924Board.org

[murphys]

"New" 1980 931 came with a 951 intercooler and RX-7 in boxes of "extra" parts. What are the pro' and con's of each? Thank you.

[ideola]

I've never actually laid hands on an RX-7 intercooler, but there's been some talk that one could potentially convert it into a top-mount setup like the Carerra GT. IMO, this is less than ideal on a street driven car because sitting at a traffic light, it'll just act like a chimney and actually add heat to the charge. If you want a top-mount, I think the better thing to do with that core would be to get it out of the engine compartment and locate it in the battery tray like Peter did.

The 951 intercooler will fit in the nose, but not before you have to grind out the welds, remove the header panel, lower the fuel dizzy, and do some other surgery to the front end. Additionaly, I personally don't think it's the greatest of intercoolers: the charge air flows the long way thru the core, which can only cause pressure loss, and the end tank designs added even more to the restriction. The core isn't bad, as it can purportedly handle ~500 CFM, but the end tank design limits it to the below 200 CFM. You can have the end tanks modified by Lindsey Racing, but their Stage II still only flows in the low 200s CFM, which isn't that great. It might be OK for a moderately boosted application, and might be better than nothing, but it's certainly not ideal. For example, if you wanted to up your boost even modestly to something like 10 or 12 psi, your intercooler should ideally be able to flow between 325-350 CFM. Consequently, IMO, the 944 FMIC isn't worth the effort required to install. It's effectively only good for lowering charge air temps with stock boost levels.

If it were me, I'd look at the batter tray option first, but you need to find out what the RX-7 can handle in terms of flow, and think about what you want to do with the car in terms of boost.
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erstwhile owner of just about every 924 variant ever made

[-nick]

There are 951 folks pushing over 400rwhp with the stock intercooler. I think it will be fine for your 931 Adding any IC is a pain on the 931 though...
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1980 931S
15psi boost, MS-II, EDIS, 951 IC, custom intake, Ford 5.0L throttle body, Forge BOV, WB o2, G31 w/LSD, 964 wheels, 968 rear sway, Bilsteins, 200# Welt. springs. A laptop, and a partridge in a pear tree.
1991 964 C4 Cabriolet

[ideola]

Nick, I don't doubt you, I'm just curious though, how are they getting enough flow? Or are the numbers on Lindsey's site incorrect?
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erstwhile owner of just about every 924 variant ever made

[bass gt]

Dan,

i have the 951 IC nose mounted in my car, and am seeing pretty good results. IC inlet temps of 100C being reduced to around 35C at the plenum. Now i have modded the end tanks, and fited the ducted extractor, but i think it is basically a good unit. I think there is a real danger of over sizing the IC for our cars, and creating all the problems that go with it. In my opinion, a 951 IC and an external IC water spray would be the perfect solution. For next season, i shall be incorporating a water mist system onto the face of the IC. Mine will be triggered by a preset MAP value, and stay on for the duration of the race, but a short shot street system would prove very effective i believe.
also, if you read the Lindsey report, they got the IC flow up to 280CFM, which they state flows more than any 951 modded head they are aware of. If a 951 head flows less than 280, then the 931 can't be over that, so surely the 951 IC is sufficient, if modified?
The Australian website AutoSpeed did a very interesting feature on these devices. http://autospeed.drive.com.au/cms/article.html?&A=0578

Regards,

Steve
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Front Wheel Drive is the Devil's work.

[ideola]

Excellent information Steve, that puts my 95i IC back on the table and will save some costs which makes me smile.

The thing I'm grappling with now are the formulas in Maximum Boost. Using the figures in Maximum Boost, keep coming up with a required airflow for the turbo system between 500-600 CFM. Here's how I arrived at those numbers:

(cid x rpm x 0.5 x VE) / 1728

So on my planned build that would be (133 x 6800 x 0.5 x .85) / 1728 = 222 CFM

Now add the requirement with boost:

CFM x Pressure Ratio

Pressure Ratio = (boost + atmo) / atmo : (21 + 14.7) / 14.7 = 2.42

222 x 2.42 = 539 CFM

Is this only the requirement for the turbo itself, and not the rest of the downstream system? If so, the book doesn't make that very clear...just noodling over all this theory and trying to figure out how it applies to reality

Thanks!
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erstwhile owner of just about every 924 variant ever made

[bass gt]

Deleted
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Front Wheel Drive is the Devil's work.

