and the best pump gas I can get here is 92 octane (it has ethanol in it too if it matters) so will I be ok or should I lower compression?

Thanks

PGM it is a little on the high side. We are going to retard the cam some. looks like a lose of 1Peak HP but a gain in the mid range.

I will shoot JimBob the link so he can come give up some of his research :D

BTW I am in Kirkland during the week and live in Monroe ;)

Yeah that could be bad....

First question is what is the IVC point (and not at .050) ?

What calculator did you use?

Gotta make sure you are really there, been there done that before.

Bret

Bret: I'm using an LT4 hotcam so I think intake closes @68* ABDC (not sure this is exactly right, but I think close) and my SCR is 11.76:1. I'm using the RB Racing calculator.

I haven't actually ordered the bottom end parts yet, so luckily I can still make some changes :)

This isn't a really wild buildup, I don't plan to spray and I'll just be using KB hyper pistons, I beam rods and cast crank so I'd really like to avoid detonation. Should I shoot for about 9.0 or less?

Thanks again guys

Way to high for that cam and pump gas.

You need something around 16cc dish piston depending upon fel-pro or gm gasket, and I assume you are using 58cc lt-1 heads? 5.7 or 6.0 rod?

Bret: I'm using an LT4 hotcam so I think intake closes @68* ABDC (not sure this is exactly right, but I think close) and my SCR is 11.76:1. I'm using the RB Racing calculator.

I haven't actually ordered the bottom end parts yet, so luckily I can still make some changes :)

This isn't a really wild buildup, I don't plan to spray and I'll just be using KB hyper pistons, I beam rods and cast crank so I'd really like to avoid detonation. Should I shoot for about 9.0 or less?

Thanks again guys

wow... not a chance of that staying together.

First thing is hyper pistons aren't gonna take the detonation. The rest of the parts aren't going to like it either.

With a 11.7:1 SCR I would want a cam over 230degs @ .050....

You should look more at another cam and a SCR 11:1-11.5:1 and a DCR of 8.3-8.5 range to be safe.... With your HotCam you really have to get that SCR down below 11:1.

Bret

Hey Bret...

Just for info here....would you favor a DCR of 8.82 and quench of .04 or DCR of 8.58 and quench of .05. That is what my engine comes out to using Kelly's calc. The same goes for Ellis's engine. We are just trying to skim the line but not go over...especially using air conditioning in the summer here, although it doesn't get over 85* in the summer... We still have time to change the head gaskets out.

4.06 bore
3.875 stroke
5.85 rods
4 cc dish
.011 piston to deck
combustion chamber of 59 CC's
gaskets of .039 and 4.125 ring (felpro) or .029 with 4.1 ring (impala)
cam Intake 289 base at 114 Lsa and Icl "78.5 ABDC by Kelly calc"

Thoughts? :bow:

Thanks Jimbob

You sure it's that big of a change in DCR? Most times the DCR changes pretty close to the same percentage as the SCR....

I like a tight quench to keep detonation away but even a .050 is ok, you can still do the racerdude trick and run a MSD timing retard box and dial it back if it's too hot for the motor.

Bret

Thanks for the info... :D

Yeah it works out to...

SCR 12.128 DCR 8.58 .039 gasket :D

OR

SCR 12.480 DCR 8.82 .029 gasket :death:


Jimbob

I would be very very worried about doing those 396 motors, especially on a .060" block. It's not worth the extra grinding and a potentially scrapped block for the extra 10 cubes. Make sure at a minimum you have the block sonic tested competently.

MachinistOne...

Man no love.......Actually, this is the engine I'v been running and racing on since 1997. Just changing heads, cam and compression (hense the DCR question). But, yes that is good info for sure....and, I would strongly agree you should have any block sonic tested if going that big....otherwise :death:followed by :cry:


Jimbob (man of low post count due to this site erasing mine a long time ago)

MachinistOne...

