Compression debate between tuners..

How much is too much? Depending on who you talk to, you get totally different answers!

First- I have Probe "10.25:1" pistons- but this compression ratio is based on a 62CC head. I have 64's. I am not sure what my final static compression ratio is because this depends on the head gasket thickness and where the piston sits in the bore. For the sake of discussion, let's assume it's 10.25:1
I'm running a TFS Stage II cam (Brian from AFM told me this is the old AFM B-4 cam)

Tuner #1, who built my engine a few years back, says DO NOT RUN BOOST UNDER ANY CIRCUMSTANCES. He's not a big "blower guy" and specializes in N/A combos. Says cam does not matter either, too much compression is too much compression and that is what I have.

Tuner#2 is a bigtime S-Trim dealer who says that many factors besides static compression ratio determine the final outcome, and says if I follow some suggestions I will be fine. He says he has much more power adder experience than tuner#1 and that they try things like this all the time to find the limits and the ragged edge.

Tuner#3 is well known for all sorts of combos, both N/A and Forced Induction/Nitrous.
He tends to be right in the middle between #1 and #2.

Couple other tangents:
On the subject of cylinder pressure- Tuner#2 said to do a cranking compression test and average out the results. He said if that average exceeds 165PSI, not to run boost. Tuner#1 says that cranking compression means NOTHING and not to listen to tuner #2
Tuner #3 says that they are BOTH right to a degree, that once your cranking compression is above 165, you really can't tell if the motor's static compression is 10.50 or 11.00 or whatever, however he said if my cranking compression, which averages 149 PSI (and is very consistant between all cylinders) that I can run boost if I follow tuner#2's suggestions.

Are we confused yet???

I already have the SQ installed, and right now I have base timing at 10 and have the BTM cranked up to 2.5 and have stuck a 5200 RPM pill in the MSD until I get a chance to sort this out.

Tuner#2 suggested to run an AFM Powerpipe (which I have) and to underdrive the blower with 3.70" pulley. Says this will result in the same boost level as the stock pulley with the crappy stock inlet tube (1994-95's have a very restrictive inlet tube from Vortech) but will result in much better efficienciency and lower the chance of detonation. He said to keep timing in check and use the BTM, run good gas, pay attention for detonation.

Tuner#1 is flat-out against running boost, does not care about tuner#2's experience or knowhow, says I am crazy and not to come crying to him if I melt down my motor (which has forged crank, pistons, and Eagle H-beam rods)

Tuner #3 says he tends to side with tuner#2 and says that since my cranking compression is only 149, it is likely that I do not have 10.25:1 compression and the only way to know is to take the motor apart.

Now in asking this question on other boards, I am told that NMRA and NSCA guys have to deal with pulley size restrictions and what they do is increase compression and tune accordingly. Have had some say that 10.25:1 is not that high compaired to what others have done. Then I read similar critique that tuner#1 gives- that this is a BAD idea under ANY circumstance. Then of course you get guys who then chime in and claim they run higher compression and a lot higher boost than I plan to run.

So by now of course I am totally paranoid to get on this thing and drive it hard for fear of a major meltdown. I have been driving it around and giving it part-throttle blasts and it runs right into the boost, 4-5 PSI but then I chicken out and let off the gas. So far no hint of a rattle or problem.

Looking for thoughtful, experienced replies and am willing to listen.


Heads are small-valve GT40X's with a AFM "stage three" port job (heavily favored exhaust side)
Running 190LPH in-tank and the T-rex. 30lb injectors and FMU, have FMS EFI Extender.

Thanks in advance, great website you guys have here!

Personally IMHO, unless you're not afraid of race gas with the S-trim I'd go with nitrous. With high compresion, and a FMU/supercharger setup sounds like sparknock city to me. I've never dealt with a blower and an engine over 9.5:1 so i have no personal experiance. Sounds like a nice setup you have btw, and welcome to the site.

You can run that compression of 10.25:1 with a blower. However, you will probably be stuck with running race gass (110 octane or higher) all the time and running very little timing to prevent detonation.

I don't think it's a good idea at this point and I thank you guys for the responses...

