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Old 09-10-2010, 11:13 PM
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377 + rx7

Hey Guys Im Steve an Im new here.

I have a race car build going on, I have a 89 rx7 gutted and were going to fo road racing in it.
The car has 4.30 gears and may get a muncie 4 speed..

We have a 377 build going on but I wanted to check a few things before we put it together..

so we have heads and valve-train to handle 7krpm, world heads ported big valves roller everything solid roller cam not specd out yet, a virgin 400 block and a torker 2 manifold and a 700-750 holley Dp.
We all ready have a cast crank and rods(unknown type) I think they are pink rods, and cast pistons or hypers(undecided) It will be balanced.
My questions are can I spin it 7k? I know some circle track guys do and they're limited so why cant I? Do I run stock 5.7inch rods and stock 400 pistons? or do I have to get a special piston? I thought eh diff in rod length and stroke is what made the difference?

Thanks again.. -Steve

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Old 09-11-2010, 12:21 AM
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So you want to use a 350 crank in a 400 block, with a standard bore this makes 372 cubic inches. We did this build many years ago. 350`s and 400`s share the same compression height pistons, so a 350 rod is used with a 400 piston. The idea behind this combo is to take advantage of the better breathing of the 400`s larger bore size, so from 3000 RPM and up it has improved power. To use the 350 crank in the 400 requires spacer bearings since the 400 block has larger main bearing saddles. However, we`ve ran them all, 350, 372, 383 and 400. Hands down the 400 made them all look bad. If It were me, I would go straight 400, use 350 rods, and get forged pistons for the 400 for use with a 350 rod. You won`t be able to rev it like you would the 372, but the gain in cubes and torque will more than make up for it, believe me. We drag raced and mud raced the engines I mentioned, the 400 dominated in both. I also wouldn`t use a torker II intake. If I was building a high rev 372 I`d use a victor Jr. If I went straight 400 I`d use a performer RPM air gap.
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Old 09-11-2010, 12:21 AM
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Piston compression height for a 377 with 5.7" rods is 1.560", same as stock 400 dimension, so yes you can use them. I would recommend forged pistons for reliability in a track car at those rpms, not hypers or plain cast. A stock 5.7" rod will be fine once ARP rod bolts are added and the big end bore resized. 7K rpm is no problem with forged pistons and good rod bolts.

The Torker II manifold is about the worst manifold you could pick, just about anything would be better.
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Old 09-11-2010, 06:37 PM
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Thanks for the advice guys, the main reason were not going to the 400 is this car weighs 2500-2600 and has 430 gears and I dont want to make it harder to drive by having more tq than I need down low. Also with gearing in mind, I can stretch it farther with the higher revs of a 377 along with that a 377 has a better rod ratio so the piston speeds are lower and causes less wear vs the 400.
I looked up the victor manifold.. should I do a victor jr.? I need the hood clearance. also how about KB forged should that be stout enough? thanks again guys.
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Old 09-11-2010, 07:21 PM
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Victor Jr is a far better choice than the Torker II. Khe KB/ICON Forged piston is a good value for the money, as good or better than the TRW/Speed-Pro forgings. Good choice for your intended build.

When you get to the point of buying bearing spacers and bearings, look at King Bearings. There is a source selling the King spacers on Ebay for around $45 a set, at that price I bought two sets a while back just to have them in the shop even though I'm not currently building a 377, just planning for the future. The King bearings are a real nice bearing, better than Clevite IMO, better finish and sizing and better materials.
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Old 09-11-2010, 10:42 PM
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Well, building an engine around a gear ratio is one way to go about it. Not how I would choose to do it, necessarily- but then it's your coin and your deal.

But in the real world, the 400 w/5.7" rods can last as good as any high-winding 377 engine- because the high revs are not needed to make good power. But then, you'd have to get the rear gear ratio in line w/the engine's power band.

BTW, the rod ratio is not what makes the piston speed faster or slower- that's caused by the length of the stroke. Rod length affects the acceleration of the piston, but the mean piston speed is still a function of stroke length.
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Old 09-12-2010, 06:51 AM
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I can understand your gear ratio issues. mostly likely not a lot of choices when dealing with a rx-7.

I did a v8 conversion in a 240 z. (2500 lbs)

a 2 bolt 350 with a forged crank (360cid), 200cc dart iron eagle heads, zero deck block, ,11:1 cr, compcam 292H, rpm intake, 750 3310 holley, full length headers, GMT5 5 speed trans from a V8 camaro, 3.90 gear (R200).

the engine pulls hard to 7000 rpm and the valves float at 7200. runs 11.7's at 120 mph and 10.7's at 135mph with a 175-200 hp shot NO2.

the engine needs a rebuild after 10 years of beating the hell out of it.

I would recommend you build a regular 350 for your project and use a manual trans with an overdrive. good heads and the proper valve springs are key.

Team G makes a short single plane intake for extra hood clearance. I tried the short TeamG, torker 2, and the RPM intake and the rpm did the best on my car, then the team G, then the torker last. however, the torker was the shortest. team g make 3 or 4 different heights.
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Old 09-12-2010, 08:23 AM
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thanks guys, I agree with building an engine around a ratio not my cup of tea either..
The reality of this build is most all these parts were cheap or free and im trying to make the best of it, we had the 400 the crank was toast, my buddie who owns a shop and is sponsoring us had all the parts for the 350 except pistons and a block so were all like lets build a 377! kinda fits perfect considering its a little revvy car stock.. anyway I appreciate all the help, I will post a bunch of pics when I get the car to my house. Also were gonna run a 4 speed due to how small it is and simplicity.
Thanks -Steve
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Old 09-12-2010, 08:27 AM
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Quote:
Originally Posted by cobalt327
Well, building an engine around a gear ratio is one way to go about it. Not how I would choose to do it, necessarily- but then it's your coin and your deal.

But in the real world, the 400 w/5.7" rods can last as good as any high-winding 377 engine- because the high revs are not needed to make good power. But then, you'd have to get the rear gear ratio in line w/the engine's power band.

BTW, the rod ratio is not what makes the piston speed faster or slower- that's caused by the length of the stroke. Rod length affects the acceleration of the piston, but the mean piston speed is still a function of stroke length.

Thanks Cobalt I didnt know that..hmm I figure with the tires were going to run with the 4.3 rear well be running 130mph at 7k I hope thats fast enough for what were doing... I cant imagine needing more for short course. the bad thing is these cars only come with the tallest gear 3.73 and with a short tire still isnt tall persay....
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