PDA

View Full Version : My Destroked 388sbc buildup (from old site)



derekf
08-20-2004, 08:20 AM
smokedtires
Registered User
Posts: 10
(1/16/04 3:00 pm)
Reply My destroked 388sbc buildup
--------------------------------------------------------------------------------
Hi all. Just thought I'd share my new engine buildup that's in process. It's a 4.125 bore with a 3.625 stroke. It's going in my 74 Corvette with a Tremec TKO & Richmond 3.90 gears.

Let me know what you think

Check it out (http://temp.corvetteforum.net/c3/mikeo/Engine/index.shtml)

nitrovette
Registered User
Posts: 40
(1/16/04 9:19 pm)
Reply Re: My destroked 388sbc buildup
--------------------------------------------------------------------------------
Well you know what ill say mike,its gone to kick azz.



smokedtires
Registered User
Posts: 11
(1/18/04 7:02 am)
Reply reply
--------------------------------------------------------------------------------
Thanks Pat. I stopped by & the block is in the honing machine now that it's been decked to height

carreranova
Registered User
Posts: 28
(1/18/04 12:13 pm)
Reply Re: My destroked 388sbc buildup
--------------------------------------------------------------------------------
It's interesting that you would compare your motor to a 302 or a 327. At what bore/stroke ratio does the 4.125" bore motor turn from a quick & high revving engine much like the old Chevy 302's & 327's from yesteryear into a low rpm torque motor like a 406? What makes a quick revving motor?

gmachinz
Unregistered User
(1/18/04 9:25 pm)
Reply okay, I'll bite....
--------------------------------------------------------------------------------
Doesn't your M motor have a taller deck height? If so, why only a 6" rod? I would imagine a shorter piston crown combined with a higher wristpin location would yield you a better rod/stroke ratio. I (only my opinion) think the rod angularity is too extreme given your stroke. I have to admit I don't understand your comparison to 302's and 327's either. They used a short stroke (3.00, 3.25) and that is why they "revved quick"....they move a lot of air very rapidly so they were great performers but you have a low RPM torque monster there! I would have opted for a longer rod for better leverage but you may not have enough room for one, either though. I like the idea of a short stroke/big bore with a long rod vs. a stroker or torquer but everyone has their favorite combos. -G

nancejd
Registered User
Posts: 87
(1/18/04 9:32 pm)
Reply Re: okay, I'll bite....
--------------------------------------------------------------------------------
I think because engine stroke and engine bore size are relative to each other. A big bore motor can run a larger stroke compared to a small bore motor and the ratio between them can still be the same size. A 400 block with a 327 crank has approximately the same bore to stroke ratio as a 327 block with 283 crank (302). I believe that this ratio is one of the governing factors in determining what the RPM limits of an engine is. Of course, with the larger stroke you also have more rotating mass.

gmachinz
Unregistered User
(1/18/04 10:17 pm)
Reply true...
--------------------------------------------------------------------------------
That is a basic determining factor for RPM limits but so is the length of the rod. While the actual length of the rod has no real effect on cubic inches, it does have an effect on RPM limits-as in higher. The problem with strokers is just what you said-more rotating mass. They are awesome at low RPM's but if you try to take them too high you beat the crap out of the bearings-not to mention valvetrain harmonics going through the roof. There is nothing wrong with strokers...but man, if only you could take all that torque with you (relatively speaking), say to 6, 7000 rpm....? What kind of numbers could you get? Now, realizing you're walking a fine line with volumetric efficiency, adding a turbo or two is nice on the low-mid range but again, what if you could take it upstairs with you-on a 383 for example? By design, you are treading on thin ice with a stroker and turbos BUT enter the land of big bore, short stroke and long rods....the game changes. A longer rod yields better rod/stroke ratios which means more stability at higher RPM's (less cylinder wall loading), moving just as much air (not by volume but by crankshaft speed) and by keeping the piston at TDC longer (hangin' out is more like it ) you can run 9:1 on pump gas w/a turbo. The issue here is stability/longevity. How to make power but keep the motor together as well. Man, I went wayyy out on a tangent-but this reminds me-I want to start a new thread........-G

smokedtires
Registered User
Posts: 11
(1/19/04 5:14 am)
Reply reply
--------------------------------------------------------------------------------
gmachinz pretty much hit it on the head. You can get a Little M in either short or tall block, mine is the short deck height. Even with the 3.625 stroke & the 6.0 rod (1.655 stroke/ rod ratio), I have only a 1.2175 compression height on my pistons. If you look closely, you'll see that my crank has the rod journals hollowed out, this is all in an effort to lighten the rotating assembly. I also believe that the EFI will help with throttle response (from what I've read, no personal experience yet ).

