Technical Dynamic compression...will this work?

I'm looking at building a 289 for my '51 shoebox, and I've got a line on a short block locally. I'm wanting to put GT40P heads from a late-model Explorer on the engine, because they're basically the best SBF head in terms of flow and will have hardened valve seats. Problem is, the combustion chambers are around 5 cc bigger than the stock 289 heads, which will harm my compression by about half a point. My question is, rather than making up the difference by milling the heads, what if I did so by getting a cam that would increase the dynamic compression back up to where it was? I've got my eye on Comp Cams #31-238-3 (262/270 advertised duration, 218/224 at .050) because of the split pattern (P heads like a little more on the exhaust, from what I understand) and the early intake valve closing (57 ATDC). According to my calculations, this would make up the difference in DCR, and actually bump it up a little past where the stock 289 was. Would this be advisable, or would it be better to not even worry about it? I'm planning to run a 600cfm carb, automatic transmission and 3.31 rear gears, if that helps. I'm also open to other cam suggestions that might be better for my application.

Thanks in advance!

 

Update: after talking to someone at Comp, he recommended the 31-234-3 which has the intake valve closing at 54 degrees ATDC. Still, the same concept applies. Any reason this wouldn't work?

 

I like your cam choice better BUT...not your choice of gear.
Also depends on the rear tire diameter.

Your cam and 3.55/3.70 gear.
Comp. Cams, cam and 3.31/3.55 gear.

Mike

 

It's a factory automatic car so the rear gear is what came with it unless I swap it out which I've been thinking about doing anyway. The guy at Comp said the first one I suggested wouldn't be very streetable with the automatic, so I think I'll go with the one he suggested. Guess I didn't post specs on it, it's 256/268 advertised and 212/218 at .050 with an intake valve closing at 54 ATDC. Powerband is 1000-5200 so I'd think it would be OK with the 3.31 gear, am I wrong?

 

What is your original cam specs?
To bump up you dynamic compression you need shorter duration and less overlap.

This also lowers the RPM range which is why higher rear-end gears are better.

The other alternative is switching to steel shim gaskets
Normal 0.041" Gaskets have 8.9cc of compressed volume
If you switched to 0.020" steel shim Gaskets .they have 4.33cc of compressed volume.

This would regain over 4.57cc of lost volume. You need to buy gaskets anyway.

It should only cost about $150 to mill the heads vs the cost of cam and new lifters

 

Big flowing heads , low compression and a none too exciting cam, this is not good. Look around for aftermarket alloy heads that will give good compression then revisit the cam choice.

 

Better still, instead of buying the 289 shortblock and fitting GT40P heads, why don't you buy a complete 5.0 explorer engine and change the cam ,intake and carb

 

It's a stock 1966 289 that I've found for $75. Here's the specs I've been able to find:

Duration intake 266 exhaust 256
Overlap 30
Intake Events
open BTDC 16 lift.004
closed ABDC 70 lift.006
Exhaust Events
open BBDC 52 lift .002
Closed ATDC 24 lift .005

Milling the heads may be an option, if that would help. It would mean, though, that I'd be back to square 1 as far as cam choice.

Maybe I'm missing something - if the RPM range is lowered, wouldn't that make higher gears OK because the engine will be more effective at lower RPM?

If I had the money, I'd just put a set of AFR's on it and call it a day; unfortunately, aftermarket heads aren't in the budget for this build.

My original plan before I found the 289 was to do exactly this - but then I found the 289 for cheaper than I'd be able to get a junkyard 5.0. Plus, if I got a 5.0 out of a junkyard, I'd just be using the short block anyway (plus the heads) but having to pay more for it and potentially going through the same parts (bearings, gaskets, rings, cam, intake, etc.). The 5.0 would, of course, also have a roller cam and a lot more proven combos, so that helps.

I ultimately haven't 100% decided on going with the 289, I was mainly wanting to see what I would need to do in order to keep it together.

Thanks everyone for your input so far, keep it coming!

