7m revability discussion

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D Walker

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Oct 19, 2005
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figgie the other things to consider are rod length, rod ratio, and bore. Porsche did extensive testing with long rod/short stroke, short stroke/short rod, long stroke/short rod, and long rod/long stroke. The short rod motors all died quickly, but made good power. The long rod short stroke motros made good power and revved extremely high. the Long rod/long stroke motors made good torque but died horribly when revved hard. I am sure there is a million page technical journal on this in a German archive, but you get the gist of it. I believe the 7Ms rod ratio to just not be optimum for revving high, not saying it couldnt, but that its just not gonna live long nor is it going to make power up there- that whole string-chopper thing comes into play here.
I do not believe revving a 7M to the redline will kill it immediately, but it definitely shortend the life dramatically. The thing to ask yourself here is- why do we need to? We have boost to make power.RPM kills motors, period. The 935 Porsche for example, made 700whp@6500 rpm from 3.2 litres back in 1978-81, with mechanical fuel injection. The 911 RSR made 325whp@8000 rpm from 3.2 litres in the same years. The difference is you got to rebuild lot more RSR motors than you did 935 motors, assuming the drivers could keep thier hands off the dont-pass-me-knob.
 
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IJ.

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Mar 30, 2005
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I only used the 350 SBC as an example of mechanical strength ie: It hangs a big heavy piston from a rod that's close to ours in size with not much less stroke and does fine.

I'm NOT saying take a stock bottom end and spin it to 8000, if anyone's been following my build thread they'll know the approach I'm taking by following ChevyEater/Brian's lead with a LOT of headwork and a built bottom end to suit.

While Rod ratio plays a bit part in all this I think/feel Cost plays a big part in why 7M's don't get spun hard too often as it's not a cheap exercise building one of these motors with this in mind.

(much prefer the discussion here much more civil)
 
Nick M

Nick M

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Sep 9, 2005
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So one of those hillbilly performance magazines decided to test the bore/stroke debate. Two engines, everything the same but bore and stroke. They had to make some consessions do to the obvious things like rod ratio. I think it was popular hotrodding. Anyway, in a nut shell, there was no notable difference. You need to change cam timing, manifolds, etc. But both motors would gain doing that. It was a strong engine too. They were suprised at the results. Short stroke is for low piston speed in a high rpm engine. It will last a little longer. But taking it 7k plus will shorten an engines life anyway. So why bother.

Horsepower is torque x rpm, so hp goes up if you increase either one. Increase the torque. More fun on the street. Figgie, you know these things.
 
F

figgie

Supramania Contributor
Mar 30, 2005
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Nick M said:
So one of those hillbilly performance magazines decided to test the bore/stroke debate. Two engines, everything the same but bore and stroke. They had to make some consessions do to the obvious things like rod ratio. I think it was popular hotrodding. Anyway, in a nut shell, there was no notable difference. You need to change cam timing, manifolds, etc. But both motors would gain doing that. It was a strong engine too. They were suprised at the results. Short stroke is for low piston speed in a high rpm engine. It will last a little longer. But taking it 7k plus will shorten an engines life anyway. So why bother.

Horsepower is torque x rpm, so hp goes up if you increase either one. Increase the torque. More fun on the street. Figgie, you know these things.
Click to expand...

yep :)

the reason for these posts is that like a stated before.. I see. I want to rev engine and the discussion goes down as previously mentioned :)
 
Nick M

Nick M

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I almost forgot. The bore/stroke power article was about peak power. The bigger bore unshrouds the valves debate. The big bore shorter stroke made no discernible power differnce to the smaller bore longer stroke.
 
I

IJ.

Grumpy Old Man
Mar 30, 2005
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Nick: As long as the Combustion Chamber isn't shrouding the valves the Bore shouldn't affect it, on a 7M when you go OS Valves the CC does Shroud the intake so it needs to be laid back some.

On our motors there isn't enough spacing to go larger than 1mm os on all 4 valves so it will work fine on our bore size but if the Valve centrelines were further apart allowing 2>3mm bigger valve we'd be running into the cylinder walls.

I remember way back seeing a certain V8 that had valve notches in the tops of the cylinders so they wouldn't hit so agree cylinder shouding mustn't have much affect.
 
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figgie

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suprabad said:
I don't mean to be critical, but why bother? :nono: Get a jz and be done with it alot cheaper...duuuh!
Click to expand...



no this is the point. With all the long stroke motors reving the shit out of thier engine. This is a well... to put it nicely a fucking stupid and worthless argument.
 
F

figgie

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ok thought i throw this into the mix

well it seems that Ryan Woon is running a Titan 3.4l stroker kit.

94mm stroke by 87mm bore

revs it to 9500 RPM.. Yes you read that right. Nine thousand five hundred RPM with 3 mm more of stroke.... time to polish up the 7m and see what it actually can do.
 
group a supra

group a supra

SPAWNING AN INTERCEPTOR
May 5, 2005
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learning to weld :) in the united kingdom
figgie said:
ok thought i throw this into the mix

well it seems that Ryan Woon is running a Titan 3.4l stroker kit.

94mm stroke by 87mm bore

revs it to 9500 RPM.. Yes you read that right. Nine thousand five hundred RPM with 3 mm more of stroke.... time to polish up the 7m and see what it actually can do.
Click to expand...
figgie that is what i am doing but take a little while yet as i dont hav a team just me:3d_frown:
 
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