Compression Ratio for a 347 Stroker w/ Vortech S-Trim

cro699 · Dec 12, 2005

I just purchased a set of Trick Flow ported and polished Twisted Wedge heads and a Vortech S-Trim for my 93 GT Convertible. I have the chance to purchase a 347 stroker short block. It has flat top pistons with valve reliefs. The seller said this should make about 9.5:1 compression and will work with my heads and supercharger.

What I need to know is: Is this the right compression ratio for a 347 with Flat Top pistons and even if not, will this 347 work well with my parts. I don't want to spend the money to buy this if the compression ratio is going to be too high for me to use the Vortech supercharger. Thanks!

Rick 91GT · Dec 12, 2005

Most flat tops in a 347 will yeild 10.5:1 or higher. (SEE BELOW, those numbers were with a normal 4cc flat top, and I used a 61cc chamber which is a stock TFS)

What cc is the flat top, what brand or pt#, are they cut for the Twisted valve?

Bore Size: 4.030
Gasket Bore: 4.100
Piston Top Land Dia: 3.995
Stroke: 3.400
Connecting Rod Length 5.400
Compression Height: 1.100
Deck Height: 8.200
Ring to Top of Piston 0.250
Gasket Thickness: 0.040
Valve Notches Volume: 4.00
Dome Volume: 0.00
Volume Head: 61.00
-------------------------------------
:COMPRESSION RATIO: 10.532
-------------------------------------

calculation variables:

Piston to Deck: 0.000
Crank Throw: 1.700
Vol. Above Top Ring: 0.90 cc
Piston to Dec Vol.: 0.00 cc
Gasket Volume: 8.65 cc
Volume Clearance: 74.56 cc
Cylinder Volume: 710.69 cc
CID: 346.815

cro699 · Dec 12, 2005

These pistons are speed pro. The seller told me they are flat tops with 6cc valve reliefs. He says they will work with my heads and I will still be able to use the supercharger. But, I was kind of skeptical because my research showed a 10.2:1 or higher compression ratio. He says it will only be 9.5:1, but this is the first motor I'm building so I don't want to just listen to what someone else says and buy something I can't use.

Thanks for the help!

cro699 · Dec 12, 2005

Also, I have a block out of a 93' Cobra. I've researched 347 stroker kits for around 8.5:1 compression ratios. They are around 1000 to 1300. If I were to buy one of them, how much does it usually run to have a stock cobra block machined for this kit. Thanks!

By the way, the block has 60,000 original miles on it and ran perfectly when I pulled it out of a totalled car.

IntenseBlue · Dec 12, 2005

How much boost are you going to run?

90mustangGT · Dec 12, 2005

Get a methanol kit and don't worry about it. I don't know this for a fact, but a 347, P+P TW's, with a good intake, cam, and exaust your looking at near block splitting power as is, 8-9psi, your going to be really putting some work in on the stock block. Be sure to have a good tune on it, high or low compression.

65ShelbyClone · Dec 12, 2005

I would run either the N/A 347 or the S-Trim, but not both. A 9.5:1 347 with TWs and supporting parts wont leave you alot of room for boost. I personally would put the TWs on the Cobra shorblock and boost it.

cro699 · Dec 13, 2005

Thanks! I'll just use the block that's in the car now as it only has 2500 miles on a complete rebuild. It's a 306 with all new internals. That way I can keep my cobra block as a spare incase I blow up the 306.

The person I bought this Vortech S-trim off of said it was putting out around 12 psi on his car. So, with the TW Trick Flow's and the 306 I have, will the compression be low enough to run this S-trim.

These heads are ported and polished and the bottoms have been milled.

Rick 91GT · Dec 13, 2005

Really depends on the pistons that are in that motor....

TheUser · Dec 13, 2005

cro699 said:
Thanks! I'll just use the block that's in the car now as it only has 2500 miles on a complete rebuild. It's a 306 with all new internals. That way I can keep my cobra block as a spare incase I blow up the 306.

The person I bought this Vortech S-trim off of said it was putting out around 12 psi on his car. So, with the TW Trick Flow's and the 306 I have, will the compression be low enough to run this S-trim.

These heads are ported and polished and the bottoms have been milled.

I'm not so sure I'd want to run 12psi on a daily driver. You should be able to get a different pulley and knock it down to 6-9psi, which would probably be plenty for the street; I'd say w/ those heads, appropriate intake and the s/c, you'd prob. be pushing around 350 w/ 6psi...does this sound about right guys?

Again, I'm not saying you CAN'T run 12psi, I'm just saying that's quite a bit...

cro699 · Dec 13, 2005

This car isn't a daily driver. I only take it out on days when the weather is nice or to cruise in the evenings.