I know that stroker motors are all the rage today, and there's no denying the performance & torque they create. I know for my own car,with the 355 I currently have in my vette I don't have a lot of traction so this was some of my mind thought in selecting a destroked engine. All of the Nascar engine's run large bores & short strokes and 377's (destroked 400's) are popular with circle track cars because of the throttle response and how that propel the cars out of the corners. I feel that by giving up some torque at the lower rpms & by tailoring the timing & fuel maps thru the injection, I'll be able to use the power down low where my traction is limited, and as the car gains speed, the engine power level will follow.

This is all new to me and my thinking may be completely wrong, but I feel this is a big step over the basically Edelbrock RPM ~400hp I currently have. My car is mostly street driven with 4-6 autocrosses and maybe 3-4 trips to the drag strip, maybe 1-2 trips to Watkins Glen each year. Should I also mention that I've driven it from Upstate NY to Bowling Green KY I'm hoping that aside from the ocassional valve lash adjustment this will be a dependable strong running engine.

carreranova
Registered User
Posts: 29
(1/19/04 11:30 am)
Reply More questions
--------------------------------------------------------------------------------
What makes an engine rev quick? Does it have something to do with the torque it produces? If torque is what makes the rpm increase, why is a low torque motor called a quick revving motor?
What's the real difference in 'dwell' between a 6" rod and a 5.7" rod? A 6" rod on a 3.625" crank will keep the piston within .250" of TDC about .325 longer than a 5.7" rod, a difference of about .6%. They will spend exactly the same amount of time at TDC. At 90, the 5.7" rod/piston is .015" further down in the bore than the 6" rod/piston.
If I buy a Little M block, which cranks are stroker cranks, and which are de-stroker cranks?

gmachinz
Unregistered User
(1/19/04 3:24 pm)
Reply reply
--------------------------------------------------------------------------------
I like to think of a "quick-revving" motor as one with a fairly large bore but small (relatively) crank journals. Less frictional loss with smaller bearings, moreover. I tend to think that the larger the main bearings, the less I would really want that crank spinning anywhere near 6000RPM, not to mention the added weight accompanied with larger stroked cranks. Now, that doesn't mean a high-revving motor is torquey-usually they are not as torquey but the RPM range is longer and is more flat/consistent so I tend to think of them moving just as much air but at a different RPM range. -G

smokedtires
Registered User
Posts: 13
(1/19/04 6:31 pm)
Reply reply
--------------------------------------------------------------------------------
My crank actually uses the small main journals and the small (327) rod journals which as gmachinz stated has less suface area for less friction, but as I stated in my earlier post, it's also about taking weight from the rotating assembly which the smaller journals also help to accomplish this. A quick revving engine is more than just light weight components though, it also takes the right parts combo, compression ratio and what not. This is all assuming we're talking about a free spinning engine though without a load on it. If you plug in other factors like the weight of a car, gear ratios and everything else that contributes to wieght to power ratio, then a 3200 lb car with 500lb ft of torque is going to rev quicker than the same car with only 300lb ft torque.

Here's a quote from www.ac427.com/html/cobra_23.shtml (http://www.ac427.com/html/cobra_23.shtml) :
Comparison to its big brother, the 350, is not really fair to either engine. The 350 is a tried and proven street engine. It's real potential for the street lies in its ability to make bottom-end torque. It can be built for high-rpm horsepower, too. But torque is its biggest potential. The 350 would be a better engine, though, if it had a larger bore, shorter stroke, and longer rods. Chevy should have built the 350 using the 327's 3.25" crankshaft, a bore of 4.140", and 6" or 6.125" rods. But that's for another discussion.

Popular Hotrodding or another similar mag did a write up on "The 350 GM should Have Built" and showed the benefits of having a larger bore and shorter stroke also. I'm simply trying to capitalize on these benefits that Nascar and other have found for my own car



gmachinz
Unregistered User
(1/19/04 8:40 pm)
Reply HR did the article.
--------------------------------------------------------------------------------
Funny you should bring it up like that. My favorite combo is using a 400 block with a DZ302 crank or a LG 327 crank. The HR mag. article used 4.9 Ford rods which are 6.209 (you could go as much as 6.300 but the block needs notched for the rod) in length-they are bigger than GM pieces and are all forged. Granted, the rod does not move down into the bore as far due to the longer rod but the crankshaft speed is so quick that air moves very rapidly and with that comes increased velocity.....then cylinder head options become critical at that point. It would be better to gear the heads towards increased airflow velocity rather than for volume. So in a sense I guess a long rod motor is the opposite of a stroker but they both do the same thing-they move a lot of air-it's just a matter of doing it by a long stroke equalling big volume, or by a faster rotating assembly meaning higher RPM capabilities. -G