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Old 09-10-2011, 11:57 AM   #16
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Default Re: What stroke to use for a big inch nitrous motor???

Great info here guys, another thing that I wouldnt even consider when building a motor. Would the comments on the previous two posts really apply to motors that big or would it also apply when building a big cube small block?
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Old 09-10-2011, 12:45 PM   #17
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Default Re: What stroke to use for a big inch nitrous motor???

Quote:
Originally Posted by Sam Vincent View Post
if its got a 1.5 rod ratio, it will be fine. lots of racers have less than that.

I dont even get caught up in rod ratio. sure either those cranks would be fine in that tall deck of engine block
Its really not even a matter of rod ratio, its actual rod length is what I was discussing with Tim @ Oliver Rods. He put it to me this way. What is going to be easier to bend under a load a 6ft 2x4 or a 10ft 2x4 when you try and support a load? It only made sense when we were talking rod length and nitrous as well. That was part of the reason for the topic. Rod ratios are completely out the window on these motors so I am really not as concerned about that as I am about how reliable that some other things will be or how good my heads will flow. I also agree that I have a challenge in the head flow department, and I have already spoken to my head guy about this ( a topic for another day.
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Old 09-10-2011, 12:52 PM   #18
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Default Re: What stroke to use for a big inch nitrous motor???

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Originally Posted by demented View Post
Its really not even a matter of rod ratio, its actual rod length is what I was discussing with Tim @ Oliver Rods. He put it to me this way. What is going to be easier to bend under a load a 6ft 2x4 or a 10ft 2x4 when you try and support a load? It only made sense when we were talking rod length and nitrous as well. That was part of the reason for the topic. Rod ratios are completely out the window on these motors so I am really not as concerned about that as I am about how reliable that some other things will be or how good my heads will flow. I also agree that I have a challenge in the head flow department, and I have already spoken to my head guy about this ( a topic for another day.
A 6ft 2X4 and a 10 ft 2X4 cant be compared to a 6.8" rod and a 7.8" rod...a better rod ratio lets the rod and piston swing in a truer up and down motion. A poor rod ratio puts tremendously more load on the cylinder walls and the rod. Racing rods are not made from pine!
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Old 09-10-2011, 01:22 PM   #19
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Default Re: What stroke to use for a big inch nitrous motor???

As said in more polite terms by Monte, don't try to reinvent the wheel. There was a reason 706s were the dominant combo on a 4.84 and this is why they went to 5 and larger bs designs. Although there have been minor improvements since those days they have not been significant enough to make it worth trying to go bigger, particularly not if your pockets are not super deep as the maintenance costs will of course be higher.

Also I would stick with an aluminum rod, even on a 706. If not other parts will need to be cycled if you don't cycle the rods...just my opinion.
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Old 09-10-2011, 01:37 PM   #20
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Default Re: What stroke to use for a big inch nitrous motor???

This was posted today in the N/A section by Scott Foxwell.....nitrous compounds this problem!

Re: 4.5 stroke in a 9.8 dart block
Not really a very good combination. Is it "doable"? Yes. Could you build a much better engine with the same money? Yes.
Windage issues are the main problem. You better plan on dry sumping this thing if you want even remotely decent oil control. It also has a terrible rod angle/rod ratio and everything that goes with that. A lot of power from those big cubic inches will be wasted on those two things.
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Old 09-10-2011, 01:45 PM   #21
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Default Re: What stroke to use for a big inch nitrous motor???

Quote:
Originally Posted by Sam Vincent View Post
if its got a 1.5 rod ratio, it will be fine. lots of racers have less than that.
It wont! The short deck engine Scott spoke of above is a 1.41 and a 5.5" stroke with 7.6" rods to have enough CH(1.27-1.29) for nitrous will have a 1.38 rod/stroke ratio.
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Old 09-10-2011, 03:00 PM   #22
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Default Re: What stroke to use for a big inch nitrous motor???

I believe rod ratio is over rated . get the best C.H. for your combo ,the put a rod that will work with that ..
On my 632 I run a 6.635 rod on a 10.200 deck 4.3/4 stroke JMO.
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Old 09-10-2011, 04:15 PM   #23
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Default Re: What stroke to use for a big inch nitrous motor???

