[Benmm]

Hi all,

I've got a 1997 1.8, M45 supercharger with 11% reduction pulley and a round-the-rad intercooler, peak boost around 8psi. I've just rigged up a second pressure sensor on the supercharger outlet and I'm getting around 5psi pressure drop between the SC outlet and the manifold at 6500rpm. I was expecting around 2-3psi loss as there are a lot of bends in my pipework and the throttle probably robs a bit, but 5psi is massive. Then I figured that most of this could be just the temp drop across the IC (temp drop -> density increase -> pressure drop) and not a massive restriction, but I'm not sure.

The all-knowing internet didn't help - I found as many people saying the temp drop would not show up as a pressure drop as those who said it was expected, so I thought I'd ask here to see if anyone else had measured their IC pressure drop?

Cheers,

Ben

 

[Shadowraven]

5psi is massive, even 2-3 is way too much. I've not really heard of anything more than around 1-1.5psi. What you've said regarding temps doesnt make sense to me either unless you are experiencing a massive temperature differential. What is your pipe diameter? and any pics of your setup would be useful, but it sounds way more likely that what you actually have is a boost leak.

 

[Zed.]

I've just rigged up a second pressure sensor on the supercharger outlet

swap sensors around pre / post ic?

also, hot air less dense than cold etc.

Rich.

 

[Benmm]

Pipes are all 2.25", but has 8 90deg bends to get around the rad. I haven't got any photos to hand but it's a pretty standard routing.

I checked the second sensor cal by plumbing it into the MAP line and they were within 1kpa, so that's all fine. No kinks in the vac lines and no weirdness in the data (apart from the 5psi difference!).

I wondered about a boost leak but I would have thought that should bring the pressures closer by relieving the upstream pressure. But I'm not 100% sure on that so I'll pressure-test the pipework.

Rich: your comment on hot air being less dense was the reason for my questioning whether the 5psi drop is actually to be expected. M45's are not efficient and I've seen 90C AITs on non-intercooled setups. If the temp drop across the IC is 60C then that temperature drop has to go into a loss of pressure or a reduced volume flow rate. That's where my thermodynamics stops!

 

[Yugen]

Boost leak most likely

 

[Shadowraven]

8 is a few but not that excessive from what I see for around the rad setups. Sorry, so which side is the lower pressure, the throttle body side yeh?

90C is really dangerous temps, you should have IATs only slightly above ambient. Aran Large has some track videos with overlays which show even under hard use IATs should be easily controlled and between 20-30C and that's with a 15% pulley.

 

[Benmm]

Yep, the manifold is lower than the SC outlet.

The 90C was on someone else's car with a 15% pulley and no IC. Not sure what boost he was making or even if his engine's still in one piece. My intercooled IAT's are within 10C of ambient, even on track. Not sure what my temps are before the IC.

 

[Shadowraven]

I'd doubt it at 90C, 15% reduction and no IC, they start to pump some heat at that. lol.

Within 10C sounds perfectly fine, it sounds like you do have a boost leak.

 

[Benmm]

Not a boost leak, the whole install held a solid 10psi for over a minute.

I’m thinking it must just be thermodynamics at work.

Anyway, 8psi for an 11% pulley doesn’t seem too far off what other people get from what I’ve read.


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[Shadowraven]

11% reduction puts the charger spinning at about 16000 rpm at 7100rpm, a 15% reaches this at 6800rpm so a 11% should reach peak boost and at red line will be comparable to most other setups we see. An efficient M45 setup will make 6-8psi tops, over this is due to nearly always due to excess heat and inefficiency. There's a lot of misconceptions and exaggeration around supercharger setups on mx5s, nearly all of them make 190-200bhp. I know of very very few that have managed to hit 200whp but these have often seen a lot of tweaking and plenty of mapping refinement on a dyno.

Just to clarify, what are your measured pressures on each side of the SC?

 

[Shadowraven]

It might be worth measuring the IAT pre and post intercooler. If your IATs pre cooler are high enough to be responsible for a 3psi pressure drop across the intercooler then I think you have a significant issue with heat soak from the exhaust manifold to the charge that you really ought to address. The fact IATs at the manifold are 10 above ambient is ok, it means the IC is coping ok, but the supercharger likely is suffering badly and at risk of seizing or rotor damage. Do you run a bypass valve for the charger as well? If not that might be a large part of the issue as the charger will still be working hard under cruise and idle, I've seen a few people seize multiple chargers this way.

 

[Benmm]

Good suggestion. It’s probably easiest to bypass the IC and (briefly) see what IATs I get. Anyone able to comment on ‘normal’ IATs with a non-intercooled hotside M45?

