Rustydawg

Hey guys, I'm looking for some input from anyone running 10:1 compression and a relatively short cam like the Comp XE250, XE 256 - cams with 206* to 212* @ 0.050" on the intake.

I'm building a 355 at 10.2:1 using Edelbrock Performer RPM heads and want to keep the cam timing conservative and the compression as high as pump gas will allow. The goal is a wide torque curve with reasonable fuel economy.

The last thing I want to do is build a combo that I can't find fuel for. Anyone running a similar high-compression / mild cam combo and not rattling it to death or forced to retard the timing?

BTW, the rest of the combo is fairly conservative; Performer, 3.42 gear, 5-speed. The shortblock will be 'zero deck'.

jfreeman74

Why are you going with such a small cam? That's not much bigger than the stock cam in my 305.

AJ_92RS

The higher static compression you run, the lower the dynamic compression needs to be.

You need a longer duration cam with a later closing point.

Something along the lines of a 280* advertised duration cam (or higher) will be needed to bleed off the cylinder pressure at low RPMs. Otherwise you will have a lot of detonation problems.

StealthElephant

10.2:1 should be fine on 93 octane. I'm running 10.7:1 with the XE268 cam and I'm having problems with timing. If I was 10.2:1 I'd probably be alot better off.

That cam does look small though, the XE268 idles nice and smooth and pulls plenty of vacuum.

ME Leigh

I'd say anything smaller then an XE-262 and your running too high of a dynamic compression ratio that wouldn't run on pump gas.

Sitting Bull

Whether you like it or not, both the XE250 and 256 are HUGE compared to the 305 peanut cam!

MrDude_1

you either need a bigger cam, or lower the compression ratio.



sounds like you want to build a truck motor..... low RPM, high MPG.... so id go with lowering the compression some.....

and id still probly go a lil larger on the cam, but im power hungery.

jfreeman74

Who said anything about a 305 peanut cam? He's building a 355. Both those cams are small cams for that engine.

RB83L69

Those are REALLY small for a 350. The 250 especially. The 262 doesn't even idle noticeably rough in a 350. The 262 is about as small as I'd go in a 350 with that much compression; otherwise, you'll have alot of trouble getting enough timing to make it not run hot and get good gas mileage and have good response, without knocking.

That's assuming of course that 10.2 is your real compression. What parts are you using? What is the block deck height? What head gasket & pistons & heads?

EvilCartman

iTrader: (1)

I ran a XE256 in my 305 lol Go bigger in a 350 man, you'll like it.

F-BIRD'88

There is a better way to adjust the running cylinder pressure than over camming the motor.

Pick a cam with the mild duration you want but with a wider lobe seperation (114+). Install the cam with no advance. (straight up). This will result in a later intake closing point
(like a big cam) This type of cam is much less likely to
ping or knock with a high compression ratio.

With aluminum heads and a tight qench clearance and 92+octane gas you should be fine.

I would not use a CompCams Extreme series cam
for this one.

try this one All 214/224 .444/.467 288/298 116 19811-21177

or this one All 210/216 .440/.454 270/276 114 19811-20844

IROCaholic

My friend had a truck running a similar setup to what your looking at and had no problems. He was running a 350 with ported cast iron L98 heads, comp cams XE256H cam, performer RPM intake, Holley 650 carb, and headers and exhaust all with 10:1 compression and he never had any problems drove it to school and work everyday. Dynoed at around 340 hp and 400 tq at the fly if I remember correctly. I dont know if he still has the truck now because he moved but he loved that setup and he was a pretty big motor head. And yes all of the statements above are true but you shouldnt have a problem if you know what your doing.I would defenitly reccomend going with the XE268 cam instead though.Good Luck.

Sitting Bull

Why you did! I quote:

"Why are you going with such a small cam? That's not much bigger than the stock cam in my 305."

jfreeman74

I don't have a peanut cam. Look at my sig. I have an '85 305.

Rustydawg

Indeed, overcamming the engine to get around the high compression and tossing away the efficiency & throttle response that compression brings is not the answer for me.

I understand the dynamic and static compression pressures at play here, I was hoping to hear some real-world limits. IROCaholic's friend was running a similar combo with iron heads and apparently having good luck with it. StealthElephant is having to retard the timing at 10.7:1 - and with a hotter cam than I'm looking at.

