I am doing some research before building a motor next year.
I origonally wanted to build a destroked 400 (377) with a 3.48 stroke and a .030 over (4.155 bore) 400 block. but this puts me over the 6.0L limit for the SCCA class I want to run in.

so I have decided to see if I can make a comparably High revving 350 block screamer. I would actually like to start with a .060 overbore which would take me to like 5.95L but I had some questions.

I am building this motor with RPMs in mind and have things like a tunnel ram and 220cc heads figured in.

what I wanted to know was -- if this thing is going to live over 7000 RPM should I build with 6" rods and shorter pistons or should I stick with 5.7" rods. I have heard that longer rods can give more power?

also how much compression can I run with iron as compared to aluminum heads? can I get away with 11.5:1 on Iron with only 93 octane or should I opt for aluminum?

Thanks for any help.

 

with that kind of compression aluminum will be a must. for that fuel. use the smallest chamber you can get for the head, preferrably around 53cc so you don't have to run a dome piston. the 6 inch rod will also be the best way to go given what your trying to accomplish. the tunnel ram, a bad idea. i'd recomend using a victor jr. if your planning to run a stick and able to keep the rpm high, stick with the 400 block at .030 over and order a 3.25 stroke crank (stroke of the original 327). this will allow for larger valve usage to get the air in by unshrouding the valves better. using the shorter stroke assuming you have a trans with fairly close ratio's and enough gear ratio, it will come out of the corner pretty strong and allow for holding the gear alot longer which can be handy on some "s" parts of the track. with a good lightweight rotating assembly you should be able to turn it 7500 with good power and up to 8000 if needed.

 

damn boy get those revs down you are getting into my range now

 

if he's going to class race it he's going to need all the power he can get. circle track guy's run that kind of rpm all the time with 406's and 434's even running 7000 to 7500. we have two local dirt tracks here and quite a few of the racers are friends. it's not as difficult as it may sound. the rpm won't be as big a problem as running a carb on the hard cornering may be.

 

thanks for the replys -- I have a few responses here.

first of all I would think that your Idea about small chambers and flat top pistons is wonderful -- just tell me where to find 53 cc chambers with 230+cc runners and 2.08I 1.60 valves and Im sold.

But isnt there something bad about running 6"rods that makes the ring lands too short or something?

Also I would love your Idea about using an even smaller crank but those are hard to come by. for now I would just stick with a 3.48 crank and some really short pistons with those 6" rods.

Why not a tunnel ram????????
I read a nice article in januarys Hot Rod on them and liked what I heard. Is it really that hard to keep carbs flowing right on a road course?

 

btt

 

If it were me- 350 block .040 over flat top forged pistons, mill block to zero deck, 76cc chamber heads angle milled to set compression where you want with your head gaskets, aftermarket rods and crank, cam with some overlap to help bleed off comp at low revs and as said before a victor jr. The aftermarket crank and rods are nice but if properly set up the stock parts are very durable also. I've turned 327's and 350's up to 7400/7600 in drag and dirt cars with little trouble if the machine work is right. Don't scrimp on the machine work...

 

you can get 64 cc heads and have them milled. some companies can get you the size you want, but you'll have to request it. the idea behind the aftermarket crank, rods, and pistons is lighter weight for more power. 6 inch rods can get into the ring lands on a 3.75 inch stroke engine, but not so bad on 3.48 stroke and smaller.

 

hehe now that I have a job I am thinking of building myself a firstgen RX-7 with either a SB 327 or build a wankel wonder that can rev up to 10krpms at least though I have heard I could prolly get around 11.5 with the buildup I am looking for

only problem is I do not know if a carb would work on this car, and the idle would be like a big *** chainsaw with a HUGE cam in it
the idle is 1500-2000

but I would have a 1.2L motor pulling around 250+ in a 2200lb car and I would prolly be putting some 5.12 gears in the rear for it as well

think I could hit 13's ?

 

what SCCA class are you going to run this in?

 

street mod

 

oh ok, solo 2. I thought it may have been A Sedan and you read the rules wrong, they have a 5 liter limit. Just checking before you spend the bucks.

 

Actually its a 6.0L limit Naturally aspirated and thats too bad because I had a 377 in mind (3.48 stroke 4.155 bore 400 block) but I think Im going to have to settle for a 60 over 350 unless I can pull a 327 crank out of my a** to run in a 400!
the process is ongoing however and I probably wont be building the motor for 6 months or so.

 

I think that cr is too high for a carbed road race car on 93 octane.
Large bore motors are also more detonation prone. I'd build for 10.5/6 for aluminum and 9.8 to 10.2 for cast iron.
The continuous heat load time in road racing demands a little less compression ratio than you can get away with on a 10 sec dyno test or quarter mile sprint. Build the motor for max horsepower output not maximum rpm. If you concentrate on this you'll have plenty of rpms.

