De-Shrouding the Valves on a 396 Big Block Chevy for more CFM & Free HP!!!
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- čas přidán 9. 06. 2022
- Un-shrouding your valves is always a good thing. I made this quick video to discuss how the smaller bore of the #396 Big Block Chevy causes significant valve shrouding, and how to correct it.
The overall width of all BBC beads within the combustion chamber is relatively the same, at nearly 4.5 inches. But the bore on a 396 & #402 Big Block is closer to 4". This means that the combustion chamber of the cylinder head is actually overhanging the edge of the block, and has roughly 1/4 of bore sticking out into the flow path of the valves.
I talk about this restriction, and show you how you can mark it out and chamfer the block at the tops of the bores to minimize the obstruction and un-shroud the valves.
This is a relatively simple task that any at home DIY'er can do themselves. All it takes is some careful measuring of the top ring height, and a steady hand with a die grinder.
I have heard some people refer to this as a block eyebrow. The engine that I use here for this example is a 396, but this is something that could apply to any engine. I always install a head onto a bare block to check the fit when building an engine.
Yes I will loose compression from the loss of the material removed, but the gain in airflow will easily offset that.
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About AutoTechNick:
Growing up in rural Oregon I was always around Hot Rods & Race Cars. I imagine my early exposure to this culture ignited a spark of interest in cars, which remains my passion to this day.
🎓 I am a proud Alumni of Wyoming Technical Institute, where I studied Automotive Technology, Performance Engine Building & Street Rod Fabrication. In addition to “Wyo Tech”, I graduated from the BMW Service Technician Education Program. More commonly known as BMW S.T.E.P.
🏎️ I spent nearly 10 years working in various BMW dealerships as a Master Technician. After a move back home to the PNW I found myself no longer servicing German-made machines, but now working with Italians. I spent nearly another 10 years as a Ferrari Master Technician, & Shop Foreman at a local Ferrari dealership.
🏆 I was selected by Ferrari as one of 12 technicians worldwide to compete in the “Ferrari Testa Rossa Awards”, consecutively for 4 years in a row. Finishing in 3rd place in 2016, & in 2019 I was able to place in 1st. Winning the competition & earning the title of the Top Ferrari Technician in the world!
🚀 As my time with Ferrari came to an end, a new career path awaited me…. Aerospace. I am currently working at Blue Origin, helping to build & develop the BE-3PM and BE-U rocket engines.
With my new career path, I needed an outlet for my automotive passion, thus the reason for this channel…
#bbc #ratmotor #bigblockchevy #454 #427 #496 #402 #chevy #bigblock #musclecar #horsepower - Auta a dopravní prostředky
I'd read about this in a David Vizard book, and I found your break down of your technique for doing it yourself with the hooked scripe very interesting. Thank you for posting this video.
I am glad that you found it helpful!!
~ Nick
Great info gotta love the old 396 motors keep the videos coming
Just purchased a 66 el camino 396. Keeping this in mind for the future 👍
Excellent tech tip! Thank you 👍🏼
Been doing this for years. For those that wonder about power, it will vary from build to build. But average of 20+ hp You will gain about 3+cc of volume, if you do both intake and exhaust. As long as you have decent compression to begin with, like 10.5 or so, it will not be a problem. Flow trumps compression at this point. Enjoy the power !
Great info! Thanks for sharing... I fully agree that in this situation that the gains in airflow outweigh the loss of compression..
~ Nick
Hello that was 1st question I had.. I was curious as to where¹ do you think the cut off would be with the compression ratios. As far as would it be worth it to do this. Im aboit little over 9 C.R and Im really limited with what I can do to it and would it be worth doing at this point if C.R. isn't there? Thanks for your videos and appreciate guys that comment so gain more knowledge from everyone.
@@supertruckertroy At that low of compression I suggest different pistons. It may be a wash at the compression you have. Maybe BOOST ! 😄
@@supertruckertroy I don’t think that their is a specific compression ratio which one can say is a cutoff point. That is the tough part
In building a 396/402. They are tough to get decent compression with, without spending a lot of money on aftermarket pistons. Each guys build and application will be different.
~ Nick
I’m just freshing up my 0.030 over 402 and think this would be an excellent add. I previously changed the heads to have 1.88/2.19 valves and ported them and gasket matched so I think this would really be helpful to get the full benefit of the bigger valves. Excellent video. Thanks!
definitely gonna look into this on the stage 1 455 we are building right now!
Great content ! Love these kinda videos . Subscriber
Thank you! Thanks for the support!!