In his book David Vizard makes over 300 HP with a 350 using well-worked production heads and the production '929' camshaft (advanced 4 deg!) at 10.1:1, however he doesn't mention what he's using for fuel. It has a broad, flat torque curve and would likely be a 13-second ride in my car.

F-BIRD'88

"In his book David Vizard makes over 300 HP with a 350 using well-worked production heads and the production '929' camshaft (advanced 4 deg!) at 10.1:1, however he doesn't mention what he's using for fuel. It has a broad, flat torque curve and would likely be a 13-second ride in my car. "

Thats because he built the engine using the exact parts
from Chevrolet's 1970 350 rated at 300bhp. Used to come in '70 Impala SS.

It used good 186 casting camel back heads, proper quench clearance 10.5:1 compression, "929" cam etc etc.
This was also the base Corvette motor in 1970.
It infact made 300 gross flywheel bhp. Nothing suprising there.
Back in the day these motors used "Premium leaded gas", not the Monkey p&^% we run today.

If you read the fine print in David's book you'll see he tweeked it further with very tight quench cleareance at .022" (not recommended) and ported the heads with large valves. The stock 19704bbl intake and carb were also tweeked for better airflow.
This same motor with one of the cams I listed would pick up 40-50hp in that same motor with much stronger torque too.

Premium leaded gas was about 96-98 octane back then.

Try one of the cams I listed. It won't be over cammed with those. I've found Accelerated Motion Cams to be "top shelf" stuff. Only in Canada, ya say... Pity.

Sitting Bull

Don't know what your specs on that TPI cam would be, but I doubt that it is as big as an L98 cam--and the L98 cams are still small compared to both the Comp XE 250 and 256.

So my point stands, even if we did have to track you down

RB83L69

I doubt those ancient generic cams will make as much power as a modern cam, when paired with good-flowing aftermarket head castings.

There is a great deal of extreme techincal misinformation in this thread. For example, wider lobe separation does not change the intake timing events. All it does, is open the exhaust sooner; just as narrower lobe separation would open the exhaust later. All of that stuff about one of those older inferior cams changing the intake closing points based on lobe separation, is wrong.

The intake lobe "location" is the thing most basic to a cam's specification. All cams are designed by their mfr to be installed at some specific rotational angle compared to crank postion. That has nothing to do with lobe separation. It's the location of the peak of the intake lobe, compared to how many degrees the crank has rotated since it passed through TDC. Most aftermarket cams are designed for the intake lobe peak to occur at 106° ATDC, which is 4° sooner than late-model factory "emissions" (71 up) timing sets will produce with any given cam.

The intake events are determined by the cam grind (duration and such); lobe separation moves the whole exhaust lobe around with respect to the intake lobe. The only way to change the intake events is to alter the "advance" or "retard", at cam installation time. Of course, if you do that, the exhaust will move in lockstep with the intake, degree for degree.

With the RPM heads, which are aluminum, you can get away with more CR than iron heads; in fact you need more. But 10.2 with a cam with that little duration is too much even with aluminum heads.

It's not a matter of "bleeding off" CR, so much as it is a question of matching your cam to the rest of the engine's RPM characteristics. The RPM heads want a peak RPM range somewhere in the 500-6200 RPM range on a 350, based on flow. Neither of those other cams will deliver that. The old generic grinds, which are perfectly good when correctly matched with untouched stock heads as they were designed for, are wrong for better-flowing aftermarket castings.

AJ_92RS

RB,

Most of what you said is true. As usual you know a lot.

However, I can't agree with everything you said about lobe seperation not having an effect on intake centerline though. It does and here's why.

A cam with 110* LSA advanced 4* will have an intake centerline of 106* (as you said). But, if you were to change the LSA to 112* and leave the cam advanced 4*, that will put the IC at 108* ATDC. That will open the intake later, and close it later, all else being equal. Not only that but it decreased overlap. This will build higher cylinder pressure at lower RPMs.

Now let's turn the table...

A cam which has the same LSA of110* and the same IC of 106*, yet more duration will open the intake valve earlier and close it later, all else being equal. This will result in more overlap and the increase of overlap will bleed off cylinder pressure (not compression ratio) at lower RPM.

I just wanted to make that clear. There's a whole slew of things that happen when you're looking for the right "combo", and you have to take all these things into consideration when building an engine. To say "wider lobe separation does not change the intake timing events" is not true unless the IC stays the same, which normally doesn't happen when using a cam ground with a wider LSA.