So far, it is as follows:

Rebuilt 306 with 2500 miles, E303 cam, ported and polished factory heads with 1.7 roller rockers, Trick Flow Upper and Lower intake, Ford Racing 24lb. hr. injectors, Holley Throttle body, Granatelli Mass Air, BBk Cold Air intake, BBk Shorty headers, H-pipe, flowmaster exhaust, 190 lph fuel pump, aluminum Ford Racing radiator, subframe connectors, 3.73 gears, new 3650 trans, eibach lowering springs, custom paint, rims, new convertible top, new leather, Saleen wing, and drilled rotors.

Currently, this car is running 12.7-12.8's on just the motor and sounds pretty bad ass. But, now I purchased these trick flow heads and the Vortech. I would like to see low 11's or high 10's if possible but still be able to cruise a little so it has to be somewhat streetable.

It's pretty fast now for a convertible, but I've owned five Trans Ams in the past and I still have two, a 2000 Ram Air and a 1988 GTA. So, it has to be faster to compete because the RAm Air is running 10.5's.

65ShelbyClone · Dec 13, 2005

cro699 said:
Rebuilt 306 with 2500 miles......(cut)...

Currently, this car is running 12.7-12.8's on just the motor and sounds pretty bad ass. But, now I purchased these trick flow heads and the Vortech. I would like to see low 11's or high 10's if possible but still be able to cruise a little so it has to be somewhat streetable.

Hell, it looks like the only thing that 306 doesn't have is aftermarket heads. I wouldnt be surprised if the TWs get you another 75hp and the blower probably 100+ @ 8psi. You'll need some bigger injectors if you want 500fwhp, though. You probably have ~275+rwhp already. Get the heads and S-Trim on there with a lower boost pulley and see where you stand.

I went to the Friday night drags last week and there was an 87-93 LX with what must have been an Incon kit. He couldnt hook to save his life(entire track!) and was doing low 12s at 128mph. Thats ~525rwhp. Hope you have slicks

cro699 · Dec 13, 2005

I'll get the heads installed and the supercharger and see how it runs. I'm going to need to upgrade to 30lb injectors though won't I? Also, any suggestions on a cam. I have the E303. Should I upgrade that or should I just leave it alone?

Wow, that car must've been spinning a lot to only run a low 13-high 12 at 128. He should be much faster with that trap speed. I think my car will be a little better hooking up. Being a convertible, it's slightly heavier and the weight distribution is better. I'm still going to buy some really sticky tires though. Any suggestions.

65ShelbyClone · Dec 13, 2005

I would expect 30s to be suitable, but personally would just go a little higher and get 36s for the extra headroom. For the cam, I think the E is alright seeing as how its a smog legal stick(usually limited overlap>better for F/I). Even if you had the same setup with a stock cam you'd be able to split the block.

I got that ET wrong; I just looked at my video and it was low 12s, but still very high 120s. I was thinking of the earlier runs where he didnt get into it until half track. Seriously, that guy would start spinning as soon as he got into boost(just past the tree) and it wouldnt stop until after the traps, even when he was pedaling it.

cro699 · Dec 13, 2005

So, I should just go ahead and order 36's instead of buying 30's. I haven't ordered either yet, so if 36's would work better I'll buy them. At the moment, it runs a little rich with the 24's.

As for the cam, I'm not worried about it being smog legal. I run dealer tags on it so I don't have to get it inspected or pass smog tests. So, if there is a better cam for my combo, I'll gladly upgrade to it. But, if it will be just fine with the E cam that's in it, then I'll save the money and stick with it.

The car sounds pretty awesome. Is that video on here anywhere. I'd love to see it.

$uperstang · Dec 14, 2005

If I were you I would run with 42lb injectors and eliminate the FMU, You will need the T-Rex inline pump as well to make it pump good fuel. With the 36's injectors you will more than likely still need the FMU.

cro699 · Dec 14, 2005

So, just use the 306 engine I have now, add the heads and supercharger, and then install 42's instead of using the FMU? I want whatever will work best and still allow it to be somewhat streetable.

Rick 91GT · Dec 14, 2005

What is the comp ratio of the current motor..... you need to know that first.

cro699 · Dec 14, 2005

The current motor is a stock rebuild bored .030 over. I'm probably going to either have to cut the proper valve reliefs in my pistons or replace them. I've been thinking of just replacing them with a set of dished ones. The Twisted Wedge heads I purchased have been milled and I've been told I will have clearance issues with the 2.02 and 1.60 valves.

So, if I'm going to need to cut valve reliefs in these pistons, I'll probably just buy new dished pistons and get the proper compression ratio for this set up. I've been thinking 8.5:1 Would this be appropriate or should I go a little higher?

Thanks!

65ShelbyClone · Dec 14, 2005

The stock 9:1 is about as high as you'd want to go on pump gas w/boost. 8.5:1 is a good number.

Oh yeah, turbo Fox3: http://www.filecabi.net/u.php?file=1134586169.wmv

Shot by me at what some may recognize as LACR.

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