I'm trying to help a fellow third genner build an engine that's suitable to run on low octane fuel.
https://www.thirdgen.org/forums/engi...-check-my.html
The performance objective is at least 1 hp/cubic inch (400 hp actually but I think that's a stretch) but the question is how much dynamic compression can a Vortec headed 350 run before you have to pull a bunch of timing out of it?
We'd like to keep the factory full dished piston and with the piston below deck at a measured .025", a .015" shim gasket (block and heads are both new) will get what flat piston top there is to within .040" of the head.
We've targeted a DCR of between 7.0:1 and maybe 7.2(+):1 but aren't entirely certain.
This build would very similar to a L98 bolt on affair.
Who's done what on regular gas?
Anyone?

 

I know lots of guys run up to 8.6 DCR on 91-93.

7.0 seems low for 87 but I've never targeted a specific DCR for 87 octane.

 

I had trouble with a DCR of 8.4 and iron heads. Had to pull lots of timing out of it and the whole thing kind of unravelled. Dropped the DCR to 7.9:1 and I can run on 91 with a full timing curve.
This current build is designed around regular grade gas and about the info I can find is Super Chevy's Agent 87. Magazine article make me nervous but there is some useful info in there.
http://www.superchevy.com/how-to/47075/

 

Our low Octane fuel is 91RON which is lower comparatively than your 91, closer in fact to US 87.

Any replies much appreciated

Thanks guys

 

8.2 dynamic. Runs fine in cool weather on U.S 87 regular unleaded. Hot weather/engine temps needs 89. Stock vortec crate L31, lunati 10030, crosswind intake. 32 total timing.

 

Use a good cooling system and a good CAI .. If possible a flat top piston and larger cc chamber would b best to optimize ur .040 quench pad .. This help with detonation more than one would think

 

The main difference is the heads.

The old bowl chambers needed lots of ign lead to run good, but that needed higher octane fuel. Best power was normally with 38 or 40* total timing but caused detonation with lower octane fuels and needed to be dialed back to 36* or so.

The Vortec heads have very fast burn chambers and do not like a lot of timing. You will make best power on 90% of vortec headed engines with 34* total or less timing.

Then it's the Iron vs. alum. heads. The iron heads holds in the combustion heat and can help to cause hot spots, the alum. heads transfers the heat away faster.

Then of course when deal with detonation and octane levels, the air/fuel ratio is important too. The EFI engines could control this much easier than a carb'ed engine.

It's gonna be hard to break down stock DCR #'s to point where you can compare apples to apples..

But I will try
Stock L31 vortec 350 truck
9.4:1 compression

Cam specs
Intake:
Opening Closing Duration
.004 14 BTDC .004 61.5 ABDC 255 Degrees
.050 13 ATDC .050 22.5 ABDC 190 Degrees
Inlet Centerline 106.25 Degrees
Max Lift .275 X 1.5 = .412
Exhaust:
.004 62 BBDC .004 24 ATDC 266 Degrees
.050 31.5 .050 16 BTDC 195 Degrees
Exhaust Centerline 115.75 degrees
Max Lift .281 X 1.5 = .423
Lobe Separation 111 Degrees

I come up with 8.112:1 DCR

Stock LS2 engines are around 8.1 DCR

I think the stock LS1 through 1999 had a 6.98 DCR

Here's a page with people's performance engine combos, SCR, DCR and octane and timing they are running. http://purplesagetradingpost.com/sum...comb-sm-1.html

It's a lot of little things that add up. My last engine (can't remember the SCR or DCR off hand) had 218 psi cranking compression (that's well into the mid to high 8s DCR) and it ran on USA 93 octane fuel BUT...

I had to have the tune perfect. Sealed to carb and hood scoop cold induction air, carb had to be pig rich, limited to 35* total timing, limit vac advance to 10*, Keep coolant temps under 195*, 2 steps colder spark plugs, etc etc.

If one of those things was 'off' it would ping

 

FWIW:
-All stock Vortec shortblock, pistons in hole .022" to .024".
-Vortec heads, uncut decks, intakes unshrouded to gasket line. Did not cc chambers.
-.015" head gaskets
-ZZ3 cam installed straight up (5 deg adv ground in; 107 ICL). Did not degree.