RB83L69

No.

If you advance or retard the cam, that will change the intake center line.

If you change the lobe separartion, that will change where the exahust events occur with respect to the intake events. Since the exhaust opens before the intake, then the larger the LS, the earlier the exhaust opens.... it has nothing to do with when the intake opens.

If you change the cam grind's lobe separation, that will move the exhaust center line with respect to the intake centerline. It will not move the intake center line. If you install the intake at 106°, the intake center line will be at 106°, regardless of where the exhaust CL is. That's just the way it is, fundamental to the design & specification of cams, not subject to discussion or argument.

If you take a cam with some particular intake lobe specs, you can vary the lobe sep all over creation, and the intake will stay the same; unless you either re-grind the cam to some other intake lobe properties, or you install the cam at some other relationship ("advanced" or "retarded") wrt the crank.

"Overlap" does not bleed off cyl pressure. "Overlap" is the period of time when both the valves are open; and as such, determines the amount of time that the intake charge can flow directly into the intake valve and out the exhaust valve. it's entirely possible to design (or install) a cam with lots of overlap, but yet still have lots of cyl pressure. On the other hand, valve opening while the piston is on one of its up strokes, does affect cyl pressure, strongly.

You're right, the combo is where it's at. But, the IC stays the same as long as the cam is installed at the same IC (duh), which is totally independent of lobe sep. Lobe sep is how far apart the exhaust events are from the intake events. The intake events are the "standard" for cam timing.

AJ_92RS

OK. I guess I have to say it.....

You're wrong. You're contridicting yourself from what you said earlier.

If you don't understand what I'm saying, fine. So be it.

I suggest you re-read what I wrote, and keep in mind that when a cam company increases LSA and keep it 4* advanced, the ICA does change.

And why would you think that increasing overlap doesn't decrease cylinder pressure at low RPM? It's called scavanging the cylinder.

RB83L69

I'm contradicting nothing.

Exhaust events (such as lobe separation) are measured by comparing them to the intake events. You don't somehow change the intake events by changing the exhasut events that are measured against them.

If you take a cam, and install it at some given ICL, then the intake events occur where they occur.

If you re-grind an identical cam except you change the lobe separation, and you re-install that cam exactly as you installed the previous cam (at the same ICL), then the exhaust lobe moves; the intake stays the same.

As long as you install a cam at the same intake center line, the intale center line does not change, no matter where the exhaust lobe is moved to. How trivial can we get here?

Overlap only changes "scavenging" within a specific band of RPMs, and is dependent on the design of the exhaust system.

I'll go back to some point in my youth 20 or more years ago when I was doing contract engineering work for cam companies (Comp in particular), and see if I overlooked anything. I doubt it though.

AJ_92RS

All you're doing now is arguing with something I didn't say. I guess you're bored? I dunno.

BTW... I'll restate something I said earlier which you obviously overlooked.

You did NOT say anything of the sort earlier. Now you have, so as usual I guess you're never wrong as long as it's worded that way YOU want it.

AJ_92RS

I think I see the confusion.

You said..... "Most aftermarket cams are designed for the intake lobe peak to occur at 106° ATDC, which is 4° sooner than late-model factory "emissions" (71 up) timing sets will produce with any given cam."

I'm saying that when Rustydawg goes to buy a cam that also has 4* advance ground into it, but has a wider LSA, then the IC is going to change. I already explained why.

What you're saying is if Rustydawg was to ORDER a CUSTOM cam and tell the cam company to keep ICA the same, then (obviously) it would remain the same, and only the EC will change.

My point is... what are the chances of Rustydawg ordering a custom cam, or even degreeing it for that matter? More than likely he will buy an off-the-shelf grind which is ground 4* advanced like 99% of the cams out there.

When he does, the IC and the EC will change. Simple fact.

That's all I'm trying to say. I'm trying to help the person buying the cam understand what will happen when he does that.

jfreeman74

Actually my cam specs are the same as the L98 cam. The '85 305 was the only 305 that had that cam. Sorry, I like to mess with people like that when they start talking about the 305's. Everyone assumes that all the 305's were created equally. My point about the 2 Comp Cams is that it seems like a lot of trouble to replace the cam with something that isn't that much bigger than a cam that comes stock in a smaller motor.

You can track me down but that doesn't mean you can catch me.