 

The cam you are proposing to use has longer duration than stock, so it will lower dynamic compression.

You're not missing anything, I wrote "This also lowers the RPM range which is why higher rear-end gears are better." which is the same thing [and sometimes better]
I personally have never witnessed a "hotter" cam swap that didn't make a daily driver a dog during rush hour traffic.
Cams , Compression, and Ratios are the Automotive "ménage à trois"

Go for the late model engine. You get the advantage of a roller cam block, hardened valve seats etc.
With the roller cam you can still keep the original lifters during a cam swap.
The roller cam profiles can ramp-up quicker so you can get higher lift without too much duration increase.
The modern engines also have a lot of longevity. I've pulled down 200,000km [120,000mile] blocks that still showed the cross hatch in the bores. They "ring and bearing up" really nice without machine work.
One piece of caution! You must keep the compatible distributor gear to match the cam.

 

The duration may be longer, but the overlap is less (intake valve closes earlier). I plugged both cams into the dynamic compression ratio calculator here: http://www.wallaceracing.com/dynamic-cr.php and the Comp cam actually seems to increase dynamic compression from stock, even with the lower static compression (9:1 vs. 9.5:1) and longer duration.

I must have misread your comment about the rear gears. I see we were saying the same thing. As far as the hardened valve seats and reusing the lifters, I can get a cam and lifters and timing set for flat tappet for the same price as the roller cam I was planning to use, so that's a wash.

Not trying to have an argument, just trying to make sure I fully understand both scenarios

 

If you rebuild the 289, most likely the deck may need to be surfaced along with the heads so you can easily get your 0.5 CR back without chasing cams based solely on dynamic compression.

 

Question: if we remove the dynamic compression requirement (i.e. by milling the heads or using a different head gasket to make up the half a point), what would be a good cam to use?

 

For a reliable cruiser in your shoebox.......... A stock roller cammed 5.0 engine with a dual plane manifold and a 500 cfm Edelbrock electric choke carb, then a nice set of thick wall headers that don't rot out when it's parked over winter.
You want to be able to "kit it in the guts" 1st thing in the morning and click it straight into gear and drive it.
Try and buy a good runner and don't pull the longblock apart

 

The GT40 heads are fine on a 289. Dont over think it. Clean everything up, make sure your push rod length is dialed in correctly and run a mild cam like the Comp guy recommends. 500 CFM carb on dual plane intake like a Weiand Stealth intake will work best.

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This. Just this.

 

Thanks - the Stealth is what I have and I've already got a 600cfm manual choke vacuum secondary carb to run on it. Just trying to figure out the rest of the package without breaking the bank, and trying to keep it at least somewhat traditional.

 

500 cfm is too small for a SBF with decent heads (GT40P heads are the pinnacle of factory heads) and a small cam. 625ish would be perfect for this combo. 600-650 would work. A 750 would work, but it's really too much for such a mild combo.

 

No, it is not.

Even if this engine were operating at 100% volumetric efficiency, with connecing rod angular thrust hovering in the failure range, and the the valves on the verge of float, this combo would require only 460cfm.

This combo could comfortably cruise with a 750, that you claim would "work", with three of the four barrels epoxied shut.

 

@gimpyshotrods what formula are you using? I've found three different formulas and they're telling me different things, all 500 or less though. Most of the research I've done seems to indicate that a 600cfm is the top end for a street-driven car, but that's for Mustangs. I've also seen that a heavier car dictates a smaller carb. So if the Mustang guys are topping out at 600 with lighter cars, then I may need to go for a 500 or 525.

 

From experience with lots of 289 work, I'll tell you 600 is too big. You'll spend a lot of time jetting it down so it runs well on the highway without being too rich, which will cause idle/mid-throttle tuning issues. You'll wind up with less vacuum than you really want, and subsequently have to change power valves to compensate. And none of the parts you'll want can come from a parts house, you'll have to order all of them because they're outside the "normal" stuff.

Not to dissuade, but if it were me, I'd be putting a 525 or 550 on it, tops.

