Fluidplay

Ignorant question, but need some education...How much of a change in compression ratio do you need to see a change in the way the engine will ultimately perform?

What's the positive or negative effect...I've heard lower CR less chance of knock on pump gas, but you lose low end torque. Higher CR more torque, but tuning is extremely critical.

Is the range below a resonable range of running our engines?

7.6 CR
8.5 CR


Have located a new set of pistons with 8:1 CR...the build continues.

Thom

Bit of a "how long is a piece of string" question... You can run whatever CR you want if you can adjust your ignition profile in function of knock threshold, turbo spool, etc.

333pg333

Are those two some pistons you have a choice of Dennis?

Fluidplay

Those are possibilities...that being said , I'm leaning toward the 7.6:1.

So here are a few questions:
1) am I giving away too much low end power
2) how does it change turbo response
3) can the tune be adjusted to compensate

Thanks in advance for your input..

Dutch944

I would choose the 8.5 (also because i've ordered my pistons with that CR)

A 7.5 is way to low in my opinion but there are way more experienced engine builders here who can provide you with the proper info!

refresh951

It really depends on what you are doing. What kind of motor are you building? What kind power? We have discussed this quite a bit lately, I would do a search.

Fluidplay

My goal is a reliable 325rwhp.

List of engine upgrades..
Blocked - bored to 101
Crank - knife edged, cross drilled
Stock rods
Wossner pistons
Head - 930 valves, ported and polished, stiffer springs
Mild cam
Super 61 turbo
48# injectors
Link ECU

Concerns with this CR: not getting the power goals and additional turbo lag

V2Rocket

and what fuel do you intend to run?

Fluidplay

with the lower CR I could run more boost on pump gas with less chance of knocking, but the question is again...am I sacrificing hp and turbo response..?

Thom

Depends on the size of the turbo, but the smallest turbo that may compensate for the loss of low end may run badly out of breathe at higher rpm, and I see the Super 61 as being too big for that.

Fluidplay

Unsure what you mean... are you saying that the Super 61 would be big enough to pull through the higher rpm's?

refresh951

I would definitely run the 8.5 CR and probably a Super 48 or Super 53 if you go LR, IMO the Super 61 will be to big for your stated goals. 7.5 CR is something I have considered but only for big power +475 hp.

67King

I'll try to reply later, but this is not a simple discussion. Quick answer is a 61 is about 50% bigger than you need, so I'd go with an appropriately sized turbo to reduce lag. Too high a CR will mean detonation, but that is usually a low RPM, high load condition. Fuel, altitude, temperature, etc. all come into play. Will try to post up more later

Fluidplay

..would love to be able to change things around, but unfortuntily this is what I have to work with...

It sounds like what you're saying is that the upgrades I do have will support much more hp than my goal. Not a bad thing but again at what cost to low end torque/power? If this engine was purely for a track car, would this would be a good set-up...?

Dougs951S

Looking at it like " my mods will support X more power than my goal, woohoo" is not really the right way to look at it. Everyone here has given their own opinions and offered you good advice and while I am not nearly as knowledgeable as some of the folks here, I will try to answer your question directly. With that low a CR and that large of a turbo, your car is simply going to be a dog if its tuned for 325 hp. What you have COULD support much more, but with 48# injectors you will never see the potential a 7.5 CR motor with a 61 turbo can reach. As stated before, with the right injectors and fuel pump that motor could see 450+ rwhp. At the very least, please tell me that super 61 is a #8 housing with a stage 3 wheel as this is the quickest spooling setup for that compressor. Get the higher comp pistons and be happy (er), or raise the bar and go full retard IE get the 7.5 comp pistons, some bigger injectors, and an upgraded clutch and shoot for 400+ rwhp and live with the lag.