Sitting Bull

No problemos! I agree with you, I wouldn't go smaller than the 256 cam amd would likely go one step further and at least use the 262.

jfreeman74

That's the cam I'm going to use.

Hype

If you're still looking for "real world" opinion...

I run a 350, zero-deck, flat-top pistoned, iron-headed (882's), with your low-buck rebuild head gasket parts (.039") gasket,

I get 8.9x:1 static compession on the calculator. Ok I'll just assume 9:1.

The heads have had nothing but a *good* 3-angle valve job, and some minor bowl work to remove casting flash and such, as supervised by some decent local gearheads who run in the high 10's.

I am using the CompCams XE262, /w all their associated valvetrain parts.

I have 34 deg. timing in by 3,000RPM, and I run on *87 OCTANE!*.

With your aluminum heads, you should have NO probs running full timing with this cam with premium fuel. It was explained to me that if your quench is setup properly, you can go a smidge further.

I get 19-20 MPG highway, and I busted off a 14.1 on the g-tech at 99mph last night in 4 degree (centigrade) weather at sea level with a manual tranny, 3.23 posi rear in a no-option '83 transam. Yeah I know its probably more like 14.2 at about 97mph, but the g-tech is a good ref. point. Heavily massaged vortecs are going on my bad-oscar in the next couple of weeks, and hopefully I'll see some low 13's with traction. Of course, I'll have to start running that $1.00/Litre 94octane..... sheeitt...

good luck.

Rustydawg

Hype, I am still open to more real-world feedback - thanks. I do understand the importance of quench like you say.

I'm sure there are more out there running a similar combo. You can't really compare them, but the modern muscle car engines are running 10+:1 with Al heads & cam profiles similar to what I'm looking at - however they have very carefully tailored spark / fuel curves run by a computer, EGR, improved cooling and combustion chambers, ports etc... It goes on and on. I would suspect that the assembly tolerances are vastly improved over the original equipment in our 80s iron too.

Incidentally, I found my G-tech ETs to be reasonably accurate and I turned in similar times at the track. The trap speeds were a bit optimistic though.

F-BIRD'88

RB ya really hate it when I recomend something other than a Comp Extreme Cam. These are not old wornout
grinds. The smaller one is a Crane grind. Both are modern computer designed proven assemetrical fast open grinds much like a Comp grind. If you re read my post you'll see by using a wider lobe separation and installing it as per the cam card or a little retarded the intake closing point is in fact later.
The basic cam timing is very simular to a Pontiac "S" cam or a 340/275hp Chrysler small block street cam. This cam timing works well with high compression and through closed exhaust (the real world) and lifts the valves plenty quick but won't tear up the valvetrain. it suites what he is trying to achieve.
Not every motor needs a cam ground on 110 and in at 106. He's looking for a good running street motor that willl get some mileage and perform well as a daily driver.
Not an all out race motor.

Either of these cams will make smooth real world power, better mileage and cruise nicer than the extreme series cams. They also will tend to not create exessive cylinder pressure at peak torque by the nature of their timing points as compared to a cam with a 106* intake c/L.
Not as likely to get detonation.
A good match for his high compression Mild/ moderate street motor. Probabily spank a simular motor with a Comp cam in it too in a street race.

AJ_92RS

Just curious...

When you say straight up, you mean 114* IC, or straight up from where the IC is ground (I.E. if it's 4* advanced)? I would be interested in seeing an engine run with that IC point, just for experience. It would be interesting to say the least.

BTW, both the cams you listed are nearly identical to what I suggested when I said at least 280* adv. duration. I left out the LSA and IC to save confusion.

I guess that didn't out work too well.

F-BIRD'88

ya most performance cams are ground so that when you install them they are about 4 degrees advanced in the motor.
I meant knock it back a bit from there to a more 'straight up position.
So a cam thats ground on 114 LSA would tpically cam card in at intake C/L of 109 or 110. I suggested moving it to at or near 114 intake C/L ("straight up") creating a later intake closing point. It's still going to have all kinds of bottom end grunt.

The other cam ground on 116 I would tweek it so the intake is at 111 to 113 C/L.

F-BIRD'88

There is nothing wrong with moving the cam around a bit in the motor (advance or retard) and seeing how the engine reacts to this change. Even a Comp Extreme Cam. When researching out a professional race motors' combination on the dyno, a good tuner would move the cam in the motor by very small amounts and note the difference in the power curve. The motor likes what it wants for valve timing points, not what's popular.

