VLS1

Someone needs to run some new PB numbers to stir up more discussion. Hurry up already!

newschool72

iTrader: (12)

OK, what is the highest DC you would run on pump gas? How much cranking compression would it take to make a rattle machine on pump 93?
My setup is above 9.0 dynamic and my cranking compression is between 225-230. Street car running 34*. Pulls from 2000 to 7000 rpms like a bullet train. Cruises at 1700 rpms , even up steep hills without a shutter, with the converter locked.
My setup tends to make me believe dynamic compression right at the edge of pump gas compatibility , builds power across the RPM sweep and eliminates the low RPM dead area that most agree you have to live with on a small cube LS3 headed build.

speedtigger

iTrader: (16)

A couple things on that topic: I am not a fan of that "dynamic compression" calculation for determining ideal compression in a performance engine. It creates a mathematical reference for determining probable cranking compression, but that is about it. Since it does not take into consideration the engine's range of volumetric efficiency, it is has little value.

Cranking compression is an important figure in assuring a car will start easily, and I would say that is pretty important, but like the dynamic compression formula is just one part of the equation.

When one considers all of the different things that are part of the ideal compression ratio forumula:
static compression
valve timing
combustion chamber efficiency
volumetric efficiency
thermal efficiency
engine load
RPM range
stroke/rod ratio (and subsequent piston speeds)
fuel characteristics etc etc etc.

It just becomes really obvious that the easiest information to use for your average hotrodder is comparative data from similar combinations. However, being familiar with the items above makes it easier to speculate which items in an example combination are the important specifications.

newschool72

iTrader: (12)

Im sure you know more on the subject than I do, but if the components are picked carefully with an eye towards compatibility , the dynamic compression has to be part of the equation . Its not the live by guide to putting a combination together, but it give a good guide line as to whether an engine will work with a certain fuel octane and for lack of a better phrase , be ready to go (peppy?) down low. A camshaft with a 240 at .050 duration wont have good dynamic compression in an engine with 9 to 1 static and will be dead down low. Take an engine with the same compression and stab a cam with 220 at .050 and it will try to jump out from under you. Taking it further, raise the compression to 12.to 1 and the engine will be very lively with that 240 cam, but be a rattle box with that 220 cam. I was under the impression that dynamic compression was the cause for those examples doing what they do. Those examples leave a lot of the players out of the equation , but you can bet as a whole ,it will be hard to change those engines personalities without changing the dynamic compression.

VLS1

I'd like to see the specs of the build to show a dynamic exceeding 9.0 for starters, remembering for dogsballs engine to reach that was with 0.100" removed from head surface, which I still think is crazy and takes away from the combo more than it adds.

Dynamic is only always a guide, but it is only as accurate as the data entered provided accurate measurements were taken in the build process.

Have seen plenty of motors with smashed up pistons in my time, and when the owner is asked if they noticed any unusual noises from the engine - they report it sounded fine at all times.

But anyway I would suggest it best for any pump gas engine to run under 8.8:1 dynamic at best, there is one big reason why lower comp can and will produce more horsepower, when built right with a good engine combo, when running pump gas.

I've seen it on engine and chassis dyno results first hand when an engine has blown to bits, and then a smarter low comp combination is put together, exceeding previous power levels and rev range in some instances.

I will let someone else speculate why this is the case with pump fuel engine building, let's see who really knows their stuff.

VLS1

However dynamic compression is only a ballpark guide to an extent, with varying factors such as LSA, duration and other volumetric efficiency factors to be considered. Such as being able to run a higher static comp with wider LSA for one example.

Some words taken from someone I know well, who reported back on their dirt track engine I helped sort out machining and head work to create more power with less static comp. "This thing is dynamite off the corner now and is already on the limiter by mid straight, water temp stays lower and is more consistent now as well.

Oh and again, this is also a racing class restricted to pump gas only. The car showed good gains on the chassis dyno as well, around 15% average through a usable rev range of 4000 - 7000 and on the track will see up to 7800rpm.

newschool72

iTrader: (12)

Mine is a factory LS3. Crate engine with nothing but cam, intake and good exhaust. Factory rated at 10.7 static compression. All the engine specs such as rod length, etc are in the specs for a factory LS3. My cam is a custom grind from Bullet Racing. We speced it to give all the usable power we could muster while staying around 16" of vacuum , with an eye towards solid low to mid range TQ, but with the rec port heads, still able to carry up into some respectable upper RPMs.
I cant give you the valve events right now, because they are in my wallet and I don't know where it is, LOL. Must be in my wifes car because the last time I saw it was Feb 26 right before I had my back surgery. Anyway, I will post it all up when I get the wallet back. Till then the numbers are 271-281 at .006, 220-230 at .050, cut on a 108 LSA with 4* of lead ground in. Lift numbers are 629 int and ex. w 1.7 rockers. Im using the Comp 26926 dual springs w tool steel retainers . The engine is very happy at every RPM from Less than 2k up to 7k, where it hits the limiter.
I built my first performance SBC when I was 15yrs old. Im 49 now. In 34 yrs of messing with this stuff, this combos average power beats all Ive seen for a setup that is absolutely street able. I think the ability to build cylinder pressure way down low with this cam is the reason for it. The engine is sitting on go from idle!