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Mike I assume you are using Aluminum rods in your motor?
Yes, GRP 5500 Pro's. GRP said 60 is plenty safe on rod recycle. I sent our last set in to be checked at 40 and they said there was plenty of life left but it was over the winter so we just threw a new set in. GRP will check rods as a free service for their customers as does Jesel for lifters.
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Old 09-10-2011, 07:34 PM   #24
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Default Re: What stroke to use for a big inch nitrous motor???

that 11.625 is a hi cam block ? if not you will never get a 5.000 +crank with alum rods to clear the cam .just did a 632 with BME 396 rods and had to clip the rods were with the ti rods you have more room i was going to build the 735 i had with pro gram 381 cnc olds heads did not care much about any thing over 7000 rpm and Nos engine was going to be built for max TQ for the street to replace it.s baby brother 632 donovan. big bores allways are better to have when you can get it but 5.000 bore spacing block and heads are far from my price range
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Old 09-10-2011, 09:21 PM   #25
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Default Re: What stroke to use for a big inch nitrous motor???

Yes the 11.625 block is a .400 raised cam block. I am really thinking that I am going to settle at a 5.375 stroke for this combo I am building and it has to do with alot of variables, not just opinions. I have taken into account many of what the guys have said and I agree with the comments about the 706ci motors being a very good combo, which uses a 5.300 stroke so the 5.375 stroke seems to be a great alternative. I also am really up in the air about the aluminum rod thoughts right now and I am really thinking of going with the steel rods for many reasons. Trust me, cost of the Oliver rods is enough to scare anyone into buying aluminum rods once you start talking over a 7 inch long length, but some of the research I have found out about the steel version is making me rethink the whole aluminum rod concept.
As far as rod ratio goes, with the deck height I have and a 5.375 stroke and a 7.700 rod I will have a 1.43 rod ratio. This is better than my 632 was with a 6.700 rod in it(which was a 1.41) and I spun that motor to 7800-8000 pretty regularly (not that I will have to with this one).
I have talked with my head guy about the breathing capabilities of my heads and he has expressed that my heads will be able to support the combination I plan on running (723 ci) very comfortably with some minor adjustments by him. His concern is actually not with the heads but with an intake that I choose. He explained that there are many sheetmetal intake makers out there that make nice looking pieces but perform poorly. I have a few that I need to check into.

Its all about homework and making sure that you cross your "t"s and dot you "i"s as well as asking the right questions. Sure the best thing would be to go and buy a 5" bore spacing block and heads combo, but last I knew I don't have Donald Trump sponsoring me so I do what I can and get what I can afford and make it all work in the end. AMEN!!!
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Old 09-10-2011, 10:17 PM   #26
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Default Re: What stroke to use for a big inch nitrous motor???

yep i know i have a 735 all parts machine work done i just need to short block it but i would like a crank grinder for the shop more then this engine .i was building BBC when a 4.500 crank was big when every thing was custom made so i know about home work .what ever rod you use will be cheaper then a 7.700 long ti rods i have over $3000. in mine but they only 735 total wt last set of BME 426 were only $1.530 for customs
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Old 09-11-2011, 12:06 AM   #27
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Default Re: What stroke to use for a big inch nitrous motor???

Quote:
Originally Posted by demented View Post
Yes the 11.625 block is a .400 raised cam block. I am really thinking that I am going to settle at a 5.375 stroke for this combo I am building and it has to do with alot of variables, not just opinions. I have taken into account many of what the guys have said and I agree with the comments about the 706ci motors being a very good combo, which uses a 5.300 stroke so the 5.375 stroke seems to be a great alternative. I also am really up in the air about the aluminum rod thoughts right now and I am really thinking of going with the steel rods for many reasons. Trust me, cost of the Oliver rods is enough to scare anyone into buying aluminum rods once you start talking over a 7 inch long length, but some of the research I have found out about the steel version is making me rethink the whole aluminum rod concept.
As far as rod ratio goes, with the deck height I have and a 5.375 stroke and a 7.700 rod I will have a 1.43 rod ratio. This is better than my 632 was with a 6.700 rod in it(which was a 1.41) and I spun that motor to 7800-8000 pretty regularly (not that I will have to with this one).
I have talked with my head guy about the breathing capabilities of my heads and he has expressed that my heads will be able to support the combination I plan on running (723 ci) very comfortably with some minor adjustments by him. His concern is actually not with the heads but with an intake that I choose. He explained that there are many sheetmetal intake makers out there that make nice looking pieces but perform poorly. I have a few that I need to check into.