~ Nick
I never thought about doing that but it makes total sense. I'm going to try that on my 396/402 rebuild to get the flow better. Already port matched the heads and intake together. I'm only going to grind the very minimum as not to lower the compression ratio. Had the heads decked .020 to try to get it near 9.25 . The heads were cc and are about 98 cc now down from about 103. Thanks for the video.
I hear what you are saying about trying to keep your compression ratio up. I know how hard it is to get the 396/402 to have a decent compression without custom pistons, but I do feel that with this modification the HP lost from compression loss, is overcome by the HP gained from the overall increase in airflow.
@@AutoTechNick thanks me too 👍
Ima try this on my rebuild
I often wondered about that but never checked it
it's not simply that the bore is bigger. although that helps with air flow, your biggest gains are from the increase in cubic inches. also, comparing to a 4.5 inch bore is more of an apples to oranges comparison than a apples to apples comparison. you have to remember, those heads that they use on a 4.5 bore have 2.3 inch valves or even bigger sometimes.
if you put a head with 2.190 valves on a 4.5 inch bore it will flow better, but gains in power from that are not going to be nearly as much as they are from the increase in displacement.
Yes, I agree. I only brought that up to help people make the connection that it is not just the displacement alone that increased power. When building an engine, displacement, bore size, valve size, AND how each of those items interact all affect valve shrouding. They are all things to consider.
~ Nick
@AutoTechNick why say what you don't mean then?
Those blocks are beautiful. How did you get them so clean??
The 396 block was hot tanked, then I painted the inside of the block with Glyptal and the outside with engine paint. The block was also decked .020” so that is why the deck surface looks so nice.
The 327 block was cleaned with degreaser, and then I used a wire wheel on a drill to clean the outside of the block.
@@AutoTechNick glyptal??? Where do you find that??
@@JmalibuJFRoadking It can be found from a lot of online retailers…. Eastwood CO has always treated me well:
www.eastwood.com/glyptal-red-brush-on-1-qt.html
how do you size the gasket after doing this do you use a gasket that is also deshrouded or do you use a gasket that matches the heads diameter
I usually select my head gasket that is the closest size to the bore to maximize compression. Now the engine I use for the example in this video is a 396 BBC. Most EVERY head gasket on the market is for a 454 or larger bore, so they are all much, much larger than they need to be when used on the smaller bore of the 396.
~ Nick
Are there specific MLS head gaskets available after doing this mod? Thx
Cometic does make custom MLS gaskets in any diameter that you specify.
~ Nick
The 402 Chev BB I have the block has the value groves from GM because it was a Hi performance engine that left GM at 425 Hp in a 1969 SS impala.
After the rebuild then put into my guide jet boat I suspect the engine could do 500 hp now.
My fishing clients love it when I show them the power in a throttle squire of gas.
What is the casting number on your cylinder heads I believe those are the same ones on my 1970 402 minor 950 casting numbers
Carl, the casting number on my heads was 3931063
~ Nick
Hey I under stand the si shrouding part but the part I don’t under stand is the gasket how is it going to seal do you use a 454 gasket or 396 thanks
You will need to check the diameter of the gasket to be sure that it does not overhang into the chamber, but I can tell you that most all gaskets are made for the larger bore sizes. It is actually very difficult to find a gasket specifically for a 396.
~ Nick
@@AutoTechNickwhat gasket would you recommend me to use I have the same setup as you heads and block thanks
Alejandro, I would recommend looking on Summit Racing. You can filter your search for head gaskets. You can select them by the bore diameter, and compressed thickness. It’s hard to get compression in a 396, esp with open chamber heads, so I try to keep the bore size and compressed thickness to a minimum. But every engine will be different. You will need to select what will best fits your engine.
~ Nick
Will I have this problem running 325hp 396 heads on a 454 block.
Not to the same extent that I highlight with the 396 block in this video, due to the larger bore of the 454. However it is somehting that I would check during any build.
~ Nick
TY for posting this. I am working on an old 72 Datsun L24 and have to do this or the larger valves will hit but bonus power is awesome!! Is there a vid that shows how to do it? The only question I have for you is what shape should it be?? Is it a square cut into the block or do you just blend it at an angle?? I have seen it done with a router as a square cut but it looks more like a tapered blend into the cyliner. Anybody that can help me clear that up would be awesome:) Thanks Again for the vid!!
You will want to avoid a square cut and blend it to an angle. This will be less disruptive to the airflow. Just be sure you know where your top ring stops its travel at the top of the bore, and stop your blend above that point!
