7m revability discussion

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figgie

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Mar 30, 2005
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Ok folks

looking at the 1jz section and the sf forums. I find it intresting that the general consensus of the 7m is that it can't rev unlike a 1jz.

The question (and hence why I put it in the tech side) is can the 7m not rev like previously thought or is this a preconcieved notion that it can't and it was believed to be truth (i never thought it truth).

discuss. :)

sidenote: IJ you best better put in your tech prowess in here.
 
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IJ.

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Wayne: Guess it all comes down to piston speed.

I think where the misconception with the 7M started is it's a long stroke motor so conventional thinking is it can't rev but there are quite a few motors with longer stroke that happily rev higher than our stock redline.

We have a very small bore for a 3 Litre motor and I believe part of why the J series went larger was to allow more breathing room for valves as you can only cram 1mm os valve in our heads due to the spacing being so close.

If you look at our Con Rods and bolts they're quite beefy say compared to a 350 SBC which has close to the same stroke (Common failure point at high RPM as they try to chuck the piston off at TDC or try to break the bolts off at 90 and 270 degrees) Our pistons are much smaller/lighter.

I've seen a BMW Motor quoted a few times in these discussions that has the same stroke and revs to 8k so it's not too much of a concern :)!

As always this all depends on good prep and no weak links. (I wouldn't be trying this on a 200.000 mile 7M)
 
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figgie

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IJ. said:
Wayne: Guess it all comes down to piston speed.

I think where the misconception with the 7M started is it's a long stroke motor so conventional thinking is it can't rev but there are quite a few motors with longer stroke that happily rev higher than our stock redline.

We have a very small bore for a 3 Litre motor and I believe part of why the J series went larger was to allow more breathing room for valves as you can only cram 1mm os valve in our heads due to the spacing being so close.

If you look at our Con Rods and bolts they're quite beefy say compared to a 350 SBC which has close to the same stroke (Common failure point at high RPM as they try to chuck the piston off at TDC or try to break the bolts off at 90 and 270 degrees) Our pistons are much smaller/lighter.

I've seen a BMW Motor quoted a few times in these discussions that has the same stroke and revs to 8k so it's not too much of a concern :)!

As always this all depends on good prep and no weak links. (I wouldn't be trying this on a 200.000 mile 7M)
Click to expand...

ahh you are awake late ;)

yes.. usually the person naming the BMW motor is ME ;)

the S54 engine more specifically. But i was looking over at the new LS7 motor from Chevy. 101mm storke with a 7k rpm redline. 104mm bore. 7 liters of displacement. 13k USD :)
 
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IJ.

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LOL it's early here I'm just surfing around a bit before I go out to work on the Supra :)! (I knew it was someone that knew what they were talking about that mentioned the BMW)

I think maybe due to the built in rev limiter and the shim over bucket design not too many people have explored the upper limits with a 7M and it all becomes an "Urban Myth" that a 7M can't rev.

I remember a guy back in the old days that had a 302 Chev (327 bore 283 crank) that was spinning it to 10K way back then when the common limit was around the 7k mark so built right I think it can be done easily!
 
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figgie

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IJ. said:
LOL it's early here I'm just surfing around a bit before I go out to work on the Supra :)! (I knew it was someone that knew what they were talking about that mentioned the BMW)

I think maybe due to the built in rev limiter and the shim over bucket design not too many people have explored the upper limits with a 7M and it all becomes an "Urban Myth" that a 7M can't rev.

I remember a guy back in the old days that had a 302 Chev (327 bore 283 crank) that was spinning it to 10K way back then when the common limit was around the 7k mark so built right I think it can be done easily!
Click to expand...

and this is what i am talking about :)

everytime i see a thread about revs.. it ALWAYS goes..

I want to rev my 7m

answer: DESTROKE to 5m crank or go 1jz!

Figgie: why destroke??? No need to destroke just pay attention to the head.

answer: STFU you know nothing!! etc hehe
 
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IJ.