Its all about homework and making sure that you cross your "t"s and dot you "i"s as well as asking the right questions. Sure the best thing would be to go and buy a 5" bore spacing block and heads combo, but last I knew I don't have Donald Trump sponsoring me so I do what I can and get what I can afford and make it all work in the end. AMEN!!!
I am going to put this as delicately as I can Fred.............lol.......what you are contemplating, is NOT a good combination. While rod ratio is not a big deal in most cases, what you are doing, with that long a rod, a steel rod at that and that poor of a ratio, it just won't be good. In my opinion, you are going to put a motor together, that is not as reliable as a 706 and also likely won't make any more power, plus, the type racing you do, is not as torque dependant as drag racing, so I just don't see all that stroke gaining you anything. I won't be surprised, if your rig is no faster with the big motor vs the 632.

Monte
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Old 09-11-2011, 12:15 AM   #28
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Default Re: What stroke to use for a big inch nitrous motor???

Quote:
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I have that same 5.5 X 4.600 combo with the olds 14 degree heads. Biggest issue with that many cubes is air. To be able to turn it 74-7500 you need a big intake valve. I don't really know how big of one you can fit in the casting but with my tiny 2.400 1.900 it just barely makes steam at 7000. A more Ideal combo for the 4.840 deal is the tried and true 706. It moves less air and the piston speed is also less. Also the 14 degree chambers are not Nitrous friendly unless they are modified.
Depending on what 14* casting you have depends on the valves you can install. Just did a set last week with 2.500 1.880 on 14* chiefs. This is on a 384 casting. On a 385 casting 14* BC you cannot go that big. The 384 casting has the guide spacing moved farther apart then the 385 casting, but I don't recall off the top of my head the amount. I have everything measured and recorded next to one of the Serdi machines at work. I can take a look Monday if you need me to.
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Old 09-11-2011, 01:13 AM   #29
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Default Re: What stroke to use for a big inch nitrous motor???

Quote:
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One thing I will say is when I talked to Tim from Oliver Yesterday he warned me about getting too long of a connecting rod whether it be a steel or aluminum with a nitrous engine and the reason was due to the unpredictably of how nitrous can be in a motor. It can have the tendency to bend really long connecting rods. This makes connecting rod construction very crucial in these long stroke/ tall deck motors.
And yes your right FoFo, we're learning!!!
Let me make easy for you.I sell you a 706 buck/Musi that just ran 4.20s on 2 stages last weekend.you can put 60 passes on it without even changeing the plugs!20,000
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Old 09-11-2011, 09:20 AM   #30
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Default Re: What stroke to use for a big inch nitrous motor???

Quote:
Originally Posted by Monte Smith View Post
I am going to put this as delicately as I can Fred.............lol.......what you are contemplating, is NOT a good combination. While rod ratio is not a big deal in most cases, what you are doing, with that long a rod, a steel rod at that and that poor of a ratio, it just won't be good. In my opinion, you are going to put a motor together, that is not as reliable as a 706 and also likely won't make any more power, plus, the type racing you do, is not as torque dependant as drag racing, so I just don't see all that stroke gaining you anything. I won't be surprised, if your rig is no faster with the big motor vs the 632.

Monte
Thanks Monte, That was pretty delicate!!! I understand what you are explaining and that is why I am asking all the questions first before I go and spend the cash on anything. Right now the only thing I actually have is the block so at the very least it will be a 706 ci motor.I have not decided on anything and I still have a lot of homework and questions to ask. However, I will say one thing, I do believe that you underestimate how much torque reliant mud racing can be, but it depends greatly on track condition. With the size of the tires, 4 wheel drive, and very high gearing, all this does pull the motor down severely about 50 ft from the start but this also depends on what tires are used. When you get some of these cars that get 60 ft times that are 1.30 or better, I would tend to think that we are getting some power to the ground, and this is on some surfaces that some people have a hard time walking on when sober.
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