 

well I dont really plan to run it on the street so I could always run avfuel (100) on the track. but a smaller compression ration with a 350 block actually makes my life easyer because there are more pistons available.
I actually also just saw a post where a guy took the bottom part of the wieand tunnel ram that I am thinking of using and converting it (with a homade plenum) to Fuel injection. I think I might be able to do this and then not have to worry about tuning the carbs for high cornering G's

 

finding a large journal crank isn't that easy for a 327, you can also use a crank from the 307 as they use the same stroke if your looking to go with a stock stroke, but for that application i recomend an aftermarket crank preferrably a lightweight one. for those thinking of questioning it, yes 307 is also a chevy engine from the late 60's to early 70's. you should be able to find a junkyard with something laying around with one in it and with only needing the crank you'll be fine. large bore engines will tend to detonate easier unless you use it to your advantage. as for the rpm, it's nice to come out of a corner with the rpm at around 4k and know you don't have to shift for another few seconds. low rpm engines are great for drag racing, but leave alot of shifting for road course racing. this is why most road racing cars are small cubic inch with the exception of the top dogs and even them only run just over 400 cubes.

 

Exactly how does detonation work to your advantage?

 

it doesn't, the large bore is what you can use to your advantage which is what i was referring to in that sentence.

 

Wether it is an advantage or not doesn't change the fact that it tends to be more detonation sensitive than a smaller bore.
This needs to be taken into consideration when deciding on the final usable compression ratio. To avoid detonation.
The big bore itself, is not an advantage to either power or torque. The fact that you can squeeze larger valves within that bigger bore, is. I'm sorry but none of what you said has any basis in fact.

 

squeezing larger valves and having less shrouding does leave room for more hp and the big bore also can be used to work with the detonation issue also. large bore engines tend to work better with longer duration cam's which bleed's off more compression at lower rpm which is where detonation under load is more a problem. the large bore again has the advantage when used with a shorter stroke in the way that longer rods will fit without getting into the ring lands which allow's for more oil control which is where some carbon problems come from causing detonation. long stroke has it's advantages when built a certain way as does large bore, but if you build them specific for a concern you can fix problems like that.

 

I run Sunoco 94 in my motor. No detonation, no pinging.

timing is 12 deg initial, 36 total.

All depends on the cam characteristics.

Good Luck.

 

Why do large bore engines have detonation problems? Is it from the amount of end gas? The distance to the endgas? Or is it from the squish distance being too great via lack of proper planning? I'd bet if the squish/quench was as tight as possible, there would be no detonation problems - that is just a guess though. Maybe since there is so much squish, the turbulence generated causes a much quicker burn rate, and ignition doesn't have to be as advanced, and old engine builders took that to mean detonation prone. I dunno.

On another note - I remember there being a steel or forged crank in the 327's that used to come in combines - you know - the thing that picks corn or beans, in fields. So, if you're near a combine junkyard, you might be in luck. I know if I find one around here, I might just pick it up.

Later,
Jeremy

 

if a large bore was a bad thing do you think they would run them in F1, lemans or other events that high rpm motors are common

most the motors you find there have a bore that is longer then the stroke

as zippy said you can fit a larger valve in there and also it will make more of the valve exposed to take in the fresh intake and to help get rid of used up gas

only real problem I see with a large bore is a bigger surface area which would make it harder to get the gas to burn nice and even so it will slow down the flame path(a problem the rotary has). that would also lead me to think that it wouldn't have as much of also the larger surface area will absorb more heat from the combustion that you would have less pressure put down on the piston (another problem with the rotary and power output).
so I would be thinking a large bore motor would be less prone to detonation. so can someoen explain that one to me cause just like RednGold86Z I am curious here

 

well the way i understand It -- wait I dont either!

why does a larger bore promote detonation?
If It has something to do with hot spots from piston or combustion chamber design that can be fixed -- but what is it about the larger cylinder WIDTH that promotes detonation?

 

Combines?? really??

think I might just stick to the performance cataloges but a combine????
you find the strangest things in the wierdest places....

 

the largest problem that causes the detonation in a larger bore engine is the quench between the piston and the part of the head that isn't combustion chamber. it tends to be the hot spot where the detonation flame begins. just working with other parts helps get rid of it.

 

ok not sure if I understand the quench part

but if there is a part of the head that is not inside the combustion chamber then I am taking it you have a sharp edge that will take give off a lot of heat?

if that is the case can't you kinda round it off or make it part of the combustion chamber without too much of a problem with a loss of cc?

not sure on this one though

 

If you have flat top pistons and 64 cc heads common on a 383
your cr would be
10.32:1 @.025" deck height ( typical)

10.94:1 @ 0.00" deck height

not 11.5

I'll go along with 10.3 or less

if your sportsman heads are in fact 68 to 72 cc's (very common)
{you cannot tell by looking at them}your cr will be even lower.
Cam timing will affect the cylinder pressure but not the heat of compression (Gas Laws). This only has a minor affect on the detonation tolerence on a said octane fuel and mosty at low rpm
below peak torque. {EGR effect}Large bore motors tend to be a little more detonation sesitive because of the large surface to volume ratio,
more sharp edges, more chance for a second spontainious
flame front to start.

 

if you are talking more surface area per cc of volume then I would think it would be easier

the rotary has one of the larges surface areas for it's volume and that least to my understand part of the reason it is not prone to detonate

but I am half asleep right now. so brain is farting on me

Kinda stinky **** right now