Advice on camshaft choice for my sbc......

So it seems that I need to raise the compression a lot for me to get the cruising sound I'm after, and in order to use a thumpr or similar I need to be up quite a bit on the compression side of things. Has anyone ever used these pistons? http://www.summitracing.com/int/parts/slp-h617cp/overview/ I'll post a new thread for vies on these as it's away from camshaft choice lol, cars is getting expensive!!!! Thanks Jason


Posted using the Full Custom H.A.M.B. App!

 

Thanks took the words out of my mouth as I typed lol, are the domed ones any good?


Posted using the Full Custom H.A.M.B. App!

 

Thanks all, you have given me great inside into what I have and what I should really get, I'm just not going to get that sound I want with the setup I have, I think I will go with the 268h although I see there's two types either extreme energy 268h or high energy 268h, what's the one I should be looking at the extreme energy has slightly higher lift values, and you are right those of you that said what's the point building an engine that sounds cool but won't drag it's sorry arse up the road lol. So just one quick bit of help on what one to go for the extreme or high energy if you wouldn't mind, thanks again to all those hambers that have took the time to help me, lol I've been gong mad thinking about this


Posted using the Full Custom H.A.M.B. App!

 

During the overlap period exhaust gases are rushing out the exhaust port as intake flow begins. The velocity of these gases and therefore the inertia is high for exhaust and low for the intake at this point. In the simplest case this velocity is largely controlled by RPM. In the more complex view it is the piston accel and decel (think R/L ratio, stroke, etc) and port and valve sizes.

Cylinder Timing Diagram:



When we are idling at relatively low RPM and low manifold pressure there is not a consistent scavenge of the cylinder and there is contamination/dilution of the charge that is not repeatable cycle to cycle. So from this we get the high performance cam erratic idle.

Another factor of high performance engine sound is the amount of energy remaining in the cylinder at EVO and the rate of Exh valve curtain area increase.

Stockers with low compression "hit hard" because they are opening the exhaust valve early and fast.

Racing roller camshafts have even faster EV opening rates, listen to a Super Stocker.

Early EVO can give you this sound.

The ultimate in fast opening exhaust are piston port engines such as most 2strokes and rotaries.

So overlap and late IVC give us the rolling idle sound and Early and Fast EVO gives the hard hit or Thump.

Compression ratio enters this equation because it controls the expansion ratio of the exhaust stroke.

This of course is a gross simplification but I think it helps understand overlap vs early EVO.

 

The extreme 268h should work and run very good for what you have. But as far as the sound you are looking for, it will idle smooth.

 

Thank you for that, very interesting read explains a lot about how the numbers match up to what's going on during the revolution of the engine.


Posted using the Full Custom H.A.M.B. App!

 

This chart demonstrates the different power levels for a cycle when we have poor mixture formation and exhaust dilution.

 

It was a rhetorical question, the answer is actually in the same post, but you would have to understand the fundamentals of how a 4 stroke internal combustion engine works to get it.
31vicky was paying attention in grade 8 shop class, he gets it. For the guys who were drawing rat finks, reading Dragtoons or gazing slack-jawed at "Parts With Appeal" during class, heres how it goes.
Increasing overlap will NOT "bleed off cylinder pressure". Overlap occurs around tdc on the exhaust stroke, roughly 180 degrees after the compression stroke has ended (remember, intake, compression, power, exhaust, in that order), it has NOTHING to do with "bleeding off cylinder pressure", and this will be readily apparent to anyone who thinks their way through whats happening as the engine cycles over.
As 31vicky has said, its all about the intake closing point, and in fact, increasing overlap increases "cylinder pressure" ( lets call it dynamic compression ratio, thats a more accurate description).