Ray87Z

Well I'm a little iffy on the compression I'm actually running at (it was supposed to be somewhere around 8-8.4:1 with 76 cc heads, and I'm running Vortecs with a rated 64 cc chamber). I run the XE262 and am pretty happy with it. Seems to make power pretty much everywhere, idle is solid at 750 rpm in drive. At slightly under 1000 rpm in park it pulls down like 17 lbs of vacuum. I don't really see going any smaller with a good set of heads like you have... You'd probably like the XE268 (I plan to mod my Vortecs to support the extra lift and either run the 268 myself or at least go with 1.6 rockers on the 262 I have...)

F-BIRD'88

"BTW, both the cams you listed are nearly identical to what I suggested when I said at least 280* adv. duration. I left out the LSA and IC to save confusion."

Good point... as long as you're talking apples and apples not apples and oranges.
Trouble is ther is no standard for "advertized seat duration"
Comp rates their hyd cams at .006" while Crane and some others are rated at .004"
Comps appear shorter and Crane's appear longer than the other.

A Comp 280Hcam would be about 230@.050" A Crane 280H would be about 224 @.050" See the difference.

Another way of looking at it is a Comp cam with 230 duration @.050" would be "advertized" at 280 seat duration. The very same cam in a Crane rating would be
"advertised" about 286-292

Also a lot of people get too hung up on the radicalness of the lobe based on how the advertized numbers make it appear. In an actual running motor the actual "running duration" and valve timing points where the valves actually start and stop moving are not the same as advertized and are effected by many variables that are not being taken into consideration.
Truth is, There is much more to it than meets the eye.

AJ_92RS

Exactly.

Plus there's hydraulic intensity to confuse even more people (a term coined by Harvey Crane).

For instance, I got a POS cam (aka LAZY cam) from PAW with my engine kit. It's advertised duration is at 280*/290* taken from .006" (I checked it myself to make sure) Yet the .050" duration is a paltry 214*/224*, 112 LSA, 108* IC, .443"/.465" lift.

Compare that to something like the Xtreme 262 which has a lower advertised duration, yet more/equal duration at .050" (218*/224*) and .462"/.469" lift.

Running these two cams through DD2000 using the advertised numbers and the difference is HUGE!!! Close to 40 ft/lbs more TQ at 3,000 RPMs for the Xtreme cam, yet 40 more HP at 5,500 RPMs for the PAW cam.

Using the .050" specs, and there's hardly a difference at all RPMs.

F-BIRD'88

"For instance, I got a POS cam (aka LAZY cam) from PAW with my engine kit. It's advertised duration is at 280*/290* taken from .006" (I checked it myself to make sure) Yet the .050" duration is a paltry 214*/224*, 112 LSA, 108* IC, .443"/.465" lift. "

Well I used that POS cam in my 350Vortec motor and the thing flew. very nice cam. ran 13 flat at 104.5 ( traction limited) which calculates out to 311 net HP. (right about 365 to 388 gross flywheel (hot rod mag horsepower)

Made 374 hp and 400+ ftlbs torque on a magazine dyno test.

it was actually a Wolverine Blue Racer, but same POS.

These "old" cams actually work really well.

the motor may have gone a little faster with the ultimate cam in it, but this one was no slouch.

AJ_92RS

That's good to hear since that's probably gonna be the cam I run with

What IC did you run it at? I'm gonna run ~10-10.4:1 compression with .045" quench. I was thinking of retarding it another 4*, but I guess I'll have to try it at 108* first just to see.

F-BIRD'88

Did you ever get to compare both cams in the motor?

F-BIRD'88

I never degreed it in. Just thru it in a buttoned 'er back up. I needed a cam and lifters to replace my first cam that lost a lobe prematurely ( A Comp solid cam) that I never got to try. The local speed shop had a bunch of Blue racer cams on sale. (the price was right)
I remembered this cam being used in a vortec 350 in a Hot rod mag article and made good power for being a mild cam so I bought it to try while I sent my Comp Cam back for Warranty.
For being an "old Generic cam" it worked very well.
was good on gas too.

AJ_92RS

I don't have both cams. I was just using the Xtreme 262 as an example for hyd intensity, because it has similar .050" specs to my lazy cam. That's all.

Thanks for the info.