~ Nick
@@AutoTechNick TY SO MUCH!! An answer I have been looking for. :)
@@AutoTechNick So another question. How do you mark the layout if you dont have access to the bottom of the block? I was going to use the gasket of course but was going to use clay to mark the valve? Any other good ideas? ThankYou
@@superz500 Having access to the bottom of the heads, up through the cylinder, is a MUST in order to do this properly. Do not use the gasket, as most All BBC gaskets are made for the later 454 & 427 blocks.
~ Nick
How much HP extra can we get from this? Lets say i have a 500hp 396.
Using this particular 396 as an example. It was in a jet boat for years, mild cam & mild port work nothing special or out of the ordanry. The boat was running a "A" cut impeller. The engine was not able to make enough power to overcome the resistance of the impeller & turn the pump past 4600 RPM. I refreshed the engine making the following changes: Block Eyebrow, Decked .020", Switched from a Torker II to a Victor JR intake. Everything else on the engine was the same. Same cam, Carb, heads, pistons etc. No Changes were made to the pump. After the work the engine was able to turn the pump to 4800 RPM. Cross referencing my impeller size to my max RPM that tells me that I was making around 350hp before the work, and around 400hp after those changes.
Jet boat pumps and Engine dyno's are very similar in operation, so knowing the impeller size and max RPM you can use it essentially as a dyno to calculate your given HP.
I know that there was more than one change and several variables in that, but it's the best I can do to giving you actual & factual information.
I hope that helps!
What is the coating in the valley
It’s a sealer/ paint called Glyptal. It’s kinda an old school hot rodding practice. Theory is that it helps the oil return to the pan quicker.
www.glyptal.com/glyptal_products.html
~ Nick
I have a 68 small block that has this unshrouding done by a previous owner that ran 62-67 Chevy 2’s.
Would this have been done by an owner, or did Chevy do this to some blocks
From my knowledge, this was never done to a small block from the factory.
~ Nick
@@AutoTechNick ok thx
How much did this reduce your compression by?
On this build I did not measure or recalculate the compression after making this change.
This engine was in a jet boat, so it was RPM limited by the water restriction on the impeller. We had a "A" impeller and the engine could only turn 4600 rpm. I refreshed the engine, and the only changes were decking the block .020" a smaller head gasket, these chamber mods, and I swapped the Torker II intake for a Victor Jr. After those changes I picked up 200PRM, which calculates out to a gain of rough 50hp.
Due its small displacement, the 396 has a hard time getting decent compression numbers as it is. So it's hard to bring yourself to reduce it further, but it is worth it in the end. The gain in CFM will make up for it, plus some.
~ Nick
@@AutoTechNick Great vid. One would think that the slightly lowered compression ratio would help with (low octane ) fuel but still bring HP numbers up .Also if using alum performance heads do you have to notch this area so the valve won't hit the block ?
Brian, I am in agreeance with you. I highly doubt that the slight loss of compression outweighs the gain in total CFM flow / HP increase. Plus, the added benefit of being a tad more friendly on pump gas, it's a no-brainer for me.
As for running aluminum heads. No, you won't need to do anything different to the block unless you are running a very exotic head that moves the valve locations. So for 99.99999% of the cases that is not the case.
But the real problem is these smaller bore engines, 396/402 need really small chambers to get decent compression. Usually 110cc or less. Most all aftermarket aluminum heads have 120cc chambers as they cater to the 454 and larger crowd. Unless you have really aggressive domes on your pistons, that large of a chamber on these smaller engines really lowers your compression. Just something to keep in mind when picking out parts for your build.
~ Nick
Why is the top deck painted
It is coated in a product called Glyptal. It is an older hot-rodding trick. The theory is that it helps the oil return to the oil pan quicker.
www.glyptal.com/glyptal_coatings.html
~ Nick
HEAD GASKETS, you forgot totally about head gaskets, now head is un shrouded block deck is un shrouded, and now who is going to make you CUSTOM HEAD GASKETS ? You didn’t show head gasket interference or gasket clearance !!!
Good catch. However, I did not forget about the diameter of the head gaskets. I did FAIL to mention it, though. I do discuss this to a degree in another video I have where I am assembling this engine:
czcams.com/video/4jANYOZstqA/video.html
Around the 14:13 point of the video, I cover the importance of the chamber diameter and compressed thickness of the head gasket.
You can clearly see how far back the gasket sits from the extended eyebrows in the block in that video. Those are standard off the shelf Fel-Pro gaskets, nothing custom.
If you look at the selection of head gaskets available for a BBC, you will find that most all are made for the larger 4.5" bore of the 454 and beyond. It is actually very difficult to find an off the shelf head gasket with the smaller bore diameter for a 396.
~ Nick