8.5:1 compression L28ET for better driveability and broader power band?

[pjo046]

Refering to what Tony D said:

 

"There is the argument of "low compression" versus "high compression"...

 

Most people use the argument that lower compression allows you to "run more boost" to make horsepower. This is true.

 

But to what extremes are we going to take the argument. An 8 or 8.5:1 compression engine will easily take 20psi of boost. And what are we making by that point? 350, 400HP? E'motive in 1983 with stone age electronics made 580HP at 7500rpm, on 20.6 pounds of boost, using racing gas, and a compression ratio ABOVE the stock ZXT of the day (considerably more if you talk with the people involved).

 

So the question becomes, in my mind at least, "How much money do you want to spend?" If you spend the money on head work and a cam, 580HP is waiting at 20-psi, but ASSUREDLY NOT at 7.4:1 compression ratio! You can get decent performance from a stock compression turbo, but for the money the drivability and spooling capabilities of a higher compression engine seem to win out when you weigh it. There really should be no trade-off in the octane scare between 7.4 and 8.5. I would personally waste my time with anything below 8.0!

 

The naysayers about "higher boost levels" might want to do a reality check and actually ask "how much boost do I terminally want to run?" For someoen going with 45psi, then yeah, I might say 7's would be in order. For someone with an inefficient intercooler? Maybe no more than 8:1. But for someone pushing a hybrid turbo and a decent intercooler on even 90 octane, I would shoot for the stock N/A drivability that 8.5:1 offers off-boost. Nothing is more discouraging to hear than some idiot with a well-tuned stocker say "I got him out of the hole till the boost come on!" Lets' not let those N.A. guys ever have the ability to say that!

 

In a roundabout way what I am trying to say isn't really about boost. It's about total mass flow through the engine. The power is, and always will be, in the HEAD of the L-Engine. A properly ported L-Head running 17# of boost will make more power than an unported engine running 20+. I have an acquaintance that did a before and after dyno test (headwork only). After retuning the TEC2 for the new flowing configuration, the SAME engine using the SAME turbo, and basically the SAME everything save for the portwork and runners in the intake manifold made more HP at 17# than he did at 21# before! (BTW, this was on an engine with 8:5:1 Slugs) So the "More Boost means more power" line of thinking is a bit corrupted. My training has (since I work in Compressed Air and Gasses for a living) always been that "pressure is the resistance to flow! The less resistance you have to flow, the more efficiently you transport air... So in some cases, LESS boost means MORE power! On the test vehicle, it assuredly did!"

 

Tony D, I really agree with what you're saying.

I am going to buy a P90 head, and have Spork port and polish it for me, and install a upgraded turbo-cam. But I also want to raise my compression, and much more than what I get by just installing a 1mm gasket. I see from my Lengine calculator that by swapping to flat top pistons instead of the dished turbo-pistons that come stock, I get a compression of 8.52:1. The only problem with this is that forged turbo pistons are expensive, and that I don't know if I am able to do the swap myself. What I was thinking doing instead is let Spork shave enough of the P90 to get me to a compression of about 8.5:1. What do you think about this? Doable? Or would it perhaps be better to go with a E88 head and a 1mm gasket? That would give me a compression of 8.46:1. But this head isn't as good as the P90. Would it be possible to make it as good by heavy porting, installing a aftermarket turbocam and perhaps bigger SS valves? What do you all say? All input on this appreciated.

 

And furthermore, if going with a compression around 8.5:1, how much boost will I be able to use when I have a good intercooler, 3" exhaust, am getting a custom intake manifold, ported exhaust manifold, and have a T3/T04B hybrid?

I have TEC3 engine management, so the timing etc can be tuned optimally.

You see, I'd rather have a engine that is really responsive and have a broad powerband, than having a low compression slug that needs high RPM's and much lag to wake up. This is going to be a streetcar, not a quartermile-runner. Can I still achieve my goal of 350-400RWHP? Will the turbo I have be to big(Since I will be using lower boost with this higher compression?)

Remember, I have 93.5 octane available at the pumps.

[Tony D]

I would watch the quench areas of the combustion chamber in the head and just knuckle under and get good forged pistons. Dished pistons aren't really optimal in design for good combustion chamber flame propogation.

I think the flat tops, and paying attention to the quench ares in the head will pay off.

 

It's all pretty academic, IMO if you are shooting for 2X stock HP in an engine, the internals should be slightly beefier! Ergo: Flat Top Forged Pistons.

