What compression ratio is safe with low boost?

[woodsbuggy1]

I am currently running a 1776 MS1 v2.2 with FI and 36-1 wheel with wasted spark, E85, intercooler and 8lbs boost at 8 to 1 compression with a W100 cam.
This is in my offroad buggy and it runs well but I lack low end power especially on long hills where I need to be in and out of the throttle.
I know that a 2387 would solve this problem, but I have everything to build a 1915 and would like to run around 9.5 to 1 compression ratio and limit boost to 6lbs if necessary to help provide more low end grunt. I also have a MS3X V3.57 for the 1915 so I have tuning options.
Any thoughts and suggestions are welcome.
Thanks
Kenric

[Wally]

I think it was mentioned before, but no CR is safe without a proper tune... Its all about the tune.

Having said that, CR is all about your cam. Your engine only feels dynamic CR and the cam largely dictates that.
So, generally speaking again, a W100 is a pretty short duration cam which yields high dynamic CR, so 8,0:1 is pretty high for static CR.
For example, if you would run a FK87 with 1:9,5 static CR, it _might_ be yielding the same dynamic CR as with your W100 and 8,0:1... just to give you an idea what I mean by the importance of the cam.
Therefore, high or low CR is relative to the valve timing which is dictated by the opening and closing events of the cam: the later closing intake valve of a longer duration cam yields lower (dynamic) compression

Hope that made any sense to you...

[Dan Dryden]

I think it was mentioned before, but no CR is safe without a proper tune... Its all about the tune.

Having said that, CR is all about your cam. Your engine only feels dynamic CR and the cam largely dictates that.
So, generally speaking again, a W100 is a pretty short duration cam which yields high dynamic CR, so 8,0:1 is pretty high for static CR.
For example, if you would run a FK87 with 1:9,5 static CR, it _might_ be yielding the same dynamic CR as with your W100 and 8,0:1... just to give you an idea what I mean by the importance of the cam.
Therefore, high or low CR is relative to the valve timing which is dictated by the opening and closing events of the cam: the later closing intake valve of a longer duration cam yields lower (dynamic) compression

Hope that made any sense to you...

Hi Wally,

I notice that modern turbocharged engines seem to make a lot more of a wooshing noise from their exhausts. (Eg. Opel Astra VXR)
I'm guesing this is due to running a longer duration cam and boost is escaping through the exhaust?
If so, do you know of the advantages of running a longer duration cam on a turbo engine?
I'm guessing it will help spool the turbo more quickly and allow the engine to achieve higher rpm's, which can only be a good thing right?

Why have we been lead to believe for so long that short duration cams are best suited to turbocharged engines?

[Wally]

Hi Wally,

I notice that modern turbocharged engines seem to make a lot more of a wooshing noise from their exhausts. (Eg. Opel Astra VXR)
I'm guesing this is due to running a longer duration cam and boost is escaping through the exhaust?

I am no expert on what the trend on cams with modern engines is headed for, but it anything, pollution control is on top of most OEM's for sure. That usually means the contrary: less duration!
More efficiency is another field to reduce emissions and twin scroll fits that bill well (BMW and others). But longer duration? Nah, most likely not..
I think you may hear the 'sound engineering' that is used more often Or the BOV of some sort?

If so, do you know of the advantages of running a longer duration cam on a turbo engine?

Actually, longer duration works for the most part about the same as on a N/A engine. The difference, at least as by my experience of using different cams on the same engine, is that the more lift that usually goes hand in hand with more duration, works the most to increase power down low rpm's.
Turbo engines really respond well to more lift I noticed. More duration is much less of a power adder in itself as the spool will be the same, but the engine does feel the extra lift!
More duration does make tuning much more easy/forgiven as it reduces dynamic CR which makes that you can run more boost or more timing or a little more of both without knock events killing it all

In that way, more duration helps quite a bit as well, but maybe in another way then you'd think.

Why have we been lead to believe for so long that short duration cams are best suited to turbocharged engines?

Maybe conservatism and unavailability of ECU's to control things better and logging and understanding engine data to make tuning better.
Its all about the tune after all

[Dan Dryden]

Thanks Wally.

