Question about head gaskets, quench and dynamic compression

I just got my 327 sbc block back from the machine shop. The bore is 4.06". I have flat top pistons that are 0.008" in the hole and have 4cc valve reliefs. The heads are 462s with 64cc chambers. I am going to run a Comp xs268s solid cam that has an intake closing of 60 degrees ATDC. my gasket set includes 0.039" head gaskets. This combo should yield a static compression around 9.8, a dynamic compression ratio a little over 8, and a quench area of 0.047". I was wondering if I went to a 0.028" if I'd get a bit more out if it or if I risk losing street ability. With the thinner gasket I get a static ratio of 10.3, a DCR of 8.3 and a quench area of 0.036". I will most likely stick with the thicker gasket but would love to hear different opinions on the matter.


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Anyone?

 

9.8 is already a touch on the high side for an iron head with your cam profile. Aluminum heads would be no problem at 9.8, but the irons might be a bit prone to detonation if you are not carefull with your timing.

I would leave it as is, run 93 octane and be mindfull of your timing numbers. Should be a nice little engine.

Good luck, -Abone.

 

The general consensus of some builders with serious dyno testing time is that a tight quench lowers the tendency to detonate more that lowering the compression. With 0.040 being the target to hit.

Yours is a tough call because it so close as it is.
If it were mine I'd look at running the thinner gaskets but that's me, and I'd look at running the next size up cam too, (?270 or 272? I think is one step up)

My SBC is 10.4 static with a 0.043 quench, I also was very careful to break all the sharp edges off the heads in the chambers to eliminate potential hot spots. This also increased the ccs of the combustion chambers slightly but I didn't mind because I started with 58cc chambers. So far it runs great on 91 fuel. But let me say that I only have 600 tied up in the whole thing and if it blew up I won't be out much.

Maybe hoop will chime in with them awesome computer dynos and have some insight for you.

 

The shop recommended I stay with the thicker gasket and I'm pretty sure that's what I will do, I was just reading about quench areas and detonation and wondered if tightening the quench some Would help out and give some more torque down low. Thanks


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Yes, it will get you some more down low and probably a higher average out put.
You'll be able to run more timing with a tighter quench too because it will be more resistant to detonation. But your gains will be small.

The other way to look at it,
Lets say you run the thinner gaskets to tighten up the quech. Youll get more resistance to detonation (x1) However This will increase your compression. But if you did some chamber work to eliminate the hard edges, there are plenty of them too, this would do twice the detonation resistance. (X2) Both because the edges are cause for hot spots and it will slightly decrease the compression ratio proportionate to the amout you removed. but the biggest plus benefactor is the tighter quench.

Run a configuration on your compression ratio if you removed some material off the hard edges and made the combustion chambers 65, 66, 67 ccs with the thinner gaskets.
This is sort of a free power out put increase

 

With today's gas I'd probably rather error on the side of slightly less compression, than more. Both of my engines run on regular with no detonation, and great performance. One BBC 454 +.040" and one SBC 327 +.040" The BBC is running Edelbrock aluminum heads and the SBC has old 2.02" camel hump heads upgraded for unleaded fuel.

 

Once I started reading more about quench area, the idea of achieving the optimal quench space became more compelling. All in all, it probably doesn't really matter if I give up a few ponies here and there as I'll be making more power than I could ever put to the ground in the roadster anyways. I'm just trying to learn and do things the beat I can as I go. Thanks for the responses


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The problem with .037 is that you would need to check each piston to make sure the stack up or squareness of the deck etc is spot on. If the .037 was the absolute tightest point and you aren't turning a ton of RPM you will probably be fine. However we are talking less than 5 average HP for the difference. Because you are a bit small on the cam side it looks like installing it at an ICL of 110 might be worth a few HP from 4500 to 6000, say 10ish.

On the graph below the CR is at 10.3, the IC is at 64 so the DCR is 8.24, the higher curve of course. (660 Carb, Dual Plane, Small Tube Headers). The 64 closing puts the DCR at 7.85 with 9.8 to 1 if you run that. So the .047 is safe, the .037 should be ok if you check all pistons. Backing the cam up 4 from the way it's ground helps.

 

Great info! Thanks


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How much would I give up down low by changing the icl?


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Not much, remember this is a simulation. Without a dyno we are talking about 1 MPH or so in the 1/8th mile. 10 HP Avg (307/317) 3000-6000.

 

That looks like its going to be a really fun car on the street.
She's gonna put you back in the seat hard off a light. Be about done around 4500 though.
If its a really light car, maybe shift the curve to the left some so you can leave without smoke and make that mountain of power after you get rolling.

 

It will definitely be fun! Model a roadster with a 4 speed and 4:11s I. The rear. I really don't care as much about absolute power as much as i do about learning while going through the process of building the car. I doubt I could get all the power down to the ground anyways.