[bass gt]

Dan,

I think we need to take a step back for a moment. You are still using boost PSI as adesired figure, not HP. I think this is making things a bit skewed. Boost is nothing more than a reflection of resistance. The ultimate aim is to make a given HP figure with the minimum of boost. As incresed boost adds increased loads and temps, the ultimate boost levels area bit of a red herring.
Work on the assumption that you need approximately 150 CFM for each 100BHP. So if your target is 300 BHP, then you need a flow of 450 CFM.
I have run my engine up through various boost levels from 12 psi to 18/19 psi. At the upper reaches, the BHP per PSI figure very quickly tailed off, as more and more heat was pumped into the system. Added to that, the turbo started to work well outside it's efficiency ccurve, so the problem became worse.
Big figures are possible on these engines, but be realistic as to the compromises you must make elsewhere.
As i said before, Joakim's engine was near the 400BHP mark, but the size of turbo needed to produce the required CFM was huge. Consequently, the low end was pretty awful. If thats OK for what you want the car to be, cool. Also, yes, for these kinds of numbers, i don't think the 951 IC is really the right choice.
Now if you are aiming for 270-300 BHP, these are figures that can be easily achieved using the smaller tubo and IC's, and will greatly improve the low end response, and subsequently, the driveability of these cars.
Look at the GTR, and the size of IC it used to control 450HP air temps. Massive. I also believe that there is a realistic HP limit of the 924/931 chassis. It's great having mega HP, but not if the chassis, suspension and brakes can't handle it. I believe that 300 BHP is the ideal HP target for these cars, in terms of outright pace and performance, allied to realistic engineering.
So, try not to think in outright terms of desired boost levels, but more in terms of desired BHP, and calculate the required components/figures accordingly. £00BHP at 21 PSI just means you have a very poor set up
With the motor you are planning, i would expect to see 300 BHP with as little as 12-13 psi of boost. All nice and easy for the motor to live with.
Also, don't forget the fuelling requirements at 21 PSI of boost!! Huge amounts of fuel required, so accordingly, you will need a decent system. (Ooops, i've strayed intot eh EFI discussion )
So, go back through your figures, but use boost as the required output, based on known figures. With that, you can properly spec the turbo, the IC, the fuel system et al.
Also, take a realistic look at your rpm value. I think you will find 6800 to be a pointless limit. Most turbo motors are done and dusted by 6K, unless you spec a turbo that has bugger all airflow below 4K.
With a stroked motor, you are building in added torque. Better to emphasise that by having a turbo that gets going at 2k, is puffing well at 3K, is steaming at 5K, and is starting to pump hot air by 6.2K
A stroked motor is not a rev hungry motor, rather it is a torque multiplier. Match your components tothose priciples and you will have one hell of a package.
http://www.turbobygarrett.com/turbobygarrett/tech_center/turbo_tech103.html
Try looking through this. It is very helpful.

Regards,

Steve
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Front Wheel Drive is the Devil's work.

[ideola]

Steve
Excellent as always...and yes, my initial goal was (and remains) 300-350 BHP. If I can hit those numbers, I'll be really really happy. Especially if I can do it at less than 1.2 bar of boost.

Even so, at 350 HP, you're still talking about 525 CFM...so it seems that the 951 IC may be marginal to hit the upper end of my BHP goals...

Regarding the turbo, I have acquired a K27 cold side (it's in transit as we speak), which I hope to match with the stock hot side. Based on what I've been able to glean about the K27, it should be more than adequate for my needs, and should just fit into the space available...
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erstwhile owner of just about every 924 variant ever made

[bass gt]

Dan,

By my calcs, you need 36lb/min, or 485 CFM for 350BHP, based on an AFR value of 11.5 and a top rpm of 6500.
As i said before, this is all working on what the turbo can produce. Lindsey Racing state on the V5 Intercooler page that the best heads they know, cannot pass in excess of 280CFM, yet these 951 engines produce major figures.
If you are going to be running on wide open throttle 90% of the time, i agree, but if this is a street car, then use the 951 unit, change the end cans, and ductit properly. Then add a simple temp/boost based water spray system, and you are good to go.
Don't forget, i am getting 250 crank HP at a simple 12 psi, and it goes VERY well, although it is a lot lighter than a road car. With 280-300 BHP, these cars fly. I recently went in a Carrera GT at a track day, and although they are only meant to be 210 BHP, this thing really flew, and had great response from 2.5K onwards. Believe me, 350 is a nice sounding figure, but i suspect the car would go just as well with 280-300, and good low end torque, which is what the stroker will give you. With 300 you are well within the design parameters of the associated components, so things like CFM/heat/lag will be a much smaller issue.

