Ok now that I have a 3.4 and can setup DIS easily enough, and discovered that 3500 heads will give me a fairly lower compression ratio of 12.33:1 I am back into looking at a hybrid build, though not till I have the car on the road of course.
But the main thing I am wondering atm, is whether or not I can run that 12.33:1 static CR with a delta 260 cam and pump gas-or do I need more cam or octane?

 

There's a nice dynamic compression calc on kb-silvolite's website that will help you. I've heard of people running that high on pump gas without problems but personally I'd keep it around 11:1.

 

Yeah but the dynamic calculater requires me to know the full specs on my cam which I don't...

 

"Intake Closing Point (degrees)ABDC @ 0.050 lift plus 15 degrees" I need that # for a comp 260 grind...

 

If you know duration and lobe centers you can approximate opening & closing points.

 

I don't know cams man, all I know is that bigger #s are better,lol. And I don't have enough specs on the cam either..

 

Well you can find out. Just give 'em a call and ask for the opening & closing points. If they won't volunteer it I wouldn't buy it.

 

I'm sure I could find out, its the cam I already bought, I just can't go calling them at 12:33am in the morning,lol.

 

Noone said you had to. Something tells me they won't pick up anyway...

Let us know when you get the specs.

 

Well delta cams is a small business...I wouldn't doubt it if the phone is his personal phone,lol.

 

Fairly LOWER compression ratio of 12.33:1? What pistons and heads are you currently running to get nearly 12.5:1 compression out of a 3.4? Didn't they have a stock C/R of something like 9.5?

However, if you're actually running that much compression, you need a cam with less duration if you're looking to run pump gas. While a cam cannot physically effect your compression ratio in any way at all, it definitely can increase your cylinder pressure, causing detonation.

This is just an example, but imagine taking in 300 CFM with your stock cam, with that 12.33:1 compression. Now, imagine taking in 400 CFM with this unknown cam of yours. Same compression ratio. But, you'd be compressing 25% more air and gasoline, to just about 8% of what it's total volume was before the compression stroke.

That my friend, may equal kaboom. That would be a serious amount of pressure.

 

I am not running them yet, I am planning ahead, going to get 3500 heads and intake setup once I get the rest of my car in order.

 

In that case, I'd highly recommend keeping it right around 10.5:1. Plus or minus. Well, unless you already went ahead and purchased pistons.

 

you want a cam with more duration, just a tighter LSA to bleed off the cyl pressure with the resulting overlap.



Is your cam a copy of the Comp 260H or the Crane H260-2? Let me know and i'll calculate the numbers you need.

 

Delta cams version of the comp 260 grind.

 

I had them do my regrind. Just ask him for the specs, I'm sure he'll give them to you.

 

They did my cam as well. I'm surprised he didn't send a cam card with yours.

 

1.5 rocker arm
212 duration @ .50
.440
112 LSA
intake open -3.4* @btdc
intake closing 33* AfterBDC
exhaust opening 41.6* beforebdc
exhuast closing-9.6 after tdc
So 48 is my magic # right?

 

Assuming my maths are right thats just shy of 11:1 CR...I'm going to need more octane or a bigger cam aren't I....or new pistons....eventually I'll put a turbo in the thing...

 

I did some poking around a while ago to make sure the numbers calculated by my Dyno2000 were at least close (dead on, actually). For the Comp 260H grind, here's what I found:

Spec... I/E
Center Angle: 110*
Intake Center: 109* (hard to find this bugger!)
Duration, @ .050": 212/212
Lift: .440/.440" (with 1.5:1 ratio rocker arms, .469/.469" with 1.6:1 rockers)
Timing Points (also hard to find on most sites):
IVO (Intake opening): -3.0* BTDC
IVC (Intake closing): 35* ABDC (bottom dead center)
EVO: 37* BBDC
EVC: -5* ATDC

HTH... I'm going to have to look up that dynamic calculator myself.

 

My dyno program is saying detonation is possible with your combo. Not probable, but possible. Again I've heard of other people running as much compression with a similar cam on pump gas without problems but I would keep it below 11:1 static.

 

Oh, and those are the specs I got from delta btw maverick, not the specs on a 260 grind, called them up since I figured there might be a little variation.

 

Possible with which octane, regular or premium? If its a matter of running premium I would rather do that than shell out for another cam...just really don't want to go through 3 cams....1 for N/A iron heads, 1 for N/A aluminum heads till i get more money for turbo setup, pistons and yet a 3rd cam...would like to keep this one till i turbo basically.

 

I'm not entirely sure you have this static vs dynamic compression thing figured out.
Large cams with long duration values require MORE staic compression ratio so as to build cylinder pressure. The longer duration tends to bleed off pressure because the intake valve is opened longer as the piston rises in the bore. Intake closing point has more impact on the engine characteristics than any of the other values.
If you want a quick read on the subject, you may want to check out this link.

http://www.empirenet.com/pkelley2/DynamicCR.html

 

You could always sacrifice a little quench and just run the .060 headgaskets...


Another option would be to get a set of 3400 pistons, get .020 shaved off the top and run the .040 gaskets. That should get you down into the high 9:1 or low 10:1 range.



bl85c, does your program take into account for combustion chamber type?

 

Yes it does. You have input for head material, coatings, chamber design, you can even make your own spark curve. I did the calc on 92 octane, so premium.

 

Sounds pretty neat. Which program is this?

 

Engine analyzer pro. They offer 3 versions now, a basic one with a realistic price but it isn't much better than DD2000, another (pro version) with more features that isn't quite so absurdly priced and the top version (version 3.9) that's geared twoards engine builders and competition stuff that has just about every variable you can imagine but you need to have a motor sitting near you to take measurements off to really use. The newest trial version is a castrated version that won't let you do much, but you can still download some of the older fully fuctional trials on some torrent sites. The pro version is the best value. The bad news is that it really takes the art out of engine building.