Man no love.......Actually, this is the engine I'v been running and racing on since 1997. Just changing heads, cam and compression (hense the DCR question). But, yes that is good info for sure....and, I would strongly agree you should have any block sonic tested if going that big....otherwise :death:followed by :cry:


Jimbob (man of low post count due to this site erasing mine a long time ago)


I am running 9.1DCR on 92 octane on the street and so are several others on the board.
A 9.1 with good cooling and tune will be OK on the street on pump gas.
If she wants to detonate put a boost timing knob on the console and turn the timing down a couple of degrees during hot days but cool nights at the track it will be right.
If you build it with 8.2/3 ya will be leaving a lot on the table and it will always be like that,if you go high ya got knob, cooling,and an upgrade to fuel if ya want.
Try to keep your quench as close to .040 as possible,but it ain't chiseled in stone.

You need something around 16cc dish piston depending upon fel-pro or gm gasket, and I assume you are using 58cc lt-1 heads? 5.7 or 6.0 rod?

I used 56cc chambers in my scr calc., because it seems like noone agrees what the stock chamber volume is--I've read everything between 54 and 58cc on this board, so I plan on measuring my chambers before I actually buy parts just to be sure. Here's what I was going to use:

Eagle cast 3.75" crank
KB .020" over -7cc dish hyper pistons, 1.433" CH
Scat 4340 5.7" I-beams
piston-deck clearance is .013" (block decked .004")
SCE Pro 0.032" gasket
stock chamber size

So if I go with a bigger cam, like the gm847, DCR is 8.77 instead of 9.17, maybe a bit on the high side still but the chambers probably really are 58cc in which case my DCR would actually be 8.56 with this cam. My only concern then is emissions. Do I have any chance of passing the sniffer with the 847?

And if I opt to lower the SCR, I don't see how I can do that without hurting quench. Doesn't a big dish piston hurt quench too?

The quench area is between the flat of the head and the flat of the piston, good aftermarket pistons have the dish located so it matches the placement of the chamber...so it doesnt matter how big the dish is, just the piston crown surface to the head surface.

There are others on this board that can better answer your SCR and DCR questions as they pertain specifically to the LT1, most of my experiance is in gen I SBC, BBC, and small block ford, the efficient reverse cooling and quality tuning make more compression possible, The highest I have tried to go on pump gas in the LT1 is 11:1 SCR with the LT4 hot cam, making 8.53 DCR, 6.0" rods and polished chambers...no detonation. I prefer to error on the safe side when many customers cant hear detonation on hot days, or put crappy fuel in a couple times thinking "it's only once...it'll be okay."

If you are going to try and push the envelope, polish the chambers, de-burr the piston crowns and keep oil out of the cylinders.....

I used 56cc chambers in my scr calc., because it seems like noone agrees what the stock chamber volume is--I've read everything between 54 and 58cc on this board, so I plan on measuring my chambers before I actually buy parts just to be sure. Here's what I was going to use:

Eagle cast 3.75" crank
KB .020" over -7cc dish hyper pistons, 1.433" CH
Scat 4340 5.7" I-beams
piston-deck clearance is .013" (block decked .004")
SCE Pro 0.032" gasket
stock chamber size

So if I go with a bigger cam, like the gm847, DCR is 8.77 instead of 9.17, maybe a bit on the high side still but the chambers probably really are 58cc in which case my DCR would actually be 8.56 with this cam. My only concern then is emissions. Do I have any chance of passing the sniffer with the 847?

And if I opt to lower the SCR, I don't see how I can do that without hurting quench. Doesn't a big dish piston hurt quench too?


If ya heads are stock they are 54CC's
Don't know about the sniffer.

8.77 will be fine and so would 9.17.
I feel with the right tuning and my set up I could have gone 9.5/6 with no problems.
Am running stock timing with the 9.1 with a .038 quench.

There are others on this board that can better answer your SCR and DCR questions as they pertain specifically to the LT1, most of my experiance is in gen I SBC, BBC, and small block ford, the efficient reverse cooling and quality tuning make more compression possible, The highest I have tried to go on pump gas in the LT1 is 11:1 SCR with the LT4 hot cam, making 8.53 DCR, 6.0" rods and polished chambers...no detonation. I prefer to error on the safe side when many customers cant hear detonation on hot days, or put crappy fuel in a couple times thinking "it's only once...it'll be okay."


Put a timing control in it and "Walk On The Wild Side".

If you are going to try and push the envelope, polish the chambers, de-burr the piston crowns and keep oil out of the cylinders.....

I do this on stuff that is pushing the limit let alone what happens on the 9:1 DCR stuff....

