DeltaElite121

I'm asking this question because I have a decent cam for the car now, but I need to bleed off some more compression in order to run 93 octane pumpgas. I have a 4200 Circle D converter, 3.42's (9 inch with different gearset going in soon to match everything a tad better), and a 10.78:1 compression iron-headed 355. With my current cam my dynamic compression is right around 8.72:1 if I recall correctly.. it's fairly high for pumpgas and I'm sure it'll ping without mixing 93 and 100 50%. I have no problems finding 100 octane to mix in with the 93 around here, but I'd LIKE to be able to drive the car anywhere ideally without having to worry about it. The reason why I am going through this is because I honestly wasn't thinking about my DCR when I ordered the cam and I told the guys at Bullet my specs and that I wanted it to run on pumpgas. After seeing the specs we had talked about (for whatever reason it didn't register in my head.. don't ask) I see the DCR is fairly high and I don't want to back out timing just to attempt correct the problem.. as that is only hampering performance and not doing things properly.




What I have currently sitting for install (this is a solid flat tappet and running 1.6 RR's, BTW):

•Advertised Duration (Int/Exh): 276/284
•Duration @ .050 (Int/Exh): 243/251
•Gross Valve Lift (Int/Exh @1.5): .518/.530
•LSA/ICL: 106/100
•Valve Lash (Int/Exh): .026/.026
•RPM Range: 2500-6800



What I am looking at, and was recommended from Steve @ Lunati:

•Advertised Duration (Int/Exh): 282/290
•Duration @ .050 (Int/Exh): 252/260
•Gross Valve Lift (Int/Exh @1.5): .541/.562
•LSA/ICL: 106/100
•Valve Lash (Int/Exh): .016/.016 (notice the tighter lash*)
•RPM Range: 3000-7500


This second cam will definitely bleed off compression, but how does a cam like this typically perform on the street and strip in comparison? Before some of you guys say it - no, I don't go jumping for the "biggest cam I can find".. I actually want to keep it smaller if I could, but I think this is the only way to bleed off that extra cylinder pressure, and it could also potentially match my motor/converter more.

Thoughts?

skinny z

Questions:
What heads are you using?
What's your quench value?
How much initial/total igniton timing do you run?
Most importantly, have you done a compression test? Perhaps that's your best indicator of the true SCR unless you have have a very detailed engine build sheet with every value measured.
As for your question, I've run cams with more advertised duration and lift albeit in a hydraulic roller (355). Idle vacuum wasn't the best at around 8" although it stilll returned decent mileage on the hiway and was manageable around town. This was with a very well tuned 750 cfm vac sec carb. Performance wise, what it lacked in low rpm torque (partially due to the 10.5:1 SCR) it made up for in top end charge.
Still, like you, I say when a choice is available the smaller of the two cams is the way to go.

cuisinartvette

With a 106 lsa it will be rowdy
Will require some fine tuning. Not sure how that gear will like the cam but the converter may have some saving grace there.

Honestly I dont get too hung up on the whole DCR thing.

Orr89RocZ

iTrader: (20)

That comp in an iron head is gonna be hard to make work on 93. have you considered a meth inj kit for heavy throttle to wot operation? Cool charge and alcohol oct will help but thats bandaiding like 100 oct mix

Carb car? How well can you control timing?

Id run the bigger cam to have a better chance at it but sneak up on max timing. Its likely gonna want less than a motor at 9-10 to 1 comp but thats not always a bad thing. Also playing with lash will help tame it if needed. Also advance it more if needed

Also play with plug heat ranges. Run abit colder plug vs what you would run on a 9-10 to one motor. It could help detonation resistance

Its gonna be abit wild and definately wont start til mid 4000 range which is good for converter you have but it would prefer more gear.

Its gonna come down to the tune. Efi would be alot better with this cam but carb can work just be alittle lazy off idle.

Motor is gonna want to scream over 7k rpm if heads can support it

skinny z

I've never understood why people tend to dismiss DCR. It's probably the most important indicator of how the engine's given SCR works with cam duration. Specifically the intake opening event.
There are countless engine builds by the ill-informed that produce poor performers because little attention has been paid to compression ratio vs cam timing.

