I have a 1.9L Mazda BP inline 4 cylinder engine. It is compound turbocharged with a 57mm/69mm turbo setup. The current motor runs 9:1 compression, around 35-40 PSI boost.

I have driven the car with 10.5:1 compression, and it felt a lot better out of boost, and felt stronger overall up to the 20-22 PSI I ran on that motor. Never got to test it at high boost.

I'm considering swapping to a significantly higher compression ratio, something like 12:1, or possibly even higher, looking for more power/tq out of boost for daily driving, and for more torque at low boost to get up on the converter quicker for drag racing. More HP up top would be nice if possible.

In theory, higher compression should yield more HP, as long as there is enough octane for a given boost level. For this setup, would 12:1 be possible with say, 30-35 PSI boost on E85? What about a higher octane race fuel? Does anyone run high compression + boost on E85?

 

People always get greedy on Boost at some point and then they hurt their stuff.
Methanol allows much higher boost levels with moderately higher compression rations.

Tractor Puller Diesels have run 100 psi of boost with 16-1 compression ratios for a few seconds.
(That being said, unless you have manufacturers GIVING YOU PARTS it gets expensive quickly
and high maint either way.

I have seen Tractor Puller diesel heads welded on cast iron blocks and wire rings to seal the cylinders.
Then I see a picture with the top of the block sheared off the block.

But in your case, high compression ratio engines and high boost are not normally done on gas (or e-85)
engines as the engine parts are not strong enough.

You might find a sweat spot between compression ratio and boost level where the parts live a while but then you will want to try a bit more of boost or compression ratio (as you are doing right now) and you will hurt stuff.

Tom V.

 

Tom,

You mentioned that the parts are not strong enough for the high boost/high compression ratio. What parts would you expect to fail? I have not bought or made any parts yet. Being a cast iron/closed deck block, would it be the rods/pistons that would likely not take the pressure?

Are you saying it's not typically done because it can't be done with much reliability, or that it's not typically done because it's not easy with readily available parts?

 

If its a street car stay with the 10.5 comp.
If straight up racing you could do 12.5. But I would recommend an Air to water intercooler. I watch the tune close.

 

Tractor pullers I know of are double or triple that pressure. Pretty neat to see the through bolts they use to clamp the block and head together. The truck in my profile picture makes a over 100psi with the factory 16.3:1 compression ratio.

 

Every part in your motor has a limit - the rods, crank, wrist pins, pistons, timing chain, block, etc. Using known limits that can be found on the interwebs would be my only source for your motor, but perhaps there are others that know how much HP your rod can hold, etc. The real issue is not the static compression ratio, but rather the effective compression ratio. This is the sum of your dynamic (cam event related), boost level, turbo efficiency, piping limits, etc. The effective compression ratio, as I'm using that term, is astronomically higher than 12.5:1 when you start adding 35-40 psi (2.5 additional atmospheres) of air. The increase in effective ratio is not a linear calculation and takes many factors into account. If you were dealing a more common engine (to me anyway) such as an LS or SBC, then finding known limits of parts becomes an easier and more sane process. I.e. making 2000 HP on a 'streetable' vehicle is a $20K+ process, but parts of known limits can be used. But in you 1.9 Mazda, hmm, I don't know to where to start looking.

Yes and yes to both questions in your last paragraph.

 

YES, I AM SHOWING MY AGE, i WAS TALKING ABOUT ENGINES THAT HAPPENED MANY YEARS AGO.
I am talking Francis Arganbright days,
(1979) 7.500 Pounds: 1-Francis Arganbright. Waterville Allis-Cholmers 219' 9".

Allis-Chalmers would drop off 10 engines at a time for him to race with.

It does not surprise me that those days were almost 40 years ago when I was advising on those tractors.
Pro Mods are a lot faster too vs the old "Fastest Street Car" guys like Danny Scott too.

Looks like you are having fun.

Tom V.

 

This is somewhat of an interesting topic, this article kind of describes what the others are talking about:

www.superstreetonline.com/how-to/engine/turp-0203-gm-racing-ecotec-engine/

They systemically replaced pieces in the engine as it went out, and spared little expense in doing such. That said, as mentioned not sure if the precedence is out there for this engine in terms of what lasts how long.

In terms of what you're doing, you'd likely gain some efficiency at the thermal level but you'd certainly be playing what is likely a

That said, my other questions become how much power are you looking for, what do you have now and what RPM you are turning at. In terms of building horsepower, you have four key components you can play with in combination; RPM, efficiency, displacement and boost. At the point you're at I'm wondering what your RPM and displacement look like. You may be able to find headroom there without even touching the boost or compression.

Also, to what purpose is this power to be used? Depending on what you're doing looking at things like aero and powertrain optimization may be better moves.

Not trying to discourage you from going the route you are, but rather wanting to be sure you're not missing out on gains elsewhere for less cost, headache and time.

 

Going backwards in more ways than one IMO.

The only benefit to going compound at such low relative boost levels is to increase the spool time. If you aren't spooling the smaller turbo instantly, it's likely sized incorrectly. Properly setup compound system should light off the small turbo quickly. If it's not I'd have to guess it's not setup correctly? The 2.0 liter mitsu compound setup I worked with would light off the 46mm 16g instantly. The larger 66mm unit would then take over and make some serious steam.

Is your cam not setup to make power at lower RPM? What kind of boost are you seeing below say 4k?

 

It's worth mentioning that comparing a gas cycle engine to a diesel cycle engine is unfair. A gas cycle engine depends on preventing detonation while a diesel engine is dependent on detonation. With that said, for the goals stated, instead of compound turbos for high and low response; might I suggest a turbo and a supercharger?

 

I have done several Supercharger Turbocharger set-ups at work over the years.
Done right they do well. The Production VW with the supercharger for low boost and low rpm response and the Turbo for higher boost and higher rpm power was almost perfect in how it drove. I worked for the Blue Oval guys so my posting that says the VW Super/Turbo was great engineering by them. No compounding just one system then the other system took over.

Tom V. .

 

Sounds like some are talking sequential while others compound. Sequential, while a little more complicated, gets you quick spool, broad boost curve, and can work on low boost. Compounds excel at high boost levels, but do also get you into boost a bit quicker than just a large single.

 

A friend had a Chebby S-10 Truck with a BB Chebby installed (long time ago) but believe it was in the 482 range. He did a compound deal and had trouble getting the drive he wanted on the street with the truck (Think Street Outlaws fun).
But he made a lot of money with that truck/compound turbo deal on the Chassis Dyno Car shoot-out events,... "Who can make the biggest dyno number?"
With the compound boost capability he was always near the top (if not the winner) in those events and collected the money. No "60 ft time" issues on the chassis dyno events.

Tom V.