So, here's what my assumptions are:
64cc chambers (based on other measured L31 chambers in 62-64cc range, and the removal of only fractions of a cc of material)
.006" IVC: 57.5 deg

....which calculates to 9.6 SCR and 8.1 DCR.

I can't tell you what actual ignition timing is. It was tweaked for best mph at the track using 93 octane. Since then, we've run 87 octane in it for daily driver use, and never a ping out of it even through the summer months.
180 deg tstat.

Maybe this is not so useful, since I'm not providing actual measured numbers on some of the parameters; however, I'm posting to give at least one example that indicates you can run 8:1 DCR and get by with low octane fuel.
Granted, this car isn't at 400 crank hp as your goal is; but, give it more cam, headers, and a proper exhaust system and I don't think it would be far from it. I'm calculating 320-330 crank hp from the weight and trap speed.
The ECU has an L69 chip in it, and the car is very snappy at low revs.

 

Which calculators are you guys using?
Plugging in 37.5 for intake closing for the stock l31 camshaft I am coming up with 8.6 dynamic for a totally stock L31 vortec.

https://www.uempistons.com/index.php...tors&type=comp

 

I use the Pat Kelley calculator.

Stock Vortec cam:
dur @ .006": 252/252.5/111.4
ICL 107.2
IVC: 53.2 deg ABDC

Those cam values are from an actual Crane Cams Campro analysis of a stock cam.

Yields a SCR of 9.40 and DCR of 8.13

Edit: So, there's example #2 of a DCR of over 8.0 running on 87 octane.

 

I also use Pat Kelley's calculator

 

Interesting results so far. I'll dig into this a little deeper when I have time. I too use Pat Kelly's calculator.
For what's it's worth, I couldn't keep out of detonation with a DCR in the 8.4 range despite excellent cooling. That's with RHS Vortec heads. Dropping the DCR to less than 8:1 hasn't presented a problem so far. I'm big on a proper timing curve too. Now I'll admit that part of my issue comes from the huge amounts of timing I like at cruise to keep the mileage up but I was having problems before with nailing the throttle from a standing start or rolling start and a (what I perceived as) massive knock.
Anyway. Keep the results coming. This is for Loopy's benefit and I'd hate to steer him in the wrong direction.
You have to take into consideration that a box stock L31 was designed to run on crappy fuel so whatever parameters they used in terms of DCR should fit the bill. Raise the SCR, increase the cam timing to keep the DCR in line and that could be the answer for a performance orientated build.

 

Based on that info, I also come up with a DCR in excess of 8:1. And that's an OEM regular fuel pedestrian vehicle.

I've been looking for a database like that. I'll have to look into it.

That build isn't too far off the 8.4:1 DCR (205-210 PSI cranking pressure) I had going on before I abandoned it. I like a lean mixture for everything except WOT and despite keeping all the variables in line (IATs, engine temps, etc) it was too difficult to tame. Even experimenting with a 50/50 mix of 110 unleaded racing fuel to 91 ethanol free pump gas, couldn't keep the knock oput of the intial hit. I tries tuning with pump shot etc but it's a street car and just wasn't working.

Seems to me that we could be on to something following the L31 architecture and a little more cam with an SCR to suit.

 

I'm going to throw a little something in the mix here that DCR alone doesn't take into account.... RPM.

DCR isn't really "dynamic". It's a static compression ratio just calculated from the point the intake valve "closes". (I put "closes" in quotes, because there are MANY different ways of stating the closing event.)

I've found that DCR is about the most worthless calculation ever, in fact. You can have a low RPM torque motor that's got a relatively low DCR and still have detonation problems. Or a higher spinning, higher compression motor that doesn't.

An engine is running state is TRULY dynamic. And one of the things that has a LARGE determining factor in whether you see detonation is the basic operating RPM. The lower it is, the easier it is to start detonation (and once it starts it's very difficult to stop). Much more difficult to start detonation at 4000 RPMs than it is at 2000 RPMs.

Take two engines. One with a small cam, modest compression and low RPM range, 8:1 DCR. Another with a large cam, higher compression and the same 8:1 DCR. The bigger cammed motor, running higher in the RPM range will be LESS prone to detonation than the low RPM motor, even with identical DCR. Why? Because it's much more difficult to start detonation in a motor that doesn't start to "work" until 4000 RPMs than a motor that starts to work at 2000 RPMs. Also, the faster the engine revs through the RPM range the less likely it is that detonation will start.

