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Charter Member
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Discussion Starter · #1 ·
I've had the Coupe on the road for awhile, and have gotten things prety well sorted, tuned, etc. I just completed a 500 mile road trip without a hiccup. The engine runs great. I've got the carb pretty well dialed in, timing is at 36 total, plugs look great. My problem is that the Coupe is quick, but not as quick as it should be. Here's my set up:

347 crate engine, professionally built.
Balanced rotating assy, aluminum pistons, 10.5 compression.
Dart heads
Custom ground cam
Dual plane intake
QFT 750 carb.
480 hp/430 tq. at the flywheel on the dyno

Tremec TKO trans

Rebuilt IRS w/ 3.31 ring and pinion

Nothing in the driveline is dragging or binding. Brakes are not dragging. In straight line acceleration my previous FFR Roadster with around 420 HP 331 was considerably quicker. My (pretty much) stock C5 Z06 is is quicker, and it shouldn't even come close to the Coupe. Any thoughts?
 

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Pro FFR Builder and Moderator
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14,823 Posts
That is too much timing. I would be curious on the cam profile and exactly what intake you have.
Feel free to give me a call and we can talk over your set up.
 

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1,178 Posts
I've learned a lot during Gordon Levy phone calls, I'd take that offer.

I think the guy that built my engine could use 30 minutes on the phone with Levy.
 

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HighPlainsDrifter
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549 Posts
Lazy

Hi,
Your car is fast it probably just feels lazy until you rev it up to 3500 rpm. Alot of cams have little power below that.
I would go with a 3.73 gear to get it up in the rpm range quicker.
Perry.
 

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Junior Charter Member
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2,087 Posts
I have a LS1 in my coupe so it is a little different, but yet exactly the same as it is also a 347. My car is brutally fast with a stock LS1 making 350 rwhp. I have trouble hooking up 315 Kumho XS's until 3rd gear and it goes through 3rd to the rev limiter really quickly (3.55 gears). With that much engine I would not go all the way to 3.73's as 1st gear will become useless and 2nd will be iffy.

Your engine is making much more at the crank than mine so that is probably not it unless something has changed. Here are a few thing I would try:

1. Try a coast down test from 70 to 30 to check for any unusual drag. Compare the times to the same test run in your Z06 as the aero drag is about the same. If the times are close then drag is not the problem.

2. Scheduale some time on a dyno. Verify just how much loss there is from the FW number you already have.

My guess is that you are seeing a huge loss from the Coupe sidepipes. They are terrible for making power. I had my passenger sidepipe slip partially off during an autocross run and I could immediately feel the additional power. The time on that run also showed the power gain as I picked up 1.5 seconds from the previous run. The standard loss for most engines is 60-70 hp due to the sidepipes, but with the big numbers your engine is making I think you might be seeing much more than that.

I no longer use my sidepipes so I tried an experiment for you. I stood the pipes on end and dropped sockets through the pipe until I got to a size the won't drop through. That socket measures 1.17" OD.
 

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997 Posts
Discussion Starter · #9 · (Edited)
Thanks guys! Gordon, I may take you up on that. Cam specs:

236/248 duration @ .050; .579/.579 valve lift; 108 LSA

I've run it at more retarded timing but this setting seems to produce the best. The timing mark on my spark plugs is mid-bend on the electrode. This is a very similar engine to my Roadster, but with a tad more HP and a broader torque band. The dyno numbers on this engine are above, so I'm a little stumped. With this set up, 0-60 ought to be in the high three/low four second range and my estimate is that it's right around five.
 

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Charter Member
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997 Posts
Discussion Starter · #11 ·
The pipes were what I was thinking of a few months ago. I bought an extra set of sidepipes that I took to an exhaust shop, had the mufflers removed and equal size tubing welded in place with no packing or core of any kind, just an open tube. I can swap pipes out in about ten minutes. Definately better with open pipes (ear plugs are mandatory!) but still nothing close to what it should be.
 

