Linear Velocity

iTrader: (1)

I was thinking down the road after I have my financial situation settled in about doing a de-stroked solid roller motor with a big turbo on it, say a 103 mm. I was thinking of a stock C5R block and a 3.268 crank truck crank, this would net a 350 cid; I would be of course doing a custom intake (LSX or other) C5R heads and a big solid roller. I would really want to stay with a C5R block since I don’t want to deal with dropped sleeves etc, I would figure it’d be easier to get a lower compression out of the 350 because of the smaller cubes (than say a C5R block and stock crank= 387 cid). So the advantages it seems to me is that you’d be able to rev the motor higher (good for solid roller), easier to get a lower compression (C5R heads have really small combustion chambers) My only question would be: is a big turbo like that RPM dependant or not, because theoretically I would spin to around 8k. Thanks for your input guys.

Pro Stock John

iTrader: (34)

Sounds interesting but the C5R heads and the 103mm turbo make the combo basically a radical setup. That turbo (and housing) is huge, think like the size of a basketball or more.

Louis

iTrader: (7)

~95#s + for a thumper

ChrisB

No way would I run a stock crank with big turbo like that - plus it would be a shame if that stock crank broke and took out the $$$ C5-R block.

How much power are you looking to make - 100mm is capable of 1200+ rwhp pretty easy.

You might check out one of the Garret GT42's - the turbo is physically huge - has a 91.5mm wheel, but because of the shape and setup actually spools up pretty quick - should support more power than a t-88 and spool up a bit quicker, only downside is it is physically huge - but if you are ocnsidering a thumper that's not an issue.


#1 I would decide on the power level you are looking for and size the turbo appropriately.

#2 Figure out how many cc's you can get out of the heads safely.

#3 Talk to your piston company and find out what kind of compression height you need for your power level. This will also limit how much of a dish you can go with.

#4 - Keeping in mind compression ratio go with as much stroke as you can put in there - i probably wouldn't go any smaller than 5.7" on rods, and you may want to talk to your engine builder regarding pulling the piston down to far with really short rods/long stroke, but with the compression ratio constraint you probably won't get that radical.

Which basically fixes all your variables - you just need to determine your intended power level, which will set your piston specs and compression ratio, which will set your max stroke, which will set your rod length.

10.5 Dave

iTrader: (5)

It sound nice.I hope you have a very fat wallet to build this beast.I would budget 35-45k to get everything done.A factory crank would be silly considering the dollars involved.

Linear Velocity

iTrader: (1)

Well I'm aware of the cost I should rehprase and say use an aftermarket crank, just with the shorter stroke. Well it's still a work in progress, I'd like to see it in the mid to low 8's, after all is said and done, I'm sure there'd by a lot of fab work to do as well. Assuming I had all the moeny to do this would it be to my advantage to go with this setup or retain a stock crank for a 387 cid.

Excal

This sounds very well thought and solid...

Sounds like u were inspired by turbo Outlaw mustangs..sine they usually have a similar set up, with stickin with less ci a a 100mm+ turbo.

This needs to be done for the ls1 too..the 5.0 crew seem to think this is exclusive to them.

Whistler

iTrader: (46)

You wont have to spin a turbo car past 7K to put it in the low 8s. Keep a 3.42 gear with a 29 or 30 inch slick and the 7,000 RPM will take you to 165-170 MPH in the 1/4

buschman

I was under the impression that for FI engines bore was more important that stroke. Due to FI's(especially turbos) ability to make torque, a longer stroke would be wasted. Plus that additional rotational drag would hurt you on the top end, limiting high RPM horsepower. I'm no pro, but could you elaborate on why you would want to maximize stroke? I would think with a turbo that big you'd want a high revving engine to build monster horsepower.

Mike

Excal

yeah...the guy atop "the boosted list" has a destroked c5r as well. and gettin a convertible to mid 8s is no cake.

ChrisB

First off you would maximize bore with respect to maintaining strength (for your intended hp level) and also possibly leaving some room on the block for a rebuild or two, etc. - so the extra displacement from that is already going to be limiting the compression ratio, which will in turn be limiting stroke.

But that said, cubic inches are always good - a larger stroke will *definitley* not hurt with respect to "high rpm horsepower" - you will make more power, etc.

More cubic inches, be them bore or stroke, will always make more power. Worst case it will just spool up the turbo quicker.

You don't need high rpm with a turbo - the turbo will work based on exhaust massflow - you can flow the same amount at a lower rpm with a bigger motor, and under full boost the difference in exhaust flow will be negligible, as most of it will result from the air the turbo is moving.

Also a larger motor will be more efficient with a blower/turbo, as you will generate a lower pressure differential at the same CFM setting, so the compressor doesn't have to work as hard, heats up the air less, etc. Or you make a good bit more power at the same manifold pressure level.