Orr89RocZ

iTrader: (20)

Well for one how does ve play into the whole equation? SCR is ratio of fixed cylinder volumes and DCR is basically effective closed valve stroke length compression since there isnt any compression when valves are in overlap phase. So by determing when valves are closed and where the piston is during that time gives effective stroke for an actual displacement of the cylinder. That can be ratioed to get the compressed volume and thus DCR

But a good engine doesnt trap 100% of cylinder volume. It can sometimes trap much more, 100+% as high as 115-120's i think for pro stock motors. So what good does DCR do in that situation when effective cylinder volume could be greater than calculated by valve events alone? Just something to think about.

If dcr is high then limit timing til it runs ok and see what it does. Think the same thing happens in a boosted motor. Scr and DCR is low but when boost is applied trapped cylinder density is increased which effectively increases volume. Effective compression ratios increase but it still runs on pump gas but requires lower timing.

skinny z

Yes, a highly developed engine or even one with the proper characteristics of overlap and exhaust pulse tuning will yield over 100% ve. However for the purposes of this discussion (and of others who might read along) I feel the DCR is very important. When someone tells me they're running a 12:1 compression ratio it doesn't mean much unless the cam specs are taken into consideration. Same applies to 9:1 scr. Combine that with a small cam and you can have 200 psi cranking pressure. That's a good indicator of an efective combination.
In my own build (and I'm in the middle of a compression test as I type) I'm running a calculated scr of over 10.25:1. That's with a cam that has similar timing events to a xr276hr. I have 200-205 psi cranking pressure across the board. Indications are my dcr is suitable for the job. Any less cranking pressure and I`ll bet my dcr is off the mark. Whether you care to include that calculation when preparing your build on paper is a matter of choice I suppose.
As far boosted applications, that I can't comment on as I have only limited knowledge. But I see your point.

Orr89RocZ

iTrader: (20)

Getting as high as you can with cylinder pressure for the fuel you run makes perfect sense to me for making as much power and torque as possible. Lots of things to look at but i agree dynamic compression is important. Depends on what you are looking for.

1gary

A DCR of 8.5 would be more accepting of the ranges of pumped gas out there.


Cuisinart.The tech has moved forward because of the pump gas. Remind me what the Elky costs up there in the great white North. I'm sure you know your engine using that fuel doesn't care as much about DCR's.

DeltaElite121

Here's the rest of my setup, for reference:

Dart Iron Eagle 206cc's with 66cc chambers (they've been lightly ported)
Airgap RPM Intake
Quickfuel 750 mechanical secondaries (annular boosters)
Harland Sharp 1.6 roller rockers
GM HEI ignition setup

I have a vacuum pump with canister because of this current cam @ 106, so we're good there. All of my parts on this motor are brand new (including the ignition and anything else that doesn't add any power whatsoever; I spent a lot to do this correctly). This motor has 1,800 miles on it total (I bought the motor off a good friend of mine). Motor has good forged pistons, but I'm working with a stock bottom-end so I'm keeping them RPM's at 6500 or so until my next build. The only thing I HAVEN'T replaced is my radiator, ironically. That will be changing soon once I find a better one I like.

This is in a full suspension Camaro behind a TH350C.

Timing isn't an issue, I can get that dialed in really well. I honestly can't remember the quench off the top of my head, but I do have it written down somewhere. I'll look for it.


This is what the motor looks like, for those of you that are curious. This is where I stopped last. It's been a long project running, heh.

urbanhunter44

iTrader: (3)

For a comparison in driveability my solid roller 248/252 @ .050 drives fantastic with very few problems in front of my TH350. Brakes work fine.

1gary

Flat top pistons with a .040 gasket@ 0 deck??.

Orr89RocZ

iTrader: (20)

Stock rods crank worries me abit with the bigger cam as it wants more than 6500

DeltaElite121

Yeah, that's kind of what I was thinking. My guess is it's going to really start pulling around 5K or so, and it probably will want to go to 7500 fairly easily to say the least. I don't intend on winding this motor up that far because that's asking to blow something up.

Gary, I'll check my records book for the gasket part #. Yes, it was decked and has flat tops.

