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ideola
Joined: 01 Oct 2004
Posts: 15548
Location: Spring Lake MI
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 Posted: Wed Aug 19, 2015 5:05 am Post subject: |
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Bootom line: you can't just bolt a 931 head on NA short block and get anything resembling usable CR. You have to purpose build the bottom end to match the top end which realistically means pulling the whole motor or having a spare short block on the bench. NA head on 931 short block is a different story but then you're stuck with lousy NA head.
There is, as the saying goes, no free lunch.
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erstwhile owner of just about every 924 variant ever made |
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friggin
Joined: 30 Jul 2015
Posts: 39
Location: Montana
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| Posted: Wed Aug 19, 2015 5:10 am Post subject: |
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I'm going today to figure out what is exactly wrong. My timing belt is fine as I don't have the cover on and can see it in plain view. If the motor is shot I'll be tearing it all apart and replacing everything there is to replace anyway. Id make my own intake manifold and use bike carbs for my ITB set up as I have some that I can get very cheap. So the intake bolts wouldn't be a problem and I can rework my header to fit the 931 head. I want to keep the CR around 9:1 so I can still do the supercharger. I don't know what pistons would be required but if I can some how make 200 rwhp with the 931 head, ITBs, and a supercharger I'd be very happy.
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1979 924 stocker soon to be autocross beast |
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Fasteddie313
Joined: 29 Sep 2013
Posts: 2596
Location: MI
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| Posted: Wed Aug 19, 2015 5:29 am Post subject: |
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| friggin wrote: | I'm going today to figure out what is exactly wrong. My timing belt is fine as I don't have the cover on and can see it in plain view.
It could slip just a couple teeth and look fine, check the marks..
I don't know what pistons would be required but if I can some how make 200 rwhp with the 931 head, ITBs, and a supercharger I'd be very happy.
Your thinking about custom building pretty much an entire engine and intake for only 200HP? Why not aim for at least 250 up to 350 maybe? Why not?
I just think all that effort you may as well do it right the first time, it's not like it would be too much more expencive to go big or go home..
What are you planning on as far as fueling is concerned? If you can ditch the NA CIS for almost anything else you're clear for takeoff..
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80 Turbo - Slightly Modified |
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friggin
Joined: 30 Jul 2015
Posts: 39
Location: Montana
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| Posted: Wed Aug 19, 2015 7:53 am Post subject: |
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i dont want to push my transmission to the breaking point and for autocross more power is not always better. Ive read that the pre 80 rearends can only hold around 250hp and i dont really want to replace that.
I only need around 240 hp to have a 10:1 power to weight ratio. So id be happy if i was getting around the 12:1 or even 11:1 ratio. Id love to have 300+ hp but ive heard that is close to impossible with the 2.0 in the 924s. and then id need to replace the transaxle as well.
If you know that the pre80 5speeds will hold more then 250 hp id look at going for more power.
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1979 924 stocker soon to be autocross beast |
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Fasteddie313
Joined: 29 Sep 2013
Posts: 2596
Location: MI
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| Posted: Wed Aug 19, 2015 9:40 am Post subject: |
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http://www.924board.org/viewtopic.php?t=38765&postdays=0&postorder=asc&start=0 = 400 crank HP on stock 924 NA internals, no boom..
Who says impossible?
I wouldn't let that tranny hold you back if I was you with plans like that..
I like how koenigsegg dude says it, something like "we don't build to a goal, we build to see how far we can go"..
You guys see there new one:1 car? 1300HP:1300KG
I like those guys..
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80 Turbo - Slightly Modified |
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friggin
Joined: 30 Jul 2015
Posts: 39
Location: Montana
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| Posted: Wed Aug 19, 2015 10:05 am Post subject: |
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New transmission is more money... I'm trying to do it on a $5000 budget... When I get more money I'd build it more. But until then 200 rwhp would be plenty. It will be a huge step up from the 60 that I was making before it died.
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1979 924 stocker soon to be autocross beast |
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fiat22turbo
Joined: 18 Jan 2006
Posts: 4040
Location: Portland, OR
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| Posted: Thu Aug 20, 2015 4:33 am Post subject: |
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The NA dogleg box is pretty strong. The weak spots are the syncros and the input shaft/TT/clutch.