You can go crazy with this, but when things go wrong they really go wrong. Even on Kaases dyno Engine Masters motors that run with 160 deg coolant and are beat on and built to detonate, not to mention extremely good chambers and reverse dome pistons he at the most is doing about 9.3:1 in a cool controled setup at sea level.

I've done the 9:1 DCR stuff, and the more stuff I do the more I pull that back, remember the better VE you get the higher your chance of detonation is for each point of DCR. Detonation proof bottom ends get expensive, detonation is hard on everything plain and simple.

Bret

I do this on stuff that is pushing the limit let alone what happens on the 9:1 DCR stuff....

You can go crazy with this, but when things go wrong they really go wrong. Even on Kaases dyno Engine Masters motors that run with 160 deg coolant and are beat on and built to detonate, not to mention extremely good chambers and reverse dome pistons he at the most is doing about 9.3:1 in a cool controled setup at sea level.

I've done the 9:1 DCR stuff, and the more stuff I do the more I pull that back, remember the better VE you get the higher your chance of detonation is for each point of DCR. Detonation proof bottom ends get expensive, detonation is hard on everything plain and simple.

Bret


Yes it is, IF ya have detonation.

I must have a couple of engines that are real screwy,cause they don't show any signs of it. I am not trying to run .050 quench either.

Has anyone realized that a LT1 is NOT a typical small block Chevrolet? Reverse flow cooling will allow higher DCR than a standard small block, how much higher will depend on how much money is spent on the cooling system and tuning. I would say the DCR "rule of thumb" does not apply to a reverse cooled LT1.

My .02


David

Yes it is, IF ya have detonation.

I must have a couple of engines that are real screwy,cause they don't show any signs of it. I am not trying to run .050 quench either.

Partly it's gas and chambers too...

For example a stock head with a stock quench (which is HORRIBLE) ain't going over 8.3 DCR in some states without detonation period. Cut the quench port the chambers etc... and you can raise it some but the gas you have in there matters the most.

The big issue is the gas... Alabama might have a good mix, I know when we got the gas here for the Engine Masters contest the first year they picked a 92 octane gas from a state that had a good Motor Octane (which is the WOT portion to make it simple, where the Research Octane is the part throttle side of the MON/RON rating) Anyways... this stuff was serious street gas. We broke the motor in and dialed in the ignition curve on the local 94 Sunoco, then once we got where we liked it switched over to 92 Unocal EM special gas and added more timing into the bottom end (brought it in earlier) to push the boundy more and it liked it..... Basically the Octane rating that each state is a certain mix of RON and MON to that states specs. Some are better than others. It would be interesting to see what states have the best gas and which ones have the worst in terms of this mix. Problem is we all don't live in Alabama boss! By the sounds of it I wish we all did!

BTW this comes from a guy who spent the last day radiusing the valve reliefs on a set of flat tops that are going in a mid 8's DCR motor... It's not pushing the limits but in 10-20K miles the motor might be, unless you want to tear these things down, clean the pistons and chambers all the time and have a perfect tune on the thing you will get octane creap... there is a good article in Hot Rod about this.. 3-4 years after we started talking about DCR on this forum... a little late to the party but a good story none the less.

Bret

Partly it's gas and chambers too...

For example a stock head with a stock quench (which is HORRIBLE) ain't going over 8.3 DCR in some states without detonation period. Cut the quench port the chambers etc... and you can raise it some but the gas you have in there matters the most.

The big issue is the gas... Alabama might have a good mix, I know when we got the gas here for the Engine Masters contest the first year they picked a 92 octane gas from a state that had a good Motor Octane (which is the WOT portion to make it simple, where the Research Octane is the part throttle side of the MON/RON rating) Anyways... this stuff was serious street gas. We broke the motor in and dialed in the ignition curve on the local 94 Sunoco, then once we got where we liked it switched over to 92 Unocal EM special gas and added more timing into the bottom end (brought it in earlier) to push the boundy more and it liked it..... Basically the Octane rating that each state is a certain mix of RON and MON to that states specs. Some are better than others. It would be interesting to see what states have the best gas and which ones have the worst in terms of this mix. Problem is we all don't live in Alabama boss! By the sounds of it I wish we all did!