ASE doc

You are really asking alot of your stock bottom end. As far as the discussion of SCR vs DCR is concerned. The high duration, higher lift cam assuming it is properly matched to the heads and intake, will increase DCR by increasing VE. You would hope this is the case, otherwise you will have an engine that idles like stink and makes no power. Orr was pretty much right on there. You need to really be aware of this as you road tune the engine. Be very sensitive to any detonation. Your SCR and that cam with iron heads is more than I would try to do on pump gas. And where do you get 93 octane? Up here in Oregon it's all 91 with 10% ethanol.

I am not a fan of big flat tappet cams on the street. At the track where we can push 12:1 CR and run 108 octane, it's a different story. Whatever power vs. drivability you achieve with that combination, you would do that much better with roller lobe profiles. (less overlap and more lift vs duration) Especially when you go that big.

DeltaElite121

1/3 of the gas stations here have 93. Generally 87,89,91 (or 93). 15 minutes from my house there is a ZX station that sells 100 Octane. I have a "favorite" gas station that sells 93 that has consistantly helped 4 different vehicles run a lot smoother/better, so I usually go there. It's five minutes from my house. I assume it has a lot to do with the quality of the gas, and not necessarily the octane by any means. Could be the additives if I had to take a guess.


ASE, I think I'll be alright @ 6500 RPM's. I don't intend on going anywhere past that currently. With the way that I drive it will rarely see those RPM's anyways since I babyfoot my car most of the time. Doesn't mean I don't romp on it (because I do) but I am not hard on my rides generally.

My concern is ultimately buying a cam that doesn't even start "going" until I'm topped out of my powerband. That's really the big issue, along with bleeding off compression to run pumpgas.

Orr89RocZ

iTrader: (20)

For kicks call Mike Jones for a cam quote as well and tell him your situation. I am curious if he can find something to work in between those recommendations.

DeltaElite121

Found his number and email on YB. I'll see what he thinks.

1gary

Ya know the best answer to your question is aluminum heads so you could run the cam you wanted to.

InfernalVortex

iTrader: (4)

Yeah after all this discussion, the tradeoffs involved, and the stock crank...

It seems to me like the answer is aluminum heads with 76cc chambers. I know you have a lot invested in your Darts or else you would have considered it... but maybe you should take a step back and reconsider? Youre talking about a MONSTER street cam with a stock crankshaft and rods. I mean at some point thats not going to end well. Does it even have ARP bolts?

skinny z

I'm not sure we're talking about the same thing. The DCR I'm referring to has nothing to do with VE or compression pressure.
That said, having properly matched SCR and cam timing events to achieve a better DCR would improve VE and increase cylinder pressure. Kind of the same but in the calculations, efficiency isn't part of the equation.

skinny z

For what it's worth, my last build (not the current one) was a more or less stock bottom end 350. Upgrades were ARP rod bolts (stock rods), hypereutectic pistons and a polished OEM cast crank. I would shift at 6500 rpm on the 1-2 (700R4) and 6000 for 2-3 when racing. That short block went many miles and made hundreds of "passes" at those rpms. My limitation ultimately became the valvetrain.
While I wouldn't recommend it, drag racing is relatively easy on the bottom end parts (as opposed to road racing) and if you keep the rpms modest then you should be ok.
It'll be real temptation to keep your foot out of it though once those rpms want to keep on going.....

cuisinartvette

I dont "dismiss" DCR Im saying some overthink their way out of a build trying to get it "perfect". So many variables to consider

Gas here? I run 11.5:1 on 91 Ca gas with 34 timing max.
If I want to up the wick I add some 130 Supreme and turn it closer to 38-40 deg around 100 oct fwiw. Comes out to about $6/gal for reference. It does use a bunch of overlap Ill admit

grumpyvette

What I have currently sitting for install (this is a solid flat tappet and running 1.6 RR's, BTW):

•Advertised Duration (Int/Exh): 276/284
•Duration @ .050 (Int/Exh): 243/251
•Gross Valve Lift (Int/Exh @1.5): .518/.530
•LSA/ICL: 106/100
•Valve Lash (Int/Exh): .026/.026
•RPM Range: 2500-6800


why not just Re-index the current cam from its current 4 degrees advanced, to 2 degrees retarded from split overlap , so its reads

•LSA/ICL: 106/108 and see how it runs (REMEMBER TOO CHECK CLEARANCES)
Thats the way Id have installed it to begin with, the change in valve timing should move the total power curve only about 250 rpm higher but move the intake close point 6 degrees retarded from its current index,dropping the DCR

http://forum.grumpysperformance.com/...+degree#p12045

http://forum.grumpysperformance.com/...gs+gears#p8680

http://forum.grumpysperformance.com/...php?f=52&t=399

http://forum.grumpysperformance.com/...hp?f=52&t=5520

1gary

Because when you fix it Grump,ya fix it and not patchwork.