Lets use valve events to illustrate this. As an example, I will use two otherwise identical cams, and vary the amount of overlap. This is just basic math here, no guesswork, no black magic.
As an example, I'll use a single pattern cam, ground straight up, with 238 duration @ 050. All valve events will be given @ the .050 check hieght. First we will do the "less overlap" example, we'll use a 112 LSA. Intake opens 7 deg before tdc, on the exhaust stroke, closes 51 deg. after BDC on the compression stroke. Make note of the intake closing point, you'll need it later.
Exhaust valve opens 51 degrees before BDC on the power stroke, closes 7 degrees after TDC on the intake stroke.
So, we add the intake opening point, and the exhaust closing point, and we get 14 degrees of overlap.
Now lets close the LSA up to 109 to get our "more overlap" example. Intake opens 10 deg before tdc, on the exhaust stroke, closes 48 deg. after BDC on the compression stroke.
Exhaust valve opens 48 degrees before BDC on the power stroke, closes 10 degrees after TDC on the intake stroke.
So we add the intake opening point to the exhaust closing point, viola, 20 degrees of overlap.
The intake closing point is now 48 degrees after BDC on the compression stroke, or 3 degrees earlier than the 112 LSA. It should be pretty obvious that closing the intake valve earlier in the compression stroke is going to trap more cylinder pressure.
Note that the change in intake closing point is the same number as the change in LSA, and half the increase in degrees of overlap. Simple math, and this relationship aint gonna change no matter what the duration is.
It just completely blows my mind that something like this BULLSHIT "increased overlap bleeds off compression" can be put in print by some halfwit magazine writer, and becomes accepted and repeated, when it wont stand up under the SIMPLEST scrutiny. The four cycles are REALLY elementary, first year autoshop stuff, and anyone who applys them to this statement can IMMEDIATELY see that its wrong, but its repeated over and over again, by people who either dont have a basic understanding of how a 4 stroke engine actually works, or cant be bothered to think it through.
I know, math and understanding the fundamentals of how this stuff actually works is BOOOORING we wanna talk about 30/30s and "thumprs".

 

That's how I understood it. Wide LSA = lowers DCR. Narrow LSA = raises DCR.

Here's a handy calculator to figure out your DCR (dynamic compression ratio). A good target for a street car is 8.0:1. A max would be 8.5:1 with iron heads

http://www.projectpontiac.com/ppsite15/compression-ratio-calculator

 

Just to throw some more crap into the conversation, I have a SBC similar to what you are doing. It has about 9:1 compression and double hump heads,Edelbrock intake and Holly carb. I am running one of the old school Crane Fireball 290 cams. It runs strong and has a lick to it when idling if you don't have real quiet exhausts. It sounds better than the 268 Comp Cams I have in another car.

 

Thats a useful link, and I am not trying to take away anything from Tjets post here, but the calculators/spread sheets really kind of hurt in some ways more that they help.
Its not that hard to understand the fundamentals, and all the calculators in the world are not a substitute for this, they are a useful time saving tool ONCE you understand the fundamentals. Theres no substitute for getting a first-year automotive text book( I dont mean you specifically T-jet, I'm assuming you already know this) and actually studying the four cycles, and when the valves are open and closed, relative to the motion of the crank/piston. These are not abstract numbers, they actually have meaning. If you cant look at a cam card, and clearly understand what would happen to the numbers on there if you advanced or retarded the cam, or if the LSA was changed 4 degrees, then you really dont understand anything about camshafts. I actually ran across a thread on here the other day where some guy was asking for specs on a cam, and Pete1 gave him the valve events. The guys response was something like "these numbers are useless, they dont mean anything." The guy had NO COMPREHENSION what the valve events even represented, and hes making cam recommendations on a forum! Its like if I hunted up a forum on knitting, and started handing out advice. Guys are throwing around cam specs, and the numbers are completely abstract to them, they have no understanding of what they actually mean. Its just numbers they read in a magazine.
For those that dont understand, the valve events, along with basic arithmetic, will give you the duration, the LSA, and even tell you at a glance whether the cam is ground straight up, advanced or retarded.
The four cycle, internal combustion engine is not rocket science. THINK about the four cycles, and what the valves are doing relative to the motion of the pistons. Learn it, to the point where you can look at a cam card, read the valve events, and actually visualize what is happening to the valves as you turn the engine over, and you will start to understand cams. Once you do that, start thinking about the motion of the piston as it moves up and down the bore, and how changing the geometry of the rod and stroke will affect that motion. Then when some magazine puts some complete BS in print, you will recognize it as BS. This advice is worth a damn site more than 10 pages of "put a XYZ cam in it".

 

Do I get a star ?

It's all about compression. Anything and everything has compression involved.

The gubment wanted NOx emissions down and the OEM responded with lowering cylinder temps to prevent NOx before it happened by dropping compression.

They also dropped the efficiency, mileage, power, drivability, owner maintainability, and the entire oem auto world into the crapper.

From the crapper you need to fish out the best turds

 

One thing is for sure - the more you know about these engines, the more you realize how truly inefficient they are. It's amazing they've lasted this long.
It's gonna be a sad day when they go away once all the non-renewable fuel source (dinosaur goo) is all used up

A modern diesel electric train can haul 100,000 lbs 100 miles using only 1 gallon of fuel

 

I gotcher star right here...

 

Man oh man- I read that post as was going to come back to it later and poof !!! It gone. There was some good stuff there, I'd a left it.

 

I'm gettin too old and tired for this shit... As the Smoke once said, "You can lead a horse to water, but sitting on his head wont make him drink."

