Hey yall currently building my first motor. Running into questions about DCR, I have a basic understanding but curious to if I'll run into problems running 93 octane.

(relevant) Parts List:
Eagle Street Rotating Assy B15455E030
Trick Flow Top End Kit TFS-K314-420-395
Edelbrock EDL-2975 (Victor Jr)

Details:
800CFM M4MC
62cc CC
195cc Intake Runner
Cam
tfs-31401001.pdf
Head gasket compressed thickness 0.04 (Currently Placeholder, TF doesnt post anything about the head gasket contained in the set, contacting them while writing)
B&S: 4.030x3.75
Deck: 0.015
Dish Piston: +5cc

Calculated Stats (gofastmath.com):
11:1 SCR
9.48:1 DCR

From what research I've done in the last 2 days it seems like a high 8 DCR is the best for 93 octane and pushing past 9 will cause problems. 9.5 obviously would be WAY too high then. Feel free to critique my setup, I'm relatively new to this. I can go further and give the full parts list, currently just threw on what pertained to the question.

Will this DCR be too high? I've heard its important and I've heard its not. I've heard that looking at SCR is enough for unserious street builds but considering im pushing the upper limits of SCR and potentially a 500hp motor I imagine DCR is a big factor. I do not want nor have much access to anything but pump gas. Obviously I can lower SCR and directly lower DCR but with the financial and knowledge constraints I have I dont want to keep mix in matching parts.

 

Check that DCR again using this calculator. I get 8.56:1 with those specs.

https://www.jeepstrokers.com/forum/calculator/

If you want to lower DCR don't do it with a thicker head gasket. Even the one you are using will give you .055" quench, which is pretty far from the ideal .040" quench. A tight quench reduces detonation potential.
Any chance you are at an altitude higher than sea level?

If you are near sea level then I would recommend lowering that DCR a bit. Typically 8.5 would be the max with aluminum heads. There are ways to do that. First CC the combustion chambers to check their actual volume, which may differ enough from advertised to solve the problem for you.
Another option is you can port the chamber on the head a bit to remove material and increase the volume. The valve cutouts on the pistons can be enlarged to increase volume. Measuring that volume is a bit trickier as it needs to be done with the piston in the block.

You can see here how it is done.
https://www.corvetteforum.com/forums...er-size-2.html

 

Im not entirely sure what I am entering wrong, this calculator actually gives me a 10.2 DCR, so probably user error. I was using the wrong value for IVC lol. But if I get apparently 8.56 DCR with this setup and reducing the HG to a more ideal 0.025 for a 0.04 quench would solve my problem then, I can also use a 7cc dish instead. Im in the midwest so altitude is not a factor.

 

Plug those numbers in and see what it gives you. Using a thinner head gasket bumps the CR and thereby the DCR up.
What transmission and rear ratio do you have?

 

Flat deck the block, use flat top pistons with a .04 head gasket. That’s a good quench.

go with a roller cam, bigger will get you a lower DCR. 230-236 intake duration.

If the valve springs support it go with 1.6 rockers.

 

What has been purchased so far? Is this still in the planning phase?
if so then many options are on the table.

 

I use Wallace Racing Calculators. Very accurate.

A Q-Jet will be difficult to tune with that low idle vac cam.
A 383 can easily make 450+ HP.
What is your target?
What year C3 and hood do you have?
A Victor Jr will not fit a C3 or a Q-Jet.

The block decking should be done after your CR, cam and pistons are picked,
How much is done already?

 

Im getting similar results from Pat Kelly's calculator. ICA of 68 and the .04" placeholder head gasket. static 10.9 CR and 8.58 DCR. I would be decking the block to remove the .015" to get to zero deck height.

The Victor JR would not fit under my '80 hood. The ZZ4 intake just barely fits with everything perfect on my 383.

Lars built me a Qjet that only needed a slight adjustment of the APT screw to make it run right. My original solid flat tappet 275/281, 240/246, 112 LSA, 4 Degrees ADV cam had a bit of overlap and that was dealt with using a little fatter AFR. I swapped the lumpy cam for a custom grind, smooth idle solid flat tappet cam now and the Qjet no longer needs the extra part throttle fuel. My 10.7 CR, DCR is 8.56 with cast iron heads and a tighter .040" quench. I have no issues at all with pump gas with either cam but my tight quench is doing its job here. Fix the quench one way or the other is my advice.

 

Just gonna answer all the questions in one post,

Nothing has been done or ordered whatsoever yet. Im working off of a completely stock L48 at the moment. I have a Super T10 (I think thats the correct 4 speed for that year). Im unaware of what the diff gear is since the numbers are back and forth from researching. I currently have the stock 76 hood (76 car) but was planning on purchasing a scrap one and cutting a hole till I can afford a more permanent L88 scoop option from dynamic corvettes.

