lock

Sorry for such a basic question. Im about to build a new 355 shortblock and plan on installing a turbo in the near future. Going all forged and now is the time to lock down my compression ratio.

biggtime

not enough info I run anywhere from 8.8 to 9.5 depending on the vehicle / its use / boost /intercooled /fuel and on and on........

lock

Mainly a weekend cruiser and several trips to the dragstrip each year. Plan on running 93 octane. Amount of boost? Well that depends on how it plays out. Im guessing 9.0 is a safe target to shoot for then? ... and I have aluminum heads on top.

83 Crossfire TA

iTrader: (2)

As low as you can possibly tolerate off boost… boost makes more power then compression any day. At some point I intend to build something in the 6.5-7:1 range and throw all the boost I can find at it. FWIW, there have been a few oem turbo 4’s in the 7.5:1 range that were plenty driveable and there were some NA V8’s in the 70’s that were in that range also and were tolerable…, so it can be done without suffering too badly. Hell, a lot of the 305’s out there as well as all the 2500 and 3500 truck 350’s through the 90’s and late 80’s were in the mid/low 8’s…

anesthes

iTrader: (12)

Problem with running soooo damn low for static is, by the time his turbo spools up about 10 seconds will have passed.

I wouldn't go lower than 8.5:1 for the turbo.

-- Joe

Doward

Yep - 8:1 to 8.5:1 is best for a street setup.

83 Crossfire TA

iTrader: (2)

Huh, I guess things like the sunbird GT and assorted saabs were totally unstreetable with compressions in the mid 7’s… and I guess the turbo porsches (some got down into the 6.5:1 range) never actually saw boost but somehow won races anway.

anesthes

iTrader: (12)

Are you trying to be the devils advocate or something Mark?

I don't think he's gonna be running enough intercooled boost to justify 7:1 compression ratio. 8.50 should be just peachy with just about any turbo he finds.



-- Joe

biggtime

Just to cause some trouble. YES the sunbirds and saabs were unstreetable. IMO I think now they are all in the crusher.

IANTI

I have a spare 305 and i am planning a turbo project also and I was wondering if there was any formulas that could calculate optimal compression?

KiLLJ0Y

just from experience and my Typhoon being stock:

Stock Typhoon is 8:35:1 pushing 14.7 lbs max boost. using a 2 Bar.

i have sense gone to a 3 bar and running 17lbs of boost.

in stock form, the turbo doesnt have much lag, course the lag is dependant on the size of the turbo itself and converter you use.

but i think 8:35:1 is probably the lowest you want to go.


i honestly dont know why more people dont build the 4.3 V6 for the turbo. i see everyone using the 3.8 and i wonder why. the 4.3 has a 4.00 inch bore which is bigger than the 3.8. its basically a 350 with 2 cylinders hacked off.

anyways im getting side tracked. sorry

anesthes

iTrader: (12)

Cuz the 3.8s are faster.

Its like the "whydont people put SBC's in hondas?" well, if most honda's didnt beat thirdgens, I supose they would.

-- Joe

TTA850

The GN's and TTA's used 8:1 compression stock. I forget what the GN's ran as far as boost but the TTA's were set to 16.5 stock.

I have just under stock compression in my motor(something like 7.8-7.9:1)and run 19-20psi intercooled boost on 93 octane with no detonation and the car is very streetable.

HTH,
Steve

83 Crossfire TA

iTrader: (2)

Huh… compression in the 7’s and only 66% the cubes… I bet your car is totally unstreetable, huh? No low/midrange, right?

B4Ctom1

A turbo porche is like 11:1 but it has a totally different combustion chamber and quench.

anesthes

iTrader: (12)

I think 8.50 is reasonable for a turbo SBC.. For the centrifigul, my setup will be between 9.14:1 - 9.30:1 (depending on what I do to the chambers, leave 'em at 62cc cast or polish to 64.).

I'm intercooled, and about 10-12psi with a vortech S-Trim.

I figure, if you can run 6-8psi on a non intercooled, 10.5:1 LT1, then I can run 10-12psi on an intercooled alum headed 9ish:1 SBC.

-- Joe

Dustin Mustangs

iTrader: (1)

That's funny, I really hope you were joking. All else being equal, the 4.3 will be faster. I realize that more people are laying down fast times with the 3.8, but that's becuase more cars came with them and a turbo from the factory so that's what the majority of people run. I'm also wondering what hondas you are talking about. The fastest one I've seen at the strip was modded to hell (turbo kit along with other things) and running a 14.9. He claimed it was supposed to be good for a 14.1; woo-hoo, here's a cookie. I also know of numerous hondas in the 16's and 17's. If you've moded your sbc thirdgen to a 14.9 then you need to sell it and go buy a honda with some altezas aand neon. I can think of a half a dozen real reasons why folks don't run SBC's in honda's, none of them being how fast they are in comparison to a thirdgen.

Man this turned into quite the rant. Sorry, I guess I just get annoyed when people take an econo-car and try to make it into a race-car and in the end they end up with neither.

Oh, and there is no optimal compression ratio for a SBC's. Way to many variables and differnt applications for there to be one magic number. In the area of 8.5:1 certainly is a popular choice though.

anesthes

iTrader: (12)

No, I was not kidding. I don't like the 4.3..

Quote:

I'm also wondering what hondas you are talking about. The fastest one I've seen at the strip was modded to hell (turbo kit along with other things) and running a 14.9. He claimed it was supposed to be good for a 14.1;

Something wrong there. Buddy of mine owns a race shop. All he does is honda. I race with a lot of his customers.. 11 and 10 second cars all day long. Few "lightly" modified cars in the 12s.

