Light-pressure turbo....random thoughts, nothing serious.

[GRMPer]

So...I'm thinking about a light-pressure turbo set-up on my car. The B engine is fairly stock....9.7:1ish compression, ported head, mild street cam.

Let's say I wanted 5 or so pounds of boost. How would I go about doing that? Basic B-motor turbo parts? My thinking is that low pressure turbo with relatively high compression would yield a long lasting/torque off-boost engine.

Based upon my in-car video, there's seconds and minutes to gain just getting up to speed on an average stage. Lots of waiting....

[Jordan]

A couple of thoughts...
Since you have already built the engine a little you probably don't want to swap too many parts out but...

The turbos (16v at least) use sodium filled exhaust valves which withstand the heat better. I'd ask around if anybody wants to part with their 8v turbo head.

I'd think you'd want some kind of better engine management/control. At the bare minimum you may want to try the SAAB APC system of boost control. If you're not familiar with it--It is a stand alone system available on any 16v turbo that uses a piezo knock sensor to reduce max boost pressure through a bypass valve. Simple but effective and easy to use.

That is a pretty high CR to be running boost, it might work but I wouldn't expect more than 3-4lbs max. Even going back to standard pistons would give you a bit more breathing room. The new SAAB LPT's with all their fancy electronics and engine management run 9.5:1 with a max boost of .5 bar ~ 7lbs.

You'd want to use a distributer with vac advance and boost retard.

You definitely want a small but efficient turbo. Unless you change your cams you will need something that will spool quickly, but not restrict the exhaust since you make power in the high rpm anyway.

I dunno, it is a tough call since you already started out in one direction, it would be an interesting experiment though!

Do you have a standard rally license or novice? Rally america prohibits turbocharges with a novice license without special permission.

[matt]

Wasn't the euro spec 900S a "light pressure turbo" setup running something like 5psi?
I'm sure you could do it with a 9.7:1 compression ratio with proper tuning, honda kids run a bunch of boost without problems (provided they don't have garbage tunes and a shti ton of detonation)

[Jordan]

Wasn't the euro spec 900S a "light pressure turbo" setup running something like 5psi?
I'm sure you could do it with a 9.7:1 compression ratio with proper tuning, honda kids run a bunch of boost without problems (provided they don't have garbage tunes and a shti ton of detonation)

Yeah the NG900s LPT run 9.3:1 compression I think.

[DeLorean]

the UK-spec C900-LPTs were just a 16V turbo motor, though it had no APC and no intercooler. Not even sure why they did this because the 2.1 motor in the US and elsewhere in Europe had almost exactly the same power output as the LPT, the LPT just had slightly more torque.

I am on-board with the LPT idea, though there may be more doing required then is immediately realized. With a modern fuel / boost controller the compression that car has should be absolutely fine. Could also run E85... With the CIS system, you might be able to get it to work, but IMO, there is easily a potential there to pop a piston, head gasket, or maybe an n/a exhaust valve.

[Jordan]

the UK-spec C900-LPTs were just a 16V turbo motor, though it had no APC and no intercooler. Not even sure why they did this because the 2.1 motor in the US and elsewhere in Europe had almost exactly the same power output as the LPT, the LPT just had slightly more torque.

I am on-board with the LPT idea, though there may be more doing required then is immediately realized. With a modern fuel / boost controller the compression that car has should be absolutely fine. Could also run E85... With the CIS system, you might be able to get it to work, but IMO, there is easily a potential there to pop a piston, head gasket, or maybe an n/a exhaust valve.

I honestly don't think I would do it to a motor with that high compression. For all the time/money/labor involved plus the added complexity,weight and reduced reliability (you don't want to be blowing motors after spending that much money in rally costs) for just a couple of extra ponies is not worth it IMHO. You could probably get more power from a standalone fuel system properly tuned N/A than 3lbs of boost.

On the other hand if you started with a 99 turbo motor, swapped your clutch/lightened flywheel, used APC/intercooler and standard 8.7:1 N/A pistons you would have a screamer.

[happyandy]

Per's compression as stated is realy not that high. 9.7:1 ratio is not to high to use a LPT set up. Some people are turbo-ing motor cycles that allready have 12:1 or higher compression and running boost pressure up into the teens, and the one that I have seen around here aren't using any special engine managment either. I think a 5 psi LPT is do-able with out to many changes, and If an engine managment system were added you could probbably boost a bit higher. An egt monitor gauge might be a good idea if you stick with the na head
PS. I know a guy around here who has a 900s w/2.1 and a turbo system from a B202 (minus the apc system) being used as a LPT, and he says it works pretty good. (I personaly can't confirm, I seen it but never been in it.
Also the reason for the euro lpt B202, instead of the B212NA, is because in europe cars over 2.0l are taxed in a more expencive class.