The clutch tends to slip somewhere above 200hp and the TT/Input shaft tend to last to 300hp or so (depending on their overall condition).
I don't think you would need ITBs to reach 200hp with a supercharger. They will make plumbing more difficult for the intercooler and having a decent sized plenum with a single throttle body tends to work better at smaller throttle openings.
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Stefan
1979 924 Carrera GTS (clone-ish)
1988 944 Turbo S (Silver Rose) |
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friggin
Joined: 30 Jul 2015
Posts: 39
Location: Montana
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| Posted: Thu Aug 20, 2015 6:33 am Post subject: |
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How difficult is it to replace the syncros and clutch?
The way my supercharger will be set up it will be a remote mounted roots style off of a Toyota van. I can't remember the model the supercharger is but they are super cheap.
I'm not worried about plumbing the intercooler as it won't be that difficult to do with the set up I'll be running.
Do you think it's possible to make 200+ hp with a factory rebuild and a supercharger? Or would it take a bigger cam and other upgrades to reach that amount of power?
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1979 924 stocker soon to be autocross beast |
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Fasteddie313
Joined: 29 Sep 2013
Posts: 2596
Location: MI
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| Posted: Thu Aug 20, 2015 7:03 am Post subject: |
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| friggin wrote: | How difficult is it to replace the syncros and clutch?
Pretty hard...
But if you're smart enough to understand and follow directions, have decent search skills, and are patient enough to do such a job without getting all frustrated, you'll be fine..
But synchros have nothing to do with power, there like little brakes in your tranny to match gear speeds when you shift, snail/dogleg trannys hav a habit of wearing out there synchros..
If your gears don't grind (yet) then you don't need synchros (yet)
You are going to pull the engine anyway right? Clutch will be right there..
The way my supercharger will be set up it will be a remote mounted roots style off of a Toyota van. I can't remember the model the supercharger is but they are super cheap.
Prolly too small, I'd say you want a charger good to 300HP on a 2.0 however you calculate that, so even if you only raise your boost to 250HP or so your not running your charger on the ragged edge..
Granted I don't really know a lot about superchargers..
I'm not worried about plumbing the intercooler as it won't be that difficult to do with the set up I'll be running.
Yeah, don't be scared of some fab..
You can move the bottom of the RAD back a few inches and fit a FMIC like I did in my "intercooler or bust" thread, or run your pipes down through the hood lock plate area, whatever.. Just do it 
If you go with low pro rad fans you'll have more room than you know what to do with I'm sure..
Do you think it's possible to make 200+ hp with a factory rebuild and a supercharger?
Piece of cake, but you need to ditch your OEM CIS prolly..
Thats going to be your big investment is EFI or upgraded CIS if you're crazy like me.. But you need good AFR control for boost, once you have good AFR control, like I said, clear for takeoff..
Or would it take a bigger cam and other upgrades to reach that amount of power?
Don't need no stinkin cam, you have the same cam as the 931 turbo and its plenty..
Want more power? Add more boost..
With good AFR control from EFI you're good for a long way.. Long way..
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friggin
Joined: 30 Jul 2015
Posts: 39
Location: Montana
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| Posted: Thu Aug 20, 2015 10:37 am Post subject: |
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Thank you so much that helps me a lot! I just wasn't sure if the stock internals would hold that much power or not. Looks like I'll be doing that next year when I get home from college!
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1979 924 stocker soon to be autocross beast |
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fiat22turbo
Joined: 18 Jan 2006
Posts: 4040
Location: Portland, OR
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| Posted: Thu Aug 20, 2015 11:07 am Post subject: |
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The problem with the clutch is that there aren't any available with more holding capability.
The splines, diameter and configuration are completely different from the 931.
It would have to be custom built. You'll also want to replace the plastic ends on the clutch cable with metal ones as the stronger pressure plate could cause the OE ends to fail prematurely.
The gearbox is similar to the one used in the 914, so you can get an idea of how hard it is to refresh by looking at some of the guides available for the 914 transaxle.
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Stefan
1979 924 Carrera GTS (clone-ish)
1988 944 Turbo S (Silver Rose) |
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friggin
Joined: 30 Jul 2015
Posts: 39
Location: Montana
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| Posted: Thu Sep 10, 2015 10:42 am Post subject: |
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has anyone done ITBs with bike carbs?
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1979 924 stocker soon to be autocross beast |
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