Some have asked me why I am going 408ci and not 427ci. It was explained to me once, but I cant explain it back to the individual asking? Something about oversquare motor? They also asked about how high a 408ci to 427ci Iron can be rev'd..? I have no clue.

 

Vince, you would need a 4.125 crank in an iron block to produce 427cubic inches. Several people have had bad luck running with that much stroke in these engines. Over stroking the motor also limits your RPMS as well. Its not a good idea. Im sure someone will chime in with a more scientific answer, but this gives you the jist of it.

 

Ron, I remember someone else on this board complaining about their old 427ci Iron block, but they complained more about the actual crank they purchased than giving a good detailed explaination on why this is not a good setup. Most talk about a 414ci Iron Block as well. In another post I read that the 427ci should make 15 to 20rwhp more than a 408ci and about the same amount of tq increase. MTI sells a 427ci Iron Block so I am sure it must be possible w/out issues, but there must be a ACTUAL downside to doing 427ci Iron.

 

The MTI427Roadster guy is the one who had problems. He had a bad crank and huge oil consumption issues.It was an MTI iron with a 4.125 crank. SPeak to Allan about it, hes not a big fan AT ALL of overstroking an engine. I think he can make the power you need out of the 408.

 

Allan explained it to me a while ago, but my work laptop crashed and I could not recover my .pst file. If MTI has a issue with this motor they might want to give away a 408ci instead a 427ci Iron.. Now to the revving of these motors.. How high would say one of these motors go with a 232/236 lsa 114 cam or a 242/256 lsa 115?

 

With a light rotating assembly, critical piston speed occurs at about 7400 to 7500rpm. I would definitely be using top name parts to go there. I know I am.

 

Now the cams I mentioned are hypothetical. I am in full guess mode. My car is going to be a FAST street car with some giggle for the other FAST cars. Idle quality is not as important to me, but I do not want to idle at 1100rpm. I want the idle to be like the 232/236 lsa 114 I had in my 346ci. I had the idle set at 850rpm and the car never stalled. I have a M6 with 3.73's and the car just rode out. I broke the motor and I am upgrading to a bigger motor. I have not talked to Futral yet about cams, but I am sure he has something in mind..

 

I would go with a bigger cam then these 232/236 lsa 114 cam or a 242/256 lsa 115 to get the most out of a 408.
From what I was told it would be better to stay around 408-410ci's I think its for clearence?

 

The only limitation with my 4.075 stroke is the hydralic cam. Lunati claims 9k rpms is possible, but not done because of valve float. Why rev a motor way past its power peak. I don't have oil consumtion or any other issues going over 4". I would stick with the 4" crank just because they are easy to come by, will be cheaper to balance, and have a better rod ratio. My idle is at 950 on an auto with 3800 stall and 242 .595 112lsa cam.

 

Vince, Alan is gonna set you up RIGHT. Let him worry about all this stuff cause he knows. Good luck brother...

josh

 

Oversquare means that your stroke is longer than your bore. A 4.125" crank would be needed to achieve 427 at the max bore of 4.060" that can be achieved on an iron block. Your 408 is at the recommended 4.030" with the recommended 4" crank.

 

414 CID = 4 inch stroke 4.060 bore, cylinder walls are kinda thin, I'd rather keep the extra meat there for strenth and let that motor have 250 shot! shoot if I were to start over, I would do a sqaure'd iron block 4 inch bore and stroke 402 CID and run 30 pounds!!!

 

Vince, I know of a 396 STOCK BORE LS1 that is using a a 4.125 crank and is producing around 480 RWHP. In THEORY one would think that this engine might not rev all that high because of the stroke but it routinely revs to 7200-7300 rpm. This is considered an oversquare motor because the STROKE at 4.125 is larger than the bore @ 3.905. This may be a case when THEORY and reality collide because this motor is VERY strong and has I beleive torque in the 450-460 range if I remember correctly. Longer stroke motors put more side stress on the cylinder walls and lower end bearings because of the angle of the rods is more severe than in shorter stroke motors. So the thought is BECAUSE of this the motor won't have the same REV capabilities in terms of higher RPMS but in fact if the motor gets it's peak power PRIOR to the theoritical REV limit (whatever that might be) then in reality it makes no difference. For instance the PEAK power on the 396 I mentioned is around 6700-6800. Since EVEN with the 4.125 stroke the motor is fully capable of reving past that then it would appear that in this case being over square is not all that bad. LONG trem one would think that you would have more bearing wear than if the motor had a smaller stroke but that as of yet has not been an issue with this motor.

 

Allan is going to build the setup of course. This thread is more for other people. I have friends that are interested in going the same route, but I cant explain to them why I am going this route other than the strength of the iron block. Now you know I am not shy about putting a big cam in the car, but I like the lope at 850-950rpm. That tells my to be victims "Danger Will Rogers, Danger!" The only thing I mentioned to Allan is I would like it to still rev and idle like my old 232/236 lsa 114 in my old 348ci, but knowing Allan this will be 90 percent of what he wants and 10 percent of what I want. In a nutshell, its going to be one FAST AZZ street machine.

 

Vince,

There is NOTHING wrong with running a 4.125 stroke and 4.035 bore in a street engine especially if you keep the revs below 7500rpm. There are race motors running 8500rpm without problems with a similar bore/stroke ratio.

I would be more concerned about running a hyd cam and also the oiling system above 7500rpm than the crank itself.

I believe there were a couple of reluctor wheel issues with certain 4.125 cranks. This may have been resolved or might still be an issue. AFAIK the lunati 4.125 cranks are fine.

Cheers,
Chris

ps- I would keep the bore around 4.035 even with sonic testing of the block. You will get several serviceable overbores/hones with the block at .035 over.

 

Thanks guys.. I have already been told 408ci is what I am getting..

 

A decently built 427 can rev higher than the limitation that's placed on you by the LS6 intake manifold anyway. If you want to routinely rev above 7K then you'll need a different intake manifold with shorter runners (custom made?). So short of that there's no reason not to go 427 unless it's cost prohibitive. You'll have better torque and more overall power.

PS. in regard to the side-loads mentioned above, this can be mitigated with longer rods which can be accomplished with pistons that move the pin location higher up.

 

So whats the OVERSQUARE thing and why do most engine builders do not like OVERSQUARE setups?

 

It's UNDERsquare that builders don't like. UNDERsquare motors have more stroke than bore. One good reason they don't like them, valve shrouding.

 

Thanks for correcting me. Why would valve shrouding be a problem with properly prepared heads?