Doug G

Even with all this math and speculation on the numbers I can't say any one answer is 100% correct.
Given a set motor and a cam we could adjust on the fly we could then see how a motor reacts to the DCR by advancing and retarding to see what it likes.
By moving the intake/exhaust closing earlier or later in the cycle you can make the motor think it's bigger than it really is due to cylinder pressure.

Kind of like the Honda Vtec ?

newschool72

iTrader: (12)

If there was such a thing it would be a cool test. You wont change the cams perceived size enough to make a difference with what we have to work with now. That is unless the cam was right on the edge of rattle, retarded, for the octane you use. Then you could see the max DCR for that particular combo.

VLS1

8.85:1 dynamic at the most, going by LS3 specs with your cam.

newschool72

iTrader: (12)

I know its right at 9.0 according to someone that ran the numbers , knowing the opening/closing points. I know the cranking compression is 225+, by my pressure tester.
What ever it is, it works great. With quiet exhaust and 34* of timing I cant hear any spark knock, but I do dip down a couple degrees from 3800 to 4800 rpms to help with piston speed at max TQ. I will try to remember to get my wallet from mama rabbit tonight when she gets home from her second job and let you run the numbers with all the info.

VLS1

No prob, your IVC @ adv. duration should be right around 59.5* going by the 108 split and 104 ICL you listed.

newschool72

iTrader: (12)

Freakin billfold is AWOL !! Hopefully I will run it down tonight. Stay tuned!

newschool72

iTrader: (12)

Billfold recovered!!! WoooHooo!
OK, my cam card gives the following info.....
duration at .050....int 220......exh 230
lobe lift.................370................370
sep....108
timing events .050...OP-6...CL-34 / OP-47...CL-3
dur at .006 ..............271.........281
gross lift 1.7 rocker....629.........629
degree int lobe to...........104

It doesn't show the OP and CL points at .006, just at .050

VLS1

The valve events at .006" are easy to work out from the advertised duration, so with all things being equal your dynamic should still work out to around 8.8:1 which is right up there for pump gas IMO, but that isn't to say that with more cam (and effectively lowering the DCR) that it wouldn't make more power.

newschool72

iTrader: (12)

There is no doubt in my mind that a longer cam would make some more power but Im very happy with the specs Tim at Bullet and I came up with. The rec port heads like "some" overlap, but from what the guys that have been doing a lot of testing on average power, the head works best with 11-13* of overlap. The big ports loose too much down low if you get a lot of reversion. With a lot of overlap, they make great top end steam, but are unhappy at 2500-3500rpms. That's no big deal for a 4500stall and 4.10 gears, but with my 2800stall and 3.42 gears, I would have a lot of dead space where I spend a lot of driving time. With my dynamic compression, I could jump up to something in the mid 220s on a 110 with about 10* of split, and not give up too much at my 1700RPM cruise speed, but I would hate to take the chance and loose what make my setup work so good for what I was shooting for when I built it. The short duration keeps the low end strong and the 108 LSA gives me the overlap to make the engine happy. The lift and lobe design allows the cam to pack every thing it can under the curve to make good power. Im excited to see what it runs , hooked up. Its already way quicker than I expected and I have a few 10ths more coming once I get the traction worked out .As far as spark knocking with 8.8 DC, on pump gas its shown no signs of it at 34* of lead. It did knock before I started tuning it with a lap top with the No. 5 pill in the box. I didn't know the box carried the Map advance along with the base curve "out of the box", and I was running 44* at one point in the curve! MSD should put a huge warning sticker on the box to let people who don't have a lap top, to add the base and Map together to determine their actual timing. No telling how many hypo pistons have broken from that oversight !

Doug G

If I did my math right and got the right numbers....I get a IVC of 61.5 on yours and with a DCR giving you....Your dynamic compression ratio is 8.64:1.

newschool72

iTrader: (12)

That's perfect! I hope you did your math right, LOL.