4.6 Dohc Build

IH8BOWTIES

Member
Jul 6, 2008
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I've been doing quite a bit of research on building this engine but one thing I can't seem to get a clear answer on is a good ballpark figure on projected HP. The block is bored .20 over. I've seen the question asked in other forums but the numbers get misinterpreted and everyone is running with .020 and claiming no HP gain. I'm talking .20 (hundredths) not .020 (thousandths). This results in 313 CID at 5.1 Liters not 283 CID and still 4.6 liters. With that confusion cleared up, I'm planning on running forged pistons and I or H rods to go with the forged crank that the 96-98 cobras came stock with. I'm not looking for an all out race car. Just looking for a fun toy that can dispatch an occasional SS Camaro with relative ease and give the Generals' finest a good run for it's money. I was looking into a Kenne Bell setup. I know this isn't intercooled but as long as I build around 9.5:1 compression and keep it around 10 lbs of boost, I should be fine on 91 pump gas with an octane booster. I'm shooting for around 500hp. Is that guess a little high or conservative? Also any other suggestions you have would be appreciated. I.E. best pistons/rods, other mods, etc. I have no interest in an eaton swap as this would be more labor intensive than I care for. Also, what thoughts are there on the IMRC deletes? I know there's more I'm not thinking of so throw everything at me. There's a lot of knowledge on this board and I appreciate you all. Thanks for any input.
 
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What block are you planning to run 0.20" over on? That is a 3.752" bore. I dont even think you could get away with that on the BOSS 5.0 with a stock 3.70" bore. I know there are MAX BORE 3.750 sleeves, and those were mainly on race engines because the water passages are so degraded, that it cant cool for normal driving.

The maximum "safe" bore, and safe is a relative term, on an iron block is like 0.060" over. A teksid aluminum block could go more than 0.060", up to probably the coyote size of 3.630" or about 0.080". On the teksid, it would only leave about 0.040" sleeve thickness, and even torquing down the heads could warp that bore.

As for 10 psi non-intercooled.....unless you are going to spray meth.....nope. KBs and any twin-screw is going to create heat....a lot of heat. KB runs their 6lb kits with no intercooler, but their 8lb kits are intercooled. You would get so much heat soak after driving 20 minutes (getting on it every now and then) you would have very high IATs and the ecu would start pullilng timing, especially in the summer.

I have a built 2v, 5.3L (stroked Boss 5.0 block) with a 2.9 whipple running 17psi on 91 octane. A built 4v with INTERCOOLED KB @ around 15 psi would be easy on 91 (with a competent tuner). 500rwhp would be simple......
 
That's good to know. I've never had a DOHC so I'm a little new to it. I bought the car already taken apart and the guy I bought it from swore it was .20. I decided to check it myself and put my caliper guage on it and yeah, it's .020 so I feel a little silly but that's ok. Sometimes that's how you learn things. Nonetheless, I don't want to do the eaton swap and I really like the looks of a roots or twin screw blower on the engine. I don't like how a centrifugal looks but I certainly won't let that be a deal breaker if I get better, more reliable performance. As far as the intercooled KB, they don't offer it for the B head cobra. I'm just trying to get some good info and get everything together before I start putting it together. Would it be easier converting to the C head with a KB setup or just go with an aftercooled centrifugal? Thanks again.
 
Go read THIS.

Really, you have 3 optoinsl.

1) If you want to go PD, and building the thing anyway, do the C heads and then throw a whipple on it. A used 2.3 whipple can be had for almost nothing these days.

2) If you want to stay with B heads, do a centri, or even a turbo.

3) If you want the KB on the B heads....change your build. Go high compression, low boost. On a 4v, 10.5 cr with a decent dcr with some blower cams (MHS or CMS). That would make nice power all the way across the band.
 
So if I chose number 2 and a centri then I would need to run a lower compression set up right? Say 9.5? I did look into a turbo and there's some good deals out there on them but I would have to drop another 5 to 600 on a tube k-member. If I didn't already have my parts to rebuild the front end, that would probably be on top of the list as a k-member isn't that big of a deal. That just wouldn't be very cost effective for me. I was looking at the vortech v3's and I think I could hit my goal with that setup. More to think about. I do appreciate it. Something else I read was that stroking these motors doesn't do much for them unless your FI. Any truth to that?
 
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9.5 Static compression really doesn't mean that much anymore. SCR can be an indication of what your peak cylinder pressures are, but DCR, or dynamic compression ratio will tell you more. You need to use a dynamic compression calculator to figure up the best combo for you. The cams you run will make a HUGE impact on what you can run, with how much boost. My engine is 9.4:1 SCR, and I can run 17psi on 91 pump. This is because I have stage 3 blower cams that bleed off a lot of my static compression.

If you lower your SCR, it will lower your DCR, and you can run more boost. How much it lowers will be based on what cams you are running. 10:1 SCR with MHS Stage 2 blower cams (I don't know the specs on these, just using this as an example, my last discussion of this said these cams closed at 44 ABDC) and 7 psi.

This would give you a 9.05:1 DCR, with a peak cylinder pressure of 186 psi. The boost effective compression ratio (EBCR) of 13.36:1 is safe on pump gas. You could probably have a bit of room to work with a little more boost, but this is seeing what would be possible with the KB that you originally discussed. So, it CAN be done, you just have to do it right.

Of the 3 options I gave you, I would lean towards 1 or 3, with 3 probably being the cheapest....especially if you find a used 2.1 KB kit.

Now, for stroking, I could point you to 1000s of articles on this. The answer is yes and no (and let the argument commence). Increasing cubic inches will always increase power, but to make use of the extra cubes from stroking, you need to add RPMS. If you just went big bore, then extra RPMS would not need to be run to see the power. To get the extra RPMS, you need to add a bigger cam to make use of the RPMS. If you stroke and add no RPMS, you are very likely to NOT see much gain.

Adding FI to a stroker can be of benefit because of the extra CI for FI cylinder volume. Again here, if you spin it a bit faster, the increased stroke will help make more power.
 
I am planning on running some 3:73's maybe 4:10's. That should get the rpm's up. I sent some info to MMR with some ideas of what I'm wanting to do and they said if I wanted to run a centri, their 4.75 stroker kit made some good power with it. What would I end up with running stock b head cams? My understanding is they like boost. I hear you on the blower cams but I'm on a budget. I'm not a big numbers crunching guy but find it intriguing. They have many "kits" and combos where someone else had already done that. I'm definitely going to cut and paste this to reference when I get started. Thanks
 
Run 4.10s minimum. The 4v's love RPMS....so throw it at it.

The stock 4v cams are not bad with blowers, but they are not optimal.

I have heard good things about 4.75 stroker, haven't had personal experience with it. A little stroke and RPMs might not be a bad thing, especially with a centri blower. Forge the bottom, leave the B heads alone, put a centri on it, and spin the hell out of it. It will be fun, and you should be able to make some decent numbers. 500+ should be pretty easy with a decent tune. The key, is forging the bottom, and keep about 9.5:1 SCR with the stock cams.
 
If you only want 500hp, I personally would just buy rods and pistons no stroker. B heads work very well with boost. I would also put a turbo on the car instead of a supercharger, but 500hp is very easy for both. A good forged engine with b heads and a 70mm turbo will make around 500 on 10-12psi easily with a very safe tune and spool very fast.