 

They needn't be as expensive as all that, Ross has a nice set for around $450 I believe. There are some vendors out there (middlemen) who rape people for little or nothing in return. Have you taken a look at JeffP's Extreme280ZXT website? I don't know if he has a shot of his pistons, they have a matched dish to mirror the combustion chamber so the quench area of the head squeezes all the combustibles into that Combustion Chamber/Dish area for combustion. With a full-on dish, you don't get that effect. Though the Euro Turbo pistons and their little "Divot" in the center of the dished piston does make on wonder what they were up to there....

 

350 to 400 will be doable, I'd worry about durability myself. You can do it on stock components, but not for very long. What you may consider is running your stock ZXT engine while learning to tune the TEC3, and eventually when the knocking from the pistons comes around, hopefully you will have mastered the art of tuning it, and have saved the money for a proper set of pistons.

This is how I'm tuning my car, doing all the learning on stock ZXT engines, and as they break, into the "core pile" they go, giving me plenty of stuff to work with when the time comes for the final build.

[Clifton]

I went from 8.4:1 down to about 7.5:1 on my 7M. I didn't notice ANY difference in off boost power or lag. If octane wasn't an issue for me I might of run 8.0-8.4:1, but I like to make as much power as I can on pump gas. I can run about 20 psi on 91 octane with out H2O, 24-25 psi with H2O, up from about 14 psi with the 8.4:1.

 

Stock L28T pistons can take 20+ psi all day long as long as you don't detonate, I do it everytime I roadrace with cast pistons, 1/2 hour at a time.

 

Electramotives numbers aren't anything magical. Port a head, more cam duration and rev it to 7500 rpm. Electamotives would be about 470 rwhp. That kind of power could be made with 7.4 if the head flowed and the turbo was up to it, just like theres was. 20 psi from one turbo may only make 300 (stock or upgraded T3) while from another it will make 500 hp.

 

Dapiper on this board made 450rwhp @ 20 psi, don't know his compression though but not far of from E motive made.

 

The size of the turbo has to match the power you want to make. If it does more boost will = more HP. Boost and rpms are the only way to make more power form a given displacement. A T3/4 won't get the job done if power is your goal, it doesn't matter how big that compressor is it's the T3 side that will limit you. It will spool fast at 2500 rpms which seems to be important for most, that is until they realize they only make 250 rwhp at 15 psi, but atleast it spools fast.

[pjo046]

Well, I guess I just have to bite the dust and realise that forged flat top pistons is my best bet. I know that's what Corky Bell says in his book as well, the only acceptable way to alter the compression ratio is through the pistons. Altering headgasket thickness is also unacceptable according to him. Furthermore he says that "Special headgaskets and O-rings are often cures to the wrong problem. They are only poor excuses for not dealing properly with detonation. If detonation is the problem, cure it, and a stock head gasket will usually perform well." So I think I'll just get myself a new stock headgasket and ARP studs to go with the ported P90 head.

 

What he also says though is that "Unless circumstances are highly unusual, street cars with stock redlines will prove more successful with cast pistons. Forgings should be reserved for the high revvers, while T6 hypereutectic alloys can cover almost all requirements. Rule: Do not rush to the forged pistons store every time a turbo engine needs preparation"

 

But I guess if buying flat top's I will get forged ones. But I don't see that happening in the near future though. I have just assembled the engine with new gaskets etc, and to take it completely apart, resetting the ignition, and going through the trouble it probably is to change the pistons, is not appealing.... Tony, which Ross were you talking about had cheap forged turbo pistons for 450$? Ross at modern motorsports? Because I didn't find pistons on his website.

 

Clifton, you say you went from 8.4:1 to 7.5:1 in your 7M without noticing any difference in off boost power or lag. Well, I think the similar compression altering on a 280ZXT engine is far more noticeable, as the 7M engine is after all a much more modern engine etc. Or is this wrong? If it is true that I won't gain much off boost response and have reduced lag etc by raising the compression on my L28T, then I won't bother with it. But I would think it should make quite a difference.

 

What I have always belived is that the lower the compression ratio, the easier it is to produce a lot of boost without detonation. The higher the compression ratio, the greater the fuel efficiency and nonboosted response, and the shorter the lag.

