Relationship between compression ratio and streetability???

[KrisHorton]

I've been getting mixed answers on this question. One engine builder said that if we run low compression (8.0:1) that the Procharger will have a "light switch" effect and not be as smooth, which would be good for the quarter mile, but crap on a road course. He suggested 9.0:1 and less than 20lbs of boost.

Another builder, namely the one who is helping with our engine, said that we will want lower compression for more street friendly driving and keeping the boost low when not at the track. He is thinking 8.0:1.

Truth be told, I would like to be able to jump in the car and go to the store if I had to. And have it be a nice simple drive. So which ratio would be better and at what boost settings?

I have never driven a forced induction motor before, so take my questions with a grain of salt...I'm still learning.

Hopefully I provided enough info for a good answer! Thanks in advance for any info!

Kris

[camcojb]

I prefer 9:1 or so on a street deal. No reason to give up the part throttle and off-boost power by dropping to 8:1. In a race application that may be different.

You can run 9:1 and 14 psi and double your power. Not saying to run that much boost unless you plan for it (piston coatings, intercooler and/or water/alcohol injection, etc). But you can make a lot of power and with the improved efficiency of a centrifugal 9:1 is where you want to be on the street.

What are your power goals and with what engine and HP (currently)?
Jody

[KrisHorton]

Current setup is a destroked 454 with a 427 crank. We are shooting for about 675-750hp and plan on running about 14-18psi, intercooled. We will be running forged pistons (though are still somewhat undecided on which ones,) AFR heads with a matched intake, a hydraulic roller cam and valvetrain from Engle, probably a set of Crane Gold series rockers, the block is also studded and O-ringed. I think we might shoot for 8.5:1 in the end because we do plan on doing a bit of racing with it, but would like to have plenty of street time with it as well.

I am used to driving a ridiculous motor around though. The 355 that used to be in the car had a really lopey idle and would barely run at idle. I just would like to have something a bit more comfortable to match the interior comforts.

Kris

[camcojb]

Well, I hate to tell you but unless that is a WHIMPY big block, you'll make a heck of a lot more than 650-750 HP with 14-18 psi of boost. I'd say 8-10 psi tops will get you to 750 HP at least. You are way underestimating the power of a good centrifugal. There is also no necessity for an intercooler at that power range, but it's never a bad idea as long as it is efficient enough. Make sure it will flow enough that you don't get a big drop in boost through it, as you're better off with no intercooler than one that is too small/restrictive.

14 psi will basically double your N/A HP, so go from there. I'd expect that 427 with those heads and a decent cam to make over 400 HP.

On the cam, make sure to get a blower specific grind. Makes boost quicker which is a nice thing on the street. No matter what though, even at 8.5:1 compression you will still have all the low end torque you want if you plant your right foot.

If you're going to run o-rings SCE makes a nice coated copper gasket that doesn't seep water. Otherwise, without o-rings I really like the Cometic head gaskets in a blown application. You ought to seriously consider having the pistons ceramic barrier coated; really allows you to run more boost and timing on pump gas. Believe me when I say you may not want more power at first, but you will eventually.

Jody

[TurboLark]

My suggestion is 9-1 with about 10-12lbs for the street. If you want more boost, drop down to 8.5-1, but no lower. Any lower and the response on the street gets sluggish. You gotta remember one thing, just putting around on the street isnt going to build alot of boost. The small opening of the throttle blades will not let much thru. you can run 20lbs of boost if you want to on the street with 87 oct gas, as long as you dont turn much rpm(like shifting at 3000).

[ProdigyCustoms]

Current setup is a destroked 454 with a 427 crank. We are shooting for about 675-750hp and plan on running about 14-18psi, intercooled.
Kris

The D1 will not blow 14 to 18 lbs of wind on that many cubes anyway. Also, O rings can be a pain in the ass to seal. They are famous for water leaks. Be sure to have reciever grooves for the o rings cut in the head (not all shops do this), and plan on a tube of Aluma seal in the radiator for saftey on a o ring street motor.
O rings are old school with Cometics now available. I would do Cometics if I were building from scratch.