I'm in the same scenario, 'bout 10.25 CR.
Did anybody explore the possiblity of using a thicker head gasket to lower compression?

1. Using a thicker gasket to lower compression with a Supercharger works great but not so on a 5.0 engine. 5.0 does not have enough clamping force(bolts) and with that much gasket you will blow them like crazy even if you don't have a problem.
2. 10.25-1 not a problem if tuned. You need larger injectors a bigger fuel system and a tuning device to even start to use the Supercharger without negative effects on 93 octane fuel.
3. Short list will get it to work but then it still adds up $2k. PMS,Water/Meth injection, external fuel pump,42lb injectors, NO FMU.
4. PSI is static compression at the end of the day. So 8-1 with 15psi or 10.25-1 with 6psi. Bonus to high compression low boost is your car runs strong at lower-mid rpms because your already building power and at 6psi the inlet temperature is much lower than the blower producing 15psi.
Everyone has a thought or formula but I always like 10-1 with a blower because your power is always on tap then when your ready to rock floor it and that 6psi is going to be 35-40% of pure raw power.

Most Real Street cars run around 10.5:1 compression on 100 octane with 18+ psi of boost. I have never blown a head gasket using a Bob Kurgan tuned PMS making 20+ PSI of boost at 8300 RPM's.

I run a 331 with flat tops and 62 cc heads. I have a SQ with a 2.95 pulley and it all works with a water/meth kit, and cometic gaskets. I drive this car daily on 91 octane I dail 1 degree of retard in. the spout is out timming set aT 30 degrees. At the track I put in 100 octane unleaded and put all the timming in. With out the water /meth kit it would be a gernade, BTW I have 6000 miles on this setup so far.

2stangs,

What kind of compression do you think you're running?
I'm looking into the water/meth kit too, got any pictures?

I run a 327 with forged internals stock block, 10.25:1 comp./I use 93 Octane pump gas with an S-Trim and 15 lbs. of boost. I use the AFM P.M.S. for the tune and I use the A.F.M. Mr. Freeze kit. I used to think with my high compression I could never put boost to it. I was way wrong as I've had no problems, but its all in the tune. BTW: I USE A .074 thick COMETIC GASKET ON A 5.0 motor and I have had no blown headgasket problems. SPEEDYTANG: It depends on the gaskets you use but it is commen knowledge that Cometics work great with the 5.0 motor and with the thick gaskets to lower the compression. You might be right with other gaskets however. Good Luck:)

wickdfast,

So how much did the cometic head gaskets lower your compression, and where and how much were they. How do you computer CR with different head gasket/combustion chamber sizes.


Ed

Well in my case my compression was only lowered by around .25-.40. I blew a head gasket running Nitrous before my blower combo. I trusted the engine builders when they set up my new cam and milled AFR's. I did however double check them using sources on-line. I will try and give you a link but you need to include this knowledge for the equation. #1The type of piston (like for instance mine are Probe flat tops with piston relief's cut in) and around how much volume is milled from the piston. #2 The installed height of the piston.(Does it meet even with the block, just below or above it?) #3 The cc of your heads (You should know that when you buy them, unless they are used and have been milled then you need to put a plate across them and fill it with fluid to find out) #4 The thickness of your headgasket after it had been compressed. #5 The bore #6 The stroke

Good luck!!!!
Here is a pretty good resource for ya!:)

http://www.fordmuscle.com/calculators/compression.shtml

BTW: Shoot me a P.M. if you need some Cometics pretty cheap. I can also get the real thick ones quicker than 2-3 weeks which seemed to be the average time when I got mine.

Don't forget that the Chevy crowd has stock engine comp. ratios at 10:1 and they run superchargers all day long.... I have no experience with their durability though.

I just want to make sure your taking your Compression readings witht he motor at operating temp. Cuz it don't matter unless your rings are sealing which they don't do when the engine is cold.
Im currently building a 331 stroker thats gonna run between 9.5-10:1 CR with a V2 SQ S-strim probably around 6-8 psi. Ill let you know how it goes. LOL

easy fix... i do ALOT of blown cars with 9.5 to 10.5 compression.. its called METH INJECTION.... works like a charm!