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figgie said:
and this is what i am talking about :)

answer: STFU you know nothing!! etc hehe
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ROFLMAO this cracked me up cos I've followed a few of these threads on SF!

I can't remember seeing a thread where a 7M has let go from being over revved!

If it floats a valve it stops making power until the revs drop enough to regain valve control (Non interference design)

Most blowups I've seen have been HG related or the more common Rod Knock then it kicks a leg out!

On a related side note I've been following the 200 mph Mk3 thread on SF have noticed the 7M can't rev myth popping up there as well! (No one has addressed the issue of the front structure failing at those speeds as all the plastics deform from air pressure)
 
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chevyeater

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Little calmer chat over here. Doing the work to make a 7M rev well is more than most mk3 folks want or can afford to do. That is pretty much what it boils down to for why a 7M can't rev.
 
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socc924

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hey figgie, i just read the thread on supraforums and had some thoughts... I completely agree with you on the fact that the 7m can rev higher, and i would bet there are some poeple that do. But looking at 7mpower, not one person has a dyno showing the car reving past around 6500rpms. My question is, is there a reason? It doesnt seem that the hp is dropping off much but the torque is dropping. Did they just stop at 6500 because the max power was already reached? Im not a profesional tuner or anything but I would think that best results might come by reving it a little further (obviously this is different for each car).

It just seems the people capable of reving higher (ie forged internals and valve work) dont really need or want to rev higher. Plus they might not want to risk their expensive set ups to be the one to test the waters. And since its expensive to go out of your way to raise the rev limiter, people that arent that deep in mods would rather spend the $ elsewhere. So that leads to misinformation and assumptions and eventually its "common knowledge" that the 7m cant rev.
It seems to me that the whole point of your posts on this subject wasnt to try to prove that the 7m is an ideal setup, or even a good one, to rev high. Its just that those people who car so much about having a "rev happy" motor so much that they go to a 1jz or a different crank could simply stick with the 7m, and achieve high reving, high torque applications.

-am I way off on this?-

by the way, my own personal experience that is probably useless info, It was a few months ago that I finally got a correctly working turbo tachometer in my supra. I found out that what I thought was 6500 was really around 5500, so I had never realy reved it to the redline. After like 2 weeks of taking a few trips to the real redline I did develope a rod knock. Im in no way saying reving it devoloped the knock, because the kid I bought the motor from turned out to be a little sketchy and the miles and "factory rebuild" done on the motor were all questionable. just thought it was wierd.
 
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figgie

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chevyeater said:
Little calmer chat over here. Doing the work to make a 7M rev well is more than most mk3 folks want or can afford to do. That is pretty much what it boils down to for why a 7M can't rev.
Click to expand...

that is correct but if IJ's information about the SBC rods vs the 7m rods is correct. I see no reason for the 7m not to be spun on stock rods.

Though osofast7m picture looks tasty for sure :) Bent rod. Never remember seeing that.

as for the stock limiter.. anyone try the recrystalization to up the rev limit?
 
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chevyeater

wastegate hose is pulled
Mar 30, 2005
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figgie said:
that is correct but if IJ's information about the SBC rods vs the 7m rods is correct. I see no reason for the 7m not to be spun on stock rods.

Though osofast7m picture looks tasty for sure :) Bent rod. Never remember seeing that.

as for the stock limiter.. anyone try the recrystalization to up the rev limit?
Click to expand...

The 7M has a longer stroke and a longer rod than the 350 SBC, never mind the 302. The LS7 is using Titanium connecting rods and a dry sump oil system among other tricks. Not sure what BMW is doing but I'm sure it isn't just a run of the mill bottom end, like the 7M has stock. Building the bottom end is a must and most folks get light headed looking at prices for real connecting rods for a 7M. Hell, I'm about to buy a pretty decent '89 Turbo for less than the price of the rods in my '90...

7k is about as hard as I'd press a stock type bottom end, in perfect condition and given I could choose any rev limit for it.
 
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