Steve
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Front Wheel Drive is the Devil's work.

[ideola]

Perfect, thanks for the validation, Steve, it's very helpful to get the real world perspective.

By the way, the LR Stage V is based on a completely new core and end tanks...not a modified stock unit. Cost is $1K. I'm wondering if I would be better off going with a completely custom design where the end tanks passed the air from the bottom and out thru the top of the core, rather than longitudinally like the 951 unit. Seems like that would be a more efficient design, less prone to pressure loss...
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erstwhile owner of just about every 924 variant ever made

[bass gt]

Dan,

My modded 951 IC is achieving 70-80% efficiency, and there is more to come once i fine tune the deflector. I think the 951 unit is fine for our purposes, it's packaging is right, and it's proven. Yes, the end cans need to be changed, but any good fabricator can do that. It also allows you to perfect the pipe work, rather than having to work around the stock OEM configuration. If you mount it in the nose, make sure it is well ducted, and seal it in, so air cannot escape around it. And as i have said, fitting a small washer jet pump, firing through a couple of little nozzles onto the front of the IC will improve the efficiency way more than just a bigger core. If you think back, this is what Subaru and Mitsubishi do on their EVO specials, and it works a treat. And whats the cost? Buttons Either trigger it manually, or via a temp and boost combination. You can mount the water bag/bottle anywhere, so packaging is much easier. Trust me, this is going to be my next mod, but i will run a full waste system, so once i hit X psi in the race, the pump will come on and stay on until i override it. As water is much more efficient at removing heat than air, this has to be the perfect solution.

Steve
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Front Wheel Drive is the Devil's work.

[emoore924]

Consider an a/w i/c from PWR...

[ideola]

I tend to agree, Eric...the smaller profile and the running characteristics of a liquid-to-air setup are ideal for a slightly modified street version of the 931. The best location for a conventional l-2-a, IMO, is the battery tray, which means relocating the battery to the hatch area (which adds anywhere from $70-$200 depending on how crazy you get with the battery relo kit).

One very interesting design that has surfaced fairly recently is Lindsey Racing's thermal hard pipe. When I last spoke to Dave Lindsey, he indicated that they were developing a second generation version, so it might be worth thinking about whether you want to invest in one of the current models or not. There was quite an argument over on Rennlist when these things first surfaced...a lot of folks don't think the design holds water (pun intended there, not literally). Lindsey has yet to produce any metrics, so some skeptism seems to be warranted at this point. I know Flosho here is using an early version, it would be interesting to get his insights as to its effectiveness. In any event, it is a very compact package, and the 6" version combined with some off-the-shelf silicone pieces would fit very nicely (if not a bit snugly) in place of the stock charge tube.

Either way, you're looking at roughly $1000 (give or take) to get into a new l-2-a setup (including the heat exchanger, all the plumbing, a secondary radiator, a pump, and a secondary reservoir). Also, don't forget, l-2-a will add more weight, and thermodynamically speaking, won't do as well as air-to-air IC in a race / autocross application.
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erstwhile owner of just about every 924 variant ever made

[924guy]

the alternative way to go is using the mazda unit, assuming its a top mount 13b unit. the inlet and outlets will have to be relocated a bit, its not bolt in, but compared to the alternatives, imo , its the easiest , least expensive way to go, and will be an improvement over stock tubing. And you wont have to relocate fuel lines or the fuel dizzy... yes you will have to deal with some heat soak, though a little creative sheilding can limit that a bit, and you will have to poke a hole in the hood for it, but hey, thats part of whats makes the cgt so cool isnt it?
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Eric
78 924
82 931 SE "smokey"
99' VehiCross
Y2K Honda Insight
http://www.cardomain.com/id/924Guy
Performance by Pasha