 

The bad thing about DCR is it's static not dynamic. It pays no attention to intake tuning and wave dynamics. Like any other tool it is good for comparing similar engines but if you are learned enough to know it's shortcomings you may want to use another tool.

If we look at Real Dynamic Compression ratio think about VE/100 times measured compression ratio.

12 to 1 compression times 106 VE means you trapped 1.06 times the cylinder volume into 1/12 the space at the RPM specified, so really dynamic.

 

It took awhile but now we have double the power, 1/100 the emissions, 4x the fuel economy and 4 x the engine life.

 

See post #46.

 

All he needs is a DBL pumper, and a dual point dist. And he will have a "runner", in the "been there, done that crowd" mind.

 

Yes, finally and in the grand scope of the industrial revolution it took an eternity to get there. However these new fangled engines are just that and have their inherent problems. I think the northstar platform is my favorite for several reasons, first because you can make them look really trick & vintage. but the head bolts give a good dose of fits to their owners. hard to beat the LS platform with the 5.3 being on top for dollar to power ratio. Funny though that my LS 5.3 does just a tiny little bit better mileage wise than my 400 tri power Pontiac did. Always wondered what a EGR and converters would have done for emissions on that old poncho. Supercharged 3.8 is a great engine and will put you back into seat hard enough to worry about the seat mounts with a little work. They run for ever with minimal maintainance but the best they can manage mileage wise is again just a little better than than 400 Pontiac. By today's standards these examples are just about obsolete.

When the OEM dropped compression they also cheapened the internals because they could and didnt need the beefy forged bottom ends. The oil companies made out like bandits too. Classic case of feed them bullshit and tell them its ice cream so they like it.

 

Sure it will run, but its not going to "run"

 

Enter the 4 pattern cams.

I started a thread that got almost laughed off of here that used VE (cfm) as a way to rate engine power and efficiency. You could have a way to effectively compare cylinder design, intake runners, exhaust tuning and cam profile against dissimilar engines.

They also call this the 5th cycle of a 4 cycle engine.

Can sit on the head but ....

 

VE certainly gauges breathing. However, efficiency would need to include BSAC. If you are at 103% VE but 6.0 BSAC we are not using the air we are breathing very efficiently.

And when we say efficient, efficient like a Pro Stocker, efficient like a Prius, efficient like F1, efficient as a means of producing joy when driving

 

Ya, didn't mean to sound so doom and gloom on my reply. I agree with 31 Vicky - lack of compression. That's the waste. Why bother to even build an 8:1 gas engine nowadays, right? You (or a car company) would be better off building a diesel engine for it.

Maybe the future of gas engines should be reserved only for high performance applications with 10:1 & up c/r.

 

We call it Compression Ratio but the key to more power from same energy is the expansion ratio which is the same number.

The more the gas can expand before EVO the more work we get from it. The 8-1 days were from very slow burning chambers (geometry and low turbulence) getting us into knock limiting. It takes 4 things to make NOx, Nitrogen, oxygen, Temperature, and time for the reaction to occur. High Turbulence and better geometry help both lower NOx and prevent spark knock with a relatively quick burn.

VVT and Direct Injection completely change the picture. Since GM has those features in a Cam in Block engine, it's only a matter of time before the overlap, DCR discussion gets a lot more fun!

 

Expansion ratio !!!
I like it

 

Thanks to everyone who has replied to this thread all comments have been gratefully received, I have been up the shop where mine is today and indeed have got 75cc Mexican heads, so I have today ordered myself an xe268h cam, followers and valve springs, they should be here early next week so hopefully can get the engine back together next weekend.


Posted using the Full Custom H.A.M.B. App!

 

Jets have 10 - 1 compression, 100 - 1 expansion (this varies a lot but you get the idea).

The greater the expansion ratio the cooler (relatively) the exhaust will be.

In the 90's when we had the 9-1 ratio Busch engines the exhaust temps skyrocketed melting plug wires.

We have kinda hijacked nitrosnutter's post but I would love to continue this discussion in it's appropriate place.

 

Yeah that 8.5:1 (max) dynamic c/r means everything else is perfect (like air/fuel to each cyl). 8 is a better target.

Below is my 355 with that XE268H cam:

Bore: 4.030
Stroke: 3.484
Combustion chamber: 64
Deck .025
Head gasket: .015
Dish: +12cc
Rod: 5.7
Intake closing 61* ABDC
Static c/r: 9.62
Dynamic c/r: 7.92

I probably could've gotten some more out of it when I built it. My goal was 9.75:1 static c/r.

For what it is (basic machine shop 350) runs pretty good. I did spend the money on the heads tho. Dart Pro1's w/200 c/c alum heads.

He will be happy with that cam

http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=86&sb=2