If you don't believe me, do a search on how RON and MON numbers are calculated. Amongst other variables, they are tested at different RPMs (600 RPM vs. 900 RPM). What you see at the pump in the US and Canada is the AKI (Anti-Knock Index) where they average the two according to the familiar (R + M)/2 method.

Long story short, for whatever you chosen cam is, you won't be far off if you back down the STATIC compression ratio of the engine to the lower end of the recommended compression ratio for the cam. In most cases that will work fine for use with lower octane (87) fuel vs. more typical higher octane fuel (91).

In almost all cases, Vortec heads will require less spark advance than earlier non-Vortec style heads. You are NOT giving up power by doing this. They are just more efficient chambers and don't need or want as much advance.

If you have to err one way or the other the stay safe, a motor with slighly too low compression but optimum spark advance will make more power than a motor with slightly too much compression running with retarded timing to stay out of detonation.

 

Thanks Damon. Very informative post and pretty much mirrors what skinny has been telling me.

This thread is interesting reading.........keep em coming.


Cheers Geoff

 

Skinny,
Your comment about preference for large amounts of timing added at cruise reminded me that I left out some information.
Despite what I said about not knowing exactly how much timing I'm running in the ZZ3 cammed Vortec, my hunch is that it's not overly tame at low MAP cruise conditions, given the fact that it's gotten over 26 mpg with 3.73 gearing.

 

This is what I like about this forum. Lots of information.
That said, I'm not sure if it's fair comment to say that "... DCR is about the most worthless calculation ever...". It certainly has merit and that's supported by stating the reducing the SCR to the lower end of a givens cam recommendation may give satifactory results. That in itself will also lower the DCR.
Of course there are a lot of variables involved in the detonation game. As you've stated, RPM is one of them and more specifically the time allowed for an "auto-ignition" flame front to be produced. Higher RPM provides less time for this to happen so it's important to take that into consideration.

I'll agree completely with:
" Long story short, for whatever you chosen cam is, you won't be far off if you back down the STATIC compression ratio of the engine to the lower end of the recommended compression ratio for the cam. In most cases that will work fine for use with lower octane (87) fuel vs. more typical higher octane fuel (91).

In almost all cases, Vortec heads will require less spark advance than earlier non-Vortec style heads. You are NOT giving up power by doing this. They are just more efficient chambers and don't need or want as much advance.

If you have to err one way or the other the stay safe, a motor with slighly too low compression but optimum spark advance will make more power than a motor with slightly too much compression running with retarded timing to stay out of detonation. "

Good stuff.

 

I've always been impressed with the fuel economy your"over-achiever" has produced.
If there's someway for you to pull the data out of your ECM to show the sparkadvance at cruise I'd like to see it.
When I'm at 2500 rpm cruise, I have 16 initial + 15 mechanical + 16 vacuum fora total of 47 degrees of advance. What I get is when trying to accelerate fromcruise, very slowly, my cruise AFRs stay very lean (15-16:1 by design) butthere's a light engine rattle while doing so. Engine vacuum doesn't drop enough to drop any vacuum advance. It's a little unsettling but sofar, hasn't shown to cause any damage. I stay out of that situation, either bygetting into the power valve circuit ( by dropping engine vacuum to less than10" and my AFRs drop to 13:1) or downshifting.

 

Unfortunately, the car now resides 3 states away with my daughter (it was her first car at age 15 ), so I won't be able to mess with it much anymore.
From a crude TDC mark I made on the front cover when assembling the engine, my best guess was about 16 or 18 deg initial timing. So, if you have a map for an L69 chip, you could figure what cruise timing is.
This car may have an advantage over your mechanical vacuum advance, in that the MAP sensor/ECM can respond to the slightly lower vacuum when just slightly leaning into the throttle at cruise ??

Edit: Found this map for an L69 chip that Fast355 had posted.
Looking at the high MAP/ high RPM portion of it, I don't see how my estimate of 16-18 deg initial timing can be right. That would be ~45 deg total timing. No way. I guess I'm back to: "no idea of what timing I'm running"