Genl Design Questions

[DoneByDoug]

Why would a 347 stroker with max RPM of 6000 ever require a carb with more than 600 cfm? Please explain with math if possible.

Why would anyone ever choose an iron head over an aluminum head, other than period correct look?

Is it true that aluminum heads can withstand an 11:1 compression ratio and still run on 92 octane?

How come no one ever talks about putting an autolite 4100 on a small block?

If you have a choice of reverse dome piston or decreasing chamber size and using a flat top to get to an acceptable compression ratio, which option is best?

Look forward to everyone's response. You guys are a great resource. Hope I can return the favor one day.

Doug

[CHANMADD]

1) You don't 347cu/2 =173.5
6000rpm/1728=3.47 173.5 x 3.47 =602
Assuming your motor is running at 100% thermal efficiency.

2)??? Don't know!!
3)I run Edelbrock heads at 11.5 :1 on pump gas
4)??
5)I would go for the flattop and chamber.It leaves a lot more for the builder to actually shape or flow etc.

[quickjack]

1. 600 cfm should be adequate.

2. price, also iron retains more heat and so will make more hp (all else being equal).

3. aluminum dissapates heat better than iron, so will tolerate more compression before detonation occurs.

4. likely because everyone knows about Holley and BG, the 4100 is more obscure?

5. not completely clear on your question here, as a reverse dome I would interperate as a dished piston? this with a std chamber might yield lower cr than the small chamber/flat top model. anyway, small chamber and lots of quench area is the way to go if looking for hp.

[DAVID GAGNARD]

Why would a 347 stroker with max RPM of 6000 ever require a carb with more than 600 cfm? Please explain with math if possible.

Holley's own formula is very simple for this;

Cubic inches X max rpm divided by 3456 = cfm needed at 100% volumetric efficency....

347 cu in X 6000 rpms= 2082000 divided by 3456 = 602.43 cfm of air at 6000 rpms...
assuming the motor is about 95% efficent, then you still only need 572.31 cfm of air at 6000 rpms..........so therefore a 600 would be perfect.....

over the years I've tried at least 6 carbs on my 350hp, 351-W, ranging form 600 cfm to highly modified 750's.... tested on the street for fuel mileage and the 1/4 drag strip for perfomance, the clear winner was a 20 year old Holley 600 double pumper in both catagories.... the second place carb was a 600 vac sec carb and the worst was the 750...................
The 600 dp gave 6 mpg better fuel ecomony on the highway and .2 a second faster in the 1/4!!!!!!!!!!!!!!!!! it suprised the heck out of me as I THOUGHT the 750 outperformed it on the 1/4, the time slip does not lie...............

David

[blykins]

1. If your engine makes power at a high peak, you need more carb. You would also need more carb in a constant WOT throttle situation, like drag racing.

2. I have no clue.

3. Yes. Especially with larger duration cams and a tight quench.

4. Why would you?

5. I think it's 6 of one, half dozen of the other. I usually select a piston based on the combustion chamber. Sometimes, you can't open the chambers up enough to get the CR down, so you have to run a dish piston. I shoot for pistons at zero deck and .040" head gasket thickness, so I get a real tight quench. As long as the quench pad is tight, I don't see a problem with big dishes. I consider it poor practice to put the pistons way down in the hole or use an incredibly thick gasket to put ratios where they should be.

[DoneByDoug]

Right on everybody. I appreciate your input. Thanks!
I'll be doing a reverse dome to get to 9.62:1 and the 600 cfm. Thanks
Doug

[Pete Munroe]

Doug,

I was a little concerned when my 347" engine build ended up at a static compression ratio of 10.7 to 1.

The block is iron, aluminum TFS heads, flat top pistons, a relatively mild cam (that doesn't have huge amounts of overlap to bleed off compression) and it still runs ok on 91 octane.

Haven't looked for a while but I think it is set at 32 degree's...seems to be just fine. I figured I can always add timing easier than rebuilding.

At 13,000 miles, no problems...I don't try to accelerate in 5th gear for sure, but when in doubt shift down.

At the track, I always try to mix with unleaded race gas...show up with a low tank and fill the difference with 100? unleaded for insurance.

I would not hesitate to go to 10+ on the compression all things being comparable...

Unless you are running an automatic with really high rear gear, should be ok.

Maybe some of the the experienced engine builders will have something to ad.

Pete