BTW this comes from a guy who spent the last day radiusing the valve reliefs on a set of flat tops that are going in a mid 8's DCR motor... It's not pushing the limits but in 10-20K miles the motor might be, unless you want to tear these things down, clean the pistons and chambers all the time and have a perfect tune on the thing you will get octane creap... there is a good article in Hot Rod about this.. 3-4 years after we started talking about DCR on this forum... a little late to the party but a good story none the less.

Bret

Bret this ain't the '60s,unlead don't leave carbon in the cyls and if you are getting oil in the chambers it's time for valve seals or rings anyway.
We don't have Sunoco down here and the stuff we get is nothing special as far as Mon/Ron it is 92 octane (and doesn't have oxygenated additives) written on the pump and the best we found was Shell at one station(maybe he don't cut it as much)

Wish ya'll all lived here,can ya imagine the racing that would take place,the beer drank,and the bench racing and war stories.Hard to visualize but it sure would be fun.

BTW this comes from a guy who spent the last day radiusing the valve reliefs on a set of flat tops that are going in a mid 8's DCR motor... Bret

Have you tried the Mahle pistons? They are awesome for the price, and I have seen 10-20 hp over comparable JE customs.

Have you tried the Mahle pistons? They are awesome for the price, and I have seen 10-20 hp over comparable JE customs.

Yeap... I push these things all the time because of this. Problem is the set I just did was a set of custom Diamonds. (not bad by any means... they advertise Anderson runs their stuff so can't be bad) The Mahles still need the radiusing, no matter what a CNC machine and a cutter cant do what ya need.

Bret

Have you tried the Mahle pistons? They are awesome for the price, and I have seen 10-20 hp over comparable JE customs.


Been using Probe stuff in my builds and have had no problems.
I would take 20HP for a piston change.

Been using Probe stuff in my builds and have had no problems.
I would take 20HP for a piston change.

Try a set sometime. Seriously nice pistons for the coin.

Bret

Bret this ain't the '60s,unlead don't leave carbon in the cyls and if you are getting oil in the chambers it's time for valve seals or rings anyway.
We don't have Sunoco down here and the stuff we get is nothing special as far as Mon/Ron it is 92 octane (and doesn't have oxygenated additives) written on the pump and the best we found was Shell at one station(maybe he don't cut it as much)

Wish ya'll all lived here,can ya imagine the racing that would take place,the beer drank,and the bench racing and war stories.Hard to visualize but it sure would be fun.

Either way you still gonna leave something in the chambers unless you WOFT all the time... yeah I know I try too.

http://www.n2performance.com/lectures/combustionlecture.pdf

Good info there too..

Even with viton seals, and some serious rings and a vacuum pump your gonna get some build up in there.

I don't even wanna know about the bench racing, drinking and war stories lol.. it could get BAD!

Bret

Either way you still gonna leave something in the chambers unless you WOFT all the time... yeah I know I try too.

http://www.n2performance.com/lectures/combustionlecture.pdf

Good info there too..

Even with viton seals, and some serious rings and a vacuum pump your gonna get some build up in there.

I don't even wanna know about the bench racing, drinking and war stories lol.. it could get BAD!

Bret


Might get faster which is never bad.

The Mahles still need the radiusing,

Bret

Bret, Can you explane this more!After hearing a lot of good stuff about the Mahle's I had to get a set,And I must say they do look nice,Plus that quote about a possible 20 extra HP sounds good.

Bret, Can you explane this more!After hearing a lot of good stuff about the Mahle's I had to get a set,And I must say they do look nice,Plus that quote about a possible 20 extra HP sounds good.

I have a good picture of this I will e-mail it for you.... if someone can host it that would rock.

Basically you round off all the hard edges left from the tools that cut the valve pockets, or make the dome or dish in the piston.

Bret

Any rule of thumb on how much DCR can be added with every step in octane rating? I get Sunoco 94 here with no problem and could even mix in a lil GT100 if needed. I'm only at about 12.0 SCR/8.9 DCR right now w/ .041" quench and no signs of detonation. That being said, the motor's not really making torque numbers that it should. Still trying to sort that out. :mad:

Steve

94 ain't bad stuff... your lucky they yanked it away from us up here.

I don't know if there is a general rule as to DCR to Octane.... kinda wish there was. There are lots of other factors that play into it more than just that.