The exhaust mani is stock and wrapped but without heat shield. The SC has foil on it to keep radiated heat out, but they are f’ing close to each other so I’ll see what I can do there. I’ll need to cut some more of the bonnet webbing out to raise it any more.


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[Benmm]

11% reduction puts the charger spinning at about 16000 rpm at 7100rpm, a 15% reaches this at 6800rpm so a 11% should reach peak boost and at red line will be comparable to most other setups we see. An efficient M45 setup will make 6-8psi tops, over this is due to nearly always due to excess heat and inefficiency. There's a lot of misconceptions and exaggeration around supercharger setups on mx5s, nearly all of them make 190-200bhp. I know of very very few that have managed to hit 200whp but these have often seen a lot of tweaking and plenty of mapping refinement on a dyno.

Just to clarify, what are your measured pressures on each side of the SC?

Measured boost pressures are 12psi after SC and before IC, 7.5psi in the intake manifold. I haven’t measured before the SC but should be close to ambient as I have a 65mm throttle pre-SC. I should add that I have dual TBs so part of the loss will be through the stock throttle in the stock location.


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[Shadowraven]

Wrap and foil should be sufficient really. Have a feel and check how hot it is by hand, I think they should be fine to touch (but be careful) if everything is working right. I know a common demonstration of the differences a bypass makes is often that the SC becomes too hot to even touch without. Your bypass valve vac line is plugged into the manifold isnt it?

You dont want to bypass the IC to test pre-charger temps if you can help it. If your issue is heat related then it would likely put the car at risk of detonation as there will then be nothing to cool down the charge air leading to a lot more heat in the engine. I duno how your IC piping is set up but if you can swap a coupler or something to move the IAT sensor it'd be safest to check that way.

I dont think pre-charger temps will be that bad as long as the filter is in a sensible location.

Dual TB wont affect the loss, all of the 5 psi pressure differential you are experiencing is occurring between the supercharger outlet and the intake manifold, and at WOT the stock throttle body isn't causing any loss. It does sound as if you have been able to isolate the drop to be clearly between the SC outlet and intake manifold, and if there is no boost leak then it's reasonable to assume it's heat related and we just need to look at why that is and how bad it is. There's very little data online about pre-cooler temps but it does sound like they are to high.

 

[Benmm]

The bypass is working as it should, fed from the mani, and the SC body doesn't get too hot (not as hot as the cam cover after a drive).

I don't want to move the IAT sensor as it'll go dangerously lean if it picks up a hotter temp than is entering the manifold. I have a spare input on my MS2, so maybe I'll look at adding another IAT sensor. It would certainly fill in a bit more missing info on what's happening on the hot side of my engine bay.

Here's a pic of the engine bay - tatty but functional! It's a couple of years old but fundamentally that's how it still is.

The Engine!

• Benmm
• May 20, 2019

 

[Shadowraven]

It all looks fine. If the charger is as cool as u say and iats seem to be fine I don't think there's much else you can do or much need for it on your setup then. Its working fine and nothing is hot enough to be at risk of seizing or det, it's just outputting warm air but it's cooling down enough at the intercooler

 

[Benmm]

Thanks for the feedback, that’s encouraging from someone clearly familiar with these installations.

The reason for installing the second pressure sensor was to help decide whether to change to an over-the-rad IC installation, but it seems the effects of the temperature drop have clouded the true measurement of pressure loss that I was hoping to get.

I’ll have a think about installing a second AIT too. If I get any more useful data points then I’ll report back.


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[Zed.]

accept that we ALL overthink things

and this is talking from experience😳

Rich.

 

[Shadowraven]

Thanks for the feedback, that’s encouraging from someone clearly familiar with these installations.

The reason for installing the second pressure sensor was to help decide whether to change to an over-the-rad IC installation, but it seems the effects of the temperature drop have clouded the true measurement of pressure loss that I was hoping to get.

I’ll have a think about installing a second AIT too. If I get any more useful data points then I’ll report back.


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Yeh i'd be interested to see the data if you do as people rarely record pre-intercooler temps.

I've done an over the rad setup as I have an M62 kit and AC so you just can't go around. It involved quite a lot of work to drop the rad, sort the pipe clearance etc, it might be easier with an M45 but as you have a functioning around the rad setup I dont think there is much to be gained from changing.

Here's mine:

 

[Benmm]

Very smart. To be honest the over-the-rad install is now feeling like the itch I need to scratch, but it does seem the gains could be minimal.