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HighPlainsDrifter
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549 Posts
tko trans

Hi,
The tko has a 2.87 or 3.27 1st gear ratio.
If you have the 2.87 gear then your 0-60 times will be slow, that's 1/2 a gear disadvantage in acceleration, 1/2 a gear to your 3.31 to 3.73 would be alot better.
Perry
 

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Junior Charter Member
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249 Posts
The group is correct on restrictive side pipes. I blew both of mine off just goofing off out front of my house.
I cut off the inlet tubes and made a single muffler that kind of resembles the original twin.
I did add a couple baffles to cut down the un-real noise at a higher rpm. They were 114db @ 4000rpm measured at 25 ft. 117 db for an extended time will cause permanent hearing loss. I totally agree on the ear plugs unless hearing is not a concern.
I'm still concerned about the amount of advance, maybe you have gone past the point where the distributor cannot change from advance>retard? I don't run any more than 32 and my engine is wild.
Have you checked the plugs for color yet?
Jeff
 

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Discussion Starter · #16 ·
Just when I get stumped (and I have to admit that's happened a fair amount with the Coupe ) you guys come to the rescue. I decided to re-check my timing after some of the suggestions and found that it was way too far advanced. The hold down on the dist. was only snug and the dist. had rotated. I re-timed it to 34 degrees total and it's considerably quicker now. Still not quite up to what I expect, but I've still got a little bit of fine tuning to do on the carb. THANKS GUYS!
 

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poor acceleration

I'd loose the 3:31 gear ratio and go with a 3:55 or 3:73. It will get the engine into its power band quicker and will make a big difference in how it pulls. I build a decent 306 and the car had 3.08 gearing. Motor didn't pull well at all. Swapped for 3.73 and it was night and day. Good luck!
 

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Junior Charter Member
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249 Posts
Lots of great info here.
I gotta agree w/most everything said.
First, loose those side pipes. I made a set of collectors that took the two 2" headers into one 3" pipe and bought 2 cobra pack mufflers from Eric Sauls @ Classic Chambered. Much nicer sound and the car feel better.
I have the Tremec 3550TKO and I'm not sure of my 1st gear but I have the 3:73 gear set. Even with that gear set, my 1st gear is not too short. No problem hooking up with Goodrich G-Force KDTAs 315. I hear they don't make this tire anymore, what a shame.
Those are some good HP & TQ numbers. Could it be that you may not be noticing the speed because your in the coupe vis roadster?
Let us know if you go w/Gordon's suggestions.
And good luck.
 

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Charter Member
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997 Posts
Discussion Starter · #19 ·
Lots of great info here.
I gotta agree w/most everything said.
First, loose those side pipes. I made a set of collectors that took the two 2" headers into one 3" pipe and bought 2 cobra pack mufflers from Eric Sauls @ Classic Chambered. Much nicer sound and the car feel better.
I have the Tremec 3550TKO and I'm not sure of my 1st gear but I have the 3:73 gear set. Even with that gear set, my 1st gear is not too short. No problem hooking up with Goodrich G-Force KDTAs 315. I hear they don't make this tire anymore, what a shame.
Those are some good HP & TQ numbers. Could it be that you may not be noticing the speed because your in the coupe vis roadster?
Let us know if you go w/Gordon's suggestions.
And good luck.
Good timing in bringing this post back up Jeff! I was still running a tad too much timing according to my plugs, so I backed it down 2 more degrees. I also installed a vacuum secondary carb. I knw there's a strong perference for DP carbs on these cars, but between the Roadster and the Coupe, I've experimented with several carbs and found a vacuum secondary to be better IF I took the time to tune it for my car. Usually, that means getting the secondaries to open on time and smoothly (earlier) and finding the right spring. I'm happy to report that the Coupe is now a rocket compared to my Z06, and traction has become a much bigger issue. :)
 

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Junior Charter Member
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249 Posts
Great to hear about the traction issue. Means its running strong.
I had some issues at the track and dumped my Barry Grant 750DP for the new Holley Ultra 750DP. WOW, if the carb was a female, I'd marry it. Just fabulous.
I also had an issue w/MSD 6al box, when I got to the 7k chip it didn't recover and stayed in a soft rev control mode. I tried several different fixes and none worked. I dumped that box for the 6al digital/programable. Locked the distributor at 32 and made the retard curve as close as I could to what I felt in the cam.
Again, WOW. Now I have an engine built for "track Only" and its MUCH more drivable on the street. I programmed in 15deg adv for start and it eases up to full advance @ 3500.
Now if it could only correct the fuel usage problem?
Jeff
 
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