Dynamic Compression Ratio w/ Iron Heads

blykins · #1 (**permalink**) 07-04-2005, 04:57 PM

What's the optimum dynamic compression ratio with iron heads and pump gas (93 octane) ?

Excaliber · #2 (**permalink**) 07-04-2005, 05:02 PM

Heres a link to a recent thread concerning this topic, GOOD stuff.

So how much dynamic compression are you guys getting away with on street pump gas?

From the calculations it looks like I'm running 7.9

Gessford spec'd out the custom piston for CC of the "dished top" and compression height based on block deck height, rod length etc. The cam profile was to work with the final compression ratio of my iron high riser heads (9.8 to 1) and my final drive ratio. As the motor runs excellent on pump gas and makes plenty of power (11 sec range in the 1/4) I'd say he "nailed it"!

blykins · #3 (**permalink**) 07-04-2005, 05:07 PM

Hey Ernie, what cam are you running?

Excaliber · #4 (**permalink**) 07-04-2005, 05:08 PM

The 282S Comp Cam (solid) flat tappet.

Recent discussions concerning Dual Pattern Cam profiles, as it concerns the FE in particular, leave me wondering if PERHAPS there is an even better choice of cam out there somewhere!

But it's hard to beat a combination (intake, cam, C.R., final gear ratio) that you KNOW works well.

blykins · #5 (**permalink**) 07-05-2005, 07:05 AM

When I mess around with Desktop Dyno, I definitely get a greater power gain from a dual pattern cam.

I used one of the DCR calculators online. It seems with my theoretical combo, I was at 7.5. However, if I bumped my static compression ratio up to 10.75, I was around 7.9. I think that's about the limit for DCR and pump gas.

So how do you know if you're right? How reliable are these calculations?

10.75:1 seems awfully high....even with the cam that I have picked out. Sure, it's got a high duration, and the intake valve stays open quite awhile.....but 10.75?

Part of me wants to try it....part of me is scared.

Excaliber · #6 (**permalink**) 07-05-2005, 07:24 AM

10.75 seems WAY to high for a pump gas motor all right, especially with iron heads. Gessford strongly urged me to stay within the 9's, so I went "high 9",

My old motor was all about high rpm, the cam came on strong AFTER 3000. By moving my torque curve down to the lower 2000+ range it doesn't "feel" like I gave up any power! The increased over all "drivability" is far better than what I had before. At the drag strip (and thats a rare occurence for me) I DO feel it "fall off" around 6000 rpm, but was surprised to see my 1/4 mile times were still pretty close. I went drag racing only a few times before and after as a means of direct comparison to performance levels.

blykins · #7 (**permalink**) 07-05-2005, 08:05 AM

Well I double-checked my calculations with another online calculator. I got a static of 10.75 and a dynamic of 7.9.

I think the cam's duration is 248 with a 73 degree intake valve setting.

Maybe it's right. I think I'd feel safer with 10.25 or 10.5.

blykins · #8 (**permalink**) 07-05-2005, 09:30 AM

Hey Ernie....

The highest peak horsepower I've found so far is with an Engel solid flat tappet cam. With all else staying the same and just comparing cams, the Engel cam showed 502 hp. I also got extremely close to that result with 2 other cams....one was an Isky solid flat tappet, and the other was a Comp solid flat tappet.

The Engel cam was a split duration/split lift cam.

The caveat....Peak was at 6500 revs....

Shift at 6700?

Keith Craft says a 2 bolt FE block will withstand 6500 revs....

I have a stump puller SBF....I was wanting something on the revvy side of the river this time....

Is it kosher with ARP studs, quality valvetrain and rocker shafts and H-beam rods?

Excaliber · #9 (**permalink**) 07-05-2005, 09:52 AM

Hmmm,,, yeah I see what you mean. You all ready got a good torquer and now looking for something different. I can dig it. I was shifting at 6500 or higher with my old motor and it was NOT "falling off" at that rpm I can tell you! It was an awesome motor and I have a feeling it was good for 7000 +, but I didn't want to risk it.

I wouldn't know what's good or bad for rpm at or above 6500. Of course ARP, good valve train, rods, etc. are a must have. Two bolt? If Keith says it will live, I guess it will. One thing I see repeated quite often is: You can have high rpm but it WILL shorten the life of your motor. Most of us don't put "that many" miles on our replicas, so maybe that's not a major issue for us?

Say you go with a BIG cam, will the heads and intake support the flow? I was (still am) running high rise heads, no doubt THEY would support the flow. My previous cam was a solid roller. On the tear down I found that all the clearances were on the "loose" side, very loose. The motor WAS built for max horse power and a short life!

blykins · #10 (**permalink**) 07-05-2005, 10:05 AM

Well I'm basing everything on research right now.....and if you speak to 10 different people, you'll get 10 different opinions...you know that. I'm sure that Gessford and Craft would tell you different things at times as well.

So I'm basing my choices on research and computer simulation software. I know better than to put all of my eggs in one basket....but Desktop Dyno has proven to be accurate to me.

I'm planning on using a 428 bore...so the largest valve I can use without shrouding or notching the cylinder decks is a 2.09 and a 1.66.

Intake....probably Edelbrock Performer RPM. I don't think the Victor is appropriate for this application...

So yes, I think the heads and intake will support the flow.

Right now I'm teetering on 6000 or 6500 rpm hp peak. I love the sound of a high-winding engine...

Excaliber · #11 (**permalink**) 07-05-2005, 11:09 AM

10 people with 10 answers! Man aint that the truth.

I agnonzied over the decisions for my motor, gut wrenching and risky. Exciting and fun as well. The "what if's" seemed to have no end. Ordering those $800 custom NO RETURJN pistons was the hardest part. "What if" I measured something wrong? I got GREAT advice from Keith as well as Gessford and members of the forum, in the end YOU have to make the final call.

I wanted to a highway "cruiser" (on pump gas) so the cam profile and compression ratio was driven more by my final gear ratio than anything else. And thats an important point, you gotta design your motor for what you want it to do. It's a series of trade off's all the way.

blykins · #12 (**permalink**) 07-05-2005, 11:46 AM

I called Comp Cams and asked them....

They recommended the cam down from the one I was eyeballing....

That makes sense. They tend to go on the conservative side for some reason.

It's that very thing that screwed me on my last motor. I told them every spec and got them to recommend me a hydraulic roller cam. I didn't birddog it well enough....

They matched my Victor Jr. intake and Vic Jr. heads with a cam that petered out at 5400 rpms.

Figure that one out. It's my fault...I should have done more research before I bought a bumpstick.

I could have picked up 30-40 more horsepower with a larger cam (not that 491 hp and 500 lb-ft is bad or anything)....

That's why I'm going through all of this now.