Bret

Ok I'l keep this good thread going... :D

So, here's what happends when you choose your cam spects by cam manufactures listed base duration. Me and Ellis finally were stabbing the cam in yesterday...

12.48 SCR = 8.82 DCR with 289 base duration and 114 installed ICL/LSA (very little loss over 110ICL), that's why the 114LSA...I figured why have a cam with 114 icl and 110LSA...Engine Analizer Pro didn't seem to mind this either...what the hay I'll try it.....

Now...after checking cam installed in engine we got exact duration on the exhaust(great), We moved to the intake duration....then a hell broke loose :cry: The .05 was actually 1 degree more in duration...fine....the ICL was pretty close....but the damn supposed 289 cam was actually 294 @.006 .That may not sound like alot but dumped the DCR down, and actual .006 was measures at 83.5 ABDC :eek: Way off...check, check and recheck everything...same numbers.......8.35 DCR urggggggg So, we sucked up the tuning issues and installed the cam 4 advanced to get atleast 79.5ABDC....Now atleast, I'll have 8.73DCR :) Moral to the story....aaa Maybe I'll think twice about having "CC" grind me a cam... :mad: and I'm glad I always degree the cam....Now if I had known this I might have got it on 108 ICL with 110LSA.....bastards.....or atleast 110ICL/LSA....

Jimbob :shrug:

Jimbob,

Few things...

First thing, cam companies have a few things that they prioritize when they grind a cam. With Comp they make sure they get the lobe profile of the cam right, and the duration specs close. Over the last few years the duration specs tolerance has gotten tighter and tighter which is a good thing, but they still emphisize getting the lobe profile correct. That might NOT sound as important to you, but the problem is you can't measure the problems in the lobe profile of the cam unless you have a Cam Dr and know what the lobe should look like. Little changes here and there can cause bad accelerations, jerks, V^4 etc.... in the valve movement that can be detrimental to valvetrain life and performance. To me i'd rather have the right valvetrain motions than worry about 1 deg @ .050, and when I was tring to push the DCR a little less is always better than a little more, especially since there is such high pressure in the port and IVC that it's doubtful that the motor sees any difference there.

BTW a degree wheel can get you close on cam specs but a Cam Dr is the only way to know... fortuneately all the cams get measured on one at Comp before they go out the door.

Second thing, just because the computer doesn't care if it's 110 or 114 LSA, the motor does.

Bret

Jimbob,

Few things...

First thing, cam companies have a few things that they prioritize when they grind a cam. With Comp they make sure they get the lobe profile of the cam right, and the duration specs close. Over the last few years the duration specs tolerance has gotten tighter and tighter which is a good thing, but they still emphisize getting the lobe profile correct. That might sound as important to you, but the problem is you can't measure the problems in the lobe profile of the cam unless you have a Cam Dr and know what the lobe should look like. Little changes here and there can cause bad accelerations, jerks, V^4 etc.... in the valve movment that can be detrimental to valvetrain life and performance. To me i'd rather have the right valvetrain motions than worry about 1 deg @ .050, and when I was tring to push the DCR a little less is always better than a little more, especially since there is such high pressure in the port and IVC that it's doubtful that the motor sees any difference there.

BTW a degree wheel can get you close on cam specs but a Cam Dr is the only way to know... fortuneately all the cams get measured on one at Comp before they go out the door.

Second thing, just because the computer doesn't care if it's 110 or 114 LSA, the motor does.

Bret


And THAT is why we were calling you saturday Bret ;)

I havent seen a forged piston that came out of the box that didnt need the radius' sanded, there should be no square or sharp edges anywhere in the chamber. This combined with correct quench allows more DCR. Along with MANY other factors.

As far as degreeing a cam, the cam may be spot on when it goes out the door coming to you, cam companies are much better at this. You degree the cam to correct the differences in crank indexing and timing chain tolerances to make sure it is timed in the engine as the cam company intended it to be.


There is no exact science to DCR and pump octane, there are MANY different fuel mixes in the pumps in various parts of the country, local weather also plays a role. What works for me, may not work for you.

The LT1 is capable of carrying more DCR than a "regular" engine with it's reverse flow cooling, which is what GM intended with it's very high(at the time in '92) 10.5 to 1 static compression.


Just my .02


David