I've often wondered if using a wilder cam in my engine would reap more power without losing driveability.

I was suckered into the marketing gimmick of using the Engle TCS 10 "turbo grind" cam when I built my engine all those years ago. I suppose I could try using 1.25 ratio rockers to increase lift but don't think they will make much difference than the 1.1's I have in there now.

I tried driving it N/A once and it felt asthmatic, only just reaching 4000rpm!
Ever since then it got me thinking "what if?"......

[Wally]

Yeah, the TCS10 is really mild.
Another N/A analogy is that turbo engine's also really respond well to good flowing heads, so the TCS10 can give great power on very good heads.

I suppose I could try using 1.25 ratio rockers to increase lift but don't think they will make much difference than the 1.1's I have in there now.

I think it would surprise you what a difference that could make. I would try that first, easy mod. Do make sure you have good springs and reset pushrod/rocker geometry.

[woodsbuggy1]

Thanks for all of input so far, sorry I did not provide all of the info on this build.
The cam is a DRD 280 cam on 112lc with .442 lift at the valve and 242 deg at .050.
The heads are DRD entry level turbo heads with 40mm Intakes and 37.5mm exh.
I will be running E85 and intercooler with a single TB and custom intake.
I have much to learn in the tuning dept but have patience and the desire to maximize my engines potential.
Thanks Again
Kenric

[turbobaja]

With regard to your compression ratio question, I would suggest running as close to the "ideal" CR if your engine were N/A, adding 7-8 psi shouldn't cause any problems with a good tune. You're looking for more bottom end, which to me means off-boost, or pre-spool pull and response. Make the engine "happy" off-boost and just don't add as much boost. You're in a lightweight buggy, right, you can afford to push the "limits" a little compared to the average setup/build.

The 94mm pistons should add a bit of torque.

Biggest red flag to me is E85 fuel and 8:1 static CR, even with the W100 cam. I ran that same cam (basically) and CR on pump premium for 2 years @ 7psi with no problems. If your goal is 8 psi on a charge-cooled setup running E85, I dare say you could run more than 8:1 static CR and expect better results. Also, the ignition advance has a lot to do with how much grunt you get down low. Ramp in the timing more aggressively, earlier (all-in by 2K for example) and see how it reacts. And you need even more advance with the slow (but cooool) burning Ethanol fuel, especially with your LOW static CR. What's your altitude? Much above sea level and you might as well bump it up some more.

I would say YES, bump it up to 9.5:1 and report back

[woodsbuggy1]

Thanks Karl, that is what I was hoping to hear. I believe that we are around 1000 ft above sea level here in flatland IN. I am sure that I am leaving some power on the table with my 1776 since I have not adjusted my timing table since switching to E85. I hate to adjust too much since she runs so well. I will have my heads flycut to reduce the 60cc chambers,
Thanks
Kenric

[Wally]

E85 makes indeed all the difference in the world and would allow more CR or boost or both

[Alexios]

I have been running 8,8 in compression with a TCS 10 cam, E85, non intercooled and supercharged with an AMR 500. Running around 0,8bar with no issues.

E85 is a miracle fuel:)

Alex

[woodsbuggy1]

Thanks again everyone for all of the great info.
Plans have changed, I have my 1915 parts for sale and will be building a 2276 probably NA for the first season, I think it will suit my needs much better.
Kenric

[Clonebug]

NOOOOOOOOOOOOOOO!!!!!!!!!

You can't give up turbo!!!!!!!!!!!

[woodsbuggy1]

I am Absolutely Not giving up on Turbos.
The 1776 will find a new home in my street buggy with a 13t turbo and my offroad buggy will still be injected with a 2276 and future plans do include a turbo for it but I need to save up for Weddle internals for the trans before it sees any boost.
Kenric

[Chip Birks]

I have been running 8,8 in compression with a TCS 10 cam, E85, non intercooled and supercharged with an AMR 500. Running around 0,8bar with no issues.

E85 is a miracle fuel:)

Alex

I just saw 27.8psi last night on e85. 9.1 comp. Can't complain about that! My cam is much bigger than an TCS10 though.