[Jordan]

Per's compression as stated is realy not that high. 9.7:1 ratio is not to high to use a LPT set up. Some people are turbo-ing motor cycles that allready have 12:1 or higher compression and running boost pressure up into the teens, and the one that I have seen around here aren't using any special engine managment either. I think a 5 psi LPT is do-able with out to many changes, and If an engine managment system were added you could probbably boost a bit higher. An egt monitor gauge might be a good idea if you stick with the na head
PS. I know a guy around here who has a 900s w/2.1 and a turbo system from a B202 (minus the apc system) being used as a LPT, and he says it works pretty good. (I personaly can't confirm, I seen it but never been in it.
Also the reason for the euro lpt B202, instead of the B212NA, is because in europe cars over 2.0l are taxed in a more expencive class.

First of all there are so many other factors that can affect pre-ignition; talking just about static compression is silly. Dynamic compression, chamber design, piston design, ignition timing, fuel quality... the list goes on and on. I'm not saying it couldn't be done or that it wouldn't be fun to try to see what could be done. What I am saying is that these guys are spending $2000+ for a weekend of serious car abuse. You want an engine that is going to be dead reliable, simple to fix and diagnose if you do have problems. Adding a turbo, piping, oil lines, intercoolers, boost controllers, knock sensors (which often don't work in rally because of all the other banging noises) and whatever else is involved just adds weight and complexity for a marginal bump in performance that could be gained elsewhere. Just my .02

[saab90089]

Ya, I'd agree with Jordan on this one. You have a reliable engine right now and there is no sense in messing that up. Plus as it was pointed out before you already took a step in the NA direction you might better off just continueing in that direction.

Ian

[GRMPer]

Thanks for the input...just random musing.

[allessence]

My 91SPG is around 9.5-1 CR or 150PSI. I have ran upwards of 30PSI on a regular basis with 93 octane gas and an A/F max boost around 11AF.

I have a 2.1 with each cylinder at 200psi compression tested and I believe this will be safe up to around 12psi or more with higher octane fuels.


From what I understand the 8V's can be made to run good boost numbers as long as fueling is correct.

[Crazyswede]

The 99 turbos had sodium filled exhaust valves. I dont think that they used sodium filled valves in the 16v turbos as they had the Stellite coating. I have put non turbo 16v heads on turbo blocks and had valve jobs done in the process. It was pretty easy to see that the Stellite coating had warn off but the valve still looked and then performed fine in the turbo afterwards.

[Sean Tennis]

Per, you need sodium exhaust valves on the 8 valve motor and different piston rings (can't think of the material off my head at this time), otherwise the rings will stick to the block, and the exhaust valves will burn...

The real problem becomes the gearbox breaking 3rd gear very quickly if you're running standard gearbox on rough surface with LSD and hard driving... The extra torque from the higher compression the turbos produces can really give troubles as is the additional heat that comes with it, and tire wear....

Better off to look at gearing, what is your axle ratio? Primary chain set-up? Taller numerical gearing will make a big difference in your cars accelleration-as I'm sure you know...Just for giggles put some low profile tires or 14" wheels on the car and go out for some 0-60 time measurements-same effect as changing axle ratio numerically higher. We naturally aspirated car guys suffer pretty good in comparison to turbos when accellerating above 70mph in our rally cars, but it's not the end of the world as our results show!

[GRMPer]

Sean, that makes sense and I know you're right on the gearbox/ratio issue...I'm running a stock early 5-speed. Most stages are done in 3rd with occasional downshifts to second (and sometimes even 1st for those STPR uphill U-turns) with occasional upshifts to 4th.

So..what's a good and reasonably cheap way to get better gearing? Change the chain gears?

[Jordan]

You can swap out primary chain gears pretty easily to change gear ratios. The mid-late Turbo 900 boxes have the tallest ratios so you probably want something a little shorter. You can mix and match primary gears to work on the desired input ratio to the transmission like a secondary ring and pinion. I made sort of a calculator a while ago, but I never finished but you can select your serial number and it will tell you some specs on it. (http://ignite-search.com/gearset-tool/t ... der/lookup).

Basically , the primary gears listed as sets (4-7, 8 if your in find a Eurobox). In fact the 3rd number of the transmission model number is the set that it contains. So a GM45610 would contain a #6 primary set while a GM45710 would contain a #7 etc. The smaller number, the shorter the overall set ratio is.

The sets are as follows listed by #teeth and final ratio given. Mixing the input/output gears will obviously give you custom ratios, or you can swap whole sets. You can't use an input as an output gear and vise versa. Also be aware that the really short sets require also swapping in a longer chain.

#4 31/30 = .97
#5 30/27 = .90
#6 31/26 = .84
#7 32/25 = .78
#8 33/24 = .73