Quote:

woo-hoo, here's a cookie. I also know of numerous hondas in the 16's and 17's. If you've moded your sbc thirdgen to a 14.9 then you need to sell it and go buy a honda with some altezas aand neon. I can think of a half a dozen real reasons why folks don't run SBC's in honda's, none of them being how fast they are in comparison to a thirdgen.

Man this turned into quite the rant. Sorry, I guess I just get annoyed when people take an econo-car and try to make it into a race-car and in the end they end up with neither.

So you think our heavy turds is better? I guess you havn't owned as many for as long as I have. Every day I wake up and consider taking a torch to the car and buying a real race car. But then I think of all the fbody specific parts I bought to make it think it could hook like a stock mustang, I throw up, and go on with my day.

Quote:

Oh, and there is no optimal compression ratio for a SBC's. Way to many variables and differnt applications for there to be one magic number. In the area of 8.5:1 certainly is a popular choice though.

True.

-- Joe

Dustin Mustangs

iTrader: (1)

Boy would I be interested to see something like this backed up with real numbers and specifics on the build(s). Certainly the lightly modded 12s hondas. You must mean 1/8 mile, right?

anesthes

iTrader: (12)

I don't understand whats so hard about this? Lighter car, more efficient engine, 300 hp = mid 12s.

Yet kids (90% of tgo) with heavy lead sleds and a modded TPI think they're the bomb.

-- Joe

IANTI

I wonder what lightly modded is defined as?

Dustin Mustangs

iTrader: (1)

What's the fastest honda run stock?? Mid to low 15's (sorry, I'm not even considering the s2000) when they're driven right, which most 'tunners' will struggle to pull off. I don't care what car or how light it is, you're not taking 2.5++ seconds off an ET with 'minor mods' unless your dropping the thing off a cliff. By the way, this:

is not what I ment by backing up your claim with real numbers. Somehow I am not suprised.

Also, I'm not naive enough to think third gens are the king of the streets. Maybe that means I don't fall under your 90% of tgo ASSumption. I do think that they are a much better starting point for something that can hold it's own at the track when compared to a 30mpg 4-banger fwd soccer mom compact. That's just my opinion, and I don't think I am alone as far as this board goes. I would imagine you are, which is fine, but if I where you I'd sell my 'turd gen' and buy my little sisters 1.6L civic so I could sleep at night.

KiLLJ0Y

the only reason they are faster is because, look at the car they come in.

GM used the 4.3 because it needed more power to push the trucks/SUV's because of weight.. the Ty weighs 4800lbs. i guarentee if you put the typhoon motor into a TTA or GN it will be faster.

GM completely underated the 4.3 in power and torque. if you go over to the www.syty.net board you can see what im talking about. there are guys there that have dyno'd the truck that was all stock and made 335hp and almost 400lb/tq (GM rates them 280hp and 345lb/tq)

push a 4800lb truck to 14.1 stock is pretty quick.. the engine responds to mods like a madman.. 12's are easily done with just tuning, a fuel pump, and a mild chip, filter, ect. no major engine mods at all.

on that board check out "recipee for 12's" its all you need.


BUT i will say that the 4.3 upper and lower intake manifold are extremely rare to find, no one makes aftermarket replacements. the throttle body is just an L98 48mm.

anesthes

iTrader: (12)

OK . I've been doing this too long to argue with you. Nice homepage by the way.

-- Joe

anesthes

iTrader: (12)

You check ebay? I saw a ton of chinese made sy-ty manifolds when I built my singleplane.

-- Joe

KiLLJ0Y

you just said it yourself.. Chinese.. which i may add not just because they are chinese, but are horribly built, usually warped.

i dont personal experience, but heard and seen from posts, people that have.


ive never seen one on ebay though, i'll start looking around, the design on the syty intake was pretty good, the faster trucks have switched to a single plane like you have. im keeping my close to stock, these trucks are way too tempramental to screw around with.


anyways im off the topic, sorry.

biggtime

a little off topic but want to clear up a cpouple of things;
I own a Typhoon and a Syclone so I know a little.

Typhoons weigh 4100 (my 93)
Syclones weigh 3750
There is no aftermarket 23* manifold made that will bolt onto a SYTY. The reason I made one for mine is I am using vortec style heads.
They did not use the buick engine in them as there was not enough room to clear the turbo placement, besides it was put together at PAS (privately held company)so it had to be a bolt in.

I have never seen a "slightly modified" honda go any faster than a 14. We have a local guy who has a totaly gutted civic with a v-tec engine and a fogger and he only musters 12, 0hs with slicks.

Now back to our regularly scheduled topic.
Automatic trans, Street, pump gas, intercooled 8.5
Automatic trans, Drag Race, race gas, Trans brake, A/w intercooler 9.5.
Manual trans, drag race, alky, A/W intercooler 7:1
Yes lower compression and more boost will make more power but with a automatic it will never make enough power to spool up the turbo to launch.
JMO

83 Crossfire TA

iTrader: (2)

What Porsche is that? Last I checked the 911, 930, Carrera… turbos all had 7:1 from sometime in the mid 70’s through about 90 and 6.5:1 prior to that. Most race Porsches, 968… don’t go higher then 6.5:1… Of course, what did old Dr Porsche know…

There are quite a few newer turbo cars running much higher compression then that, but most clearly were not designed as an all out performance setup and the turbo is more of a little bit extra at the top end rather then how the thing actually makes power across the power band. Some of the newer Porsche turbos had as high as 8’s compression but it dropped quickly as you looked at performance specific models (like the “s” models).

B4Ctom1

Im trying to think of which model. We were using the book to set the cam timing, I read it right out of the porsche book. Its hard to remember because it was 10 years ago. It was a model with the engine in the rear (VW style). I remember how it listed the turbo compression as 11:1 (which I thought was riduculous) and some snooty remark about that it was that high due to the superior combstion chamber design, but other that that I don't remember much.