I'd have to check my cam cards, but I'm fairly certain I was last running an Elgin E-1089-P, which isn't a hot cam, but performed well for a street car. About 10.5:1 compression ration running 25 mpg on the highway in OD, plenty of low-end torque, and overall fun to drive. 3:1 rear gears in a Ford 8" with ~26" tires.

Question: are you planning on having machine work done to the block and heads? If you are, why not stick with the stock heads? You'll get new guides and hardened seats if you want, and with a light cam you're not even maxing out the flow on the heads.

 

That's what I was wondering on the 600, I figured it would need quite a bit of tweaking to make it happy. I'm fine with not using it. I haven't looked at Elgin cams yet, I'll explore their options as well.

As far as machine work on the heads or block, I haven't actually purchased either yet so I don't know. I've got a line on both, but I'm wanting to make sure I have a plan before purchasing anything. The 289 I'm looking at is a short block only, so if I'm going to buy heads anyway, and don't have the budget for aftermarket, I might as well go with the best OEM option. And, if I do have to machine the heads and block, I can get my compression back that way and open up my cam choices.

 

In your application don't get too hung up on head flow. It's a bit like the carb discussion. If your short block can't out flow the heads, bigger flowing heads simply lower the flow velocity which can cause the carb to meter less accurately. And on the street you will spend 99.9 % of the time below 2500 RPM. In my misspent youth I over carbed and over cammed every engine I built.

 

That makes sense. Man, this thread got off the rails, didn't it? So it sounds like I need to start back at at square 1 - I've got the Weiand Stealth intake already, so I'm definitely using that. Not sure where to go from there. I definitely want to drive this on the street, I doubt it will ever see the track. I'd like good throttle response, and good mid-range pickup. Would like somewhere in the neighborhood of 300hp, nothing too crazy. Will the stock '66 heads support that?

 

I would run the GT40 heads, just freshened up.
They don't have crazy flow, but are the best factory head, ever.
Chasing that 1/2 compression point is pointless.

 

That's what I'm beginning to see - on the street, half a point isn't going to make that much of a difference, so I shouldn't get too torn up about it. Of course, this is all academic anyway, since I don't actually have the 289 yet. I still really like the cam they recommended with it, so I may still use that even without the requirement of the higher DCR.

 

Carb cfm formulas are close to worthless. Run what you like.

 

Follow the formula I suggested....use a 289 /302 or 5.0 roller short block dressed up like a 289. Your choice...it wont matter a whole lot for a mild street motor. Dont over cam it and dont over carb it and you'll be just fine. If you happen to rebuild the short block rebuild the stock rods with ARP bolts and have the rotating ***embly balanced. A 289 with properly prepped rods, headers, proper valve spring pressure and valve train geometry will spin 6000 rpm all day....almost regardless of heads used. 289 hipo heads were nothing to write home about in regard to flow....but were rated at 306hp in Shelby trim. The smaller cam and carb with the GT 40 heads makes for a nice snappy street motor.

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No, they are not. This has to be the most offensively ignorant comment on this thread.

Every single engine, at every desired horsepower, at every bore and stroke combination, and at every flow rate, has a very specific cubic foot-per-minute air and fuel delivery requirement.

Yes, it is more complicated than a CFM calculator that you might find on the web. Those are all over estimates of required flow.

Air and fuel delivery is not an art. It is not black magic. It is a physical science, and one that I happen to have an advanced degree in.

I have spent the last 30-years tuning engines professionally, with the past 15 using a ch***is dynomometer and, at minimum a wide-ban oxygen sensor. I build full digital engine management systems from scratch, and design them for the fifth-largest global automaker.

Try to stay in your own lane.

 

@BigChief I think that's what I'll do...maybe I've just been overthinking this whole thing. This is my first engine build so I want to make sure I have a good plan.

 

@gimpyshotrods that brings up a question I'd wondered about before...is a carbureted engine worth tuning with a wide-band, or is there not enough adjustment to get it dialed in properly?