AFAIK our drivelines wont handle much more than 500hp, apparently the diffs start to explode past that and I dont doubt that my trans will eventually go too. I would like to go all out with this build but I've been weighing my options. Im looking at at least a $6000 investment not to mention supporting items like a tremec and a rear gear swap. Between machining and every single part on my list the cost gets pretty steep.

I came across the "super" L48 build thread.
https://www.corvetteforum.com/forums...e-numbers.html

Basically a max effort build without the need to change the pistons to something a little more flavor-full. But, if im already down there to replace the old bearings, getting the block cleaned, replacing the main seals, do I go that extra mile to the 383?

I know we're getting a little more philosophical here but for a max of $4000 I can go the cheap route (low cost engine and diff rebuild) and make a decent cruise car, while also saving my money for potentially a newer and (forgive me) better corvette. Why have one semi streetable car, when I can have two and make significantly more power by opting into the C5/6 trend?

ANYWAYS, mildly off topic. Point is I would like to make this 383 build a sensible option so I can make that final decision. I do plan on keeping this car but sinking over 10k into it while in college might not be the move, but I also feel like going halfway with it wont do it justice. Like I said it opens up a whole can of worms, "what am I looking for?", "How much do I love the car?", "Where do I see myself in 5 years?" etc etc...Apologies for rambling.

It seems like the consensus is to deck it flat and get a .04 gasket and with the new stats in place im getting a 8.5 DCR, so now the last order of business would be what intake/carb combo fits under the stock hood? If the Vr Jr wont fit what are some good replacement options? I'd like to keep the Qjet if possible as it knocks down less cost but I was also looking at a Brawler carb from holley, Head ports are standard and the runners are 195cc.

 

If your bores are still serviceable, you could reuse the 350 rotating assembly. If you need to bore oversize, it will cost about the same to do the 383, so stroking is a no brainer. I would do it.

The 76 hood has a little more room than the '80 I have but still, your options are pretty limited unless you have a hood scoop. Edelbrock Performer works with the Qjet. ZZ4 works with a Qjet as well but aims a little higher in the HP range.

 

Good info!
Congrats on your first Corvette and your first engine build.


Do you have auto or manual rans?
And what rear gear?
It is the total combo that counts.
Ill give you a step by step plan, based on spending 40 years in a speed& machine shop:

1) My L48 thread is good if you want to keep the short block stock & intact.
Check the bores for cross-hatch, & measured for bore wear, and then decide from there.

2) If you are going to need to bore it anyway, then you need new pistons & you have lots of options, but you may as well stroke it and go 383.
The difference in torque on the street is amazing!

3) Do not skimp on the short block! You should plan on decking the block to "zero" deck, lets you easily get .040" quench, which is important to prevent detonation when you start increasing the compression. And also let you use the better sealing / thicker .040" head gaskets, and still get good quench. But do not actually do any machine work yet.

4) Plan you build out next. Deck height, head gasket, head chamber size, piston dome, even cam duration. 10.5 CR and 8.4 DCR max is great if you are using a 230* ish cam. If a smaller cam, you may actually want a little less for safety. With aluminum heads, with modern chambers. A little less for old school parts. No machine work yet.

5) Cam duration gets selected along with rear gear, trans 1st gear, and convertor, and desires. It's the whole combo that counts. Over 218* dur likely needs non stock rear gears.

6) Cam durations @ .050" over 220* need careful gear selection, and start costing lower end TQ. ~ 230* @ .050" is about the max limit for a reasonable street performance cam. that still has some respectable vacuum. Over 220* the idle vac starts to drop.

7) Once the pistons arrive, then you do the machine work, not before, they need to be fit.

8) Intakes are extremely height limited on C3 corvettes. You have a 76 or so so you have an extra 1" over earlier C3s with a std hood.
These are what fit without issues. Do not use a carb square to spread adapter! A Q-Jet will give you the best mileage, etc if it is a good one. Check with Lars on here. It will tolerate a 230* ish street cam with 11-12" vac if you have one of the adjustable APT ones. Otherwise a Brawler is a good choice.

9) You can easily build a 450HP+/450TQ 383 following this. AFR heads are the best, but $. A 383 wants airflow, don't choke it with slack heads.

10) You could always build the short block "stout" with your long term goals in mind, and run stock heads for now. And save up for the AFRs and do a head swap later.



Your hood is 1" taller than normal like an LT-1 hood, so an LT-1 intake, or RPM, or shorter, should fit, as long as you run a stock OEM 2.0" drop base air cleaner, not an aftermarket one. Your stock twin snorkel may also fit. If you have a functioning scoop on your year that would be good to keep.

 

Manual Gearing
2.64 1.75 1.34 1.00 2.55

Rear is likely 3.36

The Weiand was actually my go to intake on my previous list. Thank you for solving that whole problem, and for all the amazing information, I've got the intake and a good cleaner on my list now.