[Little Bob]

I suggest you get the book Maximum Boost. I read this book and it taught me alot.

[camcojb]

The D1 will not blow 14 to 18 lbs of wind on that many cubes anyway. Also, O rings can be a pain in the ass to seal. They are famous for water leaks. Be sure to have reciever grooves for the o rings cut in the head (not all shops do this), and plan on a tube of Aluma seal in the radiator for saftey on a o ring street motor.
O rings are old school with Cometics now available. I would do Cometics if I were building from scratch.



How is that? I made 16 psi with one on a 540, and although it may have been nearing it's limit it sure seemed to work for me. That was with heads that flowed 395 cfm intake and 295 cfm exhaust.

Granted an F1 or similar would have been better in my case but they weren't out when I ordered it. I think he can make 14-16 psi without issue.

I also love the Cometics, have them for my twin turbo build. However I ran o-rings on the 540 with the coated SCE copper gaskets and o-rings in the block only; never leaked a drop. I did add a tube of Alumaseal with that aluminum motor. The un-coated copper head gaskets did seep some.



Jody

[ProdigyCustoms]

How is that? I made 16 psi with one on a 540, and although it may have been nearing it's limit it sure seemed to work for me. That was with heads that flowed 395 cfm intake and 295 cfm exhaust.
Jody

Jody

Really? Everything I have been told is F1 is needed for that much boost. Procharger has been recomending F1s for everything we spec.
That is interesting, because I have a 455 we are doing now, and would rather use a D blower, but they are pushing us into a F1, and it is only for 8LBS boost?

I trust what you are saying 100%. I will see what happens when I talk to a different tech guy there. I need to follow up on that order anyway.

[camcojb]

Really? Everything I have been told is F1 is needed for that much boost. Procharger has been recomending F1s for everything we spec.
That is interesting, because I have a 455 we are doing now, and would rather use a D blower, but they are pushing us into a F1, and it is only for 8LBS boost?

I trust what you are saying 100%. I will see what happens when I talk to a different tech guy there. I need to follow up on that order anyway.


If I had it to do over again I'd use an F1 because we were looking at 1000 Hp or more. It wasn't available and the D1SC was the largest that was going to clear the stock hood without me making custom brackets to lower it.

We're making 700 rwhp in a 406 small block at 14 psi. Mine made 828 rwhp with 14-15 psi on the 540 and about 800 rwhp at 14 psi on a 468. I'm positive the F1 would be more efficient but the D1 can definitely handle it. The F1SC is the same body size as the D1SC also which makes it appealing as far as fit, but they do cost more.

There is something to be said for using a blower with more airflow than you need (to a point) and then running it slower. Air temps will likely be cooler and plenty of room to grow. But at 8 psi on a big block I feel the D1SC is plenty.

Jody

[Fuelie Fan]

Not necessarily true. I'm sure you've seen compressor maps. If you are turning a centrifugal compressor slower than it is designed for, you may be operating at a point of lower efficiency, which by definition means higher temps. Yes, temps will be lower at 8 psi than at 14 psi. But, assuming the same outlet pressure, the wheel spinning within it's designed range of speed will have lower temps than either a wheel spinning too fast OR too slow.

[camcojb]

Not necessarily true. I'm sure you've seen compressor maps. If you are turning a centrifugal compressor slower than it is designed for, you may be operating at a point of lower efficiency, which by definition means higher temps. Yes, temps will be lower at 8 psi than at 14 psi. But, assuming the same outlet pressure, the wheel spinning within it's designed range of speed will have lower temps than either a wheel spinning too fast OR too slow.

I agree. What I was getting at is there's a lot of the centrifugal guys going one step larger on the blower and slowing them down; they claim extremely promising results. I've had good luck running the size that I need.

Jody

[Fuelie Fan]

I see, sounds good. Keep the tips coming. As always, back to back tests would be even cooler, if you ever have a customer willing to try out both (or you just get bored!)