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This is fantastic. Thank you


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I'd run the tighter quench, because the engine won't need as much timing and peak power with less timing is always a good thing. Also if you run as extended a plug as possible, this will also help keep total timing lower & power/torque up!

 

Absolute power ?
To you Would that mean the high peak numbers ?
Power in a certain range?
Or overall highest average?

I like an early flat curve but balanced against highest average. That just suits my style an preference better.

That 4 speed, gears and torque curve should be a blast to drive.

Getting the power to the ground?
Lets start at the back and go forward.
4:11 gears think of that as "available torque x 4.1"
2.20 first gear in a close ratio Muncie. Think of that as "available torque x 2.2"

Remember to knock off 20% at the end for ac***ulative drivetrain loss.
Ok now @ 1500 rpm you have 340 ft lbs available torque at the crank.
340 x 2.2 or 748 at the trans out put shaft in 1st gear.
748 x 4.1 or 3066 ft lbs on the wheel hubs.
With a 30 inch tire you have to divide 3066 ft lbs by 1.3(feet) 15" radius is 1.3'
That's about 2360 where the tire meats road.
2360 - 20% will be 1888 real life torque at the tires contact patch in first gear at 1500 Rpms. That is a lot of power to get to the ground & You probably could easily start off in 3rd gear.

With that torque curve you really want a close ratio 3 speed and you'll be short shifting it to keep the Rpms in the 2400-4400 range.

Play around with some different cams and see about moveing that peak left, making the curve flatter, increasing the average over a wider band or possibly a numberically lower rear gear. You might like it

 

Pick a power band and I'll see what fits it

 

I'm thinking maybe something in the 280 advertised duration range,
So it's still pulling hard at over 6000. Bleed off some low end and let the 411 gears do their thing.

I mean this is a high compression 327 with solid lifters so I ***ume the rest of it can take the Rpms right?

Not quite sure what the exact part numbers for comp solid lifter in that range.

What do you think hoop?

 

Remember this one hoop ?
327 vortex heads in a 350 vortec block

 

yep nice combo

 

Here's a Howard 275/285 110-110

 

*****ing math! You think retarding the cam afew degrees would be in order?


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Well how do feel about 3000 Rpms @ 65 mph?
If that's too high for you think about another rear gear?

If you are ok with 3k @ 65 then change things that increase your high rpm capability by pulling some of the currently supper strong low end. You can afford a lot off the current bottom and you'll need more at the top with 4.11 gears.

What's your machinist / enginebuilder say about max rpm on your engine ?

 

I do have a set of 3.55s lurking in The garage. I was looking at old school cam grinds like isky z25, engle ep22 and a couple Schneider custom grinds that were longer on duration but shorter on lift with lasy lobes.. My machinist talked me out of them and suggested the modern grind within more aggressive ramps as he felt this would make more power with a smaller cam. This guy is very well respected in my area and builds all sort of engines from flatheads to ferraris so I trusted his experience and opinion.

 

He (the engine builder) is correct. But the smaller cam limits you to a lower RPM. The long duration lazy lobe cams make less power but can keep revvin, less efficient, maybe more fun... The 268 is ground on a 110 LSA and a 106 ICL, it looks like putting it in at 110 ICL (4 degree retard) is worth 8 HP average from 2000-6000 on the simulation and 10 from 3000-6000. Only two ways to verify that, ch***is dyno or a measured 1/8 mile the difference would be 1 MPH. You can't really lose but if you want to learn you are going to have to try, test, and tune. Simulations are to cut down on trial and error, not eliminate.

Hoop

 

I'm sure he is and I would too.

My opinion is theres Only a few reasons for 4.11 gears
1- you are moving a very heavy load, getting the load rolling is more important than top speed. Think dump truck and gobs of low end torque from the cam in the engine.

2. The engine loves high rpm because of the cam choice & really comes alive and makes its power after 3000 Rpms. As a result, it's a little short on low end power & this helps get the power to the ground because the 4.11s get it rolling without too much tire smoke and the gears also give it the Rpms on the big end.

Clear as mud right ?

 

Points well taken. The engine hasn't been ***embled yet so how does this sound? Retard the cam 4 degrees for an ICL of 110 then put the 0.03" head gaskets in to bump compression a tad and reduce quench from 0.047 to 0.038. My thinking is I will gain top end with the retarding of the cam yet recoup the low end with increased compression while reducing risk of detonation with a tighter quench. Ill wait till the car is running before swapping gears to see how much I love or hate the combo. I've really learned a lot from you guys. Thanks


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Sounds like a plan Stan! We just p*** on what others have taught....

BTW for overkill here is some info to contemplate. Chances are you can only put about 1800 pounds of force to work (Tire grip x weight) so you should be able to easily and often, as we Buick guys would say. exceed the static coefficient of friction (Make Smoke)!!

Rear Wheel Tractive Force Calculation

 

SOB I did ^^^ all that **** ^^^ in my head !