Basically #1 is strength, then compression ratio limitations, then go with as many cubes as you can, then pick the rods to fit (within reason of course).

Pro Stock John

iTrader: (34)

A Low 8 second car will have some dollars into it, you could have $75-100K into it no problem.

Excal

so destroking the motor is pointless since one shoudl maximize stroke?

won't long stroke impend top end and tractor-torque at very low rpms...

and let's say he doesn't need to rev high, but it would surely aid him..

the famous GTR-700 (skyline) does 8s on street tires, and it revs to 12500rpm.

buschman

Someone once equated stroke and bore to a simple lever and it made sense to me. In very basic terms the bore is the amount of force you can apply on the lever. Stroke would relate to the distance of the fulcrum. A longer stroke would equate to moving the fulcrum further away from you. You gain leverage(ie torque) but now have a longer travel path to move a set mass the same distance(ie rotational drag). Am I just way off base here? Would love to hear further discussion from some of the experts on the advantage/disadvantage of stroke on an FI application.

Mike

JordonMusser

iTrader: (1)

there is NO reason to destroke it. cubich inches are your friend

427

iTrader: (3)

Mike, I even understood that!
We put stroke were it needs to be for many other reasons. The gen3 c5r head engine would need around -35cc dish to get workable CR. The power level needed to beat the fastest cars is over 1200 i think. In the past we have seen people put 4 inch stroke with 5.8 long rod to get CR desired. This puts rod stroke ratio at 1.45. Many would say this is fine, however it puts more stress on cyl walls and head gaskets to make extreme cyl pressure down low. We choose a workable rpm band based on available valvetrain parts which in the c5r gen3 is 8000 rpm. At 8000 rpm the engine is stable and has reasonable maintenance needs.
I think i'm rambling on here so I will say we usually run 3.5-3.625 stroke for drag type cars, 4.0 stroke in NA engines.
Kurt

Reckless

iTrader: (9)

Kurt...you sure do have a lot of good tech advice

ChrisB

Why would a longer stroke hurt "top end" performance? What constitutes "top end" performance? Are we picking an arbitrary rpm value, say 6500, and calling that top end? If so you just need to go to a larger camshaft - you will have a similar idle, similar if not better drivability, and *much* more power to boot. Or stick with the same camshaft - your car will be much more drivable, torque/power curve will be broader, and you will still make a bit more power (albeit at a lower rpm - but it's power that defines how fast you can accelerate over a 1/4, not rpm).

The skyline has to turn a ton of rpm because it is to spool up a big turbo with a tiny engine. If you can make the same power and turn only 7000rpm why not? Your powerband will be much better, car will probably be faster, and it will be more fun to drive. Plus it's probably easier on parts to turn 7000 rpm than 12,500.

The analogy is fine - the counterpoint is - what is the problem with more frictional losses - so we loose 25hp to friction vs. 23hp? The extra cubic inches are going to make tremendously more power than is lost due to increased frictional losses.

And the same argument could be used to argue that you should go from a 347 to a 302, and then from a 302 to a 268, and then from a 268 to a 100", etc. - since there are less frictional losses, etc. Which is definitley absurd.

There are limiting constraints, but increased frictional losses, etc. are definitley not one of them, as again, the extra inches make much much much more power than are lost to any increased drag.


As 427 above pointed out there are other reasons to limit the stroke (piston comp height, compression ratio, etc.), and I think I listed those as a primary issue above - but once you have defined those I would go with as much stroke/bore as you can fit in there. R/S ratio can be an issue, and 1.45 is getting on the small side, but I wouldn't neccecarily rule it out out of hand. That's something you need to talk to your engine builder about, and also probably depends on power levels, piston clearances, skirt length, etc. Honda's run ~1.4 r/s ratios from the factory and pretty high rpm's, so it is not just totally off the wall to consider it.


What it boils down to is cubic inches are always good. Sure, with FI you can up the boost, etc - your power level will be more related to your blower cfm than anythign else - but with more cubic inches you will make the same power at a lower boost, compressor will be more effecient. It's just a matter of balancing strength vs. cubic inches, which is mainly dependent on your available compression height for the piston.

Linear Velocity

iTrader: (1)

Thanks for all the input guys, I guess it really would make sense to stay with a 387 cubes if I were to do this application. However, I have a question of how the destroked C5R 350 cid would stack up against a regular 348 like Harlan or PSJ is running. So that'd be a 4.125 x 3.268 stroke engine vs. a 3.905 x 3.622 engine. Now this would change the argument of less vs more cubes in an FI application to how we get the cubes in an FI application.

Pro Stock John

iTrader: (34)

I'd be interested in that too, my second choice as a power plant was to use this 3.27 (4.8) crankshaft I got and put it into a 6.0 truck block. I think the bore would be advantageous but real world results would be useful.