Fiat 20V turbo CR question and output

[trickymex]

Your advice needed please.

I have recently built a Fiat coupe 20v turbo engine for a friend,

I did not spec the pistons but the owner wanted higher than standard compression, STD is 8.5-1 and the new pistons are designed to be 9.0-1

This is the second time this engine has been built, the first time it had nothing to do with me

Anyway, when it was built last time he used different pistons that were 8.5-1 comp, he wanted to up the compression so he got the block decked by 0.5mm, I did not notice this until I was assembling his engine, I spoke to him and did some calcs and it turned out the compression ratio was going to end up being 9.39-1 with a squish of 27 thou….

I advised him that the comp was to high in my eyes and the squish was to tight but after discussing what he wanted he decided that after paying out for the block to be machined for the new pistons and he did not want to pay out anymore he decided to run it and see what happens and if it’s a major problem he would put in a decomp plate to put it back to 9.0-1 comp.

The car was on the dyno last week and it made about 50bhp short of what he was expecting, the mapper said that the comp was just too high and he had to retard the ignition and that is why he was not getting what he expected


The problem I am having now is the mapper wants him to lower the comp to 8.8-1 so that they can chuck in the boost and timing but I am also building another engine exactly the same for someone else (without the block being decked) but running a larger turbo, more boost and hopefully more power

My question is, is there a specific reason why we can’t run this comp-ratio apart from the obvious like detonating, and how do WRC cars run comp-ratio’s in the region of 10.0-1

And is there anything we can do to extract more ignition advance out of the engine??

They will al be running shell v-power and are road cars, they both have good intercoolers and will running between 1.4 and 1.8 bar of boost on GT28 series turbo’s

Any advice greatly appreciated

Ricky

[Guy Croft]

OK, you've built an engine for a friend, that's cool. But the end result is not good. I'm an accredited race engine professional so I going to offer this advice:

1. You never, ever raise the CR over standard on an 'uprated/tuned/higher boost unit. You lower it.
2. Don't get talked into doing that.
3. Rebuild it to max 8/1 ( me, I would use 7.5)
4. Don't be pressured by any more of that turbo-babble hearsay!
5. Trust GC.

All you get from a turbocharged engine running 'relatively' high static CR is er, slightly better 'off boost' performance. You could run a turbocharged engine at 6/1 instead of 8/1 on the same boost and not notice hardly any difference in on-boost performance. Too high CR is madness, it will just cause exceptionally high in-cylinder temperatures and lead to detonation. 9/10 DIY turbocharged engines I hear about do that and many mapped n/a ones too. Even if the engine doesn't detonate the high temperature will cause metallurgical chaos you wouldn't believe (long list).

Just for the record, readers, (and this is categorically not a 'swipe' at poor Ricky!) I don't care what anyone anywhere else on_the_planet says/does regarding turbo CR. My way is right because it is robust. As far as WRC, the budgets are so big they have access to technology we've never heard of, don't try to copy.

GC

[cantfindausername]

I definately agree with GC on this. I have a friend who builds WRC cars and some of the technology he's talked about really is more than you're even going to see or need on the road. As for the CR on them, well, thats why they're rebuilt after a 750km rally.

[pacman]

Well said, Mr Croft.
Never raise the CR on a turbo-engine rebuild unless it is far too low from the beginning.
If you are concerned about off-boost performance, set up a boost/throttle relation. Some of the engine control systems have a function where you can set boost level depending on throttle position. 20% throttle = 20% boost, 50% throttle = 50% boost, 100% throttle = Yiiihaaa!
With this set-up, and with a well-sorted turbo with quick spool-up, you will have a turbo-engine that works like a very powerful n/a engine, without the turbo-lag.

//PeterC

[trickymex]

Hi,

Sorry its taken so long to come back to you and Guy thanks for the informative reply.

i have suggested that he lower the CR, will he do that?? i cannot be sure!!!

but i have done my part and other than the BHP being less than he was expecting the engine has been perfect and still produces more power than the chassis can handle


Its a shame as most people you talk to have a differing view with regard to CR in a Turbo engine, not that i dissagree with you Guy, it just seems to be the fashion at the moment, im sure you know what its like, next week it will be the other way round.

Regards

Ricky

[Guy Croft]

Ricky, hi

no worries. The notion that it would possible to raise the CR from standard and yet still raise the boost as well has been around for quite a time. However much I say as a professional (with the credentials to actually give advice), there will always be some crackpot who 'knows better'!

I won't give advice based on guesswork. If I give advice, you can trust it.


GC

[ASF72]

Hi, this subject has been another head scratcher for me, i have a friend who owns a Mitsubishi Evo 6 with a 2.3 stroker engine in and runs a compression ratio in the region of 9's with boost upto 2.1bar, the car has been reliable and no problems what so ever (unless he's telling me lie's), i always thought you should use a lower compression for big boost applications, i know there's the problem of off boost driving but i use 7.7.1 and still very useable.

Nino (still learning)

[trickymex]

The truth is that on a road car you should never really have any real lag problems if you spec the turbo correctly.

And a race car would be less critical as it would be driven correctly and the driver would keep it in the power band

Ricky

[cantfindausername]

There is always a trade off on how a car will respond, track and road driving are obviously very different beasts, and a car suited to each one will not be suited to the other. But who really builds road cars? Fast road cars yes, but then how different are they to track cars?

A quote I saw on another forum once read...

If you have lag, you're either in the wrong gear, or the wrong car!

Obviously thats a bit general, but he's got a good point. My car is being built with a powerband of 5.5k - 8.5k RPM. And I'm not really expecting much below that, but then I didn't spec it to be responsive around town. I spec'd it for high end power. Some people might say that the turbo isn't suited for the size of the engine, but many people have had good results with a similar set up.

[trickymex]

Here is the dyno-plot of this engine

deleted,sorry


Other than that its pretty much standard

What are your views??

thank you

Ricky

[Guy Croft]

Ricky, hi

Looks plenty powerful but comparatively I don't have any benchmark because I don't know the std power/torque, boost & rpm for that motor.

GC

[trickymex]

This should help.

Standard figures are

Max BHP, 217 BHP @ 5750 RPM
Max torque, 228 lb.ft @ 2500 RPM

Standard boost is 0.9-1 bar dropping after 5k to about 0.7 bar

Unfortunatly i do not have a Dyno-plot for a standard coupe but you get the idea

Overall im rather impressed with the result, considering its running standard cams and valves.

Ricky

[Guy Croft]

No - it's VERY good!

Can't argue with those sort of figures.

Well done indeed to you and thanks for sharing your results. Doing it professionally or as a DIY'er, no matter. Keeping the very British tuning thing alive is what counts in my book.

Incidentally, you should have a go at working out exactly where the power gains cam from.


GC

[trickymex]

Guy, Thanks.

As for investigating where the gain come, im not sure where to start??

Unfortunatly the head was never flowbenched??

Ricky