I don't think a aluminum head at 76cc would be necessary. You can get more SCR out of aluminum heads.72cc or even 67cc might work. He has spent time and money doing it right and if he where to make the change,I'm not talking the cheapest pc of junk out there either.

grumpyvette

Ive never considered INDEXING a cam to maximize performance "PATCH WORK" and I don,t know any engine builders that simply throw a cam in , indexed 4 degrees advanced and think thats the only possibly index the engine may like, example Im using a CRANE 119661 HYDRAULIC ROLLER in my 383 and it runs best 4 degrees retarded from the factory DOT-TO-DOT index, or at SPLIT overlap.
you might be surprised testing current parts with a few minor changes doesn,t take much time or cash , and buying and installing new parts does.

http://www.circletrack.com/enginetec...g/viewall.html

1gary

[quote=grumpyvette;5406274]What I have currently sitting for install (this is a solid flat tappet and running 1.6 RR's, BTW):

•Advertised Duration (Int/Exh): 276/284
•Duration @ .050 (Int/Exh): 243/251
•Gross Valve Lift (Int/Exh @1.5): .518/.530
•LSA/ICL: 106/100
•Valve Lash (Int/Exh): .026/.026
•RPM Range: 2500-6800


why not just Re-index the current cam from its current 4 degrees advanced, to 2 degrees retarded from split overlap , so its reads

•LSA/ICL: 106/108 and "see how it runs" (REMEMBER TOO CHECK CLEARANCES)

Grump-the see how it runs quote is what brothers me. Then check clearances.............

Dialing in cams has been in our life style for many yrs.

Lets review:

The O/P said he really didn't want to retard his ignition timing.

That it is his hope to bleed of compression with a cam swap

That he did make a mistake in the build.(god only knows don't we all from time to time)

And even through we all know swapping heads is expensive,it is a path to him keeping what has or maybe even increasing it.

Now that is how I think about it and if you understand me,don't tell anyone because they will think your crazy too.

DeltaElite121

I think you guys are all kind of thinking what I have been thinking. I don't WANT to replace my heads, but that's kind of what it's looking like. Sometimes when you want to do things right - it's expensive getting there. I may just need to run what I have now and mix until I can afford some heads from Chad Speier.


Do you guys think it's entirely possible to gain similar effects of dissipation with an aluminum water pump (or electric pump) and a better quality 3-row radiator? Not trying to take the cheap way out, but I will eventually be getting those two things anyways. I am thinking this could potentially make some difference, but not enough to change the situation.

Orr89RocZ

iTrader: (20)

Cooler the better. Cooler plugs too. I think it all could help

skinny z

Question: How did you end up with a 10.78:1 compression ratio?
355. (4.030 x 3.48)
66 cc heads.
Zero deck.
.040" head gasket.
Flat top piston with about a 6-7cc valve relief.
I get 10:1 at the most.
Just wondering...

cuisinartvette

google wallace calculators neat stuff

Alum water pump wont really dissipate heat just save a few lbs.

You are correct the heads are where the power is yeah it hurts spending the initial hit but youll get what youre after. You can run less cam with a stellar head and still make the beans where it counts.

skinny z

Try it and tell me what you get. His calculator (yes, neat stuff) or mine can't come up with 10.78.
Is there a fundamental error here somewhere?

FtrSpeedy

10.78 is with a .100 dome piston.

skinny z

I don't think so.
Even with a zero deck and a minimum .040" quench via the head gasket you still barely get 10:1.
If the math is off, then the whole discussion is pointless. At least as far as the OP's needs are concerned.

1gary

Much could have to do with if those are two valve reliefs,four valve reliefs,or true flat tops. If they are true flat tops,it would be 10.69 SCR. I'm going to hold off in posts until he clarifies exactly what he has. No way I am going to suggest he moves the cam around until I know for sure...............