TA/30th 1002

My thoughts on the big cube timing issue are; as Dave mentioned above, Flame front has much to do with it. With a longer stroke motor, the piston will hang around TDC longer making the combustion process more subjectable(sp) to knock. Also when it does go down, piston speed will be greater than shorter stroke motor. So in theory, a less efficient, longer burning combustion process may make more power in a stroker.


Another note on timing. As the exhaust valve opens during a point when there is more cyl pressure due to lower timing angle, a tuned exhaust will scavenge a little better.

... 1002

Warbird

My ARE 422 makes the most power around 25 Deg max timing WOT. At 28-29 Degrees we saw no knock on the dyno but did loose about 15 RWHP and 10 RWTQ. With more timing torque and HP peaked sooner and dropped off quicker. At 25 Degrees both curves were broader and flatter.

I also noticed my motor ran far cleaner and power delivery was smoother and less ragged with less timing

My thoughts are simply that the bigger bore hydraulic motors need more time to burn fuel and are more efficient with less timing.

Paul

Joe Prince

Posted by Mr.Morgan,

"One other thing. The longer stroke motors have less piston dwell time."

Last time I checked, stroker motors have more TDC dwell time due to more rod angularity = more stroke and less initial and full timing requirements. What dwell time are we talking about here? It's just a question Mike.

racer7088

TA/30th 1002,

""My thoughts on the big cube timing issue are; as Dave mentioned above, Flame front has much to do with it. With a longer stroke motor, the piston will hang around TDC longer making the combustion process more subjectable(sp) to knock.""

Actually Strokers will invariably have a lower rod to stroke ratio and for any given speed they will dwell LESS at TDC.

"" Also when it does go down, piston speed will be greater than shorter stroke motor. So in theory, a less efficient, longer burning combustion process may make more power in a stroker.""

This is backwards since you would need a FASTER burn to keep up with the more rapidly increasing volume generated by the stroker's greater piston speed.

racer7088

Kenny H,

""But in theory isn't a larger engine always 'less' efficient then a smaller one, all other things being equal? ""

No, in fact it's just the opposite if your talking BSFC or how much power you can get out of so much fuel. Now if your talking about power per cubic inch then sometimes depending on the heads the smaller motor may be better but it won't make more TOTAL power and we don't have any weight breaks for street cars so you'll still lose to the "less hp per cubic inch" big motored car. This is what the ricers always say about their better efficiency except that it doesn't matter how efficient their two liter wonder is because they still only have two liters!

racer7088

Mike Morgan,

""One other thing. The longer stroke motors have less piston dwell time. So if there is more spark lead, then you may be pushing down on the piston before it needs to be. Motors with better rod/stroke ratios can run more timing.""

While I’m on a roll! The first part's usually right but the last part usually isn't. Long rodded high rod ratio motors for instance like the IRL Aurora and such CANNOT run as much static timing before TDC because they DO have more relative dwell before and after TDC and so, at say 30 degrees before TDC, would actually spend more real time between these same degrees traveling up the bore than a short rodded motor would for the same 30 degrees. For the same actual time increment before TDC you might have to advance the shorter rod motor 32 degrees for instance. These differences are not as large as some would suggest though.

Some of these IRL motors have nearly 3 to 1 rod ratios because they are sometimes forced to use production deck heights at these reduced cubic inch limits with even bigger bores than they had originally so they get stuck with an overly long and also heavier rod which they absolutely hate. Since they have longer dwell near TDC they also have more trouble with valve relief's and such so it hurts their efforts at getting compression with the small cubic inches and relatively oversquare bore to stroke ratios.

The main reason that these LS1 stokers we use need less timing is their higher compression and better higher velocity cylinder filling. Just like a blower needs less timing with more cylinder filling.

ABNRNGR (Aka Dean)

iTrader: (1)

[quote]Originally posted by Nine Ball:
<strong>Excellent replies everyone! I've never put faith in that magic "30 degrees" figure that gets tossed around here. My car has never run any better past 28 degrees. I used to have 30 degrees with just heads/cam, but it went down to 28 when I did the 382ci motor. Now you guys have me wondering if I'll pick up more power by going to 26 degrees LOL.