[frank280zx]

Intressting for sure.

as my friend Xander drove a euroflattop piston ( raised squizpad and positive deck heigt, wo are) set up at 10:1 compression, and those pistons are now for sale.... why... his headgasket kept on going at 10 PSI he made between ~210 and 240 to the wheels, ( dependable on the real weight of the car, he used road dyno) so APR studs AND a standard headgasket? i doubt it...... however, it did seem to be more responisve.., but 20psi on that combo? maybe on a 8.5:1 and 20 psi? OK given a soup bowl set up would not make the same power at 10 PSI ( Tysons Z made 209 without IC at that level)

But limitations are met sooner, when you need to bite the bullet and go for some pricey mods, aswhere the soup bowls can apparntly handle upto 20PSI on stock headgaskets, making 350 or more to the wheels!

 

Im in favor of higher compression turbo set ups, as it is the more modern approach.. Our engines where desinged in a time that turbocharging wasnt that common on roadgoin cars...

But how high? and how much PSI you can run?...

 

Driving with the soupbowls we have gives peace of mind, since allmost everyone here has been there and done that, so we know the limitations of it... so who will pioneer

[Clifton]

Clifton' date=' you say you went from 8.4:1 to 7.5:1 in your 7M without noticing any difference in off boost power or lag. Well, I think the similar compression altering on a 280ZXT engine is far more noticeable, as the 7M engine is after all a much more modern engine etc. Or is this wrong? If it is true that I won't gain much off boost response and have reduced lag etc by raising the compression on my L28T, then I won't bother with it. But I would think it should make quite a difference.

 

What I have always belived is that the lower the compression ratio, the easier it is to produce a lot of boost without detonation. The higher the compression ratio, the greater the fuel efficiency and nonboosted response, and the shorter the lag.[/quote']

 

 

The 7M is still an old engine that doesn't breathe all that much better, it is 200cc more though. In a 3500lb Supra I think the lower compression might be noticable.

 

It is true that a higher compresion ratio increases thermal effiecincy and will make a little more power off boost but it won't hurt lag which seems to be a concern. Corky also says something in there about no lag = no fun and he covers a little on turbine selection and going with the larger given a choice.

 

If 100 octane wasn't $5 at the pumps I would think differently. .

[Tony D]

I was referring to Ross Pistons a manufacturer.

Another secret is to search through pistonsizes in catalogs. You will find several common US Spec engine with proper pin and offset, that allow you to buy a set of pistons (usually 8...) for DIRT CHEAP. A set of forged slugs with rings for a Small Block Chevy is CHEAP, almost half the price of a Z set in some cases... Anyway, if you find the right combination that allows the manufacturer to work from a standard forged slug they buy in quantity, then ordering a "custom set of 4.3L V-6 Chevy Pistons with this pin heigh and these ring configurations" is usually waaay cheaper than going to any manufacturer and utteringthe word "Datsun"!

The Ross Pistons were $450 last time I checked, but Weisco and JE also have lowcost forged slugs if you BUY DIRECT FROM THEM, and know what you want! You have to do some measuring and searching to find the combos (I am in Venezuela right now and have absolutely no access to any of my notes...) that will get you pistons for far cheaper than even these prices.

 

Gone are the days when TRW Forged Piston Kits were available for the 164CID Chevrolet Corvair! Forged slugs, set of 6, $150!

Or the Chevy Vega, those were OEM pistons that were FORGED, and sold for $20 from the local GM DEALER!

 

Ahhh, the good old days. But there is stuff out there that will work, you just have to put effort into finding it!

 

Good Luck with the build!

[1 fast z]

Well Well Well TONY, now you know how I get forged pistons for all of my motors. I have all of the piston books, Sealed Power, Etc.

[Drax240z]

I went from 7.4:1 to 8.3:1 on my turbo motor and noticed a very large increase in throttle response and off boost power. It was well worth it and I'd never do another 7.4:1 turbo again given the choice.

[GrayZee]

Sounds interesting Drax, you got me thinking about swapping my engine, however I'm sure that using the higher CR would surely make the intercooler MANDATORY.

[Guest bastaad525]

Sounds interesting Drax, you got me thinking about swapping my engine, however I'm sure that using the higher CR would surely make the intercooler MANDATORY.

 

 

Intercooler should be mandatory anyways. Even at 9-10psi on a stock motor, it gets awful hot . But either way, unless you were only planning on running 9-10psi for the rest of your life on a stock motor, then the intercooler is mandatory no matter what CR you run, so who cares