Quench or no quench?

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1967DartGT

1967DartGT

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Well, I’m putting together a 351 ci stroker from parts on hand and I would like to get opinions on whether to build this for a closed chamber quench head or an open chamber non-quench head?

318 block bored 0.040 over = 3.95” bore, with 3.58” crank = 351 ci
Silv-o-lite 1278 flat-top pistons with 1.658 compression height.
Assuming a 9.6” deck height, this would put the pistons 0.029 down the hole.

So here are my options:

Leave the block unshaved, 4.06” 0.040 gaskets, and 71 cc open chamber heads = 9.43:1 compression.

Shave block 0.029 for zero deck, 4.06” 0.040 gaskets, and 62 cc closed chamber 302 heads = 11.2:1 compression. Compression is a bit high here but I don’t know how I’d bring it down and keep a proper quench distance.

Thoughts?
 
Sorry, I was going to give the cam in the description as well. I was leaning toward a Comp Cams XE268H, but might need to go bigger to deal with the compression.
 
And if it makes a difference, the elevation where I live is 4500 ft, but I would probably take the car to as low as 2200 feet.
 
with the XE268 I would stick with the lower compression. I think anymore then 10:1 with perfect quench is going to be a problem with that cam.
 
It's close, but by running a larger cam you will be fine I think. I'm not an "altitude" engine builder. But I think it will be ok with enough cam to bleed it off. The down side is, that cam will need to be much larger to bleed it... losing idle quality and low end torque.

edit: I'll add I dont think the 302s are the best choise. I'd grab a set of the EQ LA pattern magnum heads before running the 302s.
 
Yah, I figured the 302s weren't the best for this application but I have a set on hand is all. By the time I add larger valves and port match, I probably would be better off with the EQ Magnums, but I think the EQs are 63 cc and I would still have higher compression than I'd like.

How big of a cam do you think I would need to run to handle the compression?
 
You might be able to get by with 91 octane. dgc333 built a 360 with Magnum heads and 10.6:1 compression and he runs 89 octane no problem.
 
MOPEkidD-3 said:
You might be able to get by with 91 octane. dgc333 built a 360 with Magnum heads and 10.6:1 compression and he runs 89 octane no problem.
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IMHO You can run pretty high compression with the right cam and knowing how to tune it ,say 11.2, and what's your air density like?
Like the other guy's said 'cam profile is very important.

good luck.
 
AdamR said:
with the XE268 I would stick with the lower compression. I think anymore then 10:1 with perfect quench is going to be a problem with that cam.
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Adam, my 360 is at 10.6:1 with the XE268 cam and closed chamber magnum heads. It runs fine on 89 octane and if I am careful under 3000 rpm it will run on 87 in a pinch.
 
Some metal could be removed from the combustion chamber of the closed chamber heads to bring the compression down some if your budget allows.
 
A closed chamber head usually allows more compression (with-in reason). Much more efficient design. Should produce more power. My .02.
 
moper said:
It's close, but by running a larger cam you will be fine I think. I'm not an "altitude" engine builder. But I think it will be ok with enough cam to bleed it off. The down side is, that cam will need to be much larger to bleed it... losing idle quality and low end torque.

edit: I'll add I dont think the 302s are the best choise. I'd grab a set of the EQ LA pattern magnum heads before running the 302s.
Click to expand...

I agree with moper. Go with the EQ magnum heads.

I'm @ 5280' + and am running open chamber heads with a measured CR of 10.2:1 in a 5500 lb truck with a tight convertor, 3.55 gears and 33" tires, and a cam with 213 degrees of duration @ .050 and have no problem with detonation.
 
Two things I still need to factor in if I go with quench heads are any material removed from the chamber after polishing (not much removed but this should also reduce the risk of detonation) and the added volume to be had by notching the piston for valves. If polishing and valve reliefs get me another 5-6 cc, that will reduce compression from 11.2 down to 10.5.
 
You will also want to deburr the edges of the piston machinings (valve pockets, squish ring, dome, or dish edges if it has them) and chambers if this is not already a habit.
 
302s and Magnum EQ heads are both advertized with 62cc chambers. What are people seeing for chamber sizes for these head in the real world?
 
I would like to put my un educated .02 in if I might. I am thinking the 302 heads will be a little too small in runner size with 351 cui but on the other hand there will be good velocity. I am planning the same type of build but without the crank and I just got some 302's and I will stick with them! Of course I would love to put some of the EQ heads on but that would be well out of my budget. I would like to add that the Popular Hot Rodding article was a good one but I would like to finally see a 318 build without the Magnum heads! Here in Germany it is a lot harder to get magnum heads. Sorry to rant but I had to let out!
 
I run a .030 over 360 10:7 compression with 302 heads all day long on 89 octane. .474/.474 MP cam
 
mullinax95, are your 302s stock or have you replaced valves with larger ones? Any port work? Just wondering what kind of performance you get out of them on the 360.
 
1967DartGT said:
mullinax95, are your 302s stock or have you replaced valves with larger ones? Any port work? Just wondering what kind of performance you get out of them on the 360.
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Completely stock other than the valves are back cut. I wanted (velocity) torque more than anything since I will not be running this motor at the drag strip and seeing 6000 or more RPM. However I have shifted at 5500 and the motor was still there believe it or not (LOL) so I don't really know how far up it will die out. I know it can't be to much further. I have no problem doing burnouts and getting sideways. But you have to take in consideration that the 360 has a longer stroke but you already have addressed that with your motor.

I have not tried lower octane than 89 because I believe I would get into detonation problems. While you have the heads off I would make sure to loose the sharp edges in the chambers just in case. You already know this but I'm just stating what I would do.
 
11967DartGT,

This was stated in post #60 of a thread http://tinyurl.com/4vtkto



"After I get the hedders done this week I'll get a video of it and get Rob to post it for me. Then next week I should have some ET #'s tow truck gas depending. Looking for low low 7's in the 1/8 or upper 6.90's and all out of a 318 and 302 heads. This engine may be for sale later on, complete less carb and dist. and hedders."

This is a pretty good thread to just read.
 
mullinax95
Thanks, I've been following that thread for quite some time ... good stuff. It's the thread that got me to contact BJR Racing to get flow numbers for the 302s. I was originally considering rebuilding a 318 but most of the comments I got from this board suggested that I stroke the 318 with the 3.58 crank I already have. Now that I finally settled on that, I just need to decide what heads I want to build the shortblock for.
 
Can someone explain how to accuratley calculate quench with dish pistons???

Thanks in advance.................
 
Mad Dart said:
Can someone explain how to accuratley calculate quench with dish pistons???

Thanks in advance.................
Click to expand...

If the pistons have a "quench pad" like most of the modern dish pistons then its the distance between the quench pad and the closed chamber portion (flat spot) of the head.
 
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