Damn I wish LS1-EDIT for 98 cars was available! I could have some serious fun with that program and the dynojet/track. <img src="images/icons/smile.gif" border="0">

Tony</strong><hr></blockquote> Tony no kidding man, I hope the 98 version comes out soon. From the info here in the last couple of days LS1 edit is exactly what we need to optimize our setups.

McRat

The following is info for stock motors on the same subject. Sorry if some consider it off-topic.

About a year ago, I posted a series of 12 dyno pulls on a bone stock engine along with AutoTap 02 and Timing readings.

Many people made fun of me when I said exactly the same thing even though the data was overwelming.

30-31° of timing "sounds" better, "feels" better, but shows zero improvements in HP over 4500 rpm. Some gains were seen at lower RPM. Track results backed it up.

These tests were done on a 2000 SS.

Maybe higher Octane would show improvement? Nope.
The game of trying to add timing past the stock PCM settings proved pointless.

If you have a stock 2000 engine, run stock timing.

Why the "legends" of adding timing and getting big gains? I can only guess at it. My theory is the 98 PCM was very different, and everyone assumed the same things would work on newer engines.

racer7088

McRat,

I think you are very right and your not the only person who's noticed this. I know on our car at SAM at one point it didn't seem to add much at all at least at that time. I was doing autotap and we went from 28 to 31 or so from a different setting. The weather was a little different but it didn't really show any mph or anything at all. I am not sure if it ever did to tell you the truth either. I'll ask Judson if it ever really panned out at all? These cars don't seem to want much timing at least on autotap.

TA/30th 1002

racer7088:

Let me try to explain a little better. I was trying to keep my above post short.

With a high volatile fast burn combustion process, cyl pressure will peak sooner and have a shorter burn duration than with a lesser volatile combustion environment.

In a long stroke motor a high volatile combustion charge fired at ie: 28deg btdc may peak a cyl pressure of 550 psi at 8deg atdc, have a peak cyl temp of 1900degF at 20deg atdc and be pretty much fizzled out at 82deg atdc with only 50 psi left. ... fire the same charge in a lesser volatile atmosphere and burn time will be extended; resulting in lowering peak cyl pressure/temp due to the piston being farther down the bore. This extended burn will offer greater cyl pressure during the sweet spot of the crank angle expelling more force to the crank

This is somewhat of an Old School theory; "throw as much fuel at it as you can and make as much cyl pressure as you can for as long as you can"

The New School theory may be; "burn it as fast as you can starting it as late you can"

I believe we were on the same page with our thoughts, I hope this clears things up.

... 1002

racer7088

""racer7088:
Let me try to explain a little better. I was trying to keep my above post short.

With a high volatile fast burn combustion process, cyl pressure will peak sooner and have a shorter burn duration than with a lesser volatile combustion environment.""


Actually with higher swirl or "fast burn" type heads you have a less peaky pressure spike even though you burn the fuel more completely.


""In a long stroke motor a high volatile combustion charge fired at ie: 28deg btdc may peak a cyl pressure of 550 psi at 8deg atdc, have a peak cyl temp of 1900degF at 20deg atdc and be pretty much fizzled out at 82deg atdc with only 50 psi left. ... ""


Actually most engines peak later than that at around 10-18 degrees ATDC or so depending on many things but at much higher pressures. A typical 10 to 1 engines will see about 1100 psi at this point.


""fire the same charge in a lesser volatile atmosphere and burn time will be extended; resulting in lowering peak cyl pressure/temp due to the piston being farther down the bore. This extended burn will offer greater cyl pressure during the sweet spot of the crank angle expelling more force to the crank""


You adjust the timing till torque is maximized on all engines when tuning. With less compression or mixture motion you will have to run more initial timing though. Remember also that the stroker has ingested more air and fuel so it may not have any less pressure at the same dgrees down the cylinder even though that is farther down than it would be in a smaller stroke engine. It's relative to the amount of air trapped and the stroker has trapped more because of that longer stroke in the first place. It's also easier to get higher compression without bad piston designs etc. in a stroker type motor.


""This is somewhat of an Old School theory; "throw as much fuel at it as you can and make as much cyl pressure as you can for as long as you can"

The New School theory may be; "burn it as fast as you can starting it as late you can"""


Yes you'll always make more power if the piston is getting less pressure on the up side or compression stroke and yet still getting the same or more pressure on the power stroke.


""I believe we were on the same page with our thoughts, I hope this clears things up.

... 1002""


I think we're somewhat on the same page but not totally. The only down side to the stroker is that the piston speed is out running the flame front speed more, so to speak, in comparison to the smaller stroke motor BUT you usually have better flame front propogation to make up for this due to the better compression scenario and piston designs capable or possible with the bigger cubic inch designs.

racer7088

TA/30th 1002,

Your right that we're on the same page as far as the combustion process being on the frontier of what's left in getting the remaining power out of what we have succeded in getting trapped and burned with the heads and cam design we've set up. Obviously the LS1 is a bad *** example of getting this right and that's where some of the extra LSx power comes from!

John B

iTrader: (10)

Tony, another interesting data point here that really threw me for a loop was a couple of weeks ago when a bunch of people dynoed their cars at Turbo Technology I brought my laptop and ATAP'd my car and another '01 M6 LS1 F-body. What blew me away was that this guy only had crank pulley, larger y-pipe, catback and I believe MTI airlid and he pulled 335+RWHP w/350+RWTQ with only 24* total timing (with 1* KR) and his 02's were running in the .920-.935 range! He mentioned that his car has ran 12's @ 112MPH which would confirm the dyno numbers. My stock '99 F-body LS1 motor was pushing up to 29* total timing that day and all I managed was 300RWHP & 315RWTQ and I have real duals as opposed to any kind of y-pipe and low restriction mufflers and I wasn't running the A/C compressor.

It really makes me wonder if GM somehow tweaked the fuel/timing tables on these '01-'02 cars because it's just unbelievable how much more power a "truck" cam and LS6 intake can make on these cars.

02 'Hawk

This is LT1 lore but i remember talking to ed while my car was on his dyno...about where he was adding timing and fuel. he said that about 4500 (i think) that he didn't add timing, they didnt like it. from that point up, he did it with fuel. he went on to relate that to compression, and how much i was running (12.5:1) and the increased importance of fuel over timing with higher compressions motors and high RPM.

I know less than he's forgotten about tuning, but that is the info he shared with me.

Chris

TA/30th 1002

racer7088:

"Actually with higher swirl or "fast burn" type heads you have a less peaky pressure spike even though you burn the fuel more completely"

--->I don't quite see this happening. If I mix a air/fuel charge real well (high swirl), put it in a high volitile enviroment (ie: high compresion 10:1-LS1) it'll burn quick and complete. I would think I would see a higher cyl pressure peek due to the increased expansion rate of the fast Burn combustion in relation to how long the piston has had a chance to travel atdc. -- vs -- If I introduced a poorly mixed air/fuel charge into a low volitile enviroment (low compresion 7:1-farm tractor) it'll burn slower and less complete. I would think I would see a lower cyl pressure peek due to the decreased expansion rate of the Slow Burn combustion in relation to how long the piston has had a chanch to travel atdc. [both ex. engines are of the same bore/stroke, test rpm and air/fuel charge. Differant in CR/intake desighn.]<---

"Actually most engines peak later than that at around 10-18 degrees ATDC or so depending on many things but at much higher pressures. A typical 10 to 1 engines will see about 1100 psi at this point"

--->I just used a general example (ie <img src="images/icons/smile.gif" border="0"> . Although that 1100 psi seems high. ... I've worked primarily with diesels and I'm just asuming gas engines produce far less cyl pressure than what a diesel would.<---

"Remember also that the stroker has ingested more air and fuel so it may not have any less pressure at the same dgrees down the cylinder even though that is farther down than it would be in a smaller stroke engine. It's relative to the amount of air trapped and the stroker has trapped more because of that longer stroke in the first place. It's also easier to get higher compression without bad piston designs etc. in a stroker type motor."

--->Yes the stroker will displace more than the shorter stroke at 100% VE. But all variables being equal; both engines at 10deg atdc crank angle, the strokers piston will be higher in the bore makeing it volnerable to a higher peek cyl presure. ... Fix: back off the timing advance<---


Good tech chat racer7088

thanks ... 1002

racer7088

--->I don't quite see this happening. If I mix a air/fuel charge real well (high swirl), put it in a high volitile enviroment (ie: high compresion 10:1-LS1) it'll burn quick and complete. I would think I would see a higher cyl pressure peek due to the increased expansion rate of the fast Burn combustion in relation to how long the piston has had a chance to travel atdc. -- vs -- If I introduced a poorly mixed air/fuel charge into a low volitile enviroment (low compresion 7:1-farm tractor) it'll burn slower and less complete. I would think I would see a lower cyl pressure peek due to the decreased expansion rate of the Slow Burn combustion in relation to how long the piston has had a chanch to travel atdc. [both ex. engines are of the same bore/stroke, test rpm and air/fuel charge. Differant in CR/intake desighn.]<---

You are right that there will be more total pressure present on the other side in terms of area under the curve but I meant it wouldn't be as much of an actual pressure "spike" as a normal chamber causes but more of a pressure "curve."

The fast burn designs are a faster but more controlled burn that doesn't spike as much as a normal chamber does and can achieve peak cylinder pressure later than conventional heads do. You example assumes your still running too little initial timing on the "bad" engine. I've looked over cylinder pressure trace diagrams for years. 1000 psi is like a healthy street car, a race motor will see over 1600 sometimes NA. The worse or less homogeneous your mixture and mixture motion is the more chaos and detonation you will see at any given compression and fuel octane level.

When you have crappy fuel distribution in the cylinder and chamber your timing is a compromise since sometimes the cylider needs more and sometimes less so you hurting both scenarios just to span the huge range of possible combinations due to the chaos and randomness of the A/F ratios and yet still stay out of detonation. When you pair this with low compression it only makes it worse since less of the liquid fuel droplets will vaporize.

Higher compression does cause the peak pressure to be delivered earlier than later but the fast burn higher swirl design chambers will cause a less peaky burn and less of a pressure spike.

[ November 25, 2001: Message edited by: racer7088 ]</p>

racer7088

--->Yes the stroker will displace more than the shorter stroke at 100% VE. But all variables being equal; both engines at 10deg atdc crank angle, the strokers piston will be higher in the bore makeing it volnerable to a higher peek cyl presure. ... Fix: back off the timing advance<---

It's time and pressure that make detonation more possible. The stroker spends LESS time near tdc when the pressure's higher and more time near BDC because of the rod ratio differences.

Also at 10deg atdc crank angle as you say the stroker's piston will be LOWER in the cylinder at the same time and not higher. In fact at any time besides TDC the stroker's piston will always be lower in the bore at the same crank angle that's why it's a stroker.

TA/30th 1002

racer7088:

Thanks racer7088. Great info.

... 1002

2000SS

[quote]Originally posted by Terry Burger:
<strong>It's always made sense to me that a larger CI motor will require less total timing, due to a denser combustion mixture.

What is less intuitive is that you can have timing related power issues without having knock (or at least knock retard), and that you can lower timing and not lose any power. Guys who are running lower octane gas (91/92) with heads/cam should really pay more attention to this area. Doesn't hurt to test out an inverse timing tricker. If you don't slow down, leave the timing out.</strong><hr></blockquote>

thats exactly right!!!!man you guys know your stuff!!!!

<img src="images/icons/grin.gif" border="0">

wizkid

iTrader: (3)

Quick!! All you big bore owners remove your SLP ATM quick!! <img src="images/icons/wink.gif" border="0"> I couldn't resist